Optimization of Rope Sheave Groove Profile to Improve ...

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Optimization of Rope Sheave Groove Profile to Improve Synthetic Rope Lifetime Bachelor’s thesis Riihimäki – Mechanical Engineering and Production Technology Autumn 2020 Joseph Ransom Kallio-Myers

Transcript of Optimization of Rope Sheave Groove Profile to Improve ...

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Optimization of Rope Sheave Groove Profile to Improve

Synthetic Rope Lifetime

Bachelor’s thesis

Riihimäki – Mechanical Engineering and Production Technology

Autumn 2020

Joseph Ransom Kallio-Myers

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ABSTRACT Mechanical Engineering and Production Technology Riihimäki Author Joseph Ransom Kallio-Myers Year 2020 Title Optimization of Rope Sheave Groove Profile to Improve

Synthetic Rope Lifetime Supervisor Esa Murtola

Abstract The aim of this thesis is to determine whether an optimized rope sheave groove profile can improve the lifetime of a synthetic rope. Synthetic ropes have begun to see use in hoisting applications as they offer multiple benefits. Over the course of its lifetime a hoist’s rope will be replaced several times due to wear, it is important to maximise the lifetime of the rope to provide the greatest efficiency for the hoist, to improve environmental sustainability and most importantly to provide the greatest safety. The rope sheaves of a hoist represent a good opportunity to improve the rope lifetime as they are one of the main points of interaction between the load and the rope. Due to the difference in properties and behaviour between a synthetic rope and a steel rope, analysis needs to be carried out to assess whether the groove profile of the rope sheaves can be updated to optimize the rope lifetime. This work begins with a study of the interaction between the rope and the rope sheave from the wider knowledge base of steel ropes, as well as investigation into existing standards, patents and guidance related to the use of synthetic ropes in hoisting applications. Based upon this research several test pieces were selected and subjected to cyclic bending over sheave (CBOS) testing. The results of the testing show a small variation in rope life between different sheave designs and that the rope sheave groove profile does effect rope lifetime. Through further testing and refinement of the groove profile design more significant improvements could be achieved.

Keywords Groove profile, increased rope life, rope sheave, synthetic rope Pages 35 pages

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CONTENTS

1 INTRODUCTION ........................................................................................................... 1

1.1 Goal ..................................................................................................................... 2

1.2 Purpose................................................................................................................ 2

1.3 Objectives ............................................................................................................ 3

2 KNOWLEDGE BASE....................................................................................................... 4

2.1 Causes of Rope Wear .......................................................................................... 4

2.1.1 Contact Pressure...................................................................................... 4

2.1.2 Tensile and Bending Stresses .................................................................. 5

2.1.3 Cyclic Bending Fatigue ............................................................................. 6

2.1.4 Pinching ................................................................................................... 6

2.1.5 Internal Abrasion ..................................................................................... 6

2.2 Current Steel Wire Rope Groove Profiles ........................................................... 7

2.2.1 U-Groove ................................................................................................. 7

2.2.2 Undercut U-Groove and V-Groove .......................................................... 7

2.3 Sheave Factors Affecting Rope Life ..................................................................... 8

2.3.1 Groove angle ........................................................................................... 8

2.3.2 Groove Radius.......................................................................................... 8

2.3.3 Sheave Diameter ................................................................................... 10

2.3.4 Sheave Material ..................................................................................... 11

2.3.5 Groove Edges ......................................................................................... 12

2.4 Rope Factors Affecting Rope Life ...................................................................... 12

2.4.1 Rope Construction ................................................................................. 12

2.4.2 Rope Break Strength .............................................................................. 14

2.4.3 Lubrication ............................................................................................. 14

2.5 Synthetic Rope .................................................................................................. 15

2.5.1 Background of Synthetic Rope .............................................................. 15

2.5.2 Benefits/Drawbacks of Synthetic Rope ................................................. 15

3 PLANNING AND REALISATION ................................................................................... 17

3.1 Method Overview ............................................................................................. 17

3.2 Apparatus .......................................................................................................... 17

3.3 Requirements .................................................................................................... 17

3.3.1 Considerations for Test Piece Selection ................................................ 17

3.3.2 Scope and Scale of Test ......................................................................... 18

3.3.3 Relevant Research Material Related to Test Piece Selection ................ 19

3.3.4 Test Piece Selection ............................................................................... 21

3.4 Test Pieces ......................................................................................................... 21

3.4.1 Test Piece A ........................................................................................... 21

3.4.2 Test Piece B ............................................................................................ 21

3.4.3 Test Piece C ............................................................................................ 22

3.4.4 Test Piece D ........................................................................................... 22

3.5 Method .............................................................................................................. 23

3.5.1 Testing ................................................................................................... 23

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3.5.2 Controls ................................................................................................. 23

3.5.3 Analysis .................................................................................................. 24

3.6 Results ............................................................................................................... 25

3.7 Analysis .............................................................................................................. 26

4 CONCLUSION ............................................................................................................. 27

4.1 Discussion .......................................................................................................... 27

4.2 Errors/Uncertainties .......................................................................................... 27

4.2.1 Testing ................................................................................................... 27

4.2.2 Effect of Groove Profile on Rope Life .................................................... 28

4.3 Recommendations for Future Research ........................................................... 28

4.4 Final Thoughts ................................................................................................... 29

REFERENCES .................................................................................................................... 30

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1 INTRODUCTION

Synthetic fibre ropes bring many advantages over steel wire ropes; lighter weight, safer handling and greater strength, however they do offer some challenges due to their different properties and behaviour, mostly due to their greater flexibility. A hoist could see as many as hundreds of thousands of lifting cycles within its lifetime, each cycle causing bending fatigue and abrasion between the rope and sheaves and within the rope itself. A hoist (Figure 1) is a machine used to lift heavy loads, usually located in an industrial setting. The main elements of the machine are an electric motor providing torque through a gear box to a rope drum. The rope drum rotates, winding the rope around it. The rope is attached at one end to the rope drum, and the other via one or more rope sheave(s) to the hook block or the hoist itself via a rope termination. There can be as many as four rope sheaves in the reeving assembly, each one increasing the lifting capacity of the hoist, whilst reducing the hoisting speed.

Figure 1. Hoist Components – Underside View

A rope sheave (Figure 2) is a grooved wheel usually mounted on a bearing, which is then mounted on an axle. Rope sheaves allow the weight of the load to be distributed across multiple rope falls, reducing the load taken directly by the hoisting motor. The sheaves and the rope exert a large amount of force on each other and are in constant contact, so to maximise the endurance of the rope this interaction should be as smooth as possible.

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Figure 2. Rope sheave with bearing fitted

There are many factors which affect how smooth this interaction is, which will be discussed in greater detail later, however most of these variables, such as which rope is being used, have already been determined. The main element which affects the rope life that can be investigated is the shape of the rope sheave groove profile. Current rope groove designs used in the hoisting application have been created for use with steel wire rope, which has been the prevalent type of hoisting rope for almost a century. This raises the question as to whether the same factors apply to the design of a sheave for use with a synthetic rope, or whether there are new factors that must be considered when a sheave is used with synthetic rope. This brings us to the question at hand: Can an optimized rope sheave groove profile improve synthetic rope lifetime?

1.1 Goal

The aim of this thesis was to assess the effectiveness of different rope groove profiles and their ability to positively affect rope life, based on research of current rope sheave designs used in industry and factors relating to synthetic fibre ropes. Realistically the scope of this thesis only gives some indication of an optimised groove type, paving the way for further investigation into optimisation of the design. The investigation into optimised groove design can be applied to any further Konecranes products using synthetic ropes, including the future iterations of S-Hoist – using different rope diameters.

1.2 Purpose

The purpose of this study was to increase the rope life in Konecranes hoisting equipment that uses synthetic rope. This has several benefits.

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From the customer perspective increased rope life means not having to purchase as many ropes and having less downtime related to the fitting of new ropes. This can also create a small benefit to the wider society by bringing down costs of production, a benefit that may be passed onto the end user. From Konecranes’ perspective a positive result from this study gives Konecranes a more attractive product to sell to their customers and maintain a good brand image. From an environmental perspective an increased rope life would lead to less consumption of plastic, which would have a positive impact on the environment. Whilst HMPE ropes are recyclable, increasing rope life will reduce the carbon footprint of the recycling process.

1.3 Objectives

The first objective of this study was to carry out a literature study of the currently used rope sheave designs and to research the properties of synthetic ropes which may lead to differing design requirements than a sheave designed for use with a wire rope. The next stage was to identify the possible design changes to the currently used rope sheave design which would be most likely to successfully achieve an improved rope life based on the literature study and design suitable test sheaves to be manufactured. Once the test sheaves were manufactured, they were tested to see how the different designs affect the rope life. After the tests were completed, the results were analysed, comparing the longevity of the rope when used with each of the test sheaves. The ropes were also assessed to try to determine the cause of failure of the rope, as the different rope sheaves may result in different discard criteria of the rope.

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2 KNOWLEDGE BASE

2.1 Causes of Rope Wear

There are several ways that a rope can wear during use in a hoisting application. External factors can lead to abrasion of the rope, for example the rope coming into contact with a sharp edge during operation can lead to catastrophic rope failure, however this is not something that can be prevented outside of operator training and is not relevant to the scope of this thesis. The main factor that is discussed within this thesis is the interaction between the rope and the rope sheave and how the sheave design can influence the wear of the rope. This section discusses the mechanisms through which the hoisting application can cause damage to the rope over time.

2.1.1 Contact Pressure

When a rope is bent around a sheave there are two forces acting on the rope. There is the tension caused by the load attached to the end of the rope, and there is pressure between the rope and the sheave which it is in contact with. This contact pressure is dependent on the area of the rope that is in contact with the sheave, the greater the area, the lower the pressure and the greater the rope life. It would be natural to assume that the pressure between the rope and the sheave would be fairly equally distributed across the contact area, perhaps being concentrated in the base of the groove, however a study by Häberle (Feyrer, 2015, p. 201) shows that whilst the pressure is concentrated in the base of the groove, the pressure can be up to double at the points at which the rope meets the sheave and the point at which it leaves the sheave.

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Figure 3. Relative pressure k/p0 between wire rope and sheave (Feyrer, 2015, p. 209)

Figure 3 shows the distribution of pressure between the rope and sheave whilst the sheave is rotating, the groove angle shows the point around the circumference of the groove, whilst the winding angle shows the position around the circumference of the sheave. There is a higher peak at ϑ=260° because this is the point at which the rope is meeting the sheave, at the peak at ϑ=100° the rope is leaving the sheave. In this example the rope is moving around the sheave 90°, however in our case the rope would move closer to 180° around the sheave, which would give an overall reduced pressure, but would still be subjected to a similar peak pressure of double that of the rest of the sheave. This gives more of an idea of the total distribution of the force, and how different elements may affect the overall pressure. The contact pressure can be decreased, as mentioned previously, by increasing the contact area. This can be done in a few ways, by increasing the diameter of the sheave, by changing the bending angle of the ropes to increase the contact arc between the rope and the sheave or by changing the design of the rope sheave groove profile.

2.1.2 Tensile and Bending Stresses

A rope is typically designed so that each strand of the rope will carry an equal portion of the load if the rope is loaded along its axis. The strands are of equal length when the rope is straight, however in a hoisting application the rope is subjected to many bends, whether they be around the rope drum or the rope sheaves. Each of these bends results in the individual strands of the rope taking an unequal amount of the force, as the strand on the inside of the bend will take a shorter distance. The strand on the outside of the bend will have the largest distance to cover, so will be in a state of greater tension, meaning that this strand will be subjected

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to higher tensile stresses. This can cause the strands to wear faster than would be typical of the rope otherwise. When the rope is bent it will also experience bending stresses, as the strands of the rope will bend out of their original shape. In wire ropes this varies throughout the rope due to the twisted nature of its construction, each strand changing location throughout the length of the wire. In a synthetic rope of braided construction the strands stay more or less in the same position around the circumference of the rope, therefore the bending stress stays more consistent throughout the length of the strand, however due to the tighter bend radius of the strands at the base of the groove, they will experience a higher bending stress.

2.1.3 Cyclic Bending Fatigue

Due to the repetitive nature of a hoist rope’s usage, the rope is also exposed to cyclic bending fatigue. Under the bending stresses, the material on the outside of the fibre can stretch and small cracks can form. Under repeated stress cycles microscopic cracks can form in the surface of the fibre, and over times the crack can propagate and form a larger crack, eventually resulting in a broken strand reducing the strength of the rope, even without reaching the materials yield strength.

2.1.4 Pinching

Pinching occurs when the rope is subjected to excessive force from the groove flanks. This is generally caused by a groove which is too narrow to accommodate the rope, although it can be caused when there is an undercut present in the base of the groove, undercuts will be discussed in greater detail in the following section. The groove sizing can be deceptive, as an otherwise adequately sized rope groove may still cause pinching if a rope is particularly flexible as the rope can deform under tension, either being pulled further into the groove than would have been possible prior to deformation or expanding sideways as the rope flattens due to the pressure exerted on it by the rope sheave. As the rope enters and exits the rope groove the opposing forces from the groove flanks cause a pinching effect and a large frictional resistance to the natural movement of the rope, which can cause an excessive amount of abrasion.

2.1.5 Internal Abrasion

As previously mentioned, the strands of a rope move in relation to each other as they are bent around a sheave or drum. This movement between the strands of the rope can lead to abrasion between the strands, particularly when there is a higher coefficient of friction. This effect can be reduced through use of lubrication, which reduces the coefficient of friction, and therefore increases rope life.

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2.2 Current Steel Wire Rope Groove Profiles

Current rope sheave designs are mostly based on steel wire ropes, as this has been the predominant rope selection for lifting heavy loads for over a century. Below are summaries of the most common rope groove types:

2.2.1 U-Groove

Figure 4. General Guidelines on U-Groove Sheave Dimensions for Wire Rope (SFS-EN 13135:2013, p.60)

A U-groove consists of a round groove base with flanks of a height at least exceeding the diameter of the rope, usually at an angle between 30 and 60°, as shown in Figure 4. The radius of the groove base is recommended by EN standards to be within 0.52 and 0.56 times the rope diameter when using a steel wire rope, as shown in Figure 4. The tight bottom radius and a narrow angle for the flanks provide maximum support, with a contact area of almost 180° of the rope circumference between the rope and the sheave, distributing the force across the surface of the rope, leading to minimum deformation of the rope.

2.2.2 Undercut U-Groove and V-Groove

Figure 5. Left - Undercut U-Groove design; Right - V-Groove design (SFS-EN 81-1/1998, P.207)

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Both undercut U-grooves and V-grooves (Figure 5) are primarily used in traction sheaves. By reducing the portion of the groove which is in contact with the rope, they increase the pressure between the rope and the sheave. The increase in pressure allows greater traction between the rope and the sheave, however it also results in a greater pinching effect, causing increased rope wear. As such, these designs are not appropriate to this application.

2.3 Sheave Factors Affecting Rope Life

2.3.1 Groove angle

Aside from the overall shape of the rope groove, there are other factors to be considered. One of the most significant is the angle of the flanks. The smaller the angle of the flanks, the larger the surface area of the rope that is supported by the groove. In most reeving systems there is some fleet angle included – the angle between the rope and the plane perpendicular to the sheave or drum axis – which is usually limited to 4°. In addition to this, there can be additional angling of the ropes caused by sideways forces acting on the hook block, leading to the rope exiting the sheave at relatively large angles to the plane perpendicular to the sheave axis. If the groove angle is too tight, this rope angle can lead to excessive rubbing against the flank of the rope sheave, leading to a reduction in rope lifetime.

2.3.2 Groove Radius

The rope lifetime can be affected by the sheaves groove diameter. If the groove diameter was identical to the diameter of the rope, the rope would be exposed to some pinching effect despite the absence of an undercut, as the rope deforms slightly under load. The groove radius is measured as a ratio of the diameter of the rope – r/d (r – groove radius, d – rope diameter). With steel wire ropes the groove radius is frequently a ratio of 0.52 to 0.56 as seen in Figure 4, whereas for synthetic ropes the manufacturer recommendation is usually closer to 0.55 (Marlow, n.d.) as synthetic ropes are less stiff and more prone to deformation. If the groove diameter is too large the groove does not support the rope well enough, and the rope can deform too much resulting in a negatively impacted rope life.

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Figure 6. Relative pressure k/p0 for the line pressure qmax (Feyrer, 2015, p. 208)

Figure 6 shows the difference in the pressure distribution between two sample ropes giving different r/d ratios. The larger r/d ratio gives a higher peak pressure of almost double that of the smaller r/d ratio. Comparing this to Figure 3 it is quite clear that too large an r/d ratio could lead to some very high peak pressures at the point that the rope meets the sheave. Figure 3 shows a rope sheave with a contact angle close to 120°, similar to the flatter example in Figure 6. If the sheave in Figure 3 had a smaller contact angle of around 60°, similar to the sharper example in Figure 6, the peak pressure could be almost double its current level. It is quite clear that without sufficient support the rope is subjected to unevenly distributed pressure, which leads to a reduced rope life. However, as previously discussed, a groove which is too tight can cause pinching, particularly with synthetic ropes. This would lead to the assumption that the optimal rope sheave groove profile could be one that allows the rope to deform naturally, avoiding applying any unnecessary pressure to the rope, but supports the rope in its deformed shape, ensuring that the pressure applied is equally distributed.

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2.3.3 Sheave Diameter

Figure 7. Relationship between Sheave Diameter and Rope Strength (Unirope, n.d.)

As with wire ropes, sheave diameter (usually given as pitch – the distance from rope centre to rope centre on opposite sides of the sheave) has a large effect on synthetic rope life. When a rope is bent there is a higher tension on the fibres on the outer edge of the bend, causing them to stretch. This repeated stretching causes fatigue stress over time. When bending around the sheave, the fibres also move relative to each other. This causes friction between the fibres, which leads to abrasion of the fibres within the rope. In extreme circumstances the friction can lead to a build-up of heat, which in turn increases the friction between fibres, which leads to more heat. This cycle can have a catastrophic effect on rope life. A larger sheave diameter causes less severe bending of the rope, the outer fibres of the rope aren’t covering so much more distance than those on the inside of the bend. This means that the rope fibres are not having to stretch as much as they would with a smaller sheave, nor do the fibres have to move as great a distance in relation to each other, therefore reducing internal friction. This effect is relative to the diameter of the rope – the larger the rope diameter, the larger the distance between the inner fibres and the outer fibres, the larger the sheave diameter required to minimise rope wear. Due to this, rope manufacturers often provide a given D/d ratio with the ropes they produce. The D/d ratio is calculated by dividing the sheave diameter by the rope diameter. Typically, steel wire rope manufacturers may offer guidelines of a D/d ratio of between 18 and 34 (Usha Martin, 2015) depending on the construction of the rope, whereas synthetic rope manufacturers suggest D/d ratios as low as 8 (Marlow, n.d.). Following this ratio ensures that the rope is not subjected to excessive wear from bending fatigue. The effect the D/d ratio has on the strength of a wire rope can be seen in Figure 7, strength of the rope has a direct effect on rope life.

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2.3.4 Sheave Material

The sheave material can have a large effect on the rope, a coarse surface can cause greatly accelerated rope wear. This means that all rope sheaves should be manufactured with a very smooth rope groove. When using steel wire ropes with metal sheaves it is important that an appropriate material hardness is achieved for the groove base. If a soft enough material is used, the sheave will abrade rather than the rope, and the rough surface caused is not of a hard-enough material to cause damage to the rope. If a hard-enough material is used, the rope is not able to abrade the groove base, and if the groove base is smooth enough it does not cause significant abrasion of the rope. If the groove base is soft enough to allow the steel wire rope to form a wear pattern on the sheave, but is of a similar hardness to the rope, the sharp wear pattern can cause significant damage to the rope and drastically reduce rope life. Another factor relating to the sheave material is its heat conductivity. As mentioned in the previous section, heat can build up in the rope, particularly in extreme situations when the rope is subjected to repetitive bending over a short period of time. A higher heat conductivity will allow the sheave to absorb heat from the rope and dissipate it to the environment. In a study carried out by Müller (Müller, 1961, p. 249-258) it was found that plastic sheaves increased the rope life when compared to iron or steel sheaves. Sheaves of a small modulus of elasticity (plastics or other similar materials) conform to the shape of the rope more, rather than causing deformation of the rope, allowing for an increased rope life. The elastic deformation in the sheave material, combined with the increased friction and the lack of conductivity of the sheave could however lead to a build-up of heat in the rope and sheave. With a steel rope this may not cause too much of an issue in terms of rope wear due to the conductivity of the rope, however with a synthetic rope this could be detrimental to the rope life due to the lack of conductivity and the higher sensitivity to temperature. It is difficult to determine whether a synthetic rope would be more adversely affected by plastic sheaves due to the lack of conductivity, and whether this would have a greater negative impact that the benefit from the softer material of the sheaves. In Müllers test using steel wire rope, it is likely that any accumulated heat is conducted through the length of the rope, preventing any issues. In prior studies carried out by Konecranes it has been determined that sheaves made of softer materials are rapidly destroyed by the rope, whereas harder materials have not caused significant deterioration of the rope. This would suggest that the optimal material for use with a synthetic rope would be a hard material with a smooth surface finish.

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2.3.5 Groove Edges

During normal operation the rope should not come into contact with the top of the sheave groove, however this must be considered as a possibility. This means that the top edge of the sheave groove should have a rounded edge, meaning that if the rope does come into contact with it the rope shouldn’t be damaged and should more easily slide into the correct position in the groove. This principle is applied to all surfaces of the hoist that the rope could theoretically rub against during use, for instance the opening of the hook block sheave covers through which the rope exits.

2.4 Rope Factors Affecting Rope Life

2.4.1 Rope Construction

Figure 8. Twisted Wire Rope Construction (Leedem, 2014)

Rope life can be affected by the material of the rope, different materials are suited to different applications. Aramid (well-known commercially as Kevlar) rope for instance performs very well when tensile strength is the only requirement, however it does not perform so well in applications requiring cyclic bending. Steel wire rope performs well in both tensile strength and cyclic bending applications, however it is heavy and difficult to handle. Every rope material that could be applied to the hoisting application has its own benefits and drawbacks. There are two main types of rope construction, twisted (Figure 8) and braided. Twisted rope is the oldest means of construction and consists of many strands twisted in one direction, which are in turn made of many fibres twisted in the opposite direction in an ordinary lay rope construction. It is this opposite twist which prevents the rope from

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untwisting itself, however this twisting can also lead to the rope naturally untwisting under load. Ropes can also be constructed with Lang’s lay (Figure 9), in which the strands and the wires are twisted in the same direction. This gives a more rounded surface to the rope and, due to the increased portion of each wire on the surface, the rope has better wear resistance. Lang’s lay ropes are also more flexible, but they are more susceptible to crushing, kinking and birdcaging than ordinary lay ropes, and if they are not fixed at both ends the non-opposing twist results in the rope untwisting itself. Ordinary lay is usually used in hoisting applications. Twisted rope construction often consists of several strands twisted around a central core. Twisted ropes can be made from natural fibres, steel wire or synthetic fibres.

Figure 9. Ordinary and Lang’s lay comparison (Mazzella, 2020)

In a braided rope half of the strands rotate in one direction, whilst the other half rotate in the opposite direction, all the strands interweaving as they rotate around the circumference of the rope. This alternating twist results in a torque neutral rope, meaning that even under heavy load the rope does not untwist and cause the load to rotate. Braided construction leads to a hollow braided outer sheave of rope, which can be constructed around an inner core, or left hollow. Braided ropes can be made from natural or synthetic fibres, usually only cable sheaths are made of braided wire.

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Figure 10. Comparison of Braided and Twisted Rope Cross Sections

The rope construction also influences the rope life. A braided rope keeps more surface area in contact with the rope groove than a twisted rope, as the surface is more rounded (Figure 10). This of course reduces the contact pressure which in turn increases the rope life.

2.4.2 Rope Break Strength

Break strength is the amount of force required to cause a rope to fail under a tensile load. The greater the break strength of a rope, the greater its resistance to tensile stress and therefore the longer the rope life.

2.4.3 Lubrication

Lubrication can provide a much better rope life, allowing the rope fibres to move against the sheave and each other with less friction. In the case of wire rope, lubrication also serves to protect the rope from corrosion. Whilst internal abrasion is just as much of a factor with synthetic ropes, lubrication is not usually used in the same way. Typically to reduce internal abrasion of a synthetic rope the strands are coated with a proprietary coating to minimise the coefficient of friction. This operates through a process called boundary lubrication. As the strands slide against each other or the sheave, a thin layer of the coating is worn off and creates a film which reduces the friction between the strands. In this way, the first small amount of wear protects the strands from further wear.

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2.5 Synthetic Rope

2.5.1 Background of Synthetic Rope

Synthetic rope has been used since nylon cord was developed in World War II and has been used extensively in the marine industry since. It is not until more recently, however, that synthetic rope has begun to see use in heavy lifting applications. The first commercial use of synthetic rope in a heavy lifting application was in 2014 in the form of a mobile crane. Currently there are three main types of high-performance synthetic rope; Liquid Crystal Polymer (LCP), Aramid and High Modulus Polyethylene (HMPE), also known as Ultra-high-molecular-weight Polyethylene (UHMWPE, UHMW). LCP, known commercially as Vectran, is an aromatic polyester which is used in a range of applications including fibre reinforcement, professional bike tires and several NASA missions. Aramids, known as Kevlar or Technora, has been used extensively in bullet-proof vests due to their high tensile strength to weight ratio, but has also seen use in cryogenics due to its low thermal conductivity, and many sports applications due to its high strength and low weight. HMPE, also known as Dyneema and Spectra, is used in hip replacements, a wide range of sports equipment due to its high strength and low weight, and extensively in the marine industry due to its high strength and low density, allowing the rope to float on water.

2.5.2 Benefits/Drawbacks of Synthetic Rope

Synthetic ropes outperform wire ropes in several areas. Most importantly synthetic ropes exceed the tensile strength of steel wire ropes, in some cases achieving almost double the tensile strength. Synthetic ropes also offer a significant reduction in weight, with densities only a fraction of that of wire ropes. This combined with the lack of the sharp rope defects that can occur on wire ropes, such as broken strands and kinks, makes synthetic ropes a lot safer and easier to handle then wire ropes. Wire ropes are prone to rust when exposed to moisture, this is not a concern for synthetic ropes, which boast a much higher chemical resistance, which is often further enhanced with a protective coating. Both LCP and aramids do not perform well when exposed to UV light and perform poorly in in bending fatigue resistance. HMPE has a very low melting point (140-150°C) and is therefore not suitable for high temperature applications (Should not be used in excess of 65°C). Aramid ropes absorb water which could lead to contamination of the rope or corrosion of steel components.

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Figure 11. Construction of a Braided Rope with a Centre Core (Michael, Kern, Heize, 2016)

The synthetic rope Konecranes has selected for use is an 8.5mm, 12 strand, single braided, hollow rope constructed from HMPE with a proprietary coating. This rope has a very high tensile strength, exceeding that of a steel wire rope. It has better UV resistance when compared to other synthetic ropes, and its proprietary coating increases its fibre to fibre self-lubricating properties. HMPE has much better performance in bending over sheave (BOS) applications when compared to other synthetic rope materials, performing ten times better in tests carried out by Konecranes than equivalent aramid fibre ropes and twice as well as HMPE rope produced by another manufacturer. This is obviously critical to the hoisting application, as the rope will regularly be bent around the rope drum and rope sheaves during use. HMPE also has excellent abrasion resistance, which helps minimise wear from contact with the sheaves and rope drum.

The hollow braided construction (Figure 11) produces quite different characteristics when compared to a twisted steel wire rope. Due to HMPE’s high flexibility, along with the flexibility produced by the hollow braided construction, the selected rope not only has good bending fatigue resistance, but it is also prone to cross-sectional deformation under load. The rope, when new, is prone to stretch when a tensile load is applied. This causes the rope to narrow and become longer, most of this deformation is permanent, however there is also some elastic element of the deformation. This deformation does not cause any harm to the rope.

When bent around a sheave whilst under a tensile load, the rope is prone to flatten, as all the strands in the rope seek the shortest path around the sheave. This can cause issues when used with sheaves designed for use with a steel wire rope, where the groove can be too narrow and cause pinching.

The main drawback of the rope selected is its poor performance in high temperatures. Due to this restriction the synthetic rope will only be used in normal ambient temperature environments. Any hoists working in hot environments such as foundries will be fitted with a steel wire rope.

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3 PLANNING AND REALISATION

3.1 Method Overview

0

Figure 12. Konecranes’ CBOS Testing Machine - ROPE 1

The tests were carried out using Konecranes’ Cyclic-Bending-Over-Sheave (CBOS) testing machine (Figure 12). During the tests the rope was cycled backwards and forwards through the sheaves under tension. This simulates typical usage of the rope and sheave but allows for an accelerated testing time. The ropes were tested until failure and the number of cycles taken to reach rope failure (Cycles to failure – CTF) were recorded.

3.2 Apparatus

The test was carried out using ROPE H1 CBOS machine. All samples of Konecranes 8.5mm synthetic rope for each test were taken from the same batch. The test sheaves were all manufactured by the same supplier according to the mechanical drawings provided.

3.3 Requirements

3.3.1 Considerations for Test Piece Selection

One means to predict the wear on the rope is to consider the contact pressure that would occur between a rope and a sheave, the smaller the contact pressure the greater the rope life would be assumed to be. This

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leads to the idea that more supportive features, such as a narrower groove diameter and groove angle, would lead to an increased rope life. However, there are other factors that affect the rope life. Under load, a hollow braided synthetic rope flattens to form an elliptical shape, it is possible that this deformation could lead to pinching, even in a groove larger than the rope diameter. It is also possible that a rope groove that allowed the rope to deform would minimise the unequal distribution of forces within the rope and increase the surface contact, therefore increasing rope life. It could also be the case that allowing the rope to deform would lead to excessive movement of the rope strands in relation to each other, therefore resulting in excessive rope wear. During a previous study at Konecranes the groove angle for a wire rope sheave was investigated. The rope was simulated for FEM analysis and it was determined that the optimal groove angle for rope life was 53°, which works contrary to the idea that a narrower angle would support the rope more and increase rope life. It could be very useful to make a simulation of the synthetic rope to analyse how the forces are distributed internally in the rope, however this is not within the scope of this study. It is possible that, upon selection of a suitable groove type, a simulation could be used to fine tune the precise dimensions to maximise the rope life.

3.3.2 Scope and Scale of Test

In this study a U-grooved control was used with a groove r/d ratio of 0.55 and a groove angle of 45°, which made a good base point for a sheave to be used with a synthetic rope. Each test piece is a variation of this design. In Figure 13 a comparison of several different groove designs can be seen. It is quite clear that, in terms of rope life, a well-fitting U-groove design is the most appropriate rope groove design for use with a wire rope, so this makes a good starting point for this study.

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Figure 13. Breaking numbers of bending cycles for ordinary and lang lay rope in different form grooves (Feyrer, 2015, p.238)

Ideally, there would be at least 10 different sheave designs tested. In addition to a control sheave, using the same groove profile currently in use, both a larger and smaller groove diameter and a larger and smaller groove angle would be tested. To understand how these variables interact with each other, all combinations of these (e.g. large groove diameter with large groove angle, large groove diameter with small groove angle, etc.) would be tested. This would give 9 test sheaves, with the 10th being a sheave with an elliptical groove base, which would allow the rope to deform under load. Realistically, the limitations imposed by time and resources need to be considered. The number of tests including the control have been limited to four for the scope of this study, this means that the test cases that were investigated needed to be considered carefully.

3.3.3 Relevant Research Material Related to Test Piece Selection

A study carried out by Cortland Ropes (Sloan, Nye & Liggett, n.d.) investigates several factors relating to improving the lifetime of synthetic ropes. Most of the results of this study are, however, not disclosed. The only factor for which the results are not concealed is the ratio between groove diameter and rope diameter. In this study tests are conducted on sheaves with a groove r/d ratio of 0.525 and 0.75. In this study it is shown that the narrower groove diameter performs almost 20% better for rope life than the wider rope groove. This would suggest that the narrower groove diameter would be worth investigating, however it is worth noting

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that the control groove diameter of 0.55 times the rope diameter is very close to that tested in this study, so it is possible that the control could outperform both the 0.525 and the 0.75 ratios detailed in the Cortland Ropes study. ISO standard 9554:2019 Fibre rope – General specifications (ISO 9554:2019, P.12) provides guidance for general use of fibre ropes. Guidelines are provided for sheave dimensions for use with fibre ropes. It is stated that the groove diameter should be between 10 and 15% larger than the rope diameter (r/d ratio of 0.55 to 0.575), which is approximately the same as the control sheave although with a small potential to increase the diameter. It is also stated that the contact angle (this is the angle of the ropes surface in contact with the sheave, also equal to 180° minus the groove angle) of 150°, which would equate to a groove angle of 30°. This would suggest that it would be worth investigating the effects of a slightly larger groove diameter as well as a tighter groove angle. Liebherr have made a patent application (Mupende & Zerza, 2017) relating to the use of an elliptical groove for use with both rope sheaves and rope drums for synthetic rope. It is described that “the rope grooves have a flat-pressed, round groove contour which differs from the circular and which has a larger radius of curvature in the region of the groove base than in the region of the groove flanks adjacent thereto”. This would suggest that the use of an elliptical groove base would be beneficial to rope life and would be an interesting area of investigation. It has been identified (Feyrer, 2015, p. 239) that with steel wire ropes ovalisation has a negative impact on rope life; as the stiff wires are deformed, they are subjected to stresses which can cause significant wear to the rope through cyclic bending fatigue. However, with a synthetic rope the flexibility of the fibres is much greater, and it is possible that the repeated deformation of the rope structure may not have a significant impact on the rope lifetime. This is further assisted by the hollow nature of the selected synthetic rope, which allows the rope to deform without causing compression like would be observed in cases which cause ovalisation in a steel wire rope. Feyrer (2015, p. 255) states that to reach an optimal rope lifetime a reasonable balance should be found between the bending and tensile stresses. This is stated from the angle of rope selection rather than sheave groove design however, so realistically there is not much that can be altered in terms of the groove design to affect either of these stresses. There is a formula provided for calculating the optimal rope diameter for any application, however as this is determined for a steel wire rope, it is not useful in this context. It could be argued that by ovalizing the groove base and maintaining the same rope centre point, the minimum bending radius would be increased slightly which could slightly reduce the bending stresses whilst also slightly decreasing the variation in tension between the rope strands, however it is most likely that this effect would be negligible.

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3.3.4 Test Piece Selection

Based on this research it was determined that, in addition to the control, there should be tests conducted that test separately the effect of a smaller groove diameter, a tighter groove angle and an elliptical groove base. The elliptical groove base somewhat accounts for the larger groove diameter suggested by ISO 9554, allowing extra room for rope deformation.

3.4 Test Pieces

All test sheaves were constructed from machined S355 steel and the surface coated with wet paint. All sheaves featured a groove depth of 11.75mm from the outer edge to the groove bottom, and the top edge of the groove had radii of 2mm. As discussed earlier in the thesis, the D/d ratio has a big effect on the lifetime of the rope. There have been tests carried out previously with plastic sheaves of Ø136mm and iron sheaves of Ø180mm. The test sheaves for this experiment measured Ø136mm, as this matches the previously conducted tests. The material selected for use is machined steel. One reason for the material selection was ease of manufacturing. Another reason for this selection is that the steel sheaves help conduct heat away from the rope during the testing process. The previous tests showed that the Ø180mm cast iron sheaves performed better in terms of rope life than the Ø136mm plastic sheaves. This is not surprising as both the larger diameter and heat conductivity of the material would have a positive effect on the rope life. However, it is not clear how much of an affect each of these factors have on the increase in rope life. By carrying out a control test with the Ø136mm steel sheave, it gave some idea of how much of an effect each of these factors has had, by comparing the achieved rope life with that attained from these studies.

3.4.1 Test Piece A

Test piece A will use a groove dimension typically used with a synthetic rope. This will have a groove diameter larger than the rope diameter and a standard groove angle.

3.4.2 Test Piece B

Test piece B will differ from test piece A in that it will have an elliptical groove base. To determine the approximate appropriate dimensions for this groove profile a measurement was taken of the maximum width of the rope under deformation against a flat surface. As the deformation of the rope varies under different loads, the greater the load the greater the

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deformation, an ellipse was calculated with a slightly narrower width than the maximum measured, which maintained the same circumference as the rope prior to deformation. Due to advice from the machinist from the supplier of the test pieces, an approximation of this ellipse was constructed from 3 arcs. The geometry of the final groove shape can be seen in Figure 14.

Figure 14. Dimensions of final elliptical groove profile

The pitch from rope centre to rope centre is approximated based on the prediction of the rope shape under deformation, therefore the pitch should be approximately equal to that of the control sheave.

3.4.3 Test Piece C

Test piece C will match the control in every aspect except the groove angle, which will be narrowed to 30° to match the suggested value from ISO 9554.

3.4.4 Test Piece D

Test piece D will match the control in every aspect except the groove diameter, which will be narrowed to an r/d ratio of 0.525 to match the test piece from the Cortland Ropes study.

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3.5 Method

3.5.1 Testing

Figure 15. Configuration of CBOS test machine

The sheaves were mounted on the CBOS rope test machine – Rope H1 – axles, secured using a locking plate on the end of the axle. Prior to the test, the position of the upper rope sheave was adjusted using weight to set the correct rope tension, which was measured using a steel wire rope attached to a force meter. The rope was then fixed to the traction sheave by attaching a spliced eye to a rope fixing point and routing it through the rope sheaves. Once the rope was secured and set to the correct tension the test machine was activated. The test machine runs automatically, carrying out 5 cycles a minute, 24 hours a day until the rope fails. During the test the rope moves around three bending points on the rope, around three sheaves, although all sections of the rope were only subjected to one simple bend, whereby each cycle moves a section of the rope through one sheave and then back through the same sheave. There was one bend of 180°, and two bends of less than 180°, as can be seen in Figure 15. The stroke of the CBOS test machine was kept constant for all tests. The number of cycles to failure (CTF) was recorded by the test automation system and the tests were stopped once the ropes broke, as well as periodically to allow for photographs to be taken to document the wear of the rope. The sheaves were then swapped, and a new rope fitted ready for the next test to begin. The old ropes were investigated to determine the cause and location of rope failure.

3.5.2 Controls

The only free variable in this test was the rope sheave groove profile, within which there were three variations tested against the control. This means that there were several factors which had to be controlled to ensure that any results are reliable and can be attributed to the change of the rope groove dimensions.

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The sheave diameter was kept constant at Ø136 mm which, along with the constant rope diameter of 8.5mm, maintained a constant D/d ratio on all tests. There are many attributes related to the rope which could have affected the results of the test, therefore the rope used was the same as that used in production, and all tests were carried out with the same batch of rope, to ensure that there are no variations in the manufacturing. The CBOS testing machine maintained the same cycle and stroke length for all tests. The Konecranes Reliability Centre is kept at room temperature.

3.5.3 Analysis

The results were analysed using several methods. An average was taken for each test and the total sum of squares (TSS) was calculated. The TSS allows for comparison of the variability between and within the different test groups (treatments), the lower the number, the less variance between or within the treatments. The TSS can be calculated as follows:

𝑇𝑆𝑆 =∑(𝑦𝑖 − �̅�)2

𝑛

𝑖=1

where 𝑦𝑖 is the test result or treatment mean and �̅� is the mean for the treatment or the overall mean. TSS is a means to assess the variation of the test results, the greater the value the greater the variance. Due to the small number of test iterations it was useful to carry out some further analysis to determine the significance of the results and the extent to which the rope sheave groove profile influences the rope lifetime. This was achieved by carrying out analysis of variance (ANOVA). This involved making a comparison of the variance within the treatments and the variation between treatments. To determine the result of the ANOVA several values needed to be calculated; the mean of squares, F-statistic and the P-value. The mean of squares represents the variation between the means, either between the treatments or the variation and is calculated as follows:

𝑀𝑆 =𝑇𝑆𝑆

𝐷𝑂𝐹

where the 𝑇𝑆𝑆 is either applied to the variance within treatments or between treatments, and the 𝐷𝑂𝐹 (degrees of freedom) is equal to one less than the total number of tests or one less than the total number of treatments respectively.

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The F-statistic is a means to represent the ratio of two variances. With this the significance of the variation between groups can be compared to the variation within groups to determine the significance of the results. The F-statistic can be calculated as follows:

𝐹𝑆𝑇𝐴𝑇 =𝑀𝑆𝐵𝑀𝑆𝑊

where 𝑀𝑆𝐵 is the mean of squares between treatments and 𝑀𝑆𝑊 is the mean of squares within treatments. The next step in the analysis process is to compare the F-statistic with the F-critical table to determine the probability that the results are statistically significant. The F-critical value is read from the F-critical table, using the DOF within and between the treatments to determine the value. (Calvin Univeristy, n.d.)

3.6 Results

Table 1. Rope sheave groove profile rope lifetime test results

Treatment Cycles to Failure

Test 1 Test 2 Average TSS

Test A – Control 102.2 97.8 100.0 10.0

Test B – Elliptical Groove Base

93.4 99.7 96.6 19.7

Test C – 30° Groove Angle

107.0 97.4 102.2 46.3

Test D – 1.05 Groove Radius Ratio

110.6 112.8 111.7 2.6

Average 102.6 78.6

The results, seen in Table 1, are shown with the control test average as an index of 100. They show that the difference between the averages is not so great, some test pieces performing better than others, test B providing a lower average than that of the control test. Test D provided the most notable increase in the average. As the variance between the test pieces is not so large, it may be useful to compare the percentage change of the test pieces average values when compared to the average of the control test. This comparison can be seen in Figure 16.

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Table 1. % Lifetime Change vs. Control Average

Treatment Test 1 Test 2 Average

Test A – Control 2.2 -2.2 0

Test B – Elliptical Groove Base -6.6 -0.3 -3.4

Test C – 30° Groove Angle 7.0 -2.6 2.2

Test D – 1.05 Groove Radius Ratio 10.6 12.8 11.7

Figure 16. % Lifetime Change vs. Control Average

3.7 Analysis

Table 2. ANOVA table for rope sheave groove profile rope lifetime test results

Source of Variation

Sum of Squares

Degrees of Freedom

Mean Squares

F-Value

F Critical

P

Treatment 252.4 3 84.1 7.49 5.89 0.025

Resultant 78.6 7 11.2 - - -

Total 331.1 4 - - - -

The ANOVA table, Table 2, shows that there is some significance to the results. This can be stated with a certainty of greater than 97.5% based on the variance between and within the treatments.

-10.0

-5.0

0.0

5.0

10.0

15.0

Test A Test B Test C Test D

% L

ifet

ime

Ch

ange

Test Piece

% Lifetime Change vs. Control Average

Test 1 Test 2 Average

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4 CONCLUSION

4.1 Discussion

The results of this test show that the changes in rope sheave groove profile do not have a significant effect on the rope lifetime. The elliptical groove profile performed slightly worse than the control test, suggesting that this groove design is not beneficial to the rope lifetime. The narrower groove angle resulted in a very slight improvement in rope life, a smaller difference than between the test runs within each group, showing that there is no significance to this result. The smaller groove radius provided the most significant result, increasing the rope life by 11.7%. This would suggest that the most promising groove profile is one which quite tightly fits the rope. Using an ANOVA table to assess the significance of the results it is shown that whilst there is no distinct increase in the rope life for any of the tests carried out, there is some significance to the results. This shows that the rope sheave groove profile does influence rope life and that through further investigation a groove profile could be found which causes a more significant increase in the rope life.

4.2 Errors/Uncertainties

4.2.1 Testing

Despite the ANOVA showing that there is some significance to the results of the testing, the low number of tests per test piece leaves some uncertainty to the accuracy of the results. It is possible that some small variance in the production of the rope could lead to a difference in the rope lifetime. Whilst it can be stated with some confidence that the rope sheave groove profile affects the rope lifetime, the natural variance in the rope lifetime combined with the low number of tests means that it is difficult to determine how the different test pieces have performed in comparison to each other with great certainty. To achieve greater certainty further testing should be carried out with the same test sheaves so that more data can be collected to provide a more reliable average of the rope life for each sheave design. This would help mitigate any variance caused by uncontrolled variation, such as the variation of the construction of the rope. It is also worth noting that the reference material which influenced the design of the test samples were relevant to use of synthetic rope, however the construction of synthetic rope can vary a lot, giving the rope different

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mechanical properties. This means there is a possibility that there could have been other rope sheave groove profiles more relevant to the construction of the rope being used.

4.2.2 Effect of Groove Profile on Rope Life

The accuracy of these results can be called into question due to the small number of tests carried out with each test piece. It is possible that some portion of the rope has been stronger or weaker than the rest of the rope, which could cause misleading results. The decision to use a small number of tests was made due to the time constraints in place for the testing and high demand for the test machine. There is also some potential uncertainty of the results based on the small number of different test pieces. It is, for instance, possible that if two diameters of groove radius are tested with negligible difference between the results, the optimal result could lie between the two test points or greater or lower than either test point. This is particularly relevant in the case of the elliptical groove base for which there are quite many variables relating to the geometry of the groove. Due to the controlled manner of the testing there could be some elements of real-life usage that is not taken into consideration with this testing. In particular, the CBOS testing ensures that the rope is always perpendicular to the rope axle, however in real life usage the rope is subjected to some angle due to the layout of the rope drum and sheaves, and further angle can be imparted due to side pull on the hook block. This can cause increased wear as the rope rubs against the sides of the rope sheave groove and could be increased by a narrower groove radius or angle.

4.3 Recommendations for Future Research

There is an effect on the rope lifetime caused by the rope sheave groove profile, therefore it would make sense to carry out further research. Ideally there would be testing carried out on the change of groove base radius and flank angle, as well as variations of the elliptical groove profile. For the change in groove base radius there should be testing carried out on several test pieces with differing radii, both between the existing data points and to either side. Each test should be carried out perhaps 3-5 times to increase the accuracy of the results. The same process should be carried out for changes in the groove flank Future testing for the elliptical groove base could be more complicated. Further understanding of the deformation of the rope is required, this could be achieved by carrying out testing by subjecting test samples of the

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rope to several different loads whilst the rope is bent with different bending radii. This would give a greater understanding of the behaviour of the cross section of the rope under load and bending. It could be challenging to find an optimal elliptical profile as the rope cross section will differ depending on the load applied, whilst many hoists in real usage will lift loads of differing weight. It could be possible that the best compromise between the optimal cross section of the rope under different loads could be a circular groove base. Once the optimal theoretical groove profile is determined testing could be carried out on several designs similar to this to identify the optimal groove profile. Ideally there would be at least 3-5 tests carried out with each design. To achieve greater certainty of the results further tests could be carried out on different scenarios. The testing carried out looked at narrowing the groove flange angle, reducing the groove base radius and changing the shape of the groove, future testing could look at increasing the groove flange angle and increasing the groove base radius as well as testing combinations of these different changes to assess how the changes interact.

4.4 Final Thoughts

Whilst the results of the testing have not provided a conclusive solution, they do show that there is scope for further testing that may yield promising results – an optimized rope sheave groove profile can improve synthetic rope lifetime. It is possible that, with a greater number of samples per test, more accurate results could have been obtained, however given the time constraints that were present for the testing, the small number of samples was justified. It is also possible that if future tests yield a more significant increase in rope lifetime the accuracy of the results may not be so critical to determine the best optimized design. Based on the results of the research a rope sheave groove profile with a smaller base radius would provide the best rope life. If no further testing is to be carried out, then this design should be implemented to provide the best rope life for the hoist. It is clear that there is much more work that needs to be carried out to determine the optimal rope sheave groove profile for maximising the rope lifetime, however this thesis shows that a difference can be made and suggests the direction that this research could take to reach this goal.

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Mupende & Zerza. (2017). Hoist Drum and Rope Pulley for Fiber Rope Drives. Retrieved 14/02/2020 from https://patentimages.storage.googleapis.com/2d/2d/17/ca6de4e6edbaf7/US9758358.pdf Müller, H. (1961). The properties of wire rope under alternating stresses. Wire World. p. 249-258. Rice. (2017). Wire Rope Discard Standards. Retrieved 12/02/2020 from https://www.drillsafe.co.za/drillsafe-articles/wire-rope-discard-standards Rope Inc. (n.d.). Rope Handling. Retrieved 12/02/2020 from https://www.ropeinc.com/rope-handling.html SFS-EN 13135:2013 +A1:2018. Cranes, Safety, Design, Requirements for Equipment. Retrieved 17/02/2020 from https://online.sfs.fi SFS-EN 81-1 (1998). Safety rules for the construction and installation of lifts. Part 1: Electric lifts. Retrieved 22/01/2020 from https://online.sfs.fi Sloan, F., Nye, R. & Liggett, T. (n.d.). Improving Bend-over-Sheave Fatigue in Fiber Ropes. The Cortland Companies. Unirope. (n.d.). D/d Ratio and the Effect on Sling Capacity. Retrieved 21/01/2020 from: http://unirope.com/products/slings/wire-rope-slings/rigging-guidelines/dd-ratio-and-the-effect-on-sling-capacity/ Usha, M. (2015). Wire Rope User Manual. Retrieved 05/03/2020 from https://www.oceanmax.eu/images/Caitlin/User-Manual-A4_web.pdf