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POWER TRANSMISSION & COUPLINGS

 In industries power is used in many forms. Steam turbines and gas turbines are direct mechanical drives, which are used either to generate electrical power or to drive the compressors or pumps. Electrical power is used to drive rotary equipments through electric motors. In any mechanical drive system there exists a need to couple the variety of driven element with the driver and this is called the coupling and power gets transmitted through coupling. The prime mover (Electric motor, steam turbine or gas turbine) is generally a complete unit in it self and is fitted on a suitable base frame skid. Similarly the driven units like pumps, compressor, or fan etc. are also manufactured as independent units. Driver and driven units are installed on common base frame and a coupling is needed to connect both the systems.These couplings may be directly coupled between the drive and driven element or through speed reducing/speed increasing gearbox or torque converters. Selecting the right coupling is an important and complex skill because coupling failure means total breakdown of the equipment, however coupling should always be the weakest link in the system. In any abnormal circumstances, or disturbed operating conditions before brake down of the costly driver or driven equipment, coupling shall fail. Coupling shall be selected in such a way that it can absorb/withstand various varied forces and many other factors within normal range of power transmission without affecting driver or driven equipment. Primary factors that guide the selection of type and size of coupling used for any applications are listed below: - Power to be transmitted-Torque to be transmitted- RPM- Size of shafts-Type of Prime mover (Motor, Turbine, Engine etc.)-Type of driven equipment (Rotary, reciprocating, intermit ant, continuous, Cam driven etc.)- Distance between the shaft ends (Space limitation)- Maintenance and installation- Allowable misalignment- Axial movement- Dynamic Balance requirement- Criticality of equipment (safety factor)- Other specific conditions depend upon the industries The above factors are the general type of criteria however for a complex machine where the exact amount of thermal growth etc. is involved lot complex criteria’s are considered important for selection of the couplings. Classification:

 Couplings are broadly classified in two groups:

Rigid CouplingsFlexible couplings

Rigid couplings  are those in which two shafts of driver and driven equipments are connected directly through coupling hubs and reamed bolts without any flexibility between them. There is no margin for any misalignment of any kind for this type of couplings. These couplings are used in big reciprocating engines, compressors etc.Flexible couplings are those coupling though which two shafts are connected such that they have certain degree of flexibility between them. They can absorb certain degree of misalignment also. Flexible couplings may be of many kinds. In general following types of flexible couplings are used in industries:

 -         Pin-Bush type-         Gear coupling-         Spider coupling-         Tyre coupling-         Disc coupling-         Grid couplings (BIBY Couplings)-         Elastomer star couplings (LOVEJOY couplings)-         Flexible metallic membrane coupling (metaflex type)-         Magnetic couplings etc, etc

 All above type of couplings have their own advantages and disadvantages. The key advantages and limitations of any category can influence the coupling selection. The advantage of any coupling shall be judged keeping in mind the expected life and maintenance requirement during that period with respect to the application. If any equipment is required to be shut downed for maintenance of couplings only then the type of coupling is not suitable and in long run may prove to be costlier than other better options at the time of initial purchase. 

                                                          Metallic Grid Coupling (Biby Type)  Elastomer flexible element coupling:-In this category of couplings, non-metallic element are used within the coupling through which the power is being transmitted. The elastomer material may be rubber or polyamide of special grade. Again elastomer coupling may be of compression type or shear type. Typical example of compression type coupling is PIN-Bush type and Love Joy jaw type couplings.  Tyre or Molded element couplings are shear type elastomer couplings.  

           

 An Elastomer, Jaw Coupling                           Elastomers of different designsElastomer couplings fails in two basic failure modes. One, due to fatigue from cyclic loading, which can occur from misalignment or torque beyond capacity. Second, failure due to environmental factors, e.g. high temperature, chemical reactions or expired shelf life of elastomer.Advantages of  Elastomeric couplings are -Torsionally soft-No lubrications or Maintenance is required-Good vibration damping and shock absorbing qualities-Less expensive than metallic coupling-Lower reactionary loads on bearings Limitations-Usually not Torsionally stiff enough for positive displacement type of services-Larger outside dia than metallic coupling for same capacity-Not suitable for very high RPM Metallic Flexible Couplings:-These type of coupling do not use any elastomer element to transmit the torque. Their flexibility is gained through flexible fitting parts which roll or slide against each other (Gear, Grid or Chain) or through flexing/bending of a membrane (Diaphragm or Bellows). The primary cause of failure of this type of coupling is mechanical wear of the flexible element. Flexible membrane type coupling are generally more costlier then other form of metallic Flexible couplings but they  do not need any lubrication and

maintenance during the service life. Their primary cause of failure is fatigue. Advantages of Metallic couplings are:- 

-         -Torsionally stiff-         -High torque transmission capacity -         -High RPM -         -Zero backlash in many types

 

                                                 Metallic Flexible coupling                                      Misalignment Capacity 1/3 degree per packLimitations of Metallic Couplings are :- 

-         -Fatigue or wear are the main cause of failure-         -May need schedule lubrications-         -Assembly is some times complex-         -Usually cannot damp vibrations or absorb shock

 Some time selection of coupling is guided by application and criticality of equipment as categorized below 

-         Generall purpose Industrial-         Specific Purpose Industrial-         High speed-         Very heavy load-         Motion control and tensional

      It is always better to understand the type of load to be carried by the two pieces of equipment to be connected through couplings. For example if there is mild shock low cyclic load requirement that may lead to selection of elastomer element coupling and if high torques are involved then Gear coupling is likely to be found  suitable. High speed equipment will lead to selection of disc or diaphragm couplings. Diesel engine drivers need the benefits of torsional flexible coupling. Some important factors and terminology regarding alignment and couplings

        

Alignment limitsThe OEM of the equipments generally specifies alignment limits, however in industries some thumb rules are made and are acceptable by and large by all the first line supervisors for general rotary machines. These thumb rules are OK for low speed machines but for RPM of 1500 and more OEM recommendations shall always be followed.The shaft-to-shaft residual misalignment is acceptable when the intersection point of the two shafts is within the coupling area and the included angle between the shaft centerlines is small. These two criteria must be applied in two orthogonal directions, typically horizontal and vertical for convenience, and normalized to speed. That is, slow-speed machines are allowed a larger tolerance. High-speed machines are required to be better aligned. If the machines or any part of driver or driven machine is subjected to high temperature than necessarily temperature correction to the cold alignment shall be taken into account. Alignment versus vibration and Noise

Vibration shall not be used as criteria to judge alignment, even though the purpose of alignment is to reduce vibration. Other factors can also cause excessive vibration, such as structural resonances or unbalance. In a similar manner, noise and excessive bearing temperature could indicate shaft misalignment but these symptoms could also indicate other problems. Noise and excessive bearing temperatures shall not be used alone to judge alignment. Angular misalignment:-The angle made between the extension of

centerlines of driving and driven shafts is called the angular misalignment.

This is also known as axial misalignment. All couplings are designed to

tolerate some angular misalignment and these limits are specified by the

manufacturers.

                                                                            Offset or parallel misalignment 

Parallel misalignment:-  Parallel misalignment occurs when the two shaft

centerlines are parallel, but not in the same line. They are offset

horizontally or vertically (or both), displaced to the left or right, or

positioned at different elevations.

Combination misalignment :- Combination misalignment occurs when the

motor shaft suffers from angular misalignment in addition to parallel

misalignment.                                                 

                                                                                      Example of an acceptable misalignment for

an 1800 rpm machine.  

Axial Forces:- Due to axial misalignment, the driver or driven equipment can generate axial (thrust) force. In such cases the coupling will transmit these forces to the nearest bearing taking the thrust load. Bearing may not give sufficient service life in such cases. To minimize these unforeseen forces, proper axial alignment shall be done and proper couplings shall be selected to take care of such un-foreseen forces.  Backlash:-The amount of free movement between two mutually coupled rotating parts is called the Backlash. Small amount of backlash is always kept in the mating parts and it helps by providing the cushioning effect. If one half of a coupled gear coupling is held rigid and the other half can be moved to a slight amount, this is due to the play that exist between the meshing gears. This freedom of movement or looseness is called the backlash. Generally it exists in all meshing gear systems. This helps in movement of lubricant and air between the meshing gears, however in case of excessive backlash values, abnormal sound level and whistling sound will be observed in running condition which varies with speed. Reactionary loads: - If two shafts are having  parallels misalignment (offset), then the coupling’s radial stiffness will cause a bending moment to be exerted on the shafts. This force caused by the coupling due to misalignment is called reactionary loads. The quantum of force depends on the radial rigid ness and amount of misalignment. If this force is more then the bending moment of the shaft this may cause plastic bend in the week shaft. Based on the above discussion it may be noted that if the selection of coupling is not proper and / or if there is any misalignment of the shaft greater than the acceptable limits, in both cases, both the machine and coupling may fail immature due to one reason or other. Other Important Related Definitions: BORE: The central hole in the coupling hubs, that becomes the mounting surface for the coupling on the shaft is called the Bore. Close tolerances are required and OEM of the equipment normally recommends the amount of the tolerances. Depending upon the service and the speed of the equipment, tolerance limits are followed strictly to get the expected life.                   

          An example of shaft-Coupling hub hole diameter – tolerance table            

                       In normal services Coupling bores / shafts are always kept round and couplings are locked with the key and key-ways. However depending on the specific services, other mating surfaces types can be of hexagonal, square, d-shaped, tapered, and splined. In these non-standard shapes, requirement of additional Key & key-ways is eliminated. A splined bore is one with a series of parallel keyways formed internally in the hub and matching corresponding grooves are cut on the shaft. Splined bores and shafts are most commonly confirmed to Society of Automotive Engineers (SAE) standards.

DAMPING: Some couplings greatly reduce the amount of vibration transmitted between driver and driven shafts because of the damping capacity of an elastomer element in the coupling. The stiffness of the elastomer affects the rate at which vibration is damped. This property is specific in the elastomer element couplings. All-metal couplings do not exhibit this property as the most part are made of metals and have poor damping capacity.

FACTORS OF SAFETY: In rotary machines, though coupling shall be the weakest element between the driver and the driven equipment but still couplings shall be designed to perform consistently for the life of the equipment. Factor of safety is applied on the designed parts to take care of un-known forces during the actual performance. The coupling designer applies these factors to compensate for unknown elements of the product design. The factors can compensate for temperature variation, material variations, fatigue strength, dimensional variations, tolerances, and potential stress risers due to unknown reasons etc. These are generally proprietary of the manufacturers and they recommend the designed models for the specific requirements.

FAIL-SAFE: A fail-safe coupling is one that continues to operate for a period of time after the failure of the torque-transmitting element between the two hubs. This is characteristic of couplings in which some protruding ends of both halves operate in the same plane, allowing direct contact between those parts. An example of this is the jaw coupling, in which driving jaw faces push the driven jaw through an elastomer pad in compression between them; if the elastomer breaks away, the driving faces simply advance to push the driven faces directly. These type of couplings are required for very critical services where the sudden stoppage of equipment may cause up set of the whole complex. After failure of the elastomer pad, though the equipment continues to run but an abnormal metallic sound starts coming from the coupling, which indicates the failure of the coupling elastomer and sounds you to plan the equipment shutdown.

STATIC TORSIONAL STIFFNESS: A resistance to twisting action (rotational displacement) between driving and driven halves of the coupling. Stiffness is expressed in lb-inch/radian and measures the amount of angular displacement about the coupling's axis of rotation at its static torque rating. Even seemingly stiff all-metal couplings can have some degree of torsional twist.

TORSIONAL SOFTNESS: Torsional soft or hard rating is determined by dividing the dynamic torsional stiffness by the nominal coupling torque rating. Values greater than 30 are hard (very stiff). Values between 10 and 30 are torsionally flexible or soft . Values less than 10 are considered very soft couplings.

DYNAMIC TORSIONAL STIFFNESS: It is the relationship of the torque to be transmitted to the torsional angle under the loaded condition of actual operation. The dynamic stiffness will be greater than the static. The dynamic torsional stiffness can be linear, a constant value, or non-linear, an increasing value.

TOLERANCES: The amount of variation permitted on dimensions or surfaces finish of machined parts. It is equal to the difference between maximum and minimum limits of any acceptable specified dimensions.

TORQUE: In rotary motion, it is the force multiplied by the radius at which force is applied, to the axis of rotation. Torque= F x r  (Where ‘F’ is Force  and ‘r’ is radius at which force is applied)In English units (F) is in pound and (r) is in inche, expressed as in-lbs. In metric units (F) is in Newton and (r) is in meter, expressed as Newton-meters (Nm).

TORSIONAL VIBRATION: The change in torque and variation in rate of change in torque  of a rotating system cause vibration in the system. This generally happens during the start-ups and shutdown of the system.

Some other causes of torsional variation are the geometry of the rotating parts of the system e.g. internal combustion engines, cyclic and irregular torque demands of the driven equipment e.g. hydraulic press, and variations in the output of certain types of electric motors at startup etc. 

 

 

Coupling Selection

For selection of the proper couplings in industrial applications following procedure and table may be of some help. These tables are quite authentic because these have been taken from Coupling Manufacturer’s catalogue.

PerformanceThe mechanical performance required of the coupling is mainly a function of the power to be transmitted and thus of the torque and speed of the drive. Couplings are often rated in terms of the torque they can transmit at a standard speed (e.g.: 100 rev/min) and will be constrained by a limiting speed. Flexibility required in torsional and other modes should also be established. ApplicationThe application factors relate to the coupling's use in original fitting or in service. Thus size, in relation to the space available, accessibility to features requiring adjustment or maintenance, the need for lubrication and the ease of assembling/dismantling will need to be considered. GeometryGeometry factors refer to the overall space available, the tolerances required in alignment and the probability of significant misalignment

should be specified. Particularly important is the need to ensure compatibility of shaft bores with those of the driving and driven machines. EnviromentFor many couplings the ambient conditions are critical. Maximum tolerance of vibration and pollutants, and the acceptable ranges of humidity and/or temperature must be compared with the actual conditions of the system. Extremes of condition will exclude some coupling types. SafetyThe failure of a correctly selected and applied coupling is an unlikely event. However it could also be very dangerous. The reliability of the coupling selected may be difficult to establish except by reputation but manufacturers will usually quote the maximum loads and conditions. For flexible couplings the duty and total service life of the coupling are important because fatigue loading may be present. The frequency acceptable and likely manner of possible failure should be established and related to the consequences of failure in the context of the total system. CommercialAlthough price may be the most obvious commercial factor, other considerations relate to any company policy there may be in dealing with standardisation of suppliers or products, the ease and speed of obtaining the required unit and the terms of trading and reputation of the supplier.  

 Shaft Design Transmission shafts transmit torque from one machine to another machine.There is always confusion among Shaft , Spindle and Axle.Spindles are short rotating elements and donot take much torque.Axles are non-rotating elements & take only radial load.Shafts are power transmission elements which take radial as well as torsional load Shaft Loads• Torsion due to transmitted torque• Bending from transverse loads (gears, sprockets, pulleys/sheaves)_ Attachments and Stress Concentrations

Steps and shoulders are used to locate attachment (gears, sheaves, sprockets,bearings)Keys, snap rings, cross pins (shear pins), tapered pins.Use generous radius to reduce stress concentrations  Shaft Design General Considerations 1. To minimize both deflections and stresses, the shaft length should be kept as short as possible and overhangs minimized.2. A cantilever beam will have a larger deflection than a simply supported (straddle mounted) one for the same length, load, and cross section, so straddle mounting should be used unless a cantilever shaft is dictated by design constraints3. A hollow shaft has a better stiffness/mass ratio (specific stiffness) and higher natural frequencies than a comparably stiff or strong solid shaft, but will be more expensive and larger in diameter.4. Try to locate stress-raisers away from regions of large bending moment if possible and minimize their effects with generous radius and relief.5. General low carbon steel is just as good as higher strength steels (since deflection is typical the design limiting issue).6. Deflections at gears carried on the shaft should not exceed about 0.005 inches and the relative slope between the gears axes should be less than about 0.03 degrees.7.If plain (sleeve) bearings are to be used, the shaft deflection across the bearing length should be less than the oil-film thickness in the bearing.8. If non-self-aligning rolling element bearings are used, the shaft’s slope at the bearings should be kept to less than about 0.04 degrees.9. If axial thrust loads are present, they should be taken to ground through a single thrust bearing per load direction. Do not split axial loads between thrust bearings as thermal expansion of the shaft can overload the bearings.10. The first natural frequency of the shaft should be at least three times the highest forcing frequency expected in service, and preferably much more. (A factor of ten times or more is preferred, but this is often difficult to achieve).   Interference FitsComponents can be attached to a shaft without a key or spline by using an interference fit.There are two methods used to assemble these components:• Press fit• Shrink (and/or expansion) fitThe amount of interference is important. The analysis of interference follows from the equations for pressure on thick-walled cylinders. A rule of thumb that is used is one to two thousands of diametral interference per unit of shaft diameter, e.g., a shaft of two inch diameter would have 0.004 inches of interference with an attached gear hub.

Machinists use a simplified approach to this – 1/1000 of interference for each inch of diameter. However, there is a formal approach. Standards have been developed for these fits.