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Draft 3 – Section 7 - Belts February 17, 2014NOTE: This Draft is for CEMA Committee Use Only

No Further Distribution Authorized without Permission of CEMA.

CEMA Bucket Elevator BookSection 7

BELTSChair: Warren KnappV-Chair: Kris Gililland

Contacts and References

Name Company Email Address

Raul Morales, Chapter Owner

Rexnord [email protected]

Chris Tarver Maxi-Lift [email protected] Hannigan CEMA [email protected]

DRAFT / REVIEW* HISTORY*Once REVIEWED, any changes should be noted on the draft by the reviewer and documented below, then sent to CHAPTER OWNER, for approval. DO NOT FORGET TO COPY THE Chair/V-Chair, when sending reviewed material to chapter owner.

DRAFT NUMBER/REVIEWS

DATE SUBMITTED BY:

Draft 1 2/12/2013 Raul Morales

Draft 2 2/14/2013 Chris Tarver

Draft 3 2/17/2013 Phil Hannigan

REVIEW 1

REVIEW 2

FINAL



Steel Core Belts

Steel core belts for bucket elevators were developed in response to the market demands for greater elevator heights and/or larger volumetric material handling capacities, exceeding the limitations of chain and textile core belts for very tall bucket elevators.

For example, in the cement industry the requirement for greater energy efficiency has resulted in new plant designs. Multiple stage preheaters and pre-calciners make use of the waste heat from the kiln and clinker cooler to pre-heat and pre-process the kiln feed, and thereby allow for considerable energy savings. Whenever economically feasible a wet process kiln can be converted to a state-of-the art dry process production facility that includes either a multi-stage preheater, or a multi-stage pre-heater and a pre-calciner. Such transformations are usually feasible for new plants and major upgrades. Kiln systems with five cyclone preheater stages and precalciner are considered standard technology for ordinary new plants. Typical efficiency values that can be obtained with the use of multi-stage preheaters and precalcination is provided in the table below*:

Heat Consumption of Different Cement Kiln Technologies (IEA, 2007. p.145))

Process Fuel Consumption (GJ/t-clinker)

Wet processes 5.9 – 6.7

Long dry processes 4.6

1 stage cyclone preheater 4.2




5 stage cyclone preheater 3.0 – 3.1


*Source: Industrial Efficiency Technology Database; a project of the Institute for Industrial Productivity

Higher efficiency requirements are creating an increased need for multistage preheater towers that have six cyclone stages. Since preheaters operate in a downward flow each additional stage requires a taller structure and, thus taller bucket elevator to transport the raw kiln feed. These bucket elevators are amongst the tallest in the world, commonly achieving inlet to discharge lifts in excess of 360 ft (120 meters).



The center distances of bucket elevators required for preheater towers and many large grain storage facility silos exceed the maximum height and/or capacity of chain and textile core belt bucket elevators, which are constrained by several factors:

Chain elevator heights are limited by the added weight of the chain supported below, which is additional to the weight of the buckets and material being elevated. The added chain weight reduces the amount of material that can be elevated before exceeding the chain’s working load limits and requires the use of an oversized head shaft. Even though chain elevators have been successfully built and operated reliably for discharge heights exceeding 200 feet (61 meters) these might not be sufficiently tall for some of the industry requirements.

Steel core belt bucket elevators can operate at higher speeds than chain elevators, achieving speeds in excess of 500 fpm (2.5 m/s) which allow for increased capacity.

Textile core belts present elongation that is proportional to the center distance between shafts and is exacerbated under high operating capacities or the weight of heavy gauge buckets required for abrasive materials. It is not unusual for total elongation of a textile core belt in a



bucket elevator to be in the 1.5-2.0% range. For an elevator with 200 ft (61 meter) centers the belt elongation will be approximately 6-8 ft (1.8-2.4 m), thus requiring 3-4 feet (0.9-1.2 m) of take-up travel. Since this exceeds the normal range of the take-up travel it will require at least one instance of shortening and re-splicing the belt and removal of one or more buckets.

Proper belt tracking is sometimes difficult to achieve in tall textile cord belt elevators due to the inherent elasticity of the belt.

Higher lifts are possible due to the strength-to-weight ratio of the steel cord belt, which can achieve ratings of tensile ratings of 4000 N/mm, which at a safety factor of 10 could roughly be equivalent to 2,280 PIW rated working load. Textile core belts normally rate at a maximum of 1,000 PIW. It is important to note that steel cord belts are not rated using the PIW rating system customary in the USA; they are rated based on N/mm breaking strength.

Construction of Steel Core Belts

There are three main concepts utilized in steel cable configuration within the belts:

a) Evenly spaced longitudinal cables: Steel cables running only in longitudinal manner and evenly spaced throughout the belt width.

b) Longitudinal cables with cable free zones: Steel cables running only in longitudinal manner having cable free zones along the locations of the bucket attachment bolts.

c) Weft-mesh construction, which combines steel cables evenly spaced throughout the belt width with a transversal weft arrangement. Optional with longitudinal cable-free zones for bucket attachments.



Steel cable belts running only in longitudinal configuration.



Weft Mesh (steel-fabric) construction

This belt for bucket elevators is a steel carcass rubber covered elevator belt constructed with special quality, low elongation yet high elasticity steel cords in the length and cross rigid cables in the width.

Their construction and characteristics differ from those of traditional steel cable belts. They are destined for heavy duty industrial applications with long centre distances, requiring stable running and reliable belts with high safety factor.

Elevator Belts consist of a steel carcass in a solid rubber mass that cannot delaminate. The built-in elasticity allows running over slightly crowned pulleys while the cross rigid weft construction results in excellent straight tracking characteristics.

The structure of weft mesh style elevator belts is a steel carcass composed by a strong warp of steel cords with suitable elastic modulus, longitudinally aligned, such to reach the best compromise between

Typical cross section of a steel cable used in elevator or conveyor belts



low elongation and good flexibility. This characteristic makes the belt easier to be aligned than traditional steel cord elevator belt which only uses longitudinal cables.

The minimum cover used for this type of belt is 3 + 3 mm.

The most important characteristics of the metallic meshes used are:- Maximum elongation of 0.5% (0.3% for the best quality belts) at full working load, even for the longest belts. - Possibility of constructing the mesh with “free areas” (the standard construction of the mesh does not include areas free of warp cables).- The cables used in the warp direction (length of the belt) along with the weft direction cables (transverse to the belt) form a compact mesh with a structure which is resistant to deformation and offers enhanced resistance to potential damage. - The open construction of the cables allows the belt rubber to penetrate the structure, thus avoiding the potential corrosion of the cables in the case of accidental cover damage..Furthermore, the cable elasticity allows the use of smaller diameter pulleys with, depending on the requirements of belt splice (clamp) and buckets.

Two regular steel wefts placed on top and bottom cover give to the belt high transversal stiffness, necessary to assure the best stability during its operation; at the same time, it helps to significantly maintain belt integrity in the area of bucket bolts, highly increasing cutting and tear resistance and reducing the risk of bucket dislodgment.

Steel weft mesh construction showing warp (longitudinal) cables, which have a larger diameter, and weft (transversal) cables. Mesh is secured in desired configuration by the zigzag brown nylon cords.



Due to the high quality of steel cords required for the carcass construction, it is possible to design weft mesh style belts with lower safety factors than the ones required in traditional cable-only construction without cable free zones. It is critical to note that, since some cords are broken during the punching process, it is always recommended to use a minimum safety factor of 10 calculated considering the useful belt width. However, if very low elongations are required the safety factor should be doubled.

IMPORTANT: Always consult the belt manufacturer for specific recommendations regarding safety factors. The number of bucket attachment holes punched, size of these and separation between them will affect the belt strength, thus it is always best to contact the belt manufacturer for guidance on selection and ratings.

“Anatomical” display of steel weft belt construction



Belts with cable-free zones

Some elevator manufacturers prefer to utilize steel core belts with sections that are free of longitudinal (warp) cables. These cable free zones are located in the same positions as the bucket attachment bolt holes, with the intention of making the belt hole punching process easier, since it does not require cutting through the steel cables. Some manufacturers’ strength ratings of the belt are based on this construction, thus the safety factors required are usually lower than when the holes are punched through cable sections as in the standard weft construction where the minimum safety factor of 10 is used to partially offset the decreased belt ratings caused by hole punching.

One drawback of cable free zones that do not include weft (transversal) cables is the potential of the bucket attachment bolts ripping the belt longitudinally along the cable free zones, as shown in the image below.

Steel weft construction with warp cable-free zones. The transversal weft cables are maintained for integrity and uniformity.

Steel cord construction with cable-free zones. This design does not include weft (transversal) cables.



Belt Splicing

Belt Splice - Steel Cord

The belt splice is one of the most critical components in the belt elevator system. Due to the very high tension which steel cord belts are built to sustain, the belt splice must be designed and selected with the following key features in mind:

strength resistance to fatigue ease of installation

The strength of the splice comes from the clamping force that is exerted on the belt ends from the two outer splice plates and the center plate. Standard splice fasteners use locknuts to reduce the risk of bolts loosening. Because the belt will compress after initial installation and potentially will creep during operation, this requires other standard bolt designs using flat washers to be re-tightened periodically. The mechanical cord clamp at the end of the splice joins the bare ends of the steel cords to provide an added safety factor to the belt splice.



The belt rating will determine the required style of cord clamp to be utilized. In the image below the splice for the belt section is the same but the rating is determined by the length or number of sections in the cord clamp(s).



A premium design splice design is available in which the splice fasteners use disc springs to maintain bolt tension so that the fasteners do not suffer fatigue. The disc springs are designed to compensate for the inevitable creep of the rubber belting which requires other splice designs to be re-tightened periodically.

Other belt splice designs utilize molten metal or epoxy for cord end entrapment. These processes require following detailed installation procedures which must be carefully performed due to the inherent danger of handling hot molten metal or risk improper mixing of epoxy which would result in a weak bond.

Belt Splices and Splicing

Belt splices are used to connect two ends of belt and therefore allow the belt to make a continuous loop around the head and tail pulleys. Following are various types of belt splices:

1. Mechanical Belt Splice

The mechanical belt splice (ie Maxi-Splice) is a mechanical clamping device with a simple three-piece construction. Two are identical outside plates, and the third is a center plate with an elongated center hole. The outside plates and the center plate are “mating” parts with peaks and valleys opposite of each other for firm gripping. The splice functions by using the tension supplied by the belting. This tension on the belt ends pulls the outer plates apart and forces gripping pressure towards the teeth on the splice unit. The greater the belt tension, the more pressure is exerted on the gripping teeth at the forward end of the splice. The splice is held together by a ½” diameter, grade 5 bolt. The length of the bolt varies depending on belt thickness. Use 4” long bolts for belts up to ¼” thick. Use 4-1/2” long bolts for belts between ¼” and ½” thick, and use 5” long bolts for belts between ½” and ¾” thick.

Premium style splice with disc springs to ensure proper bolt tightening is maintained



Mechanical belt splices are manufactured out of two different materials, a bronze alloy (AB) and hot-dipped galvanized cast iron (CI). AB splices are typically used in agricultural and heavy duty industrial applications as they are non-sparking, non-corroding, and non-rusting and are usable on belts of up to 800 PIW tensile. CI splices are typically used in industrial applications as they can spark and can corrode and are usable on belts of up to 600 PIW tensile. Each splice is good for 2” of belt width (Ex: A 10” or 11” wide belt would use 5 splices.). Template tape that will peel and stick directly onto the belting is typically provided and shows the location of all the holes and splices to simplify installation.

For belt heavier than 800 PIW sometimes encountered at river and export facilities and 24 hour processing plants, even more substantial mechanical belt splices are utilized (ie Titan Splice). This three piece splice is constructed out of aluminum and is non-sparking / corrosion resistant. It is constructed of a central aluminum vice-grip section which is bracketed by two exterior solid aluminum clamps. Utilizing clamping force and friction to secure its load, the belt ends are bent through a 75 mm radius to a 90 degree angle. The central wedge is equipped with a rubber backing on the pulley side to prevent belt wear due to pressure and friction generated in operation. This type of splice is currently being used on 1400 PIW belting.

Excel Spreadsheets

Pictures

Maxi-Splice AB (Bronze Alloy)

Maxi-Splice CI (Zinc-Plated Cast Iron)



Titan Splice (Aluminum)

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