Thursday, January 29, 2015

Gear Coupling Tutorial - Part IV: Selection & Availability

Up to this point we have been describing the functional parts and modifications of gear couplings. Another attribute that needs to be described is the performance parameters of the unit. Those parameters include two most important items and many more secondary items. (The HercuFlex & Sier-Bath gear coupling product pages & downloadable PDF catalogs are the best place to find this information.) The important items are the bore and torque capabilities in that order. The secondary items can include a whole host of things like speed, misalignment, weight spacer length, inertia, etc. Torque in this case refers to normal operating torque that the coupling must transmit. Bore refers to the nominal shaft size where the coupling will be used. 

A.    Bore and Torque First Pass Selection 

HercuFlex CX Gear Coupling - by Lovejoy, Inc.
Hercuflex CX Gear Coupling
 The gear coupling selection in most cases will be determined by the nominal shaft size. If it fits it is okay. The second sort would be to check the torque requirement of the application versus the torque rating of the coupling. Normal operating torque is used unless a peak or cyclic torque is known. The application description is also important to see if further investigation is needed. Smooth running 1800 RPM machinery without high torque starting or stopping requirements can be selected with bore size. If the application calls for peak torque or cyclic torque, more care must be taken.

The normal or continuous operating torque is that torque value that is required for the design point operation of the machine on a continuous basis. The every day operating point if you will. Torque can be derived from the required horsepower at the operational speed. Coupling ratings are sometimes listed as HP per 100 RPM. Torque and horsepower can be derived from one another if the speed in RPM is also known. The equations for that can be found in the coupling catalog.

B.    Service Factors

Service factors are applied to the normal torque to account for application variations. The variations can take many forms, but they are usually application specific. Service factors allow selection on of a coupling for torque and perhaps life without going into the details of the situation. While a bore is finite, torque can be many different values. For application factors one can refer to the Lovejoy gear coupling section of the catalog or any other gear coupling catalog. 

Service factors for elastomer couplings may be different from gear couplings when applied to the same system, and it is critical that you pull this data from the specific manufacturer you intend to source from. (If you are looking for an overarching reference document, AGMA 922: Load Classification & Service Factors for Flexible Couplings can help and and provide a sanity check, but it should not trump the manufacturer's specified service factor when sizing that manufacturer's coupling.)

Service Factors are based on empirical data and experience. Safety Factors and Service Factors should not be interchanged or confused with each other. The former is for design work and the latter is for applications work. The method is multiply the normal or continuous application torque by the service factor to obtain the coupling selection torque. 

Normal torque and continuous torque are synonymous in this procedure. The torque developed by the motor may exceed the normal torque of the application, but should seldom exceed the coupling selection torque. It is also acceptable to select the coupling by applying a service factor to the motor nameplate horsepower converted to torque. The selection torque obtained this way can often result in oversize couplings. Sometimes gear couplings are deliberately oversized to extend the tooth life. Other complications may result from doing that.

(Note: Lovejoy has developed a great and easy to use tool for roughly selecting a coupling once horsepower, rpm, and service factor are known.) 

C.    Peak Torque, Continuous Torque and Cyclic Torque

Normal or continuous torque was defined in the previous paragraphs. The torque forces we are describing are those necessary to operate the machinery successfully. They may also be used to size the motor or driver. 

Peak torque can occur under several circumstances, but are usually transitory conditions. A typical peak could occur on starting if high inertia loads must be started, another might occur if an emergency stop happens, or blockage of the machinery occurs, or voltage spikes, or torsional resonance. Peak torque is usually accounted for by limiting the peak cycles to prevent low cycle fatigue in the coupling. 

Cyclic torque on the other hand could occur continuously for the life of the coupling. It might be that the application operates under a "cycle" of load and unload or it might be that start and stop is a routine of the application. The start and stop routine means low starting torque and non-emergency stops. They might be classified as slow and smooth. Cyclic loads are accounted for either in the service factor or in the coupling torque selection. Service or application factors are established to account for these torque variations based on past experience. If exact values are known they could be used to directly select the coupling.

D.    Stretching the Bore

The subject is included to highlight the fact that it is not recommended. That is do not exceed the bore associated with the coupling size and the key type. That means square keys have a maximum bore rectangular keys have another and metric has its own. Do not mix them. In general, the shaft to hub connection can be the weak point of the coupling. When extra shrink is requested, engineering should review the application. If an overbore is requested for low torque applications, engineering must check the application. The gear coupling is the smallest most power intensive coupling as it is designed. Stretching the limits can result in machinery failure as well as coupling failure.

E.    Other Considerations

1.    Speed.

We mentioned that high speed units need to be balanced to reduce vibration and subsequent damage to the connected machinery. Catalog ratings are often accompanied by speed limits in RPM. In the case of this coupling parameter it is possible to increase the value by balancing the coupling. That is mentioned in the catalog with the rating value. Balancing combined with special manufacturing tolerances can increase the speed even more. Even a perfectly balanced coupling will eventually have a speed limit. That limit comes from stress, friction between the teeth, and lubricant breakdown.

2.    Misalignment.

All couplings have a misalignment limit. That limit will vary by the coupling style and can also vary by modification to a standard product. The gear coupling is generally capable of 1 1/2 ° of misalignment per mesh. Special gear couplings can exceed that. The limit can go to 6° or more, however, a special design is required as is engineering involvement. High misalignment limits can reduce the torque capability of the coupling. High misalignment of some types are limited to non-operational conditions.

3.    Special Adaptations

Special couplings, many of which have been mentioned in this blog require engineering or manufacturing involvement before the selection or pricing is possible. When contacting Lovejoy engineering, it is advisable that as much information about the system and application be gathered in advance so that Lovejoy can quickly facilitate coupling selection for you.

4.    Materials of Construction

Standard gear couplings are manufactured from carbon steel, and can be from bar stock, casting, or forging depending on the size and the component. The material is subject to heat treatment to achieve the correct strength and ductility. Alloy steels are also available, and can be made from bar stock, casting, or forging just like the other couplings. Alloy steels are used to achieve higher strength and to facilitate hardening and strengthening of the teeth. Alloy steels, with the proper treatment, will enable Lovejoy to provide a coupling that has a longer service life. The longer service life is a result of hard teeth resisting wear. A finite number for the life is difficult to express. It is also possible to supply gear couplings in stainless steel. This is an unusual occurrence as it becomes an expensive proposition. (Stainless couplings are often required by the food processing industry.)

Steel can be treated in many ways to improve hardness and strength. The hardness is the key to longer life under conditions like high speed or heavy loads. Strength may be the requirement of couplings that have cyclic loads or have impact loads. Each application is reviewed on its own merits. There is no direct relationship that allows the coupling to be re-rated for a material change. Both bar stock and forging have the same strength characteristics and would hit the same basic specification. Cast steel couplings would have their own capabilities that are different and likely lower than bar or forging.

F.    Stock, Modified, & Made-to-Order

Couplings that are designated as stock units are the competitive coupling sizes from 1 to 15 for which we keep sufficient stock to supply most orders. Stock is based on forecast usage and updated regularly. While larger OEM type orders may require raw material sourcing and longer lead times, smaller MRO type order can generally be supplied from stock and are largely available for Quick Ship finishing if not.

1.    Stock Units

Stock units include HercuFlex & Sier-Bath flanged and continuous sleeve gear coupling hubs, sleeves, and accessory kits, universal hubs, mill motor hubs, and rigid half couplings. Hubs include pre-bored and keyed units with AGMA bore dimensions. Spacer pieces for predetermined spacer lengths and coupling sizes are available. (Please call your preferred Lovejoy Distributor or Lovejoy's customer service team to confirm the availablilty of the gear coupling components you seek.)

2.    Non-Stock and Modified

These units are often made from stocked components. The component stock may include both RSB hubs and partially machined parts and raw forgings ready to go to final machining once it is determined what final result is required. In this category are special bore and keyway requirements, metric bore requirements, hubs that are faced or counter bored, hubs that require clearance fits, some mill motor or taper bore hubs, and add on pieces like limited end float or vertical plates. Non-standard spacer lengths for smaller sized gear couplings are made from stock raw material. Short sliders can be made by facing hubs and making long tooth, standard length sleeves.

2.    Major Modification & Made-to-Order

Major modification couplings are a combination of standard parts, modified parts, special standardized parts and designed to order parts. Included with those designs are floating shafts, insulated couplings, long sliders or Jordan couplings, cutout couplings of two types, shear pin couplings, brake wheel couplings, and moderate speed units where careful machining and component balance will suffice. 

The line between major modification couplings and total made-to-order couplings is blurred. The size range from 9 to 15 is in the gray area, but sizes above 15 are definitely made to order. Precision high speed gear couplings are made to order. Some other gear couplings were designed for a specific application like subway car trucks or spindle rolls. (i.e. - A spindle coupling design by Lovejoy was limited to the smaller applications associated with aluminum rolling mills.)

To keep learning, go to:

Gear Coupling Tutorial - Part I: Overview
Gear Coupling Tutorial - Part II: Configurations 
Gear Coupling Tutorial - Part III: Mounting the Coupling
Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Note: This article series is an updated & modified version of a legacy Lovejoy training document. The blog writer is not the original source author.

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