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Introduction

In the power transmission industry, shaft-

mounted speed reducers provide one possiblesolution to meet the speed reduction/powergeneration needs for an application. As thename implies, a shaft-mounted speed reducer(SMSR) is a speed reducer mounted directlyonto, and statically supported by, a drivenshaftthe type of setup that may be found ona conveyor. Typically, the SMSR incorporates ahollow bore (with keyway, tapered bushing orsome other coupling mechanism) to facilitatemounting onto the driven shaft.

The SMSR may be fixed to the machine

using an output flange or, in certain instances,its housing may be bolted directly onto themachine. However, situations exist where directattachment of the SMSR is either not possibleor desirable. In such situations, the SMSR issupported only by the shaft that it is intendedto drive. When the SMSR is mounted directlyonto a driven shaft with no other externalsupport, it must have a torque arm attached toit. A torque arm is a pivoted, connecting linkbetween the reducer and a fixed anchor pointintended to resist the torque developed by the

reducer. Quite simply, a torque arm transmitsthe reaction torque produced by the SMSR

Torque Arm Design

Considerations for

Shaft-Mounted

SpeedReducersTodd R. Bobak

Figure 1Manufacturers often provide multiple mounting holes for theplacement of a torque arm, as seen in this picture of a Sumitomo Cyclo BevelBuddybox.

Figure 2Output torque capacity can be determined from a manufacturersspec sheet based on the models motor input speed rpm and gear reductionratio.

Todd R. Bobakhas worked in the gear

industry for 15 years. He has held positionsin technical service, design and development,

and quality assurance. He is a product en-

gineer for Sumitomo Drive Technologies in

Chesapeake, VA.

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into the structure of the machine, therebypreventing the counter-rotation of an SMSRduring operation. Most manufacturers of shaft-mounted speed reducers have designed, andoffer for purchase, standard torque arms fortheir products. Situations may exist, however,

where a manufacturers torque arm does notmeet the needs of a certain application (dueto space limitations, for example). In suchsituations, the machine designer may berequired to design his or her own torque armto fit within the constraints of the application.

This article provides some design guidelines forsuch a situation.

Resultant Force

In considering the design of a torque arm,first you need to understand the amount of load(or force) that the torque arm may see duringunit operation. To determine this force, youneed two pieces of critical information:

1. The torque capacity of the SMSR. As thename implies, this is the mechanical capacity(typically expressed in terms of inch-poundsor Newton-meters) that the SMSR is capableof developing at its output. While it may betempting to simply multiply the motor outputtorque by the reducers reduction ratio to obtainan output torque value, it is recommendedto consider (at a minimum) the larger of thefollowing values: the SMSRs output torquecapacity (as listed in the product catalog) or,the product of the motors output torque andreduction ratio times an additional 2.5to takeinto account the motors starting torque.

2. The dimensional location on the SMSR where the torque arm will be mounted.Manufacturers of shaft-mounted speed reducersprovide at least one mounting location for atorque arm (Fig. 1). Note that the point wherethe torque arm attaches to the reducer (eitherdirectly or indirectly through a bracket) isknown as the pivot point. Many manufacturersprovide more than one potential location forthe torque arm in order to accommodate the

various mounting configurations availablefor a given product. The location(s) of thesemounting holes/positions may be found incatalog cut sheets or through manufacturer-supplied product drawings.

Using Figure 1 as an example, lets say anapplication requires a bracket-style torque arm,and this bracket will be attached to the housingat both identified mounting holes. Using theunit model size, reduction ratio, and motorinput speed, you can determine the outputtorque capacity from the reducer catalog ratingstable (Fig. 2).

You can also use the catalog dimensionsheet to determine the location of the identified

mounting holes relative to the center of thereducers output bore (Fig. 3).

With this information, you can nowcalculate the force acting at the selected pivotpoint, given the fact that torque is the productof force and distance. You can determine theunknown force (F) using these formulas:

Torque (T) = Force (F) Distance (L), orF=T/L (1)

It is important to note that during operation,the SMSR will tend to rotate about the drivenshaft in the direction opposite to that of thedriven shafts rotation. With this in mind, it issignificant that the calculated force F (above)acts in pure tension on the torque arm. This factis especially important for torque arms that arelongif they are exposed to compressive forces,they may buckle. Figure 4a shows the correct

Figure 3Manufacturers specifications will also reveal the exact location ofthe mounting holes relative to the center of the reducers output bore.

Figure 4aIn order to maintain pure tension loading for the torque arm,the pivot point of the torque arm should run along, or parallel to, the line ofaction. The line of action (see figure 4b) is defined as the right angle formedby a line joining the pivot point and the center of the SMSRs output bore.

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A Note About Mounting HardwareIn addition to assuring that the forces

acting on the torque arm itself do no exceedthe limitations of the torque arm material, youmust also consider the effects of the force onthe mounting hardware (nuts, bolts, etc.).

When taking this into account, you mustnote that the force is not acting on the hardware

in tension, but rather in shear. Depending on thelocation of such hardware, bending movementsinduced by the force may also be acting on thehardware.

Hardware manufacturers usually provide yield and shear limitations for their products.Once again, it is important to compare thecalculated forces/stresses acting on the hardwareagainst the published limiting values to assurethat the critical points are not exceeded.

A Note About Reversing RotationIn reversing applications, the output of the

SMSR rotates in one direction for a periodof time, and then reverses and rotates in theopposite direction for a period of time. Thisapplication type requires considering thecompressive forces that the torque arm mustalso bear.

Recalling that, ideally, a torque arm ismounted so it is in tension, in a reversingapplication, the torque arm may see a period oftime when it also receives compressive forces.Such compressive forces, if great enough, couldbuckle the torque arm. For such applications,

designers may want to add a second torquearm on the opposite side of the SMSR. Thesecond torque arm would take the tensionforces generated by the reverse rotation whileminimizing the compressive forces on the firsttorque arm.

Angles Other Than 90Although it is important to maintain a

pivot point at 90 to the center of the SMSRoutput hub, it may not always be possible dueto limitations in the machine onto which theSMSR will be mounted. This is not uncommon,

and reducer manufacturers who supplyturnbuckle torque arms with their SMSR unitsoften note that the torque arm may be mounted

with an angular variation, such as 15.In such situations, the designer needs to

evaluate the effects of the resultant forces actingon the torque arm. Figure 10 shows a torquearm mounted to an SMSR at some angle ()that is less than 90.

Because the torque arm is not acting at a 90angle to the output bore of the SMSR, there aretwo forces acting on it. The free-body diagram

in Figure 11 details these forces. The resultant forces (FT and FS) are the

Figure 7A typical turnbuckle.

Figure 8A typical mounting style for a turnbuckle-style torque arm.

Figure 9A turnbuckle-type torque arm being used on a Sumitomo helicalhaft mount unit.

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Figure 12Rubber bushings installed at the anchor point and/or pivot pointwill provide float to compensate for runout in the driven shaft.

component forces of Force F acting on thetorque arm. These resultant forces place notonly a force in tension (FT) acting on thetorque arm, but also a shear force (FS). Bothforces must be considered when evaluating thestrength of the torque arm design.

The Effect of Shaft Runout

Despite a high degree of accuracy inmanufacturing processes, shaft runoutsometimes occurs. Such runout (on the drivenshaft) may cause the reducer to wobble onthe shaft during operation. This could beparticularly troublesome if the torque arm isrigidly mounted to the anchor point. A rigidlymounted torque arm attached to an SMSRdriving a shaft with a high degree of runoutmay result in (among other things) a brokendriven shaft, cracks at the anchor point, adecrease in the life of the output bearings ofthe SMSR and/or the conveyor and lubricationleakage. Introducing some float in the torquearm at the anchor point will counteract theseproblems.

Float may be introduced at the pivot pointby attaching rubber bushings to the pivot andanchor points, as shown in Figure 12.

If dual rotation of the output shaft is possiblein the application, manufacturers recommendinstalling rubber bushings on both sides ofthe pivot and anchor points so float is possibleregardless of the direction of rotation.

A loose clearance designed between thebolts and through holes in the torque armbracket may also assist in creating float.

In addition to taking precautions toeliminate a rigid mount, designers must alsoensure the torque arm is properly aligned whenmounting it to the SMSR. Misalignment mayinadvertently cause binding that could decreaseor entirely eliminate this float.

Conclusion

Although seemingly simple in concept, atorque arm is an important component whenconsidering a shaft-mounted speed reducerfor an application. Before selecting a potentialtorque arm design, designers should evaluatethe offering supplied by the manufacturer of theshaft-mounted speed reducer. In doing so, theymay determine that the standard offering fitsinto the application constraints. Additionally,some manufacturers offer more than onestandard design (i.e., turnbuckle, T-type) tomeet a variety of applications. However, ifthe existing available designs fail to meet theestablished application criteria, following themethodology detailed in this article will helpto develop a functional and robust torque armdesign.

Figure 10A torque arm mounted to an SMSR at less than 90.

Figure 11When a torque arm is mounted at an angle other than 90,force F is composed of both tension (FT) and shear (FS) forces. Both must beconsidered when evaluating the strength of the torque arm design.

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