Construction and WorkingofPivoted Motor Bases for Belt DrivesByK. P. ShahEmail: kpshah123[at] (Please replace [at] with @)Committed to improve the Quality of LifeFor more articles on mechanical maintenance, visit www.practicalmaintenance.netThe information contained in this article represents a significant collection of technicalinformation about construction and working of pivoted motor bases, also called reactivetorque motor bases for belt drives. They are widely used to automatically control the belt’stension to minimize maintenance and maximize belt life. This information will help to achieveincreased reliability at a decreased cost. Assemblage of this information will provide a singlepoint of reference that might otherwise be time consuming to obtain. Most of informationgiven in this article is mainly derived from literature on the subject from sources as per thereference list given at the end of this article. For more information, please refer them. Allinformation contained in this article has been assembled with great care. However, theinformation is given for guidance purposes only. The ultimate responsibility for its use andany subsequent liability rests with the end user. Please view the disclaimer uploaded on March 2018)1Construction and Working of Pivoted Motor Bases for Belt

Pivoted Motor Bases for Belt DrivesIn a belt drive, optimal belt tension is critical for avoiding belt slippage and excessive strainthat lead to higher maintenance costs and additional downtime. Optimal belt tension helpslengthen the service lifetime of components, such as belts and motor bearings. Two types ofmotor bases used to regulate the pressure in a belt driven system are: fixed-positionadjustable bases and tension-controlling bases.Fixed-position adjustable bases adjust belt tension via manual alteration of the centerdistance that separates a driver and driven pulleys. They allow pushing or pulling a motorinto place to install or adjust the belt. Once the belt is pulled over the pulley, single ormultiple screws force the motor away from the driven pulley until the desired tension level isattained. The mounting bolts are then tightened to complete the process. Fixed-positionmechanisms are selected due to their cost advantage over higher priced tension-controllingmechanisms.Tension-controlling bases have mechanisms that automatically alter the center distance of apulley of a running motor in response to load condition requirements. Information on types,construction and working of tension-controlling bases is given in this articleTypes of Tension-controlling BasesThe two types of motor bases used to control belt tension automatically are spring-loadedmotor base and reactive torque motor base, commonly called pivoted motor base.Spring-loaded Motor BaseSpring loaded motor bases employ built-in springs to control the belt tension/strain. In thistype of motor base, the spring force must be such that the motor is pulled back against thedrive with a force equal and opposite to the resultant belt pull, parallel to the base. Thespring must have some extra force, to overcome the friction in the base. If the mounting isnot horizontal, the effect of motor weight must be taken into account. Spring-loaded basesare most commonly used on industrial drives with motors from 5 to 10 HP, and occasionallyon larger drives.Reactive Torque Motor Base (Pivoted Motor Base)Reactive torque motor base uses the reactive torque of the motor to help tension the drive.As shown in the following figure, when a motor is delivering power to a sheave or pulley, itsframe tries to rotate in a direction opposite to that of the sheave or pulley. With the motor in a2Construction and Working of Pivoted Motor Bases for Belt

cradle with a pivot point near the motor shaft, the motor can move away from the drivensheave, tensioning the drive. As torque increases, tension increases.The reactive torque motor base must be installed so that the direction of rotation is correct. Ifthe motor shaft were rotating in the opposite direction, the reactive torque would actuallyloosen the drive, rather than tighten it. Operating in the same direction, both the reactivetorque and the motor weight act to tension the drive.Construction of Pivoted Motor Bases (Reactive Torque Motor Base)Above figure shows typical constructions of pivoted motor bases with tension spring andelastomeric tensioning unit. Depending on the construction, final belt tensioning is carriedout by adjusting the mechanical pretensioning device (the tension springs of motor base withtension spring or the pretensioning device of motor base with elastomeric tensioning unit). Incase of motor base with elastomeric tensioning unit, the pretensioning device adjusts torsionin the rubber suspension unit (often called torsion spring of elastomeric tensioning unit).Elastomeric tensioning motor bases utilize a rubber suspension unit as a pivot mount. Therubber suspension unit continuously compensates for belt stretching, hopping, fluttering andexcessive pull when a drive is started. Elastomeric tensioning motor bases are the idealtensioning solution for all belt drives from about 1/2 to 700 HP.Because elastomeric tensioning motor bases utilize rubber inserts, generally they can beused for applications operating within a 40 to 180 F ( 40 to 80 C) temperature range.3Construction and Working of Pivoted Motor Bases for Belt

Working of Reactive Torque Motor Base (Pivoted Motor Base)The following figure and text illustrates working of a reactive torque motor base (commonlycalled pivoted motor base).Automatic Tension ControlIncrease in load (e.g. starting load) activates reaction torque, which operates in a directionopposite to the rotation. This force pushes down on pivoted arms.As arms swing down, motor pivots on arc A-A, increasing center distance. This builds up belttension in direct proportion to load.When drive is at full speed, load drops back to normal, reaction torque disappears, armsswing back up, decreasing CD, and belt tension returns to normal operating condition.Automatic Belt Take-UpWith a portion of the motor weight offset away from the driven unit on the pivoted arms in anamount sufficient to provide for the initial belt tension requirement, the force of gravity givesthe pivoted motor base its automatic and continual belt stretch take-up feature (because themotor weight always acts to tension the drive). This keeps drive maintenance to an absoluteminimum.4Construction and Working of Pivoted Motor Bases for Belt

Smoothing VibrationDue to the fact that the motor "floats on the belts”, many types of minor vibration areeffectively smoothed out by the pivoted motor base.Installation InstructionsIn a pivoted motor base, pulling side (tight side) of the belt must come between motor shaftand pivot shaft.Insure that motor shaft, pivot shaft and driven shaft are parallel.As shown in the following figure, the unit’s base must be parallel to the motor’s base. If not,the unit will not have adequate adjustability.Over-tightened belts put an extra load on the mechanism bearings that is unnecessary andmay damage motor and motor base. Ideally, the belts should be just tight enough so thatthey do not slip during start-up.Follow the manufacturer’s instructions for the following:The distance that the motor is off-center from the pivot shaft must be large enough to carrythe starting and peak loads, yet be as small as possible for maximum belt and bearing life.Since ideal motor base position depends on its construction and application, the drivensheave (pulley) should be located with respect to the motor sheave as per manufacturer’srecommendation. For example, as per one screen manufacturer, the center of the motorsheave should be located within 30 degrees up or down from the horizontal center of thescreen sheave as shown in above figure.ReferencesBelt selection and application for engineers / edited by Wallace D. Erickson / published byMarcel Dekker, Inc. USA.TENS - A - MATIC Pivoted Motor Bases by Overly Hautz Motor Base Company, websiteaddress: by Lovejoy, Inc. USA, website address: http://www.lovejoy-inc.comROSTA Motor bases by ROSTA AG, website address: http://www.rosta.com5Construction and Working of Pivoted Motor Bases for Belt