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Variable Speed Drives

Overview

Note that the axial position of the belt on the flat-faced companion sheave changes as the drive moves through the speed range. This illustrates the need for a flat-faced companion sheave when using a wide variable speed belt and a one-side moveable variable speed pulley.The Adjustable Center Angle Mount Drive figure shows a more unusual mounting, with a one-side moveable pulley driving a grooved sheave. The motor base must be offset at an angle to keep the belt in line.A typical Lovejoy adjustable center drive will have a speed ratio – that is, the range of speed at the driven sheave – from 1.6 to 1 up to 3.8 to 1. For example, with the right combination of pulley and sheave, you can attain a speed range at the driven sheave of 1,800 to 600 RPM, 3 to 1 speed ratio.An alternate mounting method may be used when the spring-loaded pulley is mounted on the driven shaft, with the companion sheave mounted on the driver shaft. With this arrangement, the horsepower at the driven shaft is constant through most of the speed range, but the torque decreases in proportion to the pitch diameter of the belt in the spring-loaded pulley. Caution must be taken not to overspeed the driven variable speed pulley beyond 10,500 FPM (feet per minute) rim speed. Adjustable Center Angle Mount Drive Fixed Center Drive

2) Fixed Center Drives

This method of mounting requires no change in center distance between the driver and driven units. Because the shaft center-to-center distance does not change, this is called a fixed center drive. The usual arrangement has one pulley, the driver, which is mechanically adjustable. The adjustable pulley flanges are opened or closed mechanically, usually with a handwheel on the end of the pulley. The other pulley is spring-loaded and compensates for belt slack when the adjustable pulley is actuated. When the adjustable driver pulley is fully opened, the belt is at its minimum pitch diameter. At the same time, the spring-loaded driven pulley is fully closed with the belt at its maximum pitch diameter. This results in the slowest possible driven speed. As the adjustable pulley flanges are closed together, the driver pulley pitch diameter increases while at the same time the spring-loaded pulley is forced by the drive belt to open. This results in a smaller pitch diameter at the driven member. The driven speed then increases until the adjustable pulley is fully closed and the spring-loaded pulley is fully open (the maximum speed position). Speed change must always take place while the motor is running. The Fixed Center Drive figure shows a typical arrangement, with the adjustable pulley at minimum pitch diameter and the spring-loaded pulley at maximum pitch diameter. This is the minimum speed position.Fixed center drives are used where space is limited and/or where a wider speed range is needed. They are sometimes called “compound drives” because the ratio of the two pulleys is compounded, resulting in a greater available driven speed range. Speed ratios up to 11.5 to 1 are possible, but this is not necessarily in the form of a speed decrease. For example, with a 1,750 RPM motor and a Lovejoy HM-3/31901 drive package, the output speed at the driven pulley could be as high as 4,850 RPM.

Horsepower vs. Torque

Adjustable center drives with the spring-loaded pulley as the driver are constant torque drives. That is, for each variable speed pulley and companion sheave combination, the torque at the sheave is constant. Horsepower decreases in proportion to RPM at the driven sheave.Fixed center drives are constant torque from minimum speed to driver speed and constant horsepower from driver speed to maximum speed.Notes: N All Lovejoy pulleys and sheaves are balanced for operation at 1,750 RPM. Balancing for higher speed operation is available–consult our Engineering Department. N Lovejoy variable speed pulleys are bored to fit standard NEMA motor shafts. Consult our Customer Service Department for availability of non-standard and metric bores.
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