Multiple Drive Conveyor Requirements

Long or heavily loaded conveyors require more than one drive to keep chain tensions within allowable limits. For long life and reliability it has been found to be best to keep working tensions below 4 to 6% of ultimate strength. EXAMPLE. For a #458 chain with ultimate strength of 48,000 lb the maximum load is 2500 lb. Higher loading may be used with slow speed, few turns, and little change in elevation. Multiple drives permit use of lighter, standard construction and practically unlimited length. Surge, which would cause trouble through paint spraying, for example, is reduced by low chain pull and by strategic location of drives.

The common problem in all multiple drive applications has been control of slack chain without building up excess tensions and overloads. Conveyors passing through ovens must have means to take up elongation of chain from heating and to relax when chain shrinks on cooling. Also chains continually wear. For example, a typical 7800-ft conveyor will lengthen by 1 in. every 8 hours. The classic method requires that drives have high-slip characteristics, that is, will slow down under increased load. Any difference in speed will develop an increased load on the faster running drive and decreased load on the slower. This causes the faster drive to slow down and/or the slower drive to speed up.

Constant Speed. If chain can be kept taut at all times, even standard a-c motors will divide loads. Constant-speed conveyors can utilize high-slip (8 to 13%) a-c motors. Tests show the load on each motor will be equal even though chain tensions at the drive may be widely variable or different. High-slip motors work best with moderate drive pulls.

Variahle speed, particularly with remote control of speed changes, becomes a more complicated problem. Drives must divide loads but not necessarily equally. Drives can seldom be located at ideal points with equal loading because of clearances, ovens, process equipment, and other interferences.

Control Methods. Change of drive speeds in response to chain pull variations may be made (1) by developing excess forces at drive or (2) by using a feedback signal from a control takeup in conveyor path. The first method has been used most frequently but requires manual adjustment of elements for correct results. The second can automatically compensate for variations in load, speeds, and components automatically. The method selected depends on types of variable speed devices used on the drives.

Driving means used have been as follows:

1. Constant-speed, normal torque, a-c motors with variable speed transmissions.

2. Variable-frequency, high-slip a-c motors.

3. Direct-current motors with high-slip characteristics, either by compound winding or armature dropping resistors and shunt field control.

4. Eddy current clutch motors with' electronic control with torque limiting and adjustable slip characteristic features.