Womack Data Sheet 6: Hoisting and Cable Tensioning with Cylinders
Hoisting with Cylinders
Figure 1 - Direct Pull
When used in a direct pull, with one cable supporting the load, the ratio between load lift height and cylinder stroke is 1:1. In this example, to raise a 1000 lb. weight a height of 6 inches, the cylinder must have a 6-inch stroke and must develop 1000 lbs. pull in retraction. Since the load is supported on one cable, the cable tension will be 1000 lbs.
Figure 2 - One Cable Wrap
By using suitable idler pulleys, two cables support the 1000 lb. load, with a tension of 500 lbs. in each cable. The cylinder-to-load ratio is now changed, and a 12-inch stroke is required to lift the weight a height of 6 inches, but the cylinder force is reduced to 500 lbs.
Figure 3 - Multiple Cable Wrap
The use of additional idler pulleys and more wraps further changes the cylinder-to-load ratio. For example, with four cables supporting the load, the tension in each cable is reduced to 250 lbs. The cylinder stroke must be increased to 24 inches, but a pull of only 250 lbs. is required.
The table shows that at the same HP input, a substantial savings can be realized by reducing the cylinder bore and increasing its stroke. This is the important advantage of a multiple wrap system. You can buy more HP per pound and per dollar with small bore, longer stroke cylinders.
|Cylinder Description||Weight, Lbs.||Price|
|4" bore x 6" stroke||55||$176.00|
|2" bore x 24" stroke to do same job||31||$127.00|
|6" bore x 6" stroke||155||$385.00|
|3 1/4" bore x 24" stroke to do same job||73||$190.00|
|8" bore x 6" stroke||310||$675.00|
|4" bore x 24" sroke to do same job||95||$244.00|
On multi-wrap systems, the savings on cylinder cost by using a smaller bore cylinder may be far greater than the extra cost of mechanical pulleys. Further advantages of a multi-wrap system are: lighter cylinder weight; and the cable, while longer, can be smaller in diameter, lighter in weight, more flexible, and can be less expensive.
One disadvantage of multiple wrap is the additional friction load of pulleys and other mechanisms, and the internal friction of the wire rope. The chart below, given by one authority, shows the extra pull to be added to make up for friction of active pulleys. For each idler pulley in the system, an additional 5 to 7% may also be needed.
2-part line (2 cables supporting the load) - add 18%
3-part line (3 cables supporting the load) - add 25%
4-part line (4 cables supporting the load) - add 33%
Figure 4 - Suspended Load on a Cable.
This is a simplified method of finding cylinder force for tensioning a cable, and was derived from trigonometry, although a knowledge of trig is not necessary in order to use the method. Ratio values in the left column of the chart are "cotangent" values taken from a standard table of natural trigonometric functions. Multipliers in the right column are "cosecant" values for the same angle. For intermediate ratio values or those beyond the chart, regular trig tables may be used.
Assume the weight of the cable is relatively small as compared with the suspended weight. Tension in each leg of the cable will be equal if the weight is hanging in the exact center of the cable. Otherwise, the tension will be different in each leg. Solve for each leg.
Hydraulic Motor Torque...
When pulling tension on a loaded cable with a winch drum driven with a hydraulic motor, use procedure described above for finding cable tension. Then solve for torque on winch drum by taking tension times drum effective radius. Then calculate theoretical motor torque by dividing drum torque by ratio between drum sheave or sprocket and hydraulic motor sheave or sprocket.
Example: Cable tension = 15,000 lbs; winch drum diameter 14 inches; winch drum drive sprocket diameter= 2 inches; motor sprocket diameter = 6 inches. Find theoretical torque required from the hydraulic motor.
© 1988 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.