# Force calclulations on a drum

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## Main Question or Discussion Point

I'm doing some work were its necessary to calculate the force acting on a drum via a wire rope. If I have a known weight acting on the rope already and I want to back calculate the minimum torque necessay for a motor connected to that drum to hold that load, do I add the diameter of the rope to the radius of the drum to do so?

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Mech_Engineer
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It depends on how many wraps of cable you have on the drum. In general you will need to take into account the diameter of the wire for a couple of wraps on the drum.

Here's an example in off-road vehicle winching:

http://www.pangaea-expeditions.com/resources/winchworksheet/index.html
Winch capacity

One of the common misconceptions regarding winches is that the maximum rated winch capacity is available any time the winch is hooked up. Unfortunately, this is false. Maximum winch capacity is in fact determined by the number of layers of cable wrapped on the winch drum.
...
As you can see based upon this chart, the number of layers of cable or winch rope on the drum greatly affects the winch capacity. With the top layer losing almost half the pulling power.

What sort of hoist system is this? If a crane or elevator, then all sorts of standards and requirements apply.

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Simple design only, not for commerical review/standards. We have a motor connected to a gear box (1:1) ratio, connected to a drum that has wire rope wrapped half way across the drum on the root layer (1st layer). What I am expecting is that that if the drum had a root diameter of 100 inches (including lebus grooving) and a roper diameter of 2 inches, for calculation purposes we would have an effective drum radius of 52 inches for the first layer. Is correct?

Center of drum to center of rope. 51 inches. But if you measure carefully, the Lebus shell will probably make that number a little different. Just measure total diameter including the rope layer and divide by 2. Then subtract one inch.

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Gold Member
Confused, why subract one inch? the total diameter would be

2 X Rope diameter + Root Drum Diameter

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