How do I calculate the required torque for a drum with a wire rope load?

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Discussion Overview

The discussion revolves around calculating the required torque for a drum with a wire rope load, focusing on the relationship between the drum's dimensions, the wire rope's diameter, and the number of wraps on the drum. The context includes practical applications in hoisting systems and winching scenarios.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires whether to add the diameter of the rope to the radius of the drum to calculate the minimum torque necessary for a motor to hold a known weight on the rope.
  • Another participant suggests that the number of wraps of cable on the drum must be considered, noting that the effective capacity of a winch decreases with additional layers of cable.
  • A question is raised about the type of hoist system being discussed, indicating that different standards and requirements may apply depending on the system.
  • A participant describes a specific setup involving a motor and gearbox connected to a drum, proposing that the effective drum radius should be calculated by considering the root diameter and the rope diameter.
  • Another participant provides a measurement suggestion for determining the effective radius, emphasizing the need to account for the Lebus grooving in the drum design.
  • A participant expresses confusion regarding the rationale for subtracting one inch from the total diameter calculation, prompting a clarification on how to compute the effective diameter.

Areas of Agreement / Disagreement

Participants express differing views on the calculation methods for determining the effective radius and torque requirements, with no consensus reached on the correct approach or the rationale behind specific measurements.

Contextual Notes

There are unresolved assumptions regarding the impact of the number of cable wraps and the specific design features of the drum, such as Lebus grooving, which may affect the calculations.

wzrdstrm
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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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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.
 
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.
 
Last edited:
Confused, why subract one inch? the total diameter would be

2 X Rope diameter + Root Drum Diameter
 
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