I need some help with mechanical advantage (pulleys)

In summary, the more money you spend, the longer the duty cycle, the more likely the winch will work for years, the larger the cable drum, and the stronger the winch.
  • #1
Handysmurf
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TL;DR Summary
I think my question is about weight distribution in a pulley system. Google and YouTube have both failed me.
I'm wanting to get a winch for my truck for vehicle recovery, but they are expensive. I know if I get a couple of snatch block pulleys, I can get a "smaller" (and less expensive) winch. My problem is that I don't know if the cable and pulleys need to be rated at or above the winch capacity, or the actual weight of the load? Just to throw in some numbers, if I buy a 3,500 Lb winch + 2 pulleys, and I try dragging a 5,000 Lb truck out of a ditch. Should I be using cables and pulleys rated at 4,000 Lbs or 8,000 Lbs?
Also, this would be routed as:
From my winch, to a pulley attached to the load, to a pulley attached to the frame of my truck, to an anchor point on the load. Which I think would double the "capacity" of my winch.?
Any help would be greatly appreciated.
 
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  • #2
Search terms that will find what you are looking far: pulley mechanical advantage. Your description would not work, see the sketch:
Winch.jpg

You need only one pulley to get double the pull. Note that the winch will pull two feet of cable for each one foot that the load moves, so you will only pull half as far. Make absolutely sure that the pulley is rated for 8,000 lbs, a typical hardware store pulley is nowhere near strong enough. Also, make sure that the pulley diameter is large enough for your cable. By the time you spend the money for a strong enough pulley, and hook everything up, you may wish you had just bought the heavier winch.
 
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  • #3
The winch pulls 3500 lb on the cable. No matter how many pulleys it goes through, the cable will still have 3500 lb of tension. If you can pull a 7000 lb load with the pulley system, it is because you are pulling with 2 "lengths" of cable, each having a tension of 3500 lb. If you got 4 "lengths" of cable, it will pull 14000 lb, and so on.

Of course, in each case, the speed of the pulling action will be reduced by the same factor (twice as strong means twice as slow).

Reference: Pulley on Wikipedia
 
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  • #4
Last edited:
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  • #5
Please see “Double Line Rigging” on page 11 of this link:
https://manuals.harborfreight.com/manuals/68000-68999/68144.pdf

Dude, that's the winch I was looking at. I was try to decide between the 3,500 lbs or the 5,000 lbs. While I was looking at the manual that you linked to, I noticed the 5% duty cycle. If I cut the speed in half to get more force, but I can only run the winch for 45 seconds per 15 minute interval (so it doesn't overheat), it would take an hour to move the load about 10 feet. Perhaps I should spend a little more money on a better winch.
 
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  • #6
When buying a winch, you get what you pay for. That includes:

1) Duty cycle. More money gets a longer duty cycle.
2) Life. A cheap winch will probably pull the rated load once or even twice. But don't count on more than that.
3) Reliability. The better winches will still work a few years from now.
4) Cable drum diameter. Cheap winches have small diameter winch drums. That shortens the life of the cable. Probably acceptable if it's used only a few times.
 
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What is mechanical advantage?

Mechanical advantage is the ratio of the output force produced by a machine to the input force applied to it. It is a measure of how much a machine can multiply the force applied to it.

How do pulleys affect mechanical advantage?

Pulleys can increase or decrease mechanical advantage, depending on the configuration. A single fixed pulley does not change the mechanical advantage, while a movable pulley can double it. A combination of fixed and movable pulleys can further increase the mechanical advantage.

How do you calculate mechanical advantage with pulleys?

The mechanical advantage of a pulley system can be calculated by dividing the load (output force) by the effort (input force). For example, if a pulley system produces an output force of 100 pounds with an input force of 20 pounds, the mechanical advantage would be 100/20 = 5.

What are the different types of pulleys?

There are three main types of pulleys: fixed, movable, and compound. Fixed pulleys have a stationary axle and change the direction of the force. Movable pulleys have a movable axle and can increase mechanical advantage. Compound pulleys are a combination of fixed and movable pulleys and can greatly increase mechanical advantage.

How can mechanical advantage be used in real-life applications?

Mechanical advantage is used in many real-life applications, such as lifting heavy objects with cranes, pulling objects with winches, and even in simple machines like scissors and wheelbarrows. It allows us to use less force to accomplish tasks that would otherwise be difficult or impossible.

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