I need some help with mechanical advantage (pulleys)

[Handysmurf]

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.

 

[jrmichler]

Search terms that will find what you are looking far: pulley mechanical advantage. Your description would not work, see the sketch:

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.

 

[jack action]

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

 

[Lnewqban]

Welcome, Handysmurf

Please see “Double Line Rigging” on page 11 of this link:
https://manuals.harborfreight.com/manuals/68000-68999/68144.pdf

For future dilemmas with pulley rigs, the following link explains how to rig various mechanical advantage systems:
https://roperescuetraining.com/physics.php

The more pulleys and bends and length of rope, the less effective the theoretical mechanical advantage becomes due to some of the pulling energy being wasted in friction, stretching, etc.

 

[Handysmurf]

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.

 

[jrmichler]

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.