People think pulleys increase pulling power.

[MOguy]

After having a discussion recently I have realized many don't understand how to use a pulley to increase pulling power (mechanical advantage). I seems many people believe buy using a pulley you automatically increase the capability of your recovery setup. It can but you have to rig it correctly.

Here is something to read for those who are confused about this.
https://en.wikipedia.org/wiki/Mechanical_advantage_device

 

[camp4x4]

REALLY curious for you to elaborate on this... what "incorrect" rigging are you thinking this article clarifies? Or what misconception are you trying to disabuse people of?

 

[MOguy]

REALLY curious for you to elaborate on this... what "incorrect" rigging are you thinking this article clarifies? Or what misconception are you trying to disabuse people of?

Nothing incorrect. Just what pulleys do when used with in your set up. In order to increase pulling power your pulley has to be attached to the load or within the rigging, not the anchor. If you attach it to an anchor it only lets you change direction of the pull.

 

[camp4x4]

Cool. But adding pulleys, power is not increased....
I never learned much in school but at least I know power is defined as work divided by time.
Anyway, Technical splitting hairs of jargon, but anybody here at 'Expo already knows this.

First off, I'm not a mechanical engineer, just a guy trying to read equations on Wikipedia... very open to being corrected... but if power is work over time and work is force over distance... and the initial force used remains constant... doesn't the force at the load increasing when pulleys are added... make the power increase as well?

P = W/t = F * r/t = F * velocity

Say you start with 100N of force on 1 pulley and get 100N of force on some load. That load moves 1cm over 1sec. So power would be 100N*1cm/sec

Add a pulley at the load to double the force on the load.. 100N of force on 2 pulleys gets 200N of force on the load. So power would be 200N*1cm/sec meaning you can move a heavier load at the same velocity as the lighter load from before.

That's increased Force and Power... no?

I need an Advil...

Getting back to the original question.. I think the confusion MOguy is trying to clear up is the difference between running a winch line to a snatch block, then back a fixed anchor vs running a winch line to a snatch block, then back and being attached to the truck again. In the first case the only advantage is you're spooling out more winch line from the drum (also a good thing but not what we're necessarily talking about). There's no mechanical advantage to adding the snatch block because the length of the line from the snatch block to the final anchor doesn't change. In the second you DO get a mechanical advantage because the line is shortening from both the vehicle to the snatch block AND the final anchor point (again, the vehicle) to the snatch block. Same goes for going vehicle > snatch block (sb) on fixed anchor > sb on fixed anchor > fixed anchor: no mechanical advantage. If you go vehicle > sb on fixed anchor > sb on vehicle > fixed anchor then you get a 3x mechanical advantage because you have 3 lines lengths that are shortening...

Am I interpreting you correctly MOguy?

 

[Kmrtnsn]

 

[DaveInDenver]

We obey the laws of physics in this house, young man.

 

[MOguy]

Force upon the load will increase at the expense of increased time. Power remains the same.
For example, A winchrope doubled back to your Jeep doubles its overal pulling force. But also doubles length of winding back in, thus doubling time.
Power remains the same.

ONLY if the pulley is on the load or within the rigging.

I work for an organizations that moves and lifts things. I am not an operator, I am the guy that buys stuff. I had enough interest in this so I started paying attention, talking to vendors and operators. I can see it work, I can see the rigging and duplicate it put I don't understand the physics behind it.

 

[MOguy]

View attachment 440125

not correct. if you pulley is on an anchor point (something not moving) it is only change of direction. You last drawing will give you a 2:1 advantage.

As far as you weakest point, I don't know? OSHA requires our anchor points to be 5k. Our ropes, carabineers, pulleys etc. are rated weigh over that and our anchors to be "bullet proof". For those requirements we go buy NFPA.

 

[Airmapper]

not correct. if you pulley is on an anchor point (something not moving) it is only change of direction. You last drawing will give you a 2:1 advantage.

I think the extremely simplified image intends for it to show that as being attached to the Jeep. The Jeep moves....

 

[MOguy]

I think the extremely simplified image intends for it to show that as being attached to the Jeep. The Jeep moves....

no, extremely wrong image. of course the jeep moves but unless there is a pulley on the Jeep you gain no mechanical advantage, only change of direction.

Change of direction is important, the straighter the pull and the more evenly the cable winds back on the drum the less chance there is of other issues. But change of direction does not increase mechanical advantage.

 

[Kmrtnsn]

https://www.wired.com/2017/01/physics-of-a-compound-pulley/

 

[Airmapper]

no, extremely wrong image. of course the jeep moves but unless there is a pulley on the Jeep you gain no mechanical advantage, only change of direction.

Umm, it's a photo of a pulley in front of the Jeep. Red W is for winch. That means Winch to Pulley at fixed anchor, back to pulley on Jeep, back to fixed anchor. Two pulleys can move.

I'm not having any issues with it. It's a legit 3:1 pull. You are explaining why it's correct while calling it wrong.

 

[MOguy]

Umm, it's a photo of a pulley in front of the Jeep. Red W is for winch. That means Winch to Pulley at fixed anchor, back to pulley on Jeep, back to fixed anchor. Two pulleys can move.

I'm not having any issues with it. It's a legit 3:1 pull. You are explaining why it's correct while calling it wrong.

There is no 3:1 in that picture, only a 2:1. That picture is wrong.

Read here, they can put it into an easier to understand format.

https://www.cmcpro.com/pulleys-and-mechanical-advantage-systems/
"Pulleys perform two distinct functions in mechanical advantage systems. If the pulley is attached to the anchor, it is called a fixed or change of direction pulley. Its job is to change the direction of pull on the rope. If the pulley is attached to the load, it is a movable or mechanical advantage pulley. Its job is to increase the mechanical advantage of the system."

One of the pulleys you speak of is attached to a tree, that is an anchor point (it is fixed and does not move with the load), change of direction, no mechanical advantage.

 

[Joe917]

MOguy, you are correct in that you say you do not understand pulleys. The pulley attached to the tree is giving a 2:1 mechanical advantage with the line coming from the winch and back to the vehicle. It would be a change of direction pulley if the cable ran from the winch through the pulley and to an anchor. The key is the cable runs from the vehicle through the pulley and back to the vehicle. Just because the pulley is attached to an anchor does not make it a change of direction only pulley, it can also provide mechanical advantage.

 

[Airmapper]

Two pulleys can move.

That part I got wrong, brain fart. But still I think Joe above explains it better than I can.