How does snach blocks / pullyes work

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driftinglobo
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How does snach blocks / pullyes work

Postby driftinglobo » Wed Oct 19, 2016 4:33 am

Hi,

I am starting this topic for newbies to winching (like me).
I have seen a few winch recovery and wanted to find out more on the subject, so I am kind of cluded up by the time I'll have one.
There is a general statement that the use of a snach block reduce the load on the winch by half.
The more I think about this the more I think it is not always the case.
While I see the advantage to reduce the number of windings (layers of rope) on the winch drum, hence getting closer to the max working load of the winch.But a pulley in itself not always will reduce the load on the winch.

My physics maybe not the best, so if anybody dissagree with me please let me know, so I can learn.
So:
-If a vehicle is stuck and use it's own winch ( no pulley) to pull forward the winch need to pull with more force than the vehicle + mud +ect.
In the same senario attach a pulley to a stationary object and run the line back to the vehicle the winch still has to overcome the same force, vehicle + mud +ect, the load on the whinch is same,only the direction of the pull of the rope at the attached point has changed.
Note: since more rope is off the drum the winch is closer to its max working load.
To half the load on the winch a other pulley is needed:
Rope from winch out to pulley(1), back to pulley(2) which is attached to the vehicle, then the end of the rope is fixed near pulley1.
This set up will half the load on the winch.
-But if vehicle is winching a stuck vehicle, then attaching a pulley to the stuck vehicle, than the rope is attached back to the winching vehicle or near it to a stationary object, again will reduce the load on the winch by half.
But the tension in the line will not be reduced!

Not having much luck posting pictures but here is a youtube link it may explain better to how pulleys work:

https://www.youtube.com/results?search_ ... ulley+work

May be not everyone is agreeing with me on this, please share your views.

Also could someone tell me the torque reduction for a Safari PTO winch per winding (layers of rope). Thanks

Regards
Lobo

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Re: How does snach blocks / pullyes work

Postby catalystracing » Wed Oct 19, 2016 9:35 am

Good topic to bring up.
Here is an image that may help also.

warn-winch-rigging-technique.jpg
warn-winch-rigging-technique.jpg (8.54 KiB) Viewed 894 times

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Re: How does snach blocks / pullyes work

Postby Jafa » Wed Oct 19, 2016 11:45 am

Image

Image

Image
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Re: How does snach blocks / pullyes work

Postby drive it ... » Wed Oct 19, 2016 2:08 pm

Pulling straight line - you wind in 1m of rope and the truck moves forward 1m
Go through a block and back to truck - wind in 1m and truck only moves forward 0.5m as the 1m wound in takes o.5m off each side of the doubled rope. That is where the 1/2 the speed / double the force comes from.

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Re: How does snach blocks / pullyes work

Postby mudsurfv6 » Wed Oct 19, 2016 8:27 pm

get some small cheap pulleys and try it.

put one pulley on a branch with rope through it and down to a, say 20kg weight, see how hard it is to lift pulling down on rope.

then add a pulley on the weight and put rope through that and end of rope tied to branch next to top pulley, heaps easier.

then add another pulley on branch and undo rope tied to branch, run it through new pulley and down to weight, even easier.

https://upload.wikimedia.org/wikipedia/ ... ulleys.svg

takes longer to lift the weight the more pulleys you have but with less effort. does use a longer piece of rope the more pulleys you have.
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Re: How does snach blocks / pullyes work

Postby NJV6 » Wed Oct 19, 2016 8:31 pm

mudsurfv6 wrote: blah blah blah.... then add another pulley on branch and undo rope tied to branch, run it through new pulley and down to weight, branch falls on head as its now doing most of the work :D



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Re: How does snach blocks / pullyes work

Postby Jafa » Wed Oct 19, 2016 8:36 pm

mudsurfv6 wrote:get some small cheap pulleys and try it.

put one pulley on a branch with rope through it and down to a, say 20kg weight, see how hard it is to lift pulling down on rope.

then add a pulley on the weight and put rope through that and end of rope tied to branch next to top pulley, heaps easier.

then add another pulley on branch and undo rope tied to branch, run it through new pulley and down to weight, even easier.

https://upload.wikimedia.org/wikipedia/ ... ulleys.svg

takes longer to lift the weight the more pulleys you have but with less effort. does use a longer piece of rope the more pulleys you have.


Yeah... what he say, and here is his pic so you don't need to fawk about opening the linky :mrgreen:

Image
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Re: How does snach blocks / pullyes work

Postby mudsurfv6 » Wed Oct 19, 2016 8:39 pm

Jafa wrote: here is his pic so you don't need to fawk about opening the linky


one day I might figure out how to do that! :lol:
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Re: How does snach blocks / pullyes work

Postby Big » Wed Oct 19, 2016 9:21 pm

interesting thread actually.. as there are 2 trains of thought.. 1 thought it doubles your pulley weight..the more it goes through pulleys.. and 2nd thought it makes it less as it spreads the load across the pulleys.. which is what I thought happens.. more pulleys the easier, slower it gets which is why cranes can lift ridiculous weights from a rather small diameter rope because of how many times it goes through a pulley block.... which is it..?

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Re: How does snach blocks / pullyes work

Postby driftinglobo » Wed Oct 19, 2016 10:42 pm

Hi

This is shows my comuter skills , I just spent a fair bit of time to type up a reply, but it have dissapard into somewhere.
So here it is again:

Thanks for the awesome replys.
I am not looking for a fight just want to get my head around this. It may upset some, but I can be a bit slow to get things at times.

It seems to me that the first picture in Jaffa's first reply and figure1 in his second reply are contredicting each other.
The way I understand pullyes, figure 1 is correct therefore a pullye fixed to a stationary object does only change the direction of pull not reducing the force required. Then the statement "2:1" in picture1 is not correct, as I cannot see a difference between a line running from a winch thru a pulley and back to vehicle (horizontal plane) and a line running thue a pulley verticaly with weight on one end and a pull on the other.
Am i missing something?
In saying the Jeep picture is also true where if 2 line supports a weight, then they hold 1/2 (here 9000lb) of the total load each. This can be true as long as the Jeep is stationary, as soon as it needs to move forward each line will hold 1/2 of the total load, but the winch has to move the total load ie 18000lb. Otherwise how / what moves the other line / side of the Jeep?
The way I looking it if I turn this from horizontal to vertical, the total load has to go up if the winch only pulling 1/2 what happens to the rest?
Then my earlier statement about the "But the tension in the line will not be reduced!" is NOT correct. ( I think)

Since this does my head in, I have come up with an experimrnt to proove or disproove my theory, I just need to find some springs os spring scales.Once it done I will post my findings.
Also I am planing to talk to my friend whom is very clued up in maths and physics, maybe she can shine some light on the physics behind it for me.

If my other attemp to reply shows up as well just ignor it please.

Regards
Lobo

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Re: How does snach blocks / pullyes work

Postby sirLachlan » Thu Oct 20, 2016 12:20 am

If you can't grasp the concept from those replies then I think you need more help than we can offer .
The numbers on the 2nd figure are not related to this argument . That is for loading on each piece .

For all intents and purposes the effect of less wire on the drum is irrelevant . It's merely a beneficial by product .

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Re: How does snach blocks / pullyes work

Postby mudsurfv6 » Thu Oct 20, 2016 6:07 am

driftinglobo wrote:I am starting this topic for newbies to winching (like me).


not being mean but there is a lot of winching experience above.

but if you can prove it wrong then it would become a very interesting thread :)
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Re: How does snach blocks / pullyes work

Postby TJ » Thu Oct 20, 2016 9:15 am

https://www.gowarn.com/warn-winches/winch-tips.aspx

Double pull self-recovery halves the weight being pulled, not double the pulling power of the winch. Pull power is fixed by design and strength of the winch itself. Double pull will allow a multiplier effect of using the same pulling power but cutting the weight being pulled in half (and halves the speed at which it travels in the direction of anchored pulley block). Crane analogy was perfect for this scenario.

If you are near a Club, go on a club day and test the theory with someone.
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Re: How does snach blocks / pullyes work

Postby coxsy » Thu Oct 20, 2016 7:01 pm

used a thee pulley block to lift 80x80 mm x5 mm x8 metres long RHS ,up two stories on a building site ,hand over hand no strain at all,
it works nicely
oh a sailing ships of the line use pulley blocks in the rigging :lol:
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Re: How does snach blocks / pullyes work

Postby Smurf » Thu Oct 20, 2016 11:30 pm

driftinglobo wrote:Hi

This is shows my comuter skills , I just spent a fair bit of time to type up a reply, but it have dissapard into somewhere.
So here it is again:

Thanks for the awesome replys.
I am not looking for a fight just want to get my head around this. It may upset some, but I can be a bit slow to get things at times.

It seems to me that the first picture in Jaffa's first reply and figure1 in his second reply are contredicting each other.
The way I understand pullyes, figure 1 is correct therefore a pullye fixed to a stationary object does only change the direction of pull not reducing the force required. Then the statement "2:1" in picture1 is not correct, as I cannot see a difference between a line running from a winch thru a pulley and back to vehicle (horizontal plane) and a line running thue a pulley verticaly with weight on one end and a pull on the other.
Am i missing something?
In saying the Jeep picture is also true where if 2 line supports a weight, then they hold 1/2 (here 9000lb) of the total load each. This can be true as long as the Jeep is stationary, as soon as it needs to move forward each line will hold 1/2 of the total load, but the winch has to move the total load ie 18000lb. Otherwise how / what moves the other line / side of the Jeep?
The way I looking it if I turn this from horizontal to vertical, the total load has to go up if the winch only pulling 1/2 what happens to the rest?
Then my earlier statement about the "But the tension in the line will not be reduced!" is NOT correct. ( I think)

Since this does my head in, I have come up with an experimrnt to proove or disproove my theory, I just need to find some springs os spring scales.Once it done I will post my findings.
Also I am planing to talk to my friend whom is very clued up in maths and physics, maybe she can shine some light on the physics behind it for me.

If my other attemp to reply shows up as well just ignor it please.

Regards
Lobo


Using one pulley but anchoring the end of the rope/hook back to the stuck vehicle gives a 2:1 pull, but if you did the same but attached the end of the rope/hook to a stationary object, ie. another tree/vehicle etc. then yes as you say the single pulley only changes the angle of pull and not the reduction ratio so you are still at 1:1
If you want a double line or 2:1 pull with the rope end/hook tied off to something static/stationary, then you would need to use two pulleys to get a 2:1 reduction.
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Re: How does snach blocks / pullyes work

Postby Clint » Fri Oct 21, 2016 9:22 am

driftinglobo wrote:
Since this does my head in, I have come up with an experimrnt to proove or disproove my theory,


To make it realistic you need to get your Safari really absolutely stuck to buggery in some horrible stinky semi frozen bog with the winch under water/mud. Then break a winch rope trying to winch yourself out. Then install another rope onto the winch underwater/mud (you did bring a spare winchrope right? :shock: ) Then try again with a snatch block on a double line pull & marvel at the awesomeness of physics as your Nissan extracts itself from it's sticky predicament. :D

One thing to note is that that due to friction you never really get twice the pull, & this gets more noticeable as you go to more pulleys, But you definitely still get a bunch more pull.

To answer your question on winch drum torque, torque = force x radius. So check your drum diameter & rope diameter & you should be able to do the math. Eg. if your winch drum has a 75mm dia, 10mm winch rope, then the 2nd layer only has about 79% of the pull of the first, 3rd layer about 65% of the first.

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Re: How does snach blocks / pullyes work

Postby Pico42 » Fri Oct 21, 2016 9:50 pm

Some light reading for you OP. I found this to be an excellent guide. It refers a lot to the US Army manual on vehicle recovery.

http://www.pirate4x4.com/tech/billavista/Recovery/

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Re: How does snach blocks / pullyes work

Postby TJ » Sat Oct 22, 2016 11:30 am

Admins, make this a sticky. A lot of good info in here on the subject.
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Re: How does snach blocks / pullyes work

Postby Sheepy » Mon Nov 07, 2016 9:36 pm

TJ wrote:Admins, make this a sticky. A lot of good info in here on the subject.


This, I am also new to winching and found the pulley explanation helpful. Could have done with on on the weekend.

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Re: How does snach blocks / pullyes work

Postby driftinglobo » Mon Jan 09, 2017 6:50 am

Hi

I have finally got some springs and some time to set up my little experiments,also spent a fair bit of time and done a decent web search on the subject.
I can say it was a great sucsess. I have managed to proove my theory wrong, and got a better undestending on the subject of winching and pulleys. I still need a lot of hands on experience, but now I think I know what is happening and most importantly for me: why.

Originally this is where I wanted to finish the subject,with thanking everyone for their contributions, especially Jafa's pretty pictures as they are right on the money!
Last night I was reading the Australian 4WD magazine (issue250, page 153,) about recovery safety, and again I felt their wording is a bit missleading.
So I am going to write down the theory that I have found on the benefits of a use of a snach block.
Please note that I am doing this for other novices in winching like me, so if you know everything to know on the subject stop reading, or maybe read on and let me know if I left out anything, or got something wrong.

First a snach block does not double the torque (lb rating) of any winch, but there are other
benefits and draw backs of using a snach block during winching.

1,: Winch rating are given at the drum level of the winch.So the more rope is off the drum the closer it gets to the specified rating. After playing with different numbers for drum and rope diameter I am happy to use a 10 - 12% reduction per winding. Now this is an average number that I am happy to use,the real number of each winch will be different. (torque = force x radius)
Untill I gain more experience in winching, I have worked out how much rope per winding, so I know if I have "X" meter of rope out my winch torque is reduced by about "Y"% . I will still have to guesstimate how stuck my truck is.
2,: In a self recovery a snach block does reduces the force on the winch and also the tension in the rope by about half. Note: if the return line is connected back to the winching vehicle than the mounts / chassie of the vehicle will take the full load of being winched inc mud, ect.....but if the return line is connected to a near by object then it will take about half the load, and the winch the other half.Have to keep in mind that the angle of the return line must be minimum otherwise it is reducing the effectiveness of the set up.
3,: The above will allow a lighter vehicle recover a heavier one by connecting the return line to a nearby stationery object, tree, rock, vehicle.......as it will take about half the load,and the winch the other half.
4,:In a "slingshoot" ( front vehicle's forward facing winch assisting the second vehicle via a pulley fixed ahead and the line runing back to the second vehicle) winching the snach block does NOT halfes the load on the winch or the tension in the rope. But as more rope is of the drum the closer the winch gets to is's specified (lb) rating.
5,: During the use of 1 snach block the vehicle will move forward with about half the speed the winch is pulling the rope in.
6,: Setting up a 3 line pull (2 snach block) will 1/3 the load on the winch.With about 1/3 ing the speed the vehicle moves forward.

Hope it helps.
Now it's time for me to put all this to practice,( lots of)

Thanks Lobo

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Re: How does snach blocks / pullyes work

Postby mudlva » Mon Jan 09, 2017 2:38 pm

driftinglobo wrote:Hi


First a snach block does not double the torque (lb rating) of any winch, but there are other
benefits and draw backs of using a snach block during winching.

correct! it doesnt make the winch stronger. the block halves the load thus the same winch can pull twice as much (give on take a little bit)

1,: Winch rating are given at the drum level of the winch.So the more rope is off the drum the closer it gets to the specified rating. After playing with different numbers for drum and rope diameter I am happy to use a 10 - 12% reduction per winding. Now this is an average number that I am happy to use,the real number of each winch will be different. (torque = force x radius)
Untill I gain more experience in winching, I have worked out how much rope per winding, so I know if I have "X" meter of rope out my winch torque is reduced by about "Y"% . I will still have to guesstimate how stuck my truck is.

this is the black art of winching, also as you winch the rope will coil up more one one side affecting the ratio. then there is a winching time frame/heat build up/battery amps output etc just to throw more confusion into the melting pot
2,: In a self recovery a snach block does reduces the force on the winch and also the tension in the rope by about half. Note: if the return line is connected back to the winching vehicle than the mounts / chassie of the vehicle will take the full load of being winched inc mud, ect
correct
.....but if the return line is connected to a near by object then it will take about half the load, and the winch the other half.
wrong wrong wrong the winch cable must come back to the vehicle where the winch is to achieve a 2 to 1 pulling ratio. hooking up to a near by tree will only use more rope and recovery will still be at a 1 to 1 ratio
3,: The above will allow a lighter vehicle recover a heavier one by connecting the return line to a nearby stationery object, tree, rock, vehicle.......as it will take about half the load,and the winch the other half.
this comment is correct assuming that the lighter vehicle is not the stuck vehicle
4,:In a "slingshoot" ( front vehicle's forward facing winch assisting the second vehicle via a pulley fixed ahead and the line runing back to the second vehicle) winching the snach block does NOT halfes the load on the winch or the tension in the rope. But as more rope is of the drum the closer the winch gets to is's specified (lb) rating.
??? bit confused on this issue
5,: During the use of 1 snach block the vehicle will move forward with about half the speed the winch is pulling the rope in.
correct
6,: Setting up a 3 line pull (2 snach block) will 1/3 the load on the winch.With about 1/3 ing the speed the vehicle moves forward.
correct

Hope it helps.
Now it's time for me to put all this to practice,( lots of)

Thanks Lobo


definitely heading in the right direction thou...
excuse the pum

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Re: How does snach blocks / pullyes work

Postby mudsurfv6 » Mon Jan 09, 2017 5:32 pm

driftinglobo wrote:4,:In a "slingshoot" ( front vehicle's forward facing winch assisting the second vehicle via a pulley fixed ahead and the line runing back to the second vehicle) winching the snach block does NOT halfes the load on the winch or the tension in the rope. But as more rope is of the drum the closer the winch gets to is's specified (lb) rating.

mudlva wrote: ??? bit confused on this issue


driftinglobo wrote:.....but if the return line is connected to a near by object then it will take about half the load, and the winch the other half.

mudlva wrote:wrong wrong wrong the winch cable must come back to the vehicle where the winch is to achieve a 2 to 1 pulling ratio. hooking up to a near by tree will only use more rope and recovery will still be at a 1 to 1 ratio


same thing, but top the winching vehicle is still and second one moving, bottom winching vehicle is moving and 'near by object' is still.

both 1 to 1.
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Re: How does snach blocks / pullyes work

Postby driftinglobo » Mon Jan 09, 2017 11:50 pm

Hi

.....but if the return line is connected to a near by object then it will take about half the load, and the winch the other half.

wrong wrong wrong the winch cable must come back to the vehicle where the winch is to achieve a 2 to 1 pulling ratio. hooking up to a near by tree will only use more rope and recovery will still be at a 1 to 1 ratio


Thanks for the correction, I got that wrong again! This was the senario that started my confusion about snach blocks pulling ratios.

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Re: How does snach blocks / pullyes work

Postby coxsy » Tue Feb 14, 2017 8:07 pm

a few uses of pulley blocks
Image
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