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Snatch block loads

If the force required to move the tree is X, and a snatch block is used on another tree to reverse the pull (not shown), then what is the approximate force the winch will produce (Y) and what is the force on the tree with the snatch block (Z) ?

a) Y = 0.5 X, Z = X
b) Y = 0.5 X, Z = 0.5 X
c) Y = X, Z = 0.5 X
d) Y = X, Z = X
e) Y = X, Z = 2 X

The answers on the Facebook page were:

a) 1 vote
b) 0 votes
c) 1 vote
d) 5 votes
e) 4 votes

A few people implied that it wasn’t really worth bothering with, just hook up and start winching.  Each to their own, but if I’m dealing with potentially deadly force then I think it’d be wise to have an idea of what those forces are, as the answer isn’t obvious.

The first step to work out the forces of Y and Z is to forget all the old tales that use the word “always”, as in “snatch blocks always halve the load”.   Everything depends on how the winch and snatch blocks are rigged, and a good way to start is to imagine the load being pulled vertically upwards.

It is clear that the weight (tree trunk) requires a force of X to move.  It is also clear that there is no mechanical advantage, and the force in the diagram pulling the cable down (from the winch) must also be X.   The two Xes make, well, 2 X.  Therefore the answer is E.

The force on the tree to which the snatch block is attached is about double the force required to move the tree itself.  Which is not obvious, and if you think about, kind of dangerous if people rig things thinking the force is equal to X, or even worse, half X.   The only reason a snatch block was used in this case was to change the direction of pull. If we had needed mechanical advantage there are a number of ways to rig it, but they involve another snatch block.

All of the examples here are approximate, and ignore the fact the snatch block itself will sap some energy (around 5-8% loss).  This will not affect the force required to move the load, but it will mean the winch needs to produce a little more power to compensate.  For example, a Facebook commenter mentioned that the number of turns on the drum would be relevant.  It wouldn’t be relevant for the force required to move the load, or indeed the force required from the winch, but it would affect the energy required to operate the winch.

Another simplification is that these calculations assume the cable running to and from a block is exactly parallel.

But the basic concepts still hold true, which is that snatch blocks don’t always give you a mechanical advantage, and may increase the load on a given anchor point beyond that of the target load.

For more details refer to Chapter 34, Winches, in The 4WD Handbook which fully explores this topic, and watch this video:

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