TRobertsRN
03-09-2008, 05:37 PM
As before this still applies to a Jeep with a winch using a tree as an anchor with a snatch block on the tree and the line run from the snatch block back to the Jeep.
In the previous post I was a strong proponet of there being no mechanical advantage to this but that there would be the advantage of having less cable on the drum and therefore a stronger winch.
It is still true that without a traveling pulley there can be no mechanical advantage. At first glance there is only one pulley in this system.
Some information has come to my attention that if true may change my opinion about this.
Forget exacty where I found this first part today but it follows
The drum of the winch is the 2nd (mobile) pulley and is why the load is cut in half.
And just to clarify why the snatch block on the tree does not halve the load, for every foot of movement towards the tree there is a foot of cable through the pulley, and a foot of cable back towards the vehicle. The winch itself, namely the drum, is effectively a 2nd pulley and why a foot of cable wound back onto the WINCH, not through the snatch block, results in 6 inches of movement.
So the above got me thinking and I found this stuff below at http://www.dodgepowerwagon.com/glovebox/snatch.html
And in #2 above, if the bitter end of the line is *not hooked to the winched vehicle* but is hooked to a tree or rock next to it, for each pound of winch force applied, the block will be under *twice* the load, while the line strain will be equal to the line pull. So if you apply a full 12,000 pounds of pull, you are putting 24,000 pounds of stress on the block/anchor. If, however, the bitter end *is* attached to the winched vehicle, the block becomes a "traveling block", even though it is attached to a solid object, because the vehicle itself is actually doing the "traveling", and the load on the *line* is halved, since you are using two lines to "support" the load. In this case, the mechanical advantage is 2/1, so you get 2 pounds pull for each pound of winch effort. Now, since you have two lines splitting the load, let's say it's 10,000 lbs. (you're *really* stuck), the line load is halved to 5000 lbs per line, but the *block* load is still 10,000 lbs because of the parallel line angle factor of 2. As you can see, while the load on the winch is cut in half, as is the line speed for winding, the load on the block is *not*, and is equal to the total line load.
If this is true, and my mind begins to think it may be then using a snatch block on a tree as in our original example would indeed have a 2:1 mechanical advantage plus the greater pulling power of not having as much cable on the drum.
It appears it is possible that in this situation there are 2 pulleys, one of which is the winch itself.
I am not convinced but I am now more open than when I absulutely viewed this as a one pulley system.
One very important piece of information was that in this set up the load on the snatch block pulley would be double the load and a very dangerous accident can happen with overload.
In the previous post I was a strong proponet of there being no mechanical advantage to this but that there would be the advantage of having less cable on the drum and therefore a stronger winch.
It is still true that without a traveling pulley there can be no mechanical advantage. At first glance there is only one pulley in this system.
Some information has come to my attention that if true may change my opinion about this.
Forget exacty where I found this first part today but it follows
The drum of the winch is the 2nd (mobile) pulley and is why the load is cut in half.
And just to clarify why the snatch block on the tree does not halve the load, for every foot of movement towards the tree there is a foot of cable through the pulley, and a foot of cable back towards the vehicle. The winch itself, namely the drum, is effectively a 2nd pulley and why a foot of cable wound back onto the WINCH, not through the snatch block, results in 6 inches of movement.
So the above got me thinking and I found this stuff below at http://www.dodgepowerwagon.com/glovebox/snatch.html
And in #2 above, if the bitter end of the line is *not hooked to the winched vehicle* but is hooked to a tree or rock next to it, for each pound of winch force applied, the block will be under *twice* the load, while the line strain will be equal to the line pull. So if you apply a full 12,000 pounds of pull, you are putting 24,000 pounds of stress on the block/anchor. If, however, the bitter end *is* attached to the winched vehicle, the block becomes a "traveling block", even though it is attached to a solid object, because the vehicle itself is actually doing the "traveling", and the load on the *line* is halved, since you are using two lines to "support" the load. In this case, the mechanical advantage is 2/1, so you get 2 pounds pull for each pound of winch effort. Now, since you have two lines splitting the load, let's say it's 10,000 lbs. (you're *really* stuck), the line load is halved to 5000 lbs per line, but the *block* load is still 10,000 lbs because of the parallel line angle factor of 2. As you can see, while the load on the winch is cut in half, as is the line speed for winding, the load on the block is *not*, and is equal to the total line load.
If this is true, and my mind begins to think it may be then using a snatch block on a tree as in our original example would indeed have a 2:1 mechanical advantage plus the greater pulling power of not having as much cable on the drum.
It appears it is possible that in this situation there are 2 pulleys, one of which is the winch itself.
I am not convinced but I am now more open than when I absulutely viewed this as a one pulley system.
One very important piece of information was that in this set up the load on the snatch block pulley would be double the load and a very dangerous accident can happen with overload.