Self recovery with a snatch block pulley
Jan 2, 2011 | 03:07 PM
#21
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From: South Ga
K I'm gonna go nerdy here lol:
Is the load on the snatch block/shackle actually doubled? I've always had trouble trying to think it through... But the way I try to see it, it's like pulling yourself towards a tree with one arm, it's hard, but then use a snatch block and tie the rope to your other arm. You have to use half the force because the pulling with your one arm is also pulling your other arm.
So that leads me to believe that it's not doubling the load, it's cutting the force required in half. So we should be safe with good ol' 3/4" shackles i think...
So it doubles the winching capacity in a given situation by reducing the force required by half... But no matter what, your winch will never magically pull more than it's rated for.
Is the load on the snatch block/shackle actually doubled? I've always had trouble trying to think it through... But the way I try to see it, it's like pulling yourself towards a tree with one arm, it's hard, but then use a snatch block and tie the rope to your other arm. You have to use half the force because the pulling with your one arm is also pulling your other arm.
So that leads me to believe that it's not doubling the load, it's cutting the force required in half. So we should be safe with good ol' 3/4" shackles i think...
So it doubles the winching capacity in a given situation by reducing the force required by half... But no matter what, your winch will never magically pull more than it's rated for.
Jan 2, 2011 | 04:29 PM
#22
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From: longmont, co
that episode was pretty amazing, big ass humvee unstuck with physics and man power
Jan 2, 2011 | 05:38 PM
#23
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From: New Bern, NC
This is why forums are awesome - great discussion.
So you guys are right - for a set force, using a pulley or fulcrum decreases that force. So if you have a 10K lb resistance force, and you use a snatch block, there is 5K on each "arm" of the winch cable, but still 10K on the SBP/recovery strap system. Same with a fulcrum - the longer the fulcrum (or moment arm), the less force is needed at the working end, but there still would be a 10K force on the system.
Here's the thing - the whole purpose of using the pulley system, and why most straps and pulleys are rated so high, is so if you are stuck, and it requires for example a 15K lb force to pull yourself out, and you only have a 10K winch, you can set up the pulley system, which puts 7.5K on the winch and winch cable b/c you've halved the force. But there is still a 15K force on the SBP/strap system.
Those high-capacity shackles are awesome - I'll probably pick one up. Who knows if I will ever need more than 10K, but it will be nice to have the piece of mind to know I have full system integrity! Thanks again for the discussion and ideas.
So you guys are right - for a set force, using a pulley or fulcrum decreases that force. So if you have a 10K lb resistance force, and you use a snatch block, there is 5K on each "arm" of the winch cable, but still 10K on the SBP/recovery strap system. Same with a fulcrum - the longer the fulcrum (or moment arm), the less force is needed at the working end, but there still would be a 10K force on the system.
Here's the thing - the whole purpose of using the pulley system, and why most straps and pulleys are rated so high, is so if you are stuck, and it requires for example a 15K lb force to pull yourself out, and you only have a 10K winch, you can set up the pulley system, which puts 7.5K on the winch and winch cable b/c you've halved the force. But there is still a 15K force on the SBP/strap system.
Those high-capacity shackles are awesome - I'll probably pick one up. Who knows if I will ever need more than 10K, but it will be nice to have the piece of mind to know I have full system integrity! Thanks again for the discussion and ideas.
Last edited by wndrboy777; Jan 2, 2011 at 05:40 PM.
Jan 5, 2011 | 04:49 PM
#24
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From: New Bern, NC
Trying to bring this thread back to life with another question - I'm more focused on the system maximums, but I was just thinking about the vehicle maximums. What is the maximum force you can put on the frame of the vehicle? For instance, if I need to get towed out by a trailer hitch shackle, what is the maximum force that can be placed on the hitch? A class IV hitch is only rated to 5K lbs, correct? If that is the case, is there any reason to get a receiver shackle setup rated to > than 5K lbs (or to be safe, maybe 10K lbs)?
I've also heard of people mounting their winches to their receiver hitch - is that a reasonable practice if the hitch mount itself is only rated to 5K? Thanks for the input.
I've also heard of people mounting their winches to their receiver hitch - is that a reasonable practice if the hitch mount itself is only rated to 5K? Thanks for the input.
Jan 6, 2011 | 12:21 AM
#25
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From: Victorville, CA
I think trailer hitch ratings are just the weights of the trailer, not the loads they can produce. Think of a big rv trailer or a 28 ft boat behind your pickup and run the brakes full, Im willing to bet more then the 6000lb of initial weight is pushing against that hitch.
Probebly a crappy logic but i"d say the frame might be the weak link in most factory hitches
Probebly a crappy logic but i"d say the frame might be the weak link in most factory hitches
Jan 7, 2011 | 09:05 PM
#26
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Hitches have two ratings, tongue weight and load rating. Tongue weight rating is far less the load rating. Class 5 load rating is 12k (14k w/weight distribution) and tongue rating 1.4k
So you point is spot on about using the receiver as a recovery point
For vehicle frame ratings you need to look up the towing specs for the vehicle the hitch is attached to.
So you point is spot on about using the receiver as a recovery point
For vehicle frame ratings you need to look up the towing specs for the vehicle the hitch is attached to.
Jan 8, 2011 | 12:41 AM
#27
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From: I live in New Tripoli Pa out in the woods
It all comes down to the strength of the bolts or welds of the receiver .
I know there are charts somewhere on shear strength of bolts for the size and grade .
Welds are around 5000 pounds per inch of weld but I could have mis remembered that
Also remember that towing is alot different then recovery.
I can see no reason to winch forward unless one must get over or through an obstacle .
That is why I can swap the winch front to back when needed. Getting out backwards if needed.
I know there are charts somewhere on shear strength of bolts for the size and grade .
Welds are around 5000 pounds per inch of weld but I could have mis remembered that
Also remember that towing is alot different then recovery.
I can see no reason to winch forward unless one must get over or through an obstacle .
That is why I can swap the winch front to back when needed. Getting out backwards if needed.
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