Weakest point in winch and rigging?
- Thread starter jcbrandon
- Start date
Bongo Boy said:
There was an article several years ago in a Jeep mag, and they had a tensionometer and did a bunch of scenerio type pulls. They were not as high as you might think. Pulling out of a mud bog I think was the highest and near the winch capacity. Obviously a mired Unimog will take more of a pull to free than a mired TJ, but for most 4000-5000 pound trucks, the quoted figures are what I remember. It goes along with my seat of the pants gestalt too, but I don't live or wheel in a muddy environment.
The WLL are for overhead lifts, yes. Breaking strength is MUCH higher. I always have figured that a rated hook or shackle will not break in a recovery type situation. The winch line is virtually always the weak point since it isn't typically rated, and if it were, the rating on 5/16 cable (the most common size) would be in the 2500 pound range!
Antichrist
Expedition Leader
Yes, proven to do what they claim. That said, there are some good technical articles available that go very deeply in to the physics of crashes, deceleration, inertia and other factors in how they relate to trauma on the human body. The upshot is that there is negligible difference, in a crash and resulting injury or lack thereof, between an airbag compliant bumper and one that isn't.NothingClever said:
Another important factor to consider is the law in your country. In Australia for example, where the ARB is made, my understanding is that it's against the law to replace the bumper on a vehicle that had an OEM airbag bumper, with one that is non-airbag compatible. Contrast that with the US where there are no laws regarding bumpers as they relate to personal injury. The laws in the US only govern the area of car makers and limiting the $$ value of vehicle damage in an accident. The laws in the US were passed as a result of too much body damage during low speed impacts.
AndrewP said:
True as far as you go. However, overhead lifting generally produces static loads, so that having an industry normal 4X break rating constitutes an adequate safety margin only in a static lift.
But when a static load rated device is used in a dynamic situation, all bets are off. First, it is not that difficult to produce 20,000 pounds of energy with a 5,000 pound vehicle using nothing more than a snatch strap. Typical shackles are rated at 3.5T, and some at 4.5T unless you use the military monster shackles intended for recovering 6x6 trucks. Our typical shackles are rated for 7,000 to 9,500 pounds of static load, without regard to assumptions about safety margins. Obviously it is not impossible to place that much load on the shackle with an ordinary winch of the 8,000 to 9,500 pound rating, even if the load is basically static. If we were working with a lifted load, we would not routinely exceed the working limit of the equipment. Why is it okay to do so with a dynamic load?
Now use the same shackle to capture a sheave in a double line pull. Although the winch cable itself may be under no more strain than the rated pull of the winch (say 8,000 per line), the shackle and whatever you use to attach down line from the pulley is going to have twice the stress. Now you are looking at potentially 16,000 to 19,000 pounds of stress, even if we assume the load to be static. This places the strain on the shackle somewhere well into the safety margin of a 3.5T shackle.
However, vehicle recoveries are never static unless everyone has walked away from the job. Mass in motion causes accelerated strain on the system, and does so in ways that, without measuring equipment, we can not really determine or predict. I would expect that dynamic stress added to a recovery situation is never more with a double line pull than it would be with a single line pull, but even so, if you add 10,000 pounds of momentary strain to an already fully loaded double line recovery, you are very closely approaching the ultimate failure point of a 3.5T rated shackle, even though the winch cable is still within its rated capacity.
It is not always the winch line that fails first, and it is never safe to make assumptions about safety margins. In vehicle recovery we are always using dynamically equipment that is rated statically, and we can not control the variables. Whether the breaking point of a device is or is not MUCH higher than the WLL rating is, in my opinion, not relevant. It is not safe to work above the rating of your equipment.
madizell said:
So what are you saying? Stay within the WLL of your shackles? That's 4 3/4 ton and so more than enough for most recoveries. For the rare times it isn't less than 4 3/4 tons, the breaking strength is above 4 x 4 3/4 which is about 19 tons which is 38,000 pounds. I would worry far more about the part of the truck the shackle was attached to, rather than the shackle itself. I don't think there is any practical recovery point on a jeep that would even approach this kind of strength..ie the vehicle would break before the shackle.
Having recovered numerous vehicles and seen countless more recovered by others, The common 3/4 shackles we all use have never been an issue. Winch lines, yes, pull straps, yes, but never shackles. I doubt it would even be possible to generate the type of force that could threaten the breaking strength of a 3/4 shackle in the usual offroading environment. There is always something else in the system that would fail first.
And an update__I just went out to the garage and checked the 3/4 inch shackles on the rear bumper of my FJ40. The right shackle is rated at 6.5 tons (Made in USA Breaking strength above 52,000 pounds). The left side is a made in China shackle and rated at 4 3/4 tons. Being practical about this, neither of these is likely to ever fail in use, but interesting there is that big a difference in the ratings.
With all of that said, there are non-rated shackles sold and in use. I've seen them at Tractor Supply. I would avoid those entirely.
The different rating is attributable to the quality of the metal.
Not saying you should not ever work above the WLL. What I am saying is that because we can't control the variables in a dynamic recovery, it is presumptively not safe to work above the rated limits of the equipment, and in particular, it is not safe to make blanket assumptions based on manufacturer's built-in margins for failure. The margin for safety is only that when the equipment is used as intended. In a static lift, a 4X safety margin accounts for minor shock loading associated with a gradual lift, traverse, and lowering of a load, and also accounts for wear and tear, metal fatigue, and whatever else might arise in use when lifting heavy loads. Still, I don't believe anyone would argue that it is appropriate or safe to lift loads in excess of the WLL of the weakest component when making overhead lifts.
If it isn't safe to do so in an essentially static vertical load, how can it be safe in an uncontrolled dynamic recovery?
So, I am not saying you can't recover a load heavier than the WLL, and I used shackles as an example only because we all use them and they all have a rating stamped on them that is easy to find. All I am saying is that ignoring the possibility of breakage, or assuming that if anything breaks it will be the rope or cable, is not safe practice and may not always bear out. Just something to consider rather than presume our personal safety. False assumptions about safety margins might some day lead to carelessness. I exceed the safe limits of my recovery equipment quite often, but not without thinking about it, and I guarantee I get way out of the way when I know I am pushing the envelop.
I also disagree that you can't find a spot on a vehicle which will sustain loads in the 30,000 plus range, as long as you are willing to consider dedicated off road toys, and not just OEM bumpers. Or if that seems too extreme, loads in the range of 20,000 pounds are not hard to carry with properly constructed and placed recovery points, even on overland vehicles. Perhaps one of the engineering or mathematically privileged in the crowd can provide either the formula or the answer to the question of how much load/force/strain is created by a 5,000 pound vehicle going 5 miles an hour when it hits the end of a non-dynamic restraint. I suggest a non-kinetic strap or chain example only because it eliminates from consideration the force-over-time component of an otherwise kinetic situation, but I don't think it significantly changes the answer either way.
Having snatched any number of vehicles weighing equal to or more than my 5,300 pound trail weight, and knowing the verve with which these heavy vehicles have become unstuck (bordering on airborne), I assume the apparent force at the end of the strap is significant, and since we all seem inclined to occasionally connect such straps to ours and others' vehicles using the odd steel shackle (we shouldn't but we do), it might be interesting to know just how much force we generate under the circumstances.
Not saying you should not ever work above the WLL. What I am saying is that because we can't control the variables in a dynamic recovery, it is presumptively not safe to work above the rated limits of the equipment, and in particular, it is not safe to make blanket assumptions based on manufacturer's built-in margins for failure. The margin for safety is only that when the equipment is used as intended. In a static lift, a 4X safety margin accounts for minor shock loading associated with a gradual lift, traverse, and lowering of a load, and also accounts for wear and tear, metal fatigue, and whatever else might arise in use when lifting heavy loads. Still, I don't believe anyone would argue that it is appropriate or safe to lift loads in excess of the WLL of the weakest component when making overhead lifts.
If it isn't safe to do so in an essentially static vertical load, how can it be safe in an uncontrolled dynamic recovery?
So, I am not saying you can't recover a load heavier than the WLL, and I used shackles as an example only because we all use them and they all have a rating stamped on them that is easy to find. All I am saying is that ignoring the possibility of breakage, or assuming that if anything breaks it will be the rope or cable, is not safe practice and may not always bear out. Just something to consider rather than presume our personal safety. False assumptions about safety margins might some day lead to carelessness. I exceed the safe limits of my recovery equipment quite often, but not without thinking about it, and I guarantee I get way out of the way when I know I am pushing the envelop.
I also disagree that you can't find a spot on a vehicle which will sustain loads in the 30,000 plus range, as long as you are willing to consider dedicated off road toys, and not just OEM bumpers. Or if that seems too extreme, loads in the range of 20,000 pounds are not hard to carry with properly constructed and placed recovery points, even on overland vehicles. Perhaps one of the engineering or mathematically privileged in the crowd can provide either the formula or the answer to the question of how much load/force/strain is created by a 5,000 pound vehicle going 5 miles an hour when it hits the end of a non-dynamic restraint. I suggest a non-kinetic strap or chain example only because it eliminates from consideration the force-over-time component of an otherwise kinetic situation, but I don't think it significantly changes the answer either way.
Having snatched any number of vehicles weighing equal to or more than my 5,300 pound trail weight, and knowing the verve with which these heavy vehicles have become unstuck (bordering on airborne), I assume the apparent force at the end of the strap is significant, and since we all seem inclined to occasionally connect such straps to ours and others' vehicles using the odd steel shackle (we shouldn't but we do), it might be interesting to know just how much force we generate under the circumstances.
Crookthumb
Adventurer
Frame Bending
Here is a video of a recovery when the frame was the weakest link. Right around the 7:50 minute mark is when it goes bad. Sure glad that wasn't me.
http://www.youtube.com/watch?v=BEWHqIOGsVo
Now if he was going to daisy chain it like that he should have hooked up to a rear reciever. This way the angles would have been in line with each other rather than using the boom or whatever the triangulated thing is called.
Here is a video of a recovery when the frame was the weakest link. Right around the 7:50 minute mark is when it goes bad. Sure glad that wasn't me.
http://www.youtube.com/watch?v=BEWHqIOGsVo
Now if he was going to daisy chain it like that he should have hooked up to a rear reciever. This way the angles would have been in line with each other rather than using the boom or whatever the triangulated thing is called.
Here's the short version of that two truck driver's bad day:
http://www.youtube.com/watch?v=gqCy7ZxVdgI&feature=related
http://www.youtube.com/watch?v=gqCy7ZxVdgI&feature=related
Crookthumb
Adventurer
kerry said:
I agree. The tow truck driver operator seemed pretty clueless.
Redline
Likes to Drive and Ride
But it sure shows the force involved when vehicles are stuck in mud/slush etc. Getting some speed up and yanking on a strap puts the strength of rigging in question, as Madizell points out.
I know I have been lucky in the past when momentum seems to work when nothing else will.
I know I have been lucky in the past when momentum seems to work when nothing else will.
Seems like some folks will go to any lengths to get on TV. Did anyone see any of these guys do anything right?
The tow truck driver looped steel chain around the ball hitch of the pick-up in front.
Failing to pull a mere Disco out of the mud from the front, would it not have been prudent to do it from the back, the direction in which the vehicle entered the obstacle, and the direction in which resistance to recover would be least? The guy drove out eventually by passing just behind the Disco so obviously he could have accessed the Disco from the rear.
Eventually two vehicles were recovered, but each was pulled out with (what appeared to be) chain attached to a tow hitch on the tow vehicle. One was seriously snatched. This can only lead to damage to either or both vehicles.
The guy driving the chipper truck never got out to verify how his tow rig was attached, and the kid fiddling with the tow chain did so repeatedly while the chipper truck was in motion. Who needs fingers?
The daisy chain idea was to anchor the tow truck because it was sliding. The anchoring vehicle in such a situation should never attempt to move while under tension, much less drive, much less hit the end of a chain while in motion, and the guy in the tow truck should not have allowed such shenanigans. And I would agree that because the towing vehicle was a tow truck with a boom, it should not have been anchored because of the angular stress. It was also more likely that it was the truck bed that failed, not the frame, but it could be both.
Neither of the stuck vehicles was deeply stuck. The difficulty shown in the recovery of each was due more to the inexperience of the guys involved and the inadequacy of their equipment than in the condition of the stuck vehicle.
This is like some kind of Jeff Foxworthy skit. If you think these guys have it together, you could be a redneck.
The tow truck driver looped steel chain around the ball hitch of the pick-up in front.
Failing to pull a mere Disco out of the mud from the front, would it not have been prudent to do it from the back, the direction in which the vehicle entered the obstacle, and the direction in which resistance to recover would be least? The guy drove out eventually by passing just behind the Disco so obviously he could have accessed the Disco from the rear.
Eventually two vehicles were recovered, but each was pulled out with (what appeared to be) chain attached to a tow hitch on the tow vehicle. One was seriously snatched. This can only lead to damage to either or both vehicles.
The guy driving the chipper truck never got out to verify how his tow rig was attached, and the kid fiddling with the tow chain did so repeatedly while the chipper truck was in motion. Who needs fingers?
The daisy chain idea was to anchor the tow truck because it was sliding. The anchoring vehicle in such a situation should never attempt to move while under tension, much less drive, much less hit the end of a chain while in motion, and the guy in the tow truck should not have allowed such shenanigans. And I would agree that because the towing vehicle was a tow truck with a boom, it should not have been anchored because of the angular stress. It was also more likely that it was the truck bed that failed, not the frame, but it could be both.
Neither of the stuck vehicles was deeply stuck. The difficulty shown in the recovery of each was due more to the inexperience of the guys involved and the inadequacy of their equipment than in the condition of the stuck vehicle.
This is like some kind of Jeff Foxworthy skit. If you think these guys have it together, you could be a redneck.