Advanced winching physics

[twisted32]

Just trying to my head around this. With a single block you move the vehicle half the distance for the length of rope. So torque is doubled, correct? That means if the winch stalls at 9500lbs then it is pulling 19,000lb of force with a single block.

Ok my head hurts. What you are saying is a strain gauge will still show 9,500lbs of force on the rope, and part of my brain agrees as the winch can not pull more the 9,500lb regardless the number of pulleys. But at the same time you are putting 19,000lbs of force to shift the vehicle.

My brain is stuck in loop. One side says you have doubled the pulling torque but the other is saying you can not increase the tension on the rope. So what am I missing?

Oh hang on I get it now. With a single pulley you actually have "two ropes" pulling 9500lb so one side of the pulley has two forces of 9,500lb and the pulley single attachment point has 19,000 lbs. Finally got there. The more attachment points the more the torque applied. So when tying pullies to a vehicle use different points to spread the load.

Correct. If you put a force gauge in any leg/section of cable it would show X Newtons, if you put the same gauge between the pulley and its mount ie the car, it would show 2X Newtons

[RoyHarvey]

Correct. If you put a force gauge in any leg/section of cable it would show X Newtons, if you put the same gauge between the pulley and its mount ie the car, it would show 2X Newtons

Hmmmm...... Going to have to keep a lookout for a cable like that, must be a use for a cable that can be variably stressed along it's length.

[Kumabear]

I am not entirely sure I agree with the power draw comment.

While yes the total amount of energy expended to recover the vehicle will most certainly not change due to conservation of energy, the rate at which that energy is expended will be reduced in line with the net gain of mechanical advantage.

This is important because the charging system is much better able to keep up with a halved current draw for twice as long.

I love snatch blocks, saved my ass once when I got stuck hard enough to stall out my winch, which I was surprised by as it is a good 12000lb winch. Started to flatten the battery then it stalled, I got out and rigged up a 2 line pull and it just walked it out.

[nj swb]

Hmmmm...... Going to have to keep a lookout for a cable like that, must be a use for a cable that can be variably stressed along it's length.

Have another read. Nothing has been said about the cable being "variably stressed".

[nj swb]

I am not entirely sure I agree with the power draw comment.

Which "power draw comment"?

Whether single, double or triple pull, if the stuck car is dragged the same distance using the same force on the stuck car then the winch has (nominally) done the same "work". If the double or triple line pull takes longer, then same work over longer time equals lower power.

Total energy drawn from the battery is a different matter. Lower power for longer can be same total energy consumption. Less current can be less resistive losses, so greater efficiency. Then, as you pointed out, winching for longer gives the alternator a better chance to keep up, so the battery should be in a higher state of charge when it's all over.

[Kumabear]

I read it wrong lol ignore me, I was only half way through my coffee

The important take away is, snatch blocks are awesome. And if you have a winch you should have some

[RoyHarvey]

Have another read. Nothing has been said about the cable being "variably stressed".

My apologies, I did indeed misread it.

[old Jack]

I am not entirely sure I agree with the power draw comment.

While yes the total amount of energy expended to recover the vehicle will most certainly not change due to conservation of energy, the rate at which that energy is expended will be reduced in line with the net gain of mechanical advantage.

This is important because the charging system is much better able to keep up with a halved current draw for twice as long.

I love snatch blocks, saved my ass once when I got stuck hard enough to stall out my winch, which I was surprised by as it is a good 12000lb winch. Started to flatten the battery then it stalled, I got out and rigged up a 2 line pull and it just walked it out.

I had a look at your Runva EWX 12,000 winch specs and checked amperage draw;

At 0 lbs load the motor draws 100amps
At 4,000 lbs /1818 kgs load the motor draws 220 amps so that is an extra 120 amps.
At 8,000 lbs / 3636kgs load the motor draws 330 amps so that is an extra 110 amps.
At 12,000 lbs / 5545kgs load the motor draws 450 amps so that is an extra 110 amps.

What the website does to detail is if these figures are on the first wrap on the drum, I suspect it is as this gives the lowest amp draw for a given load and this is the most common method of presenting this information.

Also need to consider that with each extra layer on the drum you lose mechanical advantage and for the same amps draw you pull less weight.
1st layer 12,000 lbs / 5545kgs (100% capacity) 450 amps.
2nd layer 9,653 lbs / 4388kgs (80% capacity) 450 amps.
3rd layer 7949 lbs / 3613kgs (66% capacity) 450 amps.
4th layer 6801lbs / 3091kgs (57% capacity) 450 amps.

Therefore the use of a pulley block especially for short winching distances enables you to work with the first 2 wraps where your winch is at 100% and 80% capacity.

As pointed out the slower the winch operation the less amps drawn from the battery per minute and the alternator is able to replenish some of these. A Gen 4 Pajero has a 120Amp alternator but the DID engine requires about 40 amps just to run so at best you only have 80 amps of replenishment.
Best practice is not to winch continuously at high loads but to winch and then pause this gives the alternator time to makeup the amperage shortfall. A 50% winch time and 50% pause time will give you 160 amps, 18% winch time and 82% pause time will give you 450 amps based on an 80 amp input and this will not kill your battery. Remember as the battery is drained the voltage decreases so the amperage increases for the same amount of load, another consideration is as the winch motor heats up the internal resistance of the motor increases so this increases amperage draw again.

Secret of successful winching particularly if it is a long distance, high load recovery is a multiline pull, use the first 2 layers of the drum and do not operate the winch continuously.

OJ.

[Ent]

This little exercise in physics has highlighted an issue for me. I first brought the Supercheap recovery gear package. Yeap, while the extra strong snatch strap sounded the go, I later better understood the physics of snatch straps so got another one (Australian made) that is better suited to the weight of vehicles in our convey (with the Challenger being the heaviest at say 2.8 tonne and Suzuki lightest at 2.1 tonne, so now 6 tonne rather than 11 tonnes strap). Other one might wind up for a friend's Landcruiser 200, if that shopping trolley ever winds up going off road.

The issue is the winch extension rope. It is 4.5 tonne rated which means it matches the 9500lb winch, but with a block then it is underdone, unless it forms the reverse on the loop. But that will not work as it is a strap not designed to go around the block. Basically rather limited solution.

I for the Tirfor had brought a 10mm or was it 12mm Dyneema rope of fifty metres which was on advise of the seller (Australian) made into at 30 metre and 20 metre length. The 30 metre can go through the block, so say 22 metres of winch rope and 30 metres of that gives 27 metre reach, so there is my double pull. The triple pull comes from the 20 metre one and second snatch block. I brought two "rope" shackles as well as metal ones to complete the kit. Like the rope shackles as they weigh next to nothing, and in the worse case lot less potential kinetic energy compared to the steel ones.

So looks like the Supercheap recovery kit that I brought will largely dumped, except for the tree protector, two 4.7 tonne shackles, and bridal strap. Once again I have learnt with my wallet But the extension rope did save my bacon when the road gave way so we could get a 45% side pull thanks to a friendly tree.

Now I understand better the opening post, as that setup means say 9500lbs x 2 pull on the tree and 9500lbs on the recovery vehicle with 28,500lbs on the stuck vehicle. Of course the strain on the stuck vehicle could do damage such as twist the chassis, so that vehicle would need a straight pull.

Back when recovering DooSo from a bog hole the rope with the pulley would have been very handy, but luckily the straight pull of the 1.6 Tirfor, plus diff locks and four Maxitraxs saved the day.

So recovery gear nowadays is/will be, two 4.7 tonne shackles (for vehicle attachment as vehicle recovery points can have sharp edges, two rope shackles to join the rope and 30 metre plus 20 metre Dyneema rope and rope blanket along with 6 tonne snatch strap and two blocks, one eight tonne and other nine tonne. Probably will chuck in a dedicated nine tone tow rope. Not excessively heavy or space intensive as the snatch strap and shackles live under the passenger seat, as will one of the blocks. Rope and other stuff can be loaded in for a trip.

Tirfor rather redundant now, but for play trips may come along, as handy for pulls where the front winch will not work.

[Kumabear]

Keep in mind regarding the 4.5T strap/rope that with a 9500lb winch you are still fine even when using the snatch block, apart from the obvious inability of the strap to be passing through the block.

As has been mentioned, the actual tension on the rope/winch line/extension strap is not increased and will not have more weight on it than the winch is capable of pulling. The increased torque comes from having multiple lines to the car pulling with the same force and so multiplying the torque applied to what you are pulling. So your 4.5 ton extension strap is perfectly fine to match the power of your 9500lb winch.

That being said even a rope extension thinking about it would be quite a pain to get through the block as you need to have some way to join it to the end of your winch line like a D-shackle which wont pass through the block so you will still need to back the tension off and manually put through. I am hoping that by the time I pull them or my self in enough to have the strap hit the pulley that I can just remove the strap and have the winch line be long enough.

[nj swb]

Yes, the winch line and anything in series with it cannot be subject to more force tan the winch can exert.

What about the shackle attaching the snatch block? That is subject to the rope pulling twice.

Stall your winch on the bottom layer with a double line pull, and the poor shackle holding the snatch block will be looking at close to 9T.

[Kumabear]

Stall your winch on the bottom layer with a double line pull, and the poor shackle holding the snatch block will be looking at close to 9T.

...That is a very good point

I wonder if I should look at buying a couple of bigger shackles for the snatch blocks. I should also perhaps change the one I am using on my winch as it is a 4.75T one on a 12000lb winch

I see warn make some nice ones like this

http://www.roadrunneroffroad.com.au/92093-Genuine-Warn-Premium-Bow-Shackle-9t-34-22.html

Anyone know a more affordable option?

[old Jack]

Shackles are rated with a SWL (safe working load) but their breaking load can be 3x to 5x the SWL.

12,000lbs is 5454kgs and this is under the GCM of a Gen 4 Pajero however you are not lifting the weight so even with friction/suction of the gooiest mud the winch load will be less than 5454kgs unless you are buried up to the floorpan in a bottomless hole. Most winches stop pulling because you have exceeded the load for the number of wraps on the winch drum, gone out on thermal overload because the motor is too hot and/or you have not done any digging or ramping to make the winch load less.

If you keep upgrading the strength of the components then eventually the winch attachment will be the weakest point of the car and it will fail. Much better to have the winch stall than something let go.

OJ.

[Ent]

Keep in mind regarding the 4.5T strap/rope that with a 9500lb winch you are still fine even when using the snatch block, apart from the obvious inability of the strap to be passing through the block.

As has been mentioned, the actual tension on the rope/winch line/extension strap is not increased and will not have more weight on it than the winch is capable of pulling. The increased torque comes from having multiple lines to the car pulling with the same force and so multiplying the torque applied to what you are pulling. So your 4.5 ton extension strap is perfectly fine to match the power of your 9500lb winch.

That being said even a rope extension thinking about it would be quite a pain to get through the block as you need to have some way to join it to the end of your winch line like a D-shackle which wont pass through the block so you will still need to back the tension off and manually put through. I am hoping that by the time I pull them or my self in enough to have the strap hit the pulley that I can just remove the strap and have the winch line be long enough.

Yes I agree providing the strap/rope is the loop side of the pulley. But if it is from the block to the stuck vehicle then it will be subject to the multiplied strain. So that is why I assume blocks tend to be rated twice as much as the rope that they are designed for.

When DooSo was in the bog hole it was about six foot deep with water just at door seal level and paddle wheeling on his bash plates. Tirfor 1.6/3.2 had no trouble dragging DooSo on the flat but struggled on the 45% plus ramp out until the tyres got some grip. The rear bumper was probably acting as an anchor digging in. Once wheels got traction he drove out.

Yes, would much rather winch stall than twist the chassis. TJM is happy to publish diagrams with winch to pulley then attached back to one of their recover points. But only rate their bars for 9500lb winches. This means that their bar likely can handle the pressure of one block but multiple block might push the friendship. But as OJ points out unlikely you will ever need more than 19,000lbs of pull. That would suggest a vehicle wedge into the ground or rock somehow.

But multiple blocks do make sense for hand winches as getting full 3.2 tonne pull needs a very strong and fit person. I have yet to read of one person breaking their safety pins, and even two people have not succeeded. I did read of three people breaking the safety pins, but they indicated that they were more than happy they broke as they were wrecked by the effort.

Read lovely story about a farmer recovering a fifteen tonne piece of equipment with the bigger Tirfor and series of blocks. By all accounts the fence flies told him he could not do it and to get a bulldozer. Turns out he was ex Royal Army Engineer and within an hour had it out. The block and wire setup was pretty impressive.

Might dump one block and work on the principal if I need two then I am likely rather stuck beyond my experience to extract the vehicle.

One trouble with the Tirfor is the cable is solid core to handle the crimping action and this needs a 210mm block according to specifications. Too many people use the more common 120-125mm block so the cable suffers. Did try to get a 210 block and found apart from massive cost it made the winch look light. So went the rope option to feed through a standard 8-9 tonne block. Yet to have a play with that setup.

Oh yes, also noted with block TJM have two blankets, one for each length.

[Kumabear]

True so I guess I wont be too worried beyond due caution about my 4.7 t working load shackles breaking.

I have updated the picture I posted at the start of the thread in this post to reflect what we have discussed and made it correct to avoid misinformation.