Pulling versus lifting capacity for winches

Actually referring to hand winches - when you're stuck standing next to a cable under tension, it pays to know these things. I was looking at getting a 4000kg winch extension strap until it was pointed out that this could be potentially overloaded with a hand winch and a snatch block. Extension straps seem to have bugger-all safety factor built in - their ratings are the breaking strain, as the 4WD Monthly tests demonstrated.

I was looking at getting a 4000kg winch extension strap until it was pointed out that this could be potentially overloaded with a hand winch and a snatch block.
Please explain .

Mechanical advantage. Say your hand winch is rated to 2500kg, and you add a snatch block - you can now pull 5000kg, as you have two cables between the anchor and the vehicle, each with a 2500kg max pull. Double the effort, but double the pull. Same principle as being able to lift an engine out of a car with a block and pulley by hand. Stick a 4000kg *breaking strain* winch extension strap in line, and if your vehicle is really hung up or bogged, you could exceed the 4000kg rating of the strap. The strap could let go before the safety pin in the hand winch. I guess any wear and tear on the strap from use could also slowly lower the breaking strain of the strap too, making it more likely with an older strap.

It's unlikely, but the safety factor on paper is so low...

How exactly is a hand winch rigged up with a snatch block ? Thats what i cannot follow .

The only time you'd get mechanical advantage is if the hand winch was attached to the vehicle, then the cable pulled through a snatch block and then back to the vehicle. Simply by putting somehing through a snatch block will not give you any advanatge (other then perhaps a change in direction.)

Yes - that was the type of snatch block usage that I was referring to, e.g. hand winch connected to one recovery point (via a short strap, e.g another tree trunk protector), out to the snatch block on the end of a winch extension strap attached to an anchor (e.g. tree), and back to a second recovery point on the vehicle. You've doubled the potential pull that the hand winch can exert on the winch extension strap.

One might ask why use a snatch block if you need the extra length of an extension strap, but you might need the extra pull, and it seems to me that this is really halving the actual load that the wire cable might have otherwise been subjected to, so it would be adding a margin of safety in terms of wire cable breaking?

Thanks - that makes perfect sense.

Just on factors of safety, as a qaulified and experienced mechanical engineer I feel I need to say something here. Just because your winch may have a design factor of safety of 5 or 6, I would strongly advise against taken this as an invitation to allow your winch to start pulling/lifting loads greater than it is rated too. Factors of safety (FOS) are there to allow for manufacturing defects, design calculation assumptions, deterioration of the winch over its life etc. The original design FOS may be 5, but on your winch it may be less than this when the above-mentioned factors play a part. And short of having it destructively tested in a lab there is no way of knowing exactly what the real FOS is. Having been part of incident investigation where a worker was killed by a winch cable letting go, I can tell you that the results are quite horrific. Stick to the manufacturers guidelines, ensure your equipment is well maintained and you won't go wrong.

And don't put an extension on the handle. These hand winches (Tirfor type) Have a specially hard wire rope and snatch blocks should be 33x diameter of the rope

Craig

Try ... http://www.standards.com.au/catalogue/script/search.asp

If they don't meet AS standards don't touch them.

Cheers

Bugger it I was going to say that earlier but could not convice myself I should know better

Richard

Hmmm I don't think that's the reason - I reckon the higher safety factor for lifting makes more sense.

You could have a 2400kg mass, bolted to the floor, and a 10,000 kg mass on a trolley - I'll bet the winch will move the 10,000kg mass before it moves the 2400kg mass, regardless of pull rating.

The safety factors certainly go to the actual numberthey give

BUT it takes less force to pull 1 tonne horizontally than it takes to lift the same weight (against gravity).

So the amount of force on the components of the winch is greater in a lift than in a pull for exactly the same weight ( or mass)

force = mass x accelleration ( or at least it did when I went to school!) gravity adds to the accelleration as you are working against it. ( so does friction - buts lets not go there - it's too late)

But I have been known to be wrong.... so we can happily agree to disagree...;-)