Mounting winches amidships?

Sounds like the old Scammell Pioneer - used by REME, I believe, for tank recovery purposes. I 'borrowed' one once to extract my S11A which was stuck in a bog, down to the headlights & still going down. The Scammell winch rope did not even pull taught before the Series came out, albeit with trivial chassis damage caused by this irresistable force dealing with a seeming immovable object. I learnt a fair bit about driving in swamps that day/night ...

Karen

Looking at the forces applied to a pulley doing a 90 bend I reason that there are two forces. The load pulls one way and the winch pulls another. Doing the old parallelogram of forces thing I get 1.4 time the load applied to the pulley spindle. Of course if it was a 180 turn back on itself it would be twice the load.

We are looking at more than drill a hole and pop a bolt in.

nigelH Physics geek

On Thu, 6 Apr 2006 09:54:10 +0100, Ian Rawlings wrote (in article ):

Sorry, Ian, but you're illustrating a fundamental lack of understanding of the laws of mechanics. No energy is used in the application of any force unless that the object to which the force is applied force moves. It might be counter-intuitive, but it's a basic rule of physics!

The force exerted on the roller is passed to its mount, and unless the mount moves, no energy (and hence no power) is used in the application of that force. However, the rolling motion of the rope over the roller does introduce friction, both in the bearings of the roller and (especially) in the flexing of the cable. The more tension in the rope and the tighter the bend around the roller, the greater the bearing and flexing friction will be, so to that extent you are correct that some power is lost in the roller chain, but none of the losses are as a result of the force exerted on the chassis of the vehicle by the roller (unless the mountings of the roller, or the chassis, bend).

It's probably helpful to think of this in terms of the tension in the winch cable. If you have a straight pull directly off the winch drum, the tension in the cable is obviously the same at the load end as it is at the winch drum. If you put the cable round a pulley, the tension is still the same unless you loose some of it in making the roller turn. In practice this is what happens - the forces required to turn the pulley and bend the cable round the pulley absorb some of the cable tension so the tension in the cable length between the winch drum and the pulley is slightly higher than the tension in the length between the pulley and the load while the load is moving (they are the same if the load is static).

The 'wastage' in the pulley/roller will be a function of how well the cable rolls over it and how tight the turn is in relation to the diameter of the cable. Tight turns distort the cable more so they create more friction.

The moral of the story is to make the roller/pulley as big as possible, and mount it as rigidly as possible. This way you will minimise the losses in the cable transport system.

Nick.

Hello Nigel, what I was originally wondering was not how much of the load the pulley would have to handle, but how much harder the winch would have to work in order to pull via a 90-degree bend pulley than it would if it were pulling with a straight cable. Can you apply your physics geekery to that? Would be useful info.

People Hi,

I believe some of you may know the IBEX vehicles. For a few years now you can order your IBEX with a very interesting centraly mounted winch which can operate both from the front and the rear of the vehicle.

As for the discussion related to the loss of power or forces generated on the pulley and in relation to the angle the wire rope has to sustain in order to have its pulling direction altered by 180 degrees I believe that we could also discuss about the synthetic winch ropes which tend to be a far better choice for such applications due to their extremely good flexibility.

Take care Pantelis

On or around Thu, 6 Apr 2006 11:55:58 +0100, Ian Rawlings enlightened us thusly:

not much. The rate at which the load moves is the same and the losses created by bending the pull are, as someone else said, frictional. There's a sideways force on the pulley, due to the angle in the cable, which will try to straighten. This force doesn't use energy unless the pulley starts to move... If the bearings are good and the rope fits the pulley nicely then it still pulls the same, near enough.

consider the case of using a pulley in the roof to lift a load from the floor by pulling downwards. The downward pull needed is the weight of the load plus any frictional losses to balance the load and anything more than this weight will lift the load.

Sure, there's a force holding the pulley to the ceiling, but you get that "free", it's supplied by the structure. You don't have to pull twice as hard as you would if simply lifting the load direct, you just pull in the opposite direction.

This is not to be confused with using pulleys to multiply the force at the expense of distance moved. If , when pulling a vehicle, you attach a pulley to the load, run the cable around that and hook it back to the winch vehicle, then you get double the pulling power but half the distance. The difference is that the pulley moves with the load. If you have 2 pulleys, and another on the winch vehicle, you can get 4x the load by going around twice. This is how your traditional rope crane manages to lift 10 tons on a cable that's only good for 1T on a straight haul - the use about 10 runs of cable, by going around 5 pairs of pulleys, and the byproduct of that is that the winch hauling it only needs 1T capacity as well, although it needs a big drum and miles of cable.

the thing altering forces applied is vectors: it applies, for example, if your load is a vehicle in deep ruts and you're pulling off-line. The load is constrained to move in the direction of the ruts, so your most efficient pull is in that direction. If you pull off-line, you waste a part of the force applied by the cable proportional to the angle between the optimum direction and the cable direction. If that angle gets to 90 degrees, then the forward component of the applied force is zero.

In addition to this you may add drag to the load, by pulling it sideways against the walls of the ruts, thus further reducing the force available to move it forwards.

Same point applies to a vertical misalignment of cable on load, although if the thing's stuck in deep mud, then adding a vertical-up component to the force may in fact help free it.

As I said it is an elegant solution to providing some tug fore or aft and I think it would get you out of most difficulties. It also has the advantage of keeping the winch relatively safe.

If you really do need to re route it then there are a number of options which come from timber winching. One is the idea of tag lines, you have a soft eye in the winch wire to which you attach a short length of wire rope with your hook on. One tagline with hook does the rear recovery and the other does the front, the front tag line is long enough so that while the hook is wound in as far as the front bumper the joint is at the end of the route to the rear. The idea is that the tagline never gets wound in far enough to spool on the reel.

This system requires that all the fairleads are rollers so that the joint can run past them. Proprietary joints with C hooks are available but are too bulky, I prefer two opposed soft eyes and a wire quoit made up between the two, this quoit is expendable and can be cut with an axe in an emergency. NB this is for pulling only, not for any suspended loads without a full secondary back up.

Indeed, it's a bad habit of mine. I wouldn't have bought a Lotus and a Landy if I knew about the laws of mechanics, e.g. the chances of both vehicles being off the road being proportional to the amount of rain.

OK, that makes sense, Austin agrees too so that backs up an already sensible explanation.

Thanks muchly.

Thanks Austin, nice explanation, I hope your fingers don't smart too much after all that typing.

I've just shelled out the money for the pinzgauer so hopefully once the bank account has calmed down a little after such trauma I'll be able to look at putting the ideas into practice. Both the pinz and the Landy would lend themselves well to having a central winch if I can swing it.

The sensible way to go is an electric motor on each wheel at least that is the way my mums four wheel drive four wheel steer offroad electric wheelchair worked.

For something powered by a couple of traction batteries it had a lot of oomph.

Did you get a 6 wheeler?

I remember seeing one of these off on an expedition to the Antarctic, or the other one, it cost GBP12k, about 24 times my annual wage but I was determined to have one. In the end I settled on a 1965 Haflinger which promptly failed its MOT on a suspension bush. I just could not get the bronze bush out of the aluminium housing and just wasn't nouse enough to solve the problem as I would now. I asked a mate to fix it and it took about a year of asking till I was fed up, went round to check and he had sold it! It was then I learned about the difference between theft and illegal conversion :-(.

AJH

Yes I did :-D

It's sat out on the drive, making the 110 look very very small indeed, despite them being about the same size. I need to convert the rear lights to have separate brake lights, indicators and sidelights, then get a class 7 MOT IIRC, then have some fun!

the grin will be rather forced tomorrow when I'm doing the finances, but it's a lot of vehicle for the money..

On or around Fri, 7 Apr 2006 19:20:15 +0100, Ian Rawlings enlightened us thusly:

.JPEG!

Heh, I'm rewiring the lights at the moment (after de-butchering the rear wiring, when they replaced the body many years ago they took out all the multinational wires and just left the swiss), then I'll park it next to the Landy and take a few pics.

In the meantime, just imagine a big green box with six wheels and that's about it!