Thread: portable electric winches

Frank is correct if the hook end of the winch rope was anchored to a non-moving object such as another tree. The snatch block would merely be redirecting the rope.

If the hook end of the winch rope is attached to the vehicle (load), then a mechanical of 2 is obtained. This is because both sections of rope are doing effort. This is what SteveG and RR P38 are saying. The snatch block does not have to be moving, as it doesn't care if it is attached to the fixed or moving anchor.

As with everything, you can't gain something without losing something else. In this case we lose out in the velocity ratio of ½, meaning the load only moves ½ the distance the effort does.

Originally Posted by isuzutoo-eh

Frank is correct if the hook end of the winch rope was anchored to a non-moving object such as another tree. The snatch block would merely be redirecting the rope.

If the hook end of the winch rope is attached to the vehicle (load), then a mechanical of 2 is obtained. This is because both sections of rope are doing effort. This is what SteveG and RR P38 are saying.
With this set up you do lose out in another area-the velocity ratio of ½ means the load only moves ½ the distance the effort does.

Noooooo, Thats how I thought it was too

I just got my head around it & now you throw a spanner in the works,
I'm pretty sure thats not what Frank is Saying.

The snatch has to be moving for there to be MA. If the block is on one tree it dosent matter if the Hook is on another tree next to the car, or fixed to the front of the car itself, there is no MA .

Thats why pulling myself out of a bog with the block on a tree & the hook back on the car seemed harder than I thought it should have,
And why pulling a scented gum out in the back yard with a block on it was easier than I thought it would have been !!

oh hang it, I'g going up the bush to have a play & find out for real.

The snatch block doesn't know which end it is on, whether it is on the fixed anchor or on the load. All it knows is the winch is at the other end. Hell, for all it knows, the winch is pulling the tree towards the 4WD. It just knows it has 1xload from the anchor it is fixed to, and two lots of ½xload pulling the opposite direction. The tension in each direction of rope is half the load as two ropes are supporting it in (roughly) parallel.

If you compare it to a crane, the boom is the vehicle as that is where the winch is fitted. The snatch block is on the load. free end of the winch rope is attached on the boom again. This has the snatch block moving with the load, giving a mechanical advatage of 2. Load and effort are switched compared to 4WDs, but the bits in the middle are all the same.

Thanks Mark - that's a better example than my garble.

Steve

Steve & Mark

this concurs with what you are saying,

How to Use a Winch Snatch Block « « ProMark Offroad Blog ProMark Offroad Blog
(the middle drawing is the scenario I'm interested in)

Of course the winch dosent know the tree is not moving, As far as the winch is concerned the Snatch block IS moving, The winch does not necessarily know its bolted to the front of a car. so the MA would be 2

So now I'm back at square one.

Which brings up back to the original post, the portable boat winch - with snatch block on the tree !!!

I think I'll still take my Tirfor 15kg plus rope 15kg vs 20kg boat winch , might not always have 12v in a recovery situation

Originally Posted by steveG

Your example of lifting a weight from a single pulley on a beam is a different scenario than a winch on a vehicle with the line anchor point attached to the same vehicle (which effectively is pulling in the direction of travel).

If you took the winch off the vehicle and anchored it to a tree - then you would have the same scenario as your beam lift. But that's not the case.
The winch on the front of the vehicle IS pulling to advantage - if you stopped the snatch block pulley from rotating the winch would pull the vehicle forwards.
With the pulley free, the tension from the winch is felt on the vehicle via the fixed end of the line - the same as it would if the winch was attached to another anchor point (not the vehicle).
So there are 2 forces pulling the vehicle forward. The winch pull, and the pull from the line coming back from the block.

Is that not correct?

Steve

Steve I have explained it to the best of my ability, you seem to think that all this is my idea, it's not.
I was taught this basic setup by ticketed riggers, TAFE college teachers, and it is available for all to read and digest in the many Tafe text books or the OH&S riggers guide available through Google or send me your email address and I will email it to you.
I don't want to argue with you, so have a look at the references available and argue with their authors, sorry I can't explain it better to you, Regards Frank.

Reading this thread took me back to seamanship school. Rigging to advantage or disadvantage is another concept Rigging or reeving a tackle to advantage when the hauling part comes away from the moving tackle. Conversely when the hauling part comes away from the fixed tackle it is reeved to disadvantage.
What you use, how you use it rig and how you reeve all impact to some extent.

Single Whip (fall through a single fixed block)- no MA
Runner (rope through a single moving block)- MA 1.82
Double Whip (two single blocks cannot be rove to advantage)– MA 1.67
Luff (double and single block) - MA 3.08 or 2.3 depending if rigged to advantage or disadvantage.
Two fold purchase (two double blocks) MA 4.37 if rigged to advantage or 2.86 if disadvantage


Did MA just get a little more confusing? “Admiralty Manual of Seamanship Volume 1” page 238 on would match the riggers guide fairly well I guess and I guess as every sailor got one for donkeys years they should be as cheap as chips if you can find one. Mine is a 1979 version.

Originally Posted by Tank

Steve I have explained it to the best of my ability, you seem to think that all this is my idea, it's not.
I was taught this basic setup by ticketed riggers, TAFE college teachers, and it is available for all to read and digest in the many Tafe text books or the OH&S riggers guide available through Google or send me your email address and I will email it to you.
I don't want to argue with you, so have a look at the references available and argue with their authors, sorry I can't explain it better to you, Regards Frank.

I'm not arguing with the theory Frank, just that I believe you have applied it incorrectly in this case.
I've read a number of rigging references, but failed to find one that shows the line anchor point on the winch. I emailed you earlier for a copy of the rigging guide but haven't received it yet.
I don't have your experience or knowledge, but that doesn't mean you're correct all of the time.

Firstly, you're giving conflicting information. You say that there is no mechanical advantage to using a single pulley with a winch, but you give a scenario with the 10kg bucket that shows a 10kg line pull over a single pulley results in a 20kg beam loading. That transposes horizontally to the winch situation and gives a pulley anchor point loading of double the line pull - which goes against your no MA statement.

To test for myself, I've just been out on my sloping driveway with a spring balance, some rope, a snatch block, my garage creeper and my fat ass.
With a direct pull on the rope, it takes about 13kg to get/keep me moving up the driveway, but if I tie the rope around my waist and pass it through the snatch block and pull on the free end it only takes about 8kg. Plenty of variables there, but if you were correct the line pull through the snatch block would be the same or slightly higher then a direct pull as there is some additional pulley friction. The reality is that its closer to half the load.

Since this is the recovery section, I can't sit back and let something go if I feel its wrong as I consider it my responsibility to point it out (as I know you would ).
If someone were to take your advice that there is no mechanical advantage, and uses a double line pull with an 8000lb winch, proceeds to anchor the block with something rated to 8000lb its likely that the anchor would break. I'm not comfortable with that.

Steve