simple machines for complex problems

June 29, 2007 12:15 PM

Help me help a friend lift heavy things using pulleys and a ramp.

A friend owns an old garage/shop that features a 30 degree ramp the goes from one level to another. He would like to lift heavy things up said ramp using pulleys. While we're both familiar with the idea of pulleys, the actual practical application eludes us. For example, does pulley diameter matter? Will the rope used need to be able to hold the total weight of the object or some fraction of the weight depending on the number of pulleys? Is there a website or other resource that a physics novice could use to work out different pulley/weight scenarios?

While I've found lots of elementary school experiment-type information, I'm really looking for something more practical.

Any guidance would be greatly appreciated. Thanks!

posted by zem to home & garden (13 comments total)

If an end of the rope is tied to the heavy thing, then the rope is holding the whole load. If the rope passes around a pulley which is connected to the heavy thing, then each part of the rope - the free end and the standing end - carry half the weight.
posted by notsnot at 12:19 PM on June 29, 2007

The google term for you is "block and tackle". Don't make one yourself, you can buy them.

When you're lifting heavy things, the foremost thing in your mind should be, "if the rope/cable/chain fails right now, will I be killed?". Keep that in mind, and you probably won't be.
posted by jellicle at 12:33 PM on June 29, 2007

Not if it is pulling sideways up a ramp, then the weight of the load is mostly on the floor.
posted by Pollomacho at 12:34 PM on June 29, 2007

The basics of a block and tackle. Wikipedia article on block and tackle, with photos. Field rigging chapter from manual of Naval Construction Force, with illustrations and formulas.

But here's the deal: Rigging is a dangerous business for amateurs. Especially if you're affixing your force multipliers to architectual elements of a building. You can easily rig simple arrangements of ropes and pulleys that you think should move something, and find that when you try it, what moves is walls and roofs. If this is something that involves more weight than you can lift by hand, have an experienced rigger look it over. You can generally find one in the Yellow Pages, and a few hundred dollars for some rigger's time is cheap compared to the damage you can do experimenting.
posted by paulsc at 12:42 PM on June 29, 2007

Look into what's called a "comealong",
something like this

The basic concept is, you pull the lever a short distance, and it pulls the line or chain in a little, and then ratchets so that it can't slide back. A single person can move incredible amounts of weight like this, and because it can hold the load statically between moving, you can ratchet it up a few feet, readjust, and go some more. I've moved very heavy loads using nothing but one of these, a steel cable, with the load resting on 3 1"dia steel pipes.

You could probably use of of these in combination with a pulley.

I don't know too much about rope strengths, I usually use coated steel wire (wire that has like a plastic sleeve on it, very strong) or chains.

Some people I know who do a lot of moving stuff around in their place have installed permanent anchor points in their foundation. Basically they drilled deep holes in the foundation and set bolts into them, and drilled through the bolts to provide a place to attach a comealong, or to thread cable through. You might consider places you could add permanent tools like this to your ramp.
posted by RustyBrooks at 12:45 PM on June 29, 2007

To answer your example questions:

• Pulley diameter does not matter, except that the friction at the pulley axle will be multiplied by the amount it has to rotate under load.
• The rope needs only to be able to hold a fraction of the weight.

I recommend putting wheels under the things that are going up the ramp. Three simple machines are always better than two :)
posted by nicwolff at 1:04 PM on June 29, 2007

Paulsc makes several great points. You CAN do it, but you must have a thorough understanding of the elements in the system, including your anchors (i.e., the building!).

Other items that you should have some comprehension of... mechanical advantage, coefficient of friction, weight of object to be moved, strength of rope/cables/couplings.

Pulley diameter is important for multiple reasons. Taken to a minimum extreme, it impacts bearing size and consequently, strength. Larger diameters reduce the pulley-contributed bearing frictions and increase the bend radius of your rope/pulley. From a mechanical advantage standpoint, they have no further impact... that's mostly about the number of movable pulley elements in the system. You can get significant mechanical advantages using compound pulleys, but not so large as those available from a Chinese windlass, for instance. There are a kazillion ways to leverage energy to accomplish your goals.

Find a good anchor, though, as paulsc recommends. Don't wind up in New of the Wierd with a Darwin award.
posted by FauxScot at 1:15 PM on June 29, 2007

Buying a comealong or chainhoist to do this is probably a lot simpler and probably a lot safer than trying to rig it up yourself. Not nearly as much fun though.

If it's a permanent install, and something that will be done regularly, I'd probably set up a electric winch to do the work..Even a hand winch would be safer, since most are ratcheting types. Ropes and pulleys kind of suck when you lose your grip, or the rope breaks, or you just get tired and let go. Ratcheting and/or auto-braking winches are a lot safer.
posted by alikins at 1:21 PM on June 29, 2007

Having once pulled a building down instead of a flatbed press up, I would just like to chime in here to echo paulsc's advice. If four of you can't lift it by hand, call a rigger.
posted by Floydd at 1:25 PM on June 29, 2007 [1 favorite has favorites]

Depending on what the layout is, a more elegant solution might be an overhead beam, trolley, and chain hoist. You install a long metal I-beam on the ceiling; a wheeled trolley carries the load back and forth; you can buy manual chain hoists or electric ones to lift the objects. Here is a photo of a scale model; here and here are a bunch of trolleys and hoists as examples. (For really light loads, you can repurpose barn door channels and trolleys, although I wouldn't do that if there is even a slight possibility that a falling load could land on a person.)

If you do insist on using a come-along, please buy a really good quality one, not a cheap stamped one at the hardware shop (especially not if there is any chance that an "oops" could have a person downhill of the load). Tirfor/Griphoists are really nice, although pricey; the More Power Puller isn't quite as spiffy, but is very robust and is made by a small company in Ohio. (Note that neither of these should really be used to lift things over your head -- if you are going to be doing that, buy something specifically designed for overhead loads.)
posted by Forktine at 1:43 PM on June 29, 2007

This is a reference for recovering stuck vehicles, but it links to a buttload of rigging manuals and is in itself a great introduction to safety factors, equipment names and terminology, and other basics.
posted by Skorgu at 2:35 PM on June 29, 2007

To be clear I wasn't recommending that particular comealong, it's just an example of what one is.
posted by RustyBrooks at 5:15 PM on June 29, 2007

In addition to paulsc's warnings, never wrap the rope around your hand or fingers to get a better grip. If there is a sudden shift in tension, it can lead to amputation. An acquaintance lost his thumb this way in a river rescue operation. Also consider face protection. If a rope or connection snaps, it can fly into your face like a bullet. Another friend lost an eye when a rope parted while trying to free a broached raft.
posted by JackFlash at 6:09 PM on June 29, 2007 [1 favorite has favorites]