Lifting threads?

Not quite model engineering, but please bare with me....
I'm in the process of building a myself a single sided post lift for a
motorcycle - just at the testing phase. It is almost complete, working
and I have done a quick test of it.
Basically it is an L shaped fixed main frame, an H shape under the bike
to keep the L vertical - with a sliding section attached to a main
horizontal lifting beam. Attached to the later are a couple of forks
enabling the whole thing to be slid under the bikes wheels to then
raise it by cranking the sliding part up the main vertical post. This
all made from welded box section steel.
The actual lifting is done by means of an ordinary mild steel threaded
rod with a captive nut under the slider and a welded nut on top. Turn
the rod and up it goes, the rod being under tension from a bracket at
the top of the post. The OD of the rod is approximately 20mm, weight of
bike plus lifting system about 250kg and the distance it raises it from
the ground to a maximum of 30".
It works, but involves quite a lot of effort cranking the rod around to
raise it the 30". I have not fitted any sort of bearing under the top
nut, just a couple of washers and liberal amount of grease. It has been
suggested that a square thread would be much less effort, rather than
the V thread I am having to use at the moment. Is that correct?
If it is correct, where could I source a (cheap) length of square
threaded rod plus a pair of matching nuts, able to fit through a 20mm
hole with a length of around 32"?
I know hydraulics would be much easier, but it has to work from almost
ground level up to its maximum height - not even space to get a car
trolley jack in.
Reply to
Harry Bloomfield
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If you are lucky you may find at Model engineering shows like Harrogate second hand ball screws about 30" long that would do the job better. EBay is another likely source. You perhaps could also put a simple 2:1 reduction gear drive to reduce the effort? Just more winding though!
Reply to
Alan Marshall
Alan Marshall laid this down on his screen :
Thanks for that.
More winding would not be a good idea - unloaded it is taking a battery drill 30 seconds to move it the 30 inches. Loaded it needs the use of my socket set and its ratchet - a long slow process.
I'm not far from Harrogate, have you a URL to the date(s) please? Are there any specialist machine parts scrap dealers around Yorkshire perhaps?
Reply to
Harry Bloomfield
You are using a moly grease, aren't you? It would be better with a proper thrust bearing (ideally a ball bearing) under the nut.
Reply to
An worn leadscrew & nut from a lathe would do the trick - for example a 3/4" dia. one that's no longer accurate but still intact. Although lifting a motorbike upwards at 8 turns/inch or so won't be much fun...
hth Guy
Reply to
Guy Griffin
Hydraulics would be an easy solution, envisage how a forklift works. By using a cable or chain anchored on one side of the ram and the other end passing over a sprocket/pulley fixed to the ram end, the cable end being then fastened to the carriage to be lifted. Because of the 2 : 1 purchase, for every inch of ram movement, the carriage will rise 2 inches. Your 30 inches would require a 15 inch stoke ram, cheap chinese long stroke jacks abound these days, engine hoists have them as well. However if you wanted to be cheap you could use a 7.5" stroke jack and cable. With a double sheave mounted on the ram and one underneath, the diameters being big enough that the cable cleared the jack body, you would have a 4 : 1 advantage to lift to 30". Be aware that there are no free lunches and a 2 ton jack will only lift 1/2 ton in this instant.
Reply to
One of my early ideas for the lifting part was..
A wire attached to the slide part - up to a winch mounted on top of the post. I have such a geared hand cranked winch with a ratchet mechanism. I presently use it to winch my caravan up the slope of my drive it is probably capable of lifting half a ton at a guess. I tested it pulling my 2 ton car up the 1:10 slope which it managed without a problem.
I copied the basic idea for the lift from a commercial product which uses a threaded rod of some sort, so I assumed the threaded rod would be workable and it appeared to be a safer method than a winch.
Reply to
Harry Bloomfield
Er, no...
I just slapped some grease on from the nearest tin I could find - some HMP grease. It does seem almost as difficult to wind down as up, so might be worth persevering by getting some moly grease and a bearing for the head.
Reply to
Harry Bloomfield
So, are we saying that despite the 'higher gearing' of a leadscrew it would be produce less frictional losses?
Just to give you an idea of what it is like at present, with plain washers taking the load and HMP grease on the thread...
I can run the unloaded lift up and down fairly easily using a battery drill to turn the threaded rod.
When I put the bike on the lift, I need to use an 1/2 drive ratchet to turn the rod to raise the lift. It is not that difficult, but it will be quite tiring over the number of turns to get it all the way to the top. Going down I can turn it with the 1/2" drive speed brace, but not the drill which lacks the torque.
Reply to
Harry Bloomfield
Harry Bloomfield presented the following explanation :
The winch is one I was given many years ago. The only details I could find on it was - '542 winch Fulton co., Milwaukee'. I have it fixed via a bracket welded on top of a short section of scaffold pole. I simply drop the pole into a hole drilled in my drive, then run its wire down my drive to winch the caravan up the drives slope backwards.
I have not been able to trace the winch manufacturer to find out what it is rated at, so I have just used it for pulling rather than actual lifting duties.
Reply to
Harry Bloomfield
It should be smoother than a nasty snaggly-threaded MS bar.
Compare the ratio of threads-per-inch to estimate the ratio of force required. Say you have 16tpi threaded bar; changing to an 8tpi leadscrew will mean twice the manual force, if friction is the same, but the bike will go up twice as fast.
Reply to
Guy Griffin
An acme thread, with a roller trust bearing at the top should work. This was a common arrangement on aftermarket American car jacks. These were bumper jacks, which could support well over 2,000 pounds. They had a simple crank at the top end of the screw. I still have one somewhere!
Steve R.
Reply to
Steve R.
I thought it might be worth doing some sums on this and I found the numbers so surprising that I've probably got them wrong but here they are anyway.
If the thread pitch is 2.5mm (metric coarse at 20mm dia) then for each rotation of the thread you lift the load 2.5mm (or 0.1inch). The end of a 12inch long ratchet wrench will move 12*2*PI inches (about 75 inches) giving a theoretical mechanical advantage of 750:1. If it were a metric fine thread (1.5mm) the advantage would be about 1270:1.
If we neglect friction and consider a 500 pound load then the force needed to move the end of the ratchet wrench would be about 0.66 pounds for the coarse thread and 0.4 pounds for the fine thread.
If you used a speedbrace with a 4 inch throw then the loads would be 2 pounds and 1.2 pounds.
If I've got those sums right then there's a lot of friction in the system and I've been trying to get my head around why friction on the thread should be greater going up than down and I don't think it should be. You don't say much about the sliding arrangement and it strikes me that there will be a lot of out of balance forces as it's a single post lift. Is there anything you can do to reduce friction there. I think that may be the main difficulty. Is there any arrangement to ensure that the nut doesn't go out of alignment under load as that would increase friction dramatically too.
I hope these thoughts help. Even if the sums are wrong.
Reply to
Standard threaded bar is very ineffecient at transmitting power, and from what I remember, effiency is less than 50%, and possibly somewhere around 30%.
Reply to
Not particularly cheap, but marchantdice, an ebay seller, does 20x4 mm acme threaded rod at £1.86 per 100 mm, so a meter would cost £18.60. They also do nuts, but these are expensive at £19.50. BTW, why would you need two nuts?
Put a bearing on the thrust end of the rod though.
Reply to
Peter Fairbrother
The jack of the Deux Chevaux was like that with bevel gearing so you didn't swing the wheelbrace into the body. Long while before the wheel you were changing left the ground. About 2ft lift.
Yep, got one here, bit rusty - lot of cobwebs. Can deliver in that condition. I'll be travelling Redditch to Sittingbourne area via M40/M25/M26/M20 tomorrow or Friday, shall I drop it off somewhere?
Reply to
Chris Heapy says that adding a proper thrust bearing to a vice tightening screw is so effective you have to watch for stripping the thread.
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