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aceofspades
9th Jan 2006, 07:57 AM
Hi, bit of a novice here, im designing & building a bike rack for my garage thats also part of my A-level course at college. It will hold 2 bikes, lifting them up from the ground high enough to get the car underneath. I'm going to make the rack telescopic, the top section (which holds the bikes) will move up outside the main 'shaft'. The problem is how to get the two rectangular tubes to move over each other with as little friction as possible, whilst supporting the weight of 2 bikes on one side...

The rack will obviously dig in on one side and theres no way it will slide up and down as it is.
At the moment I'm thinking of cutting slots in the inner tubing and fitting some sort of roller(s) to both sides (probably drilled out nylon rod cut to length). From the measurements I have taken of the stuff in the workshop - the gap between the outer and inner walls will be 2.5mm on each side so I'm planning on fitting 5mm diameter nylon 'rollers' on the inner tube.
Bearing in mind I have the run of the college workshop (including MIG welders/brazing torches, CNC machines etc) - What is the best way of doing this??
I'm trying to make the design as simple as possible, I'm pretty sure I might have to scale the whole thing up a bit! I'm also not sure on how best to fit everything together, It all exists in my head and on paper at the moment so im fully open to any suggestions you might have, any help with this would be much appreciated, Rob

E. maculata
9th Jan 2006, 08:13 AM
Have done similar things on an industrial scale before and have used a variety of plastic & brass compounds in thin strips as simple "sliders" some are even self lubricating.

rodm
9th Jan 2006, 11:46 AM
Keep your design simple and don't get carried away with bearings and rollers. Slides rather than rollers are easier to set up and there is much less chance of failure down the track.

Ashore
9th Jan 2006, 01:45 PM
Mate your description is good BUT a drawing would be better .
Post one if you can as verbal descriptions often leave things out

To ease the slide and weight have a think about using aluminium square section

Waiting to see the drawing

Rgds

Andy Mac
9th Jan 2006, 02:20 PM
Hi Ace,
I scored some offcuts of a white nylon material, small slabs of it, which gets used in engineering works for bushings and slides. I really don't know the trade name for it, but works a treat. I cut it with a hacksaw and drill it, but doesn't abrade easily, as in file or sander, and even drilling is ... interesting! Have to overcompensate by drilling larger than finished size, if that makes sense. I reckon you could use the stuff as small section slides if you can work out how to locate it, because glueing is impossible. Maybe some on the inside of the outer (fixed) tube near the mouth, and some at the far end of the inner (sliding) tube...as per attached drawing.

Cheers,

knucklehead
9th Jan 2006, 03:25 PM
Do a google search on UHMW.
This stuff is really good as bushes and can often be used to replace rollers.

I have bought this stuff off the local bearing dealer. They have several profiles on hand and a huge number available to order. Of course if it is going to be used in a situation that is not very demanding good old cutting boards make a passable substitute.


http://www.ultrapoly.com/Pages/pr_uhmw.html

2shane
3rd Feb 2006, 04:11 PM
The simplest and easiest way is usually the best.

Do what I do.

Do a rough mental calcuation of:

a) what it has to hold,

b) how many "in / out" or slide cycles it will face in it's life time

c) the most basic system or arrangement you can figure out
(a few drops of oil on plain shafts)

d) the best and most sophistocated sytem you could ever contrive
(teflon pads and sealed needle rollers etc..include much awsome mystical drivel)

Then balance out how much time and effort, costs and all the rooting around, for something that on balance - if:

1) The whole thing carries almost no weight

2) It's hardly ever likely to be used much any way - as in it would be used say X times per week for X years.

3) The remainder of your useful life expectancy

4. And if it was plain steel shafts - lightly oiled or greased, would probably start showing some wear after 500,000 insert and withdraw cycles.

5. vs. all the buggerising around for what ever degree improvment obtainable, at what ever increase in time, effort and cost in creating the best thing possible

then either

a) go with the simplest system you can devise,

b) Go with the most sophisticated system you can devise or

c) compromise some where between the two.

- cause some small differences can make huge improvements and some huge differences can make small improvements...

It's your choice and your decision.

You know - I won't buy a "Deluxee Bread Knife" for $50 from the posh shop, when I can pick up a perfectly good one for 50C from the opp shop.

It's just a retracting bike rack, not an 80,000lb thrust jet engine.

Keep it simple - design it simple

troppolad
26th Mar 2006, 07:37 PM
G`day guys
i`m a motor body builder by trade, at work we have our migs on raised booms so we can get trucks/trailers under them
they have to hang anything up to 100 kgs permanently while retaing the ability to raise and lower and swing from side to side
i have always liked to use the K.I.S.S principle (keep it simple stupid! lol)
these frames are raised and lowered ten times a day on the lower end of the scale up to 7 days a week so the are reliable and long lasting, they use two rollers placed on oposing sides of the main beam that work on the leverage of the weight (hope that makes sence)
i`ll try to do a simple text based diagram of the setup for you


http://i14.photobucket.com/albums/a324/troppolad/weldingboom1.jpg
these are simple and highly reliable, with a couple of pins (one top and bottom) you cam make it rotate around a central axis too
cheers guys
troppo

justrite
28th Mar 2006, 08:33 PM
Hello people,
I am a Consulting Mechanical Engineer and I would suggest you view the following sites. If you can use a length uf this up the wall and then run the standard trolleys in the section. You may have to fab a bracket to run two trolleys in the track.
http://www.unistrut.com.au/order_catalogue Metal Framing section. They provide Trolley assemblies that run in their Unistrut channel
Also
http://www.cowdroy.com.au/ Door hanging systems. This could meybe be used in a vert orientation

troppolad
29th Mar 2006, 12:53 PM
G`day just
that unistrut is great stuff, we use it and the trolleys in our taughtliners
i`ve even used it for sliding doors on a mates shed, very easy to install and use
cheers
troppo