indykid
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| posted on 1/2/08 at 01:25 PM |
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safety factors on lifting devices
i'm currently woking on a project for uni designing a mechanism to lift a vehicle.
just wondering if anyone worked on designing lifting equipment and if they could give me some sort of idea of what safety factor i should be working
with.
at the minute, it's just over 2, which doesn't seem so brilliant, but thinking of safety margins of jacks and stuff in production, i
can't imagine a jack booked at lifting 2 tonnes lifting 4 without real protest....
it's supposed to have a mechanical lockout too though, so needs to be safe to work under in the lifted position without extra support.
any help would be much appreciated
tom
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twybrow
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| posted on 1/2/08 at 01:53 PM |
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I had to do an analysis for an overhead travelling crane for a uni project. It is governed by a Brisitsh Standard. Normally you have different safety
factors for different parts depending upon the severity if it was to fail. For a crane, the highest safety factor will be on the overhead beam/wall
fixing, and the lowest will be the hook. That way if it breaks, the crane doesn't fall off the wall! Check out the BS website - might find some
info there.
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indykid
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| posted on 1/2/08 at 06:57 PM |
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cheers for the hint on the british standards.
my housemate had downloaded them before but he said he'd not found anything in them. just checked them for myself now and vehicle lifts should
be tested to 115% dynamic overload and 150% static overload.
so i guess a safety factor of 2 satisfies the system then.
cheers,
tom
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JoelP
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| posted on 1/2/08 at 09:31 PM |
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its an interesting problem, cos obviously if it cant lift something it doesnt need to be able to support it - but if excess weight should fall onto
it, ie a second winch failing, then it would also have momentum to deal with, which is much more likely to overload it.
I suppose the detailed answer depends on your mechanism - if its a winch and cable then you can easily pick the factor on the cable, and then its just
motor, lock and mounts that can fail. If it were like a hydraulic jack then you need to consider some fat bastard jumping on the lever to lift excess
weight etc.  easy enough to control with a pressure valve.
I guess a key part of the project is actually picking the safety factor itself, since too low is dangerous and too high is inefficient. A key part of
the writeup might be about your research into margins. Maybe suggestions for destruction testing too!
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indykid
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| posted on 2/2/08 at 01:15 PM |
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it's a scissor lift driven by a lead screw and motor. it has to be that way as there'll be a pair of units and they need to be
syncronised. it's designed to lift on the sills like a 2 post ramp, but has to lift all 4 wheels at once. the brief states to lift a car of 2
tonnes, so a tonne each. i've designed it working on lifting 2 tonnes on each lift.
we decided against hydraulics for the syncronisation issues, though if the units could be lifted independently, it would probably be a better
system.
it's to be used in a domestic garage with a flat floor, and no outside influences like wind or suchlike.
the biggest problem is trying to find steel section that'll allow the lift to drop sufficiently low to get under the sills while still having
the strength to withstand the bending moments. the second moment of area of 60x40 (hxw) isn't too far from 40x80 though, so it can be lower, but
has to be wider.
just re read that and it's a load of rambling shite, but you should be able to pick most of it out
tom
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