Frame on castors

[colinmseries]

Hi. Not sure if this should be in the structural section or here, so please refer me if it should.
I'm wanting to build a mobile frame to support two small cars, something I can push around a workshop. They weigh about 1000kg each and the hope is to place one above the other on a frame mounted on castors. The new workshop where they will live has been designed around this idea and so one way or another I have to make it work. Plan A is to use the sort of framing and cross beams used in warehouses, because used they can be bought cheaply and are rated to carry far in excess of the cars' total weight. When assembled the frame would look fairly box like at about 60"longx70"widex50"high, basically two side frames with two pairs of cross beams. If it were to stand on the floor I'd be happy it's being used as designed albeit unconventionally and get on with it. However, it's the castors that worry me and I'm struggling to find a source of help. I can see the bottom of each frame leg (4no.) will have a bending load from the castor trying to roll away and I'm at a loss to quantify this. Obviously the frame legs have bolted plates and normally they sit flat on the floor. I know enough to know better than to just press on. Can you help suggest where I go next please?
Any thoughts radical or otherwise appreciated, Colin.

 

[Jboggs]

Colin,
Maybe you haven't gotten any responses because your idea, with the limited information you have provdied, scares the h___ out of experienced engineers. Not saying it couldn't be done. Just saying the MULTIPLE possible failure modes all lead to potential human injury or death.

For example, the structural framing you are considering may have an adequate rating, but that rating is based on a stationary application like a warehouse shelf, not one with the unknown forces and shocks associated with rolling around.

Or maybe its because the center of gravity of the whole arrangement is high enough to warrant concern.

Or maybe because one of the likely failure modes, commonly called "parallelogramming", involves the inability of the corner joints to withstand the moment loads created by moving, starting, and stopping the whole frame.

Or maybe because we don't know how you plan to load and unload this device.

Or maybe because we don't know how you plan to move it (forktruck, manpower, etc.)

As far as casters, there are several manufacturers you could consult for design and selection help (swivel, rigid, wheel material, face width, bearing type, etc.) I've worked with Colson, Hamilton, and others over the years.

I recommend you contact a good local structural or mechanical engineer or reputable machine shop for assistance.

Above all - be aware you have a potential killer there. When it fails it will be sudden and it will be too late.

 

[Screwman1]

The OP also has to worry about flat spotting any soft wheels on those casters, once that happens he'll never move that thing again. This idea is scarey for so many different reasons. You can buy two and four post lifts that are designed for just this purpose (lifting one car over another for storage; why not use one of those?

 

[MiketheEngineer]

Just noticed an article in the Oct issue of Popular Mechanics which has some info on this -- commercially available - I think. Check it out.

 

[rb1957]

sorry, i don't understand the concern. castors are rated, yes?

so you need 4 500 lbs castors at a minimum, 4 1000 lbs would be better, or something inbetween; no?

other questions about the frame strength haven't been asked (presumably to OP is comfortable with the frame)

 

[colinmseries]

Thanks to both of you. In the last day the issues about parallelogramming had sort of occured to me. The racking is very secure in the front plane but less so in the side. I can see I'm overlooking the start an stopping issues too. I had intended to use solid wheel castors to avoid flat spots but that's only one issue I know.
The idea of using conventional lifts would mean I couldn't move the two cars around and was why I hadn't gone there, and compared to the frame work are very bulky. I was planning to lift and crib the cars into position using two sets of beams off the frame work using scissor jacks in 3 or 4 lifts.
The whole idea was to conserve floor space and for the whole assembly to be moveable and not in the way when needing the workshop for something else. I've even included doors tall enough (2.4m high) to wheel the lot outside if necessary.
You are obviously right, I need to contact a professional and get it thought through and designed. What sort of title person am I looking for, a structural engineer or a mechanical person or?
Thanks for helping, Colin.

 

[colinmseries]

Just seen the other comments, thanks also, and thanks for not laughing. I built the workshop with this idea in mind but without knowing exactly how it could be done. A few months ago I was wondering around B+Q and saw the huge loading on their racking and assumed surely 2000kg would be a breeze. The frames and beams I'd seen available second hand, I was told, had been holding several tons of paper records. I had intended to be sure that was true and it does look substantial but I accept I'm a bit lost as to how to be certain. I don't want an accident.
I will look in Popular Mechanics thanks for the tip. I've been in scrap yards and seen cars stacked on crossbraced folded sections so thought this would be easy. I did't consider the rolling and moving shocks.
Colin.

 

[kingnero]

several 4-post lifts have factory-fitted castors under the four legs - either as an option or standard issue.
that leaves you to put the other car "below", either by driving or putting her on dollies, when moving the lift.

 

[hendersdc]

A couple of lifts here: Greg Smith Equipment say they come with caster kits included. I think these might fit your situation well since you are mostly looking for storage. The website mentions that they are not a good fit if you need to raise and lower many times a day.

Doug

 

[racookpe1978]

How flat and level are your floors?

"Rolling" resistance of highly-loaded casters on asphalt will dig in and not turn (pivot) the castors.

On even mildly sloped concrete (as if for floor drains) a sloped floor might get away from you and crash the moving load. Cracked concrete will stop the motion, or prevent the casters from pivoting as you expect.

 

[colinmseries]

Hi again and thanks for the suggestions including off the shelf lifts. The option of a proprietary lift for storage had been considered but abandoned due to the space the lifts occupy. The building is only 3.0m internal width so they would render it impossible to move around or squeeze past. Also planning issues have left me with limited head room where the two cars are to be stored. So as a compromise I designed the building and its door openings so the cars could nest so long as the frame was compact.
I appreciate your interest in commenting and don't want to go off topic on a serious forum. Firstly, to address the castor issues. A manufacturer has helped so far in suggesting either the frame sits on a suitably stiff bogie carrying 4no. large castors or, each leg is positioned centrally over a cluster of smaller castors fastened to a suitable plate. They suggest steel wheels to avoid flat spots and sized for tripoding over non-level sections of floor. The floor is a power floated concrete surface and pretty flat. The only obstacle is the main door threshold plate to move it all outside. This would need timber fillets making though going outside would be a very rare occurrence.
Looking through your comments I feel the parallelogramming issuse is the biggest problem at the moment. I think the beam to frame joint is very stiff and strong due to the section of connector they use, but the bottom joint to the castor arrangement would be I guess where the highest bending might be.
My interest in the racking framing was that I saw a way of using two beams one under the other as a way of lifting the car, one as a jacking beam and the other as a crib. The frame section has punched slots at regular intervals already there and so 4no. scissor jacks would allow the jack, crib, and reposition method in a number of lifts until the cribing beam was in the final position.
I'd like to employ a professional to look at it really, I've a reasonable idea but not enough to be sure it's toally safe. I live in Urmston, Manchester near the Trafford Centre. If anyone rreading this is interested in helping me work up something that can be fabricated please get in touch. Failing that just advice on getting the right person on board. I have no option but to make this work as the proprietary alternatives either won't fit or prove impractical.
Colin.

 

[Jboggs]

One other issue many don't consider - the force it takes to make a swivel caster actually swivel while stationary. The reason is the friction of the tire on the floor. Unless the caster is rolling, its entire area of contact with the floor is being forced to slide under load. You're a mechanic. You know power steering linkages do that all the time, but can you imagine manually pushing on the side of the tire to get it to rotate while under load?

 

[colinmseries]

Taken your advice and used a pro. Actually, the chassis designer of the cars involved, he was really interested to help! Furthermore the design which at least to me is very original is most likely going into production. I'm just having the prototype. Thanks for the suggestions, Colin.