caster wheel loading into industrial floor, two or four points?

[PeterJ1982]

Hi.

My customer wants to transport a very heavy machine part inside an old factory using rectangular frame that has caster wheels under all four corners and probably in the middle as well. Machine part weight 300 kN. It is an old factory so that floors are not completely level. The question is, how to calculate caster wheel point loads for checking the floor punching and temporary supporting structures? I already told customer that, if there is no spring damping in the caster wheel, you have to consider the load to 2 point only, and other two corners only help a little bit. But if, and only if there is a spring under the floor, i would calculate the load in 4 corners. Does air wheels help also so that they can be considered somehow elastic support?

Do you think that my idea is correct and not an overkill?

--Pete

 

[PeterJ1982]

I already found this thread... I feel like I am right now and the wheel load must be considered in two points only in worst case scenario.

http://www.eng-tips.com/viewthread.cfm?qid=117973

 

[Lomarandil]

You certainly wouldn't be overly conservative to consider load into 3 wheels, and 2 (or mostly on 2) may be reasonable.

Air wheels will limit the reaction to the air pressure x contact area, but depending on your situation that will only get you some distance between counting load on 2.0 wheels and counting load on 2.9 wheels.

----
The name is a long story -- just call me Lo.

 

[12345abc6ttyui67]

I have seen it done a long the lines of a load split of 75% across two diagonally opposite wheels, and 25% on either of the other wheels.

E.g. front left at 37.5%, back right at 37.5% and either the front right or back left at 25%.

This assumes a central COG, and one pair of diagonal wheels takes the majority of the load, with the load then 'balancing' itself by 'tilting' onto either of the other two wheels. Of course, at the instant before the load 'balances' it could be on only the two corner wheels...

Counter intuitively it is often 'better' to use only three wheels (assuming the package remains stable) as then you can place the wheels at positions to guarantee a 33.3% share of the load - less than 37.5% I note above, and clearly much less than 50% if you consider just two wheels.

If you have an uneven surface I would think the uneven load split can happen with any kind of wheel, although it will certainly be more pronounced with hard wheels / no suspension. I would steer well clear of using a central caster - potentially you might get all load on that one central point. It would be far fetched, but not outwith the realms of possibility...

An alternative is also the use of air skates (they work like a hovercraft and use compressed air to 'raise' the load just of the floor). I'm not too familiar with these, but they have a much larger surface area (less shear / bearing problems) and I think the 'cushion' effect of the air will mean you can be more generous with sharing the load to each skate.

 

[Ron]

It depends on the wheel spacing. The controlling stress is usually tension at the bottom of the slab; however, if the stress patterns overlap,tensile stress at the top of the slab in reverse bending can control. Since the contact area for hard casters is very small, check punching shear as well.