Castor Spacing for Moveable Cabinets

[scruffmeister]

I am planning some base cabinets for my workshop, and need to put them on castors. To fit the space, I would ideally make the unit about 180cm (72") wide so that actual cabinets and this is what leads to my question.

I do want to avoid sag over time as the surface above the cabinets will be used for tasks that need a "flat" surface. My instinct is that I would want at least six castor on this to ensure the span is supported midway or should I be considering 8 castors?

Appreciate there won't be a fixed answer, really looking for opinions and photos if you have them of what has worked for longer base cabinets on wheels.

 

[Sgian Dubh]

Six or eight castors would surely lead to the cabinet pivoting vertically on one or more of the mid-point castors as you move it about, and perhaps when stationary. What you need are four castors, one at each corner rated to carry whatever load the contents of the cabinet are meant to carry, plus the weight of the cabinet itself. Secondly, you need to build the whole cabinet strong enough to maintain upper surface flatness: that's down to technical construction choices you make for the cabinet, i.e., bracing, supporting beams, intermediate support, triangulation and the like rather than the four points of rolling contact at the floor. Slainte.

 

[Orraloon]

My thoughts too execpt he just said it better than I could. Only other thing I would add is is that anything that you are going to work on like a bench or assembly table will be better to work on if the feet are on the floor when working so perhaps look at jack up wheels to move it. There are a few different ones out there. Ebay, Amazon plus tool sellers and hardware places.
Regards
John

 

[ajs]

A couple of thoughts, although I suspect more points to consider than practical suggestions. I'd have a look at the compensating beams used on locomotives to distribute weight between axles on potentially uneven track. In short pairs of axles are connected on their own subframe which is then attached to the main frame on a pivot. This allows the castors to move up an down in a controlled manner while supporting the load above. This also allows the load to be redistributed between the wheels by altering the position of the pivot with respect to each wheel pair. I can certainly see how you could form that in wood but I suspect a welded steel subframe would be a more natural choice.

However that solves equalising the loading between wheels but does nothing to prevent sag except move the load bearing points a bit inboard. To solve that I suspect you'll need some form of Vee members between the main frame and the castors. I've sketched out both options in the attached PDF, hope they make sense as they are ony two minute rough drawings.

Of course potentially you could combine both approaches, having pivoted "bogies" (although railway bogies tend to be though more as to allow for curves than as compensation units, in reality they do both) connected to Vee supporting structures, but this is beginning to look non-trivial.

Whe you say movable how movevable in reality? For semi-permanent positioning I would consider simply four fixed castors on perhaps a 3'6" to 4' wheelbase and supporting on as many raisable feet as you consider appropriate.

 

[peter-harrison]

I had exactly your problem. I had an assembly bench/cabinet with 6 legs which I decided to make moveable. The obvious solution was to put a castor on each leg. It led to the problems described above and was also quite hard to push. I ended up taking out the middle two castors, and beefing up the frame to stop it sagging.

 

[Spectric]

I think @peter-harrison has your solution, rather than try and maker the castors stop any bow, make the frame more substantial and use heavy duty castors.

 

[scruffmeister]

Thank you all, this has been extremely valuable - it's much cheaper making these changes in CAD while I plan than once I've bought birch ply!

 

[HOJ]

Ditto as @peter-harrison my table is 2000 X 1000, now only has a wheel on each corner, made with 4" x 2" stretches etc , oh, and a TORSION BOX MFT type top.