Linear Bearing Question

[karthur]

I am wanting to have a rectangular base plate move on two parallel linear rails. The base plate will have a linear bearing in each corner. The plate will be driven from the center.

My question: Is there a common (rule-of thumb)ratio of the width Vs. length for a bearing arrangment like this?

I am concerned that if the bearings are too close together and the distant between the rails is excessive, the base plate will tend to "Bind" and not move freely.

Thanks

 

[strokersix]

You may wish to allow the bearings on one end of the plate to float in the direction toward the other bearings. This will reduce the fabrication accuracy required in the assembly. As far as a rule-of-thumb goes, I'm not aware of one. I suggest in addition to the above suggestion you consider the rigidity of the rails and bearings under the expeced load conditions and make a judgement call. If your bearings are roller or ball bearings the reduced friction will help a lot.

 

[karthur]

Stroker,
The bearings will not be allowed to float, but rather be fixed by a shoulder to increase the rigidity of the system. The rails/bearings will handle the loads just fine.

Thanks

 

[strokersix]

Take a look at machine tool way construction for an example. One common design uses a vee way on one side for alignment and support and a flat way on the other for support only. Of course this is not the only method, just one successful example. Providing redundant constraints as you propose may be more rigid but will almost certainly be more difficult to produce.

 

[MJuric]

Generally I dictate the distance of the bearings by the load.

If the system is bearing the load vertically thru the rails than the only load left that would cause binding would be that of the force need to move the load. Most manufactures list this load capability in there catalog. With out seeing any numbers I suspect that your design is fine and will not bind considering...

a) Its fairly dificult to make one runner block bind by applying torsional force. (Note it's really easy to lock em up if your rails aren't running parrellel)
2)By having two runner blocks on the same rail you have effectively spread any load across that distance (Most MFG's of runner block rail systems can get you all of the formulas for calculating loads for their runner blocks. I had a neat few pages that gave all of this info but can't seem to locate it presently )
3) Assuming your load is centered on the runnerblocks and your pushing in the center of the load, your torsional load should be little or nothing.

I guess that's as close as I could come to a rule of thumb. Although I've seen a number of bushing style plates lockup due to binding of this sort, I've yet to see a runner block and rail lock up under these conditions, wear prematurely, yes, lockup no.

~Matt

 

[gbednar]

Check out

http://www.pacific-bearing.com

. I used their bearings and shafting to move an Induction welder to from the weld box on a tube mill. My guess is the welder weights around a ton - ton &1/2.