View Full Version : Sideways Open Bearings
22-09-2012, 10:55 PM
Has anyone tried opposing SBR open bearings on the long axis of their machine? I've only seen it done with profile rail or on the shorter axis'. Is there a reason not to do this?
If you did use opposing SBR bearings on the long axis, do you think it is better to have the bearing oriented as in picture A or B, with the weight resting on top?
22-09-2012, 11:06 PM
It may make aligning the X-axis rails tricky - that is getting them parallel and preferably horizontal.
B since with A only one row of balls is counteracting the force of gravity.
Those bearings are rated for the highest force when the bearing is pushed 'down' on to the rail and the least in the opposite direction.
24-09-2012, 04:41 PM
I would think B as your putting weight between 2 set of bearings then
24-09-2012, 09:55 PM
I've answered previous posts on sideways mounted open bearings by pointing out that they will open up a bit with load in that direction. This time however I decided to carry out some stress analysis on a simplified part. The results were quite interesting.
For a 16mm open bearing:
1N load was applied vertically to inner bearing whilst restraining the outer face (the area you would clamp your gantry to).
Displacement is 1.85e-5 mm, therefore 1.85e-5 mm/N.
So for a 30kg gantry spread over 4 bearings thats 73 N per bearing, which is about 0.00136 mm or 1.36 microns.
Initially that seemed pretty small so either there was an error in the calcs or I was worrying over nothing - but then realised that I should really model the supported rail and include that since I think it would be quite significant. Will have to wait for another night . . .
Here are the pretty pictures for the open bearing under 1N vertical load. Stress on the left in N/m2 and deflection on the right in mm.
The FEA discussion ensuing from this post is here: http://www.mycncuk.com/forums/general-cad-cam-machine-control/5141-fea-software.html
27-09-2012, 05:08 PM
Just to let you know, I tried this today and it seems to work very well :)
However, as Jonathan said it is tricky to get the exact spacing so that the bearings on the gantry and the rail on the frame align. I made the gantry about 1mm less wide than the frame/rails and then used shim plates (see here (http://uk.rs-online.com/web/p/shims/0831876/)) to fill the space between the bearings (fixed to the rails) and the gantry side. With a firm wedge of shims in place, the bearings ran smoothly on the rails. At first I just used washers but the gantry moved with some resonance wobbling about. Having got the full shim plates in place today which support the bearings across the whole surface, it runs very nicely!
It was also damn hard getting the gantry in place to begin with since I could not attach the bearings and shim plates until the gantry was perfectly aligned to accept them. I did it by putting the machine bed in place and stacking up random bits of wood until the gantry was perfectly supported. That way even without the bearings affixed, the screw holes would remain in correct position allowing me to slide the bearings in and out trying different shims.
---END OF REPORT--- :applause:
27-09-2012, 05:30 PM
This is where an expanding joint arrangement a.k.a something like a gib strip would be helpful...
27-09-2012, 06:18 PM
You mean like a slightly sprung plate?
27-09-2012, 07:42 PM
no, a gib strip is a tapered piece that slides against a similarly tapered piece so as they are pushed together they exert an outward force. The diagram explains all...
27-09-2012, 11:03 PM
Ahh I see. That is an elegant way to do it, but seems rather tricky to implement in practice. None-the-less the job is done now!
27-09-2012, 11:29 PM
Not at all, its standard practice on pretty much all milling machines and lathes where you have 2 moving parts that you want accurately controlled levels of 'tightness' between them
30-09-2012, 05:16 PM
tapered gibs are hard for the home guy to do a straight gib would be easier and you could make it from ground flat stock
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