16mm open linear bushings on 5/8" rails.

[IZen]

Hi Everyone.

I was wondering if anybody can tell me how flexible these 16mm open linear bearing bushings from VXB might be:

http://www.vxb.com/Merchant2/merchan...t_Code=Kit7443

I'm hoping I could squeeze them a bit to reduce the diameter to 15.875mm to fit the four supported 5/8" rails I just scrounged from scrapped plotters.

My blocks would be designed to vary the clamping pressure around the bushings.

The price difference for 16mm vs 5/8" is $7.95 USD compared to $21.95 USD each and I'm thinking, I might use 12 pieces if I can score more rails, so the savings is big.

The free rails are 56" long and, along with the garage opener screw drive sections I also picked up for free, will probably dictate my design. My machines main function will be milling wood and it looks like the design will be a 4'x4' addressable area.

I can't afford the 5/8" linear bearings right now. If the 16mm won't work I'll build a skate bearing truck system until I can upgrade but it would be sweet to start out solid from the start.

Any observations on this type of bearing or any other advice would be greatly appreciated. I also have some ~175oz-in steppers to start off with and I just started building the drivers.

Thanks,
IZen

[ger21]

Originally Posted by IZen I can't afford the 5/8" linear bearings right now. If the 16mm won't work I'll build a skate bearing truck system until I can upgrade but it would be sweet to start out solid from the start. Any observations on this type of bearing or any other advice would be greatly appreciated.

Set aside $10 a week and start designing and building. In two months, you can buy the 5/8" bearings. I don't think anyone will tell you that the 16mm will work, although they might. But if you try it and they don't work, you'll have spent a lot more.

[IZen]

Thanks for the advice Gerry. That's what I expected to get back, but I'm looking for some real world observations.

I had enough trouble justifying the cost of driver ICs to my "funds manager." This is supposed to be a "no-cost" scrap built machine, constructed during a break in employment. The $264 for twelve 5/8" bushings won't be in the budget until I can show results.

What I was considering was spending $38 with shipping for 4 bearings. I can build a test gantry around them and measure any slop and deflection before finalizing my design. If they turn out not to work, I have some 16mm rods I might use on my Z, so they can be reused if I don't ruin them.

The following calculations are simplified by ignoring clearance:

The difference in diameter is .125mm or 5 thousandths of an inch. In relative terms this means that I would have to reduce the diameter by 0.8%.

Others have commented on the need to support these type of bearings from opening up under load. If they are indeed this flexible I have no doubt I could sufficiently constrict them.

What I don't know is if the resulting constriction to the ball races will bind them. The interior circumference will also be decreased by about .8% which is 0.393mm. In the worst case scenario where all of the deformation occurs at the three races each race gap would shrink by ~.131mm. This is just over 5 thousandths and may be too much.

I cant find the spec for ball size or clearances so I can't calculate the relative amount of pinching.

I'm probably going to try it out. I'll have to do some initial measurements and tests on the shorter 16mm rails so I can compare the rolling resistance. As long as they don't bind and wear my rod I think they might be OK for now.

-Scott

[mactec54]

Hi IZen

The .0049 may close up ok but these bearings are loose to start with so you may need to start with a bore .008 smaller in your mount & just keep boring out a little untill you get the right fit, now this will only work with the open bearings & not the closed