[Azalin]

Is it acceptable using ball thrust bearings instead angular contact ball bearings in X axis ball screw support?

Problem is that OD of the angular contact ball bearings is too large and that makes the support too large. That blocks the X axis on the ++ direction. Instead I'm thinking of using ball thrust bearings to have a smaller support.

Thanks in advance,
Suat

[handlewanker]

Hi.......The screw is only experiencing end forces as it tries to push back or forth in the nut so that will mean you only need to have thrust bearings to resist the screw movement.

Any radial moves are contained in the nut which will always centralise the screw in it.

The screw will not revolve fast enough to need any radial restraint from whipping........a milling machine is a different matter as the axis screws can whip if it's not prevented from whipping up and down under fast traverse conditions.

You will need a shoulder in the housing for the bearings to press against from either side........seals are a very big necessary too.........angular contacts can come complete with seals, but the housing design will be different for both types of bearing.
Ian.

[Azalin]

Say, In the support block I have 2 thrust bearings and one standard ball bearing for side loads. Is it then OK?

What really worries me, how tight the nuts should be. If I over tighten the thrust bearings will not work as smooth as a pair of angular contact b. bearings. If I make it loose then I'll have a backlash.

[handlewanker]

Yes that will work.....you won't get side loads.......this is more complicated than just 2 thrust bearings and you still have the radial,bearing diam like the angular contact ones.

You could use a needle bearing between the thrust bearings if you think the side loads are an issue.........the screw turns slowly so you won't need to harden the shaft for the needle bearing to run on.

For the adjustment nut I would use a single Nylock type instead of 2 lock nuts or one with a grub screw.
Ian.

[Azalin]

Unfortunately I can't use a needle bearing because my ID is 10mm and OD is only 18mm. There is no needle bearing that meets the criteria.

[handlewanker]

Hi......a needle bearing would be nice but a plain brass bush would do the same job as there is no significant side thrust to wear it......it only acts as a centralising item ......if you have two thrust bearings with deep groove tracks as opposed to plain flat faces then they would do the centralising all in one, provided that one race OD is a good fit in the housing and the other race is free to revolve with the screw..

Using thrust bearings you need to have the bore of the housing slightly bigger than the OD of the races.......one will be static against the face of the inner body and the other race will rotate with the screw......same for the other side.........a simple sketch will make it clear.........adding seals will complicate the design but are VERY necessary.
Ian.

[Azalin]

Hi Ian,

I forgot to mention, the axis is belt driven. There is side load.

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[handlewanker]

Hi......if you have room to use ball thrust races then you can fit a sleeve to the screw end and run a needle bearing on it to cater for the side loads that are now significant.

There are needle bearings that have angular contact races incorporated in their design.....cost a bit more than plain ball thrust bearings but could be the answer.

Attached is a pic of such a bearing.
Ian.

[Azalin]

Thank Ian.

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[handlewanker]

Hi, the reality is that even a plain thrust bearing will be almost as big in diam as an ang contact bearing and you still need a housing for it too.

This is one drawback to having a thrust bearing on the far end of the screw as opposed to at the front end.

At the back end the cross slide will pass over the bearing whereas at the front end the slide will probably not go that far back unless you are turning large diams.
Ian.