[2_many_hobbies]

Ok so I am building a spindle and mounting my acme screw and would like to know if my setup is right.

Acme Leadscrew:
On the far end of my 1/2"-12tpi screw I have a single 608ZZ DG bearing for support and on the motor end do I use just one 708A AC bearing or two? I figure I would only need one as usually one would just use another 608ZZ and a thrust bearings and the 708A provides both right?

ER16 Spindle:
This is the complicated one, I hear mounting the AC bearings back to back is best but what side is the back?? Im guessing ill mount them like this ( tooling end - |O\| - |/O| ) the bearings im using are 6204RS DG and 7204B AC

So the entire spindle would look like this:
Tooling - |O\| - spacer (outer race) - |/O| - spacer (inner race) - |O| - nut

So in words let me explain what I think is going on, the collar of the ER16 shaft is seated against the inner race of the first AC bearing so when pressure is applied it will seat the bearing. Their is a spacer between the outer races of the two AC bearings so the inner races can be loaded independently. The next AC bearing has the inner race pointing up and a spacer that slides over the ER16 shaft makes contact with the AC inner race and the inner race of the deep groove on the top so when you tighten the nut on the very top its preloads the AC bearing. (you would set this so that their is no wobble in the spindle but so the spindle still spins freely so as not to overheat the grease)

Is this the correct method? I don't want to press in the AC bearings as their damn hard to get out. Currently I warm the bearings indirectly and put the ER16 in the freezer to make them go together easier.

Thanks!

[ger21]

There's a pic here showing a pair of AC bearings for a ballscrew mount.
http://www.5bears.com/cnc04.htm

[escott76]

Ok so I am building a spindle and mounting my acme screw and would like to know if my setup is right.

Acme Leadscrew:
On the far end of my 1/2"-12tpi screw I have a single 608ZZ DG bearing for support and on the motor end do I use just one 708A AC bearing or two? I figure I would only need one as usually one would just use another 608ZZ and a thrust bearings and the 708A provides both right?

ER16 Spindle:
This is the complicated one, I hear mounting the AC bearings back to back is best but what side is the back?? Im guessing ill mount them like this ( tooling end - |O\| - |/O| ) the bearings im using are 6204RS DG and 7204B AC

So the entire spindle would look like this:
Tooling - |O\| - spacer (outer race) - |/O| - spacer (inner race) - |O| - nut

So in words let me explain what I think is going on, the collar of the ER16 shaft is seated against the inner race of the first AC bearing so when pressure is applied it will seat the bearing. Their is a spacer between the outer races of the two AC bearings so the inner races can be loaded independently. The next AC bearing has the inner race pointing up and a spacer that slides over the ER16 shaft makes contact with the AC inner race and the inner race of the deep groove on the top so when you tighten the nut on the very top its preloads the AC bearing. (you would set this so that their is no wobble in the spindle but so the spindle still spins freely so as not to overheat the grease)

Is this the correct method? I don't want to press in the AC bearings as their damn hard to get out. Currently I warm the bearings indirectly and put the ER16 in the freezer to make them go together easier.

Thanks!

Two AC bearings opposing each other for each ball screw (one end on each screw, the plain bearing on the other end is fine) . Otherwise you will only be able to take thrust loads in one direction.

[2_many_hobbies]

Thanks for the feedback, I have plenty of AC bearings so ill be putting two on my screw. I saw in the 5bears picture that the bearings have a spacer on their outer race just as i thought they should so im glad ive got the general idea down.

For the spindle is putting a spacer around the collet chucks shaft to transfer pressure to the inner AC bearing the proper way to keep a spindle loaded? I notice all the Chinese mills just have a lower roller and a regular and thrust bearing at the top.

[2_many_hobbies]

Ok found some info on how to install AC bearings here

Does anybody use this heated method and then crushing the inner races together for preload or is my method above of the inner spacer that would provide an adjustable preload preferred for home built spindles??

[SCzEngrgGroup]

Ok found some info on how to install AC bearings here

Does anybody use this heated method and then crushing the inner races together for preload or is my method above of the inner spacer that would provide an adjustable preload preferred for home built spindles??

It is important for both the inner and outer races to be a very snug fit. This can be accomplished by either making the shaft very slightly oversize, and the outer race recess slightly undersize, so both are a light press fit, then press them into place. Or, heat the bearing so it drops onto the shaft, and heat the outer race support so it also drops onto the bearing. Either approach will work just fine. Unless you're using ground pre-loaded matched bearings (think $$$$$), then you should use shims to set the pre-load. Without adequate pre-load, you WILL get end float. I would not recommend making the preload adjustable by the shaft nut, as it will be very difficult to get it set right. Too loose, and you have end float (which leads to backlash). Too tight, and the bearing life will be compromised. The only question is how much preload is "enough". Without any hard data, I would suggest shimming such that you are at the point where the slightest additional pre-load noticeably increases torque requirement. Since it can be hard to come up with very thin (less than 0.001") shims, you might find it easier to space the bearings apart, and use machined shims (hollow tubes) that you custom machine to the required length. This is how the spindle bearings in many large mills are setup.

Regards,
Ray L.

[philbur]

If you use a matched pair I don't think you need shims. The preload is preset by the fact that they are a matched pair.

Phil

Unless you're using ground pre-loaded matched bearings (think $$$$$), then you should use shims to set the pre-load.

[2_many_hobbies]

I would not recommend making the preload adjustable ... use machined shims (hollow tubes) that you custom machine to the required length

Thank you for the detailed explanation. The AC bearings I am using are the VXB 7204B
They are sold as a set but I doubt they are a 'ground set' but probably higher quality than sourcing individually.

I was thinking of the adjustable preload idea from this thread and reading posts by the participants I noticed they where using an inner sleeve that sits on the top AC inner race to the rear DG inner race and then a nut on top the DG and figured it was for preloading but maybe I mistook it and its just to taking up any extra play in the assembly.

You say to just try and test with different shims (the shims go against the inner races right?) but if its all a press fit (I use the heating method as I have no press) I might need to take it apart quite a few times. Is this method acceptable?

Heat bearing and drop on shaft, drop on shim for inner races, drop on last AC bearing (the shim is large enough their is a slight gap between the outer races). Then heat the housing and drop the bearings in. Repeat for rear DG bearing. Place assembly on metal surface to cool down then:
Check for axial wobble and any looseness if their is any heat the lower assembly and with a machined cup that seats over the ER16 and rests flat on the bottom AC bearings race give it a tiny tap with a brass hammer. Let it cool and look for wobble again. The second its all gone stop and they should theoretically be perfectly set for rigidity and lifespan.

Hopefully my bearings will be here this week and Ill post pictures as I get it together. Thanks!

[SCzEngrgGroup]

Note that those VXB bearings are *not* a matched pair, simply a pair. They will require careful setup to achieve proper pre-load. Matched pairs will require no shimming whatsoever. You can simply stack them, and bolt everything up tight, and the preload will be correct.

If you stack those two bearings, whether face-to-face or back-to-back, and press each pair of races together you will almost certainly find either one pair of races cannot be made to touch, or the inner races are loose, or the bearings bind up. You can determine the required shims by test-fitting on a test shaft with a slip fit, then do the final assembly on the final press-fit shaft, but the test-fitting must be done with the bearings compressed axially to create the pre-load.

Regards,
Ray L.

[stevespo]

Here's another link to a helpful post on angular contact bearing preload:

http://cnczone.com/forums/showpost.p...1&postcount=26

I just did this with some 7200B AC bearings and it worked out great. Very happy with the results.

Steve