Multiple Bearings in Spindle

Looking good! And pretty pictures too. (Wish I could bring myself to re-learn CAD. I had an early version of TurboCAD - it drove me nuts. Ever since, it's been back to pencil & paper...).

I think the most challenging aspect of manufacture is getting the spacer-to-spindle fit right, if one is going for a snug fit in the hope that it will add rigidity. Perhaps someone could tell us how snug the fit has to be to be effective, for this geometry. I'd imagine that significant hoop stress has to be induced in the outer member, to effectively pre-load the inner, so it would need to be a press fit, which rather screws up the whole idea. The idea is more effective when the spindle is really skinny, and the spacer can increase the overall diameter a lot. It's probably an unnecessary gilding of the lilly here, because the spindle is of relatively large diameter already, and the spacer can't easily be much larger. Please can someone advise?

So, perhaps the spacer can be freed from its obligation to add stiffness, tolerances can be relaxed and simplification is achieved..

More simplification. If the spindle's 7.5 mm diameter section between the bearings is made only a little larger than the ID of the tail end bearing (say 7.1 mm), the dark green thrust washer (?) twixt spacer and inner track of tail bearing can be deleted. Also, I'd delete the groove in the shaft behind the nose bearing pack. If needed (and it looks unnecessary from your drawing, a small internal chamfer could be added to the spacer tube to clear the transition between spindle diameters here

Don't forget the pulley, the nut being outboard of it. Keep us posted!

Edited By Kiwi Bloke on 23/07/2023 23:56:38

Are you concerned about stiffness? (Of the spindle!)

For best stiffness, the front and rear bearings should be separated by 3-3.5 x the diameter of the shaft through the front bearings (so 24 to 28mm in this case - see page 66 of the SKF document I linked to earlier). The major contributors to the lack of stiffness of my original CNC spindle were the large tool overhang combined with an excessive distance between the spindle bearings, allowing the spindle shaft to flex between them.

I try and explain in this video (from about 12:45 if it doesn't go straight there).

For my improved spindle, I also considered (and rejected) a spacer sleeve to clamp both front and rear bearings to the spindle at the same time. I wasn't convinced that I could make the sleeve to fit well enough that it was guaranteed to: a) run concentrically; b) Not bind up on the shaft; and c) end up perfectly parallel so that it didn't induce a bend in the spindle shaft when everything was clamped up.

Edited By Andy_G on 24/07/2023 00:30:35

The old skf catalogue gave lots of info about abutment dimensions. I made the mistake of binning mine on the basis it would all be on the Web. You could make the 14.5 bore 14.1 to increase the abutment. I

The spacer tube only needs to be a good fit at the ends, so if you machine the spindle so the the 8 diameter is proud of the bearing to the left by a few mm then make the dark green bit part of the long spacer it makes life easier, most of the length of the bore can be clear of the spindle, just the right hand end has to fit the 8 dia. It doesn't want to rattle about, a nice slide fit at the ends.

You need a means of locking the right hand bearing adjusting nut. Unless you actually need axial thrust both ways you could put a wavy washer in and tighten the nut fully, or take other's suggestion and get a matched set of bearings. I still think you'd be wise to draw it out to scale, I don't think it is at the moment

I’m not sure, Duncan … but this might be your lucky day !

**LINK**

https://kntu.ac.ir/DorsaPax/userfiles/file/Mechanical/OstadFile/dr_asgari/skf/3-GeneralCatalogue.pdf

MichaelG.

Thank you for all the input.

I will do some revised drawings with a simpler spacer. When it comes to abutment sizes etc. I've found this information for the bearings i intend to use so I can incorporate it into my drawings (which are to scale, by the way)

I know what you mean about the stiffness. I had a look at the SKF link. This spindle has bearing spacing of 6x diameter which is nearly twice recommended.

I need the length as this particular one will need to be able to slide at least 20mm in a 50mm long bush/sleeve. It will only be used in the vertical position for light drilling so I think I'll get away with it.

I intend to make a shorter one as well for my lathe cross slide. That one will have to do some light milling so will be stiffer. I will watch your linked video later.

Steve

This time I've drawn the shorter version, it's easier to see the whole thing. Bearings are 3.23 x diameter apart.

I've taken on board suggestions for simplifying the spacer tube and the dimensions have been adjusted so all abutments are to the SKF spec. for those particular bearings.

There were also dimensions (0.2mm) for the fillet radius for abutments. I have inserts with that tip radius so that should work.

Not sure how to lock the adustment nut. I can't find any wavy washers that fit such narrow confines. Suggestions welcome.

Steve

Edited By Steve Crow on 24/07/2023 15:09:38

OK, more simplification. Probably.

Make the 'nose nut' without the flange you've drawn, so that, when tightened, it sticks out say 3mm beyond the end of the housing. The nut's OD is threaded along its entire length (easier to machine), and it has a couple of axial holes on its exposed face, for a pin spanner. When boring and screw-cutting the housing, make the threaded section longer, then part off say 3 mm. This parted-off section then screws onto the exposed threads of the 'nose nut' (taking the place of your flange), locking it. You'll need to make a couple of pin spanners (or a C spanner for the locking nut, if its face is too skinny for spanner holes) to tighten the nuts. (Sorry for that complication). I can think of other locking strategies, but this one seems the most elegant - so far...

I'd be more concerned about the M7 nut on the other end. Although both look like candidates for loctite.

M7 Nyloc?

That nut can be done up tight, but it should be left hand thread for a clockwise spindle by rights (or backed up by a locknut).

I think that the concern for the front clamp ring is because it will be used to set the bearing preload, so will need to be locked at the chosen position, rather than being done up tight.

Edited By Andy_G on 25/07/2023 10:01:17

I recently have replaced the spindle bearings in the head of a 8mm Leinen lathe. The arrangement of the bearings and spindle were as a cartridge using simple tubular spacers of slightly different lengths to provide bearing pre-load. Although the spindle was supported by only 2 bearing the same arrangement can work for 3 but, for the simplest arrangement, they would all need to be the same diameter or at least have the same OD.

The spindle would have a single diameter for its whole length the same as the bearing ID with an abutment at the front where the ER8 collet fits. There would be large tubular spacers matching the OD of the bearings and others with ID's matching the OD of the spindle. The two spacers between the front bearing have slightly different lengths that set the pre-load of the front bearings and the different lengths of the 2 longer one set the pre-load, if any, for the rear bearing. The outer tube would be be symmetrical with internaly threaded ends to retain the cartridge.

I think the attraction of this arrangement is that all the parts are very simple to make and alter if pre-loads need changing.

Some special bearings are paired with the width of the out race being different to the inner race so when assembled with equal length spacers the correct pre-load is provided. This was the case with the Leinen bearings which were £250 a pair and had to fitted the correct way around.

However it is relatively easy to shave a few thou/microns off the sleeves by lathe or stoning the inner or outer until you get it right.

CS

I may well be incorporating the rear nut into the pulley yet. That way won't the torque keep the nut locked?

Once I'm happy with the rest of the spindle design, I'm going to look at the drive etc.

I like the nose nut lock suggestion and will do a drawing but I've also found a wave washer that fits so will draw one for that as well.

I would rather not use loctite on this. No objection to it, I use it all the time, bu I'd rather not on this spindle.

Steve

Steve

It is common for ultra accurate spindles to have a separate bearing(s) for the drive pulley to take belt side loads and an axial spline or pin coupling to the spindle. This is the arrangement of the Hardinge lathe and my Bridgeport M milling head. On the Hardinge this done so that no belt/pulley rumble gets transmitted to the spindle allowing super surface finishes to be obtained.

Given the size of the cutters you are likely to be using most of the power will be needed to drive viscous friction loads of the bearing lubrication. The are some excellent brushless DC motors cheaply available that could be mounted axially provided you have the space.

CS

Two more drawings. The first is the nose lock nut suggested by Kiwi Bloke.

The second one has a wave washer in the nose. It's shown loaded on the drawing dimension wise.

This one looks kind of complicated. I'm sure it can be simplified.

Steve

Hi Clive, that Leinen system sounds simple and elegant but I should imagine it's fiendishly difficult to match the lenghts of the tube for an optimal result.

I have seen that "remote pulley" system on a few machines. I was thinking of useing something similar for my longer spindle.

Steve

I would consider a spacer for the outer races, then the nose nut can be locked up tight.

To create the right preload is a case of making the inner race and outer race spacers different lengths.

if you have access to a surface grinder then you can make them initially at the same time - this creating a

paired length, and then selectively lap the inner one shorter.

On the back nut a pair of lock nuts should do.

Dave

Nah, don't like the wave washer. You'd still need to lock the nut. The wave washer does provide spring pre-load to the bearing pair, but it also destroys axial stiffness.

I'd rather forgotten about the other end... If you're driving this with a pulley, it can be screwed on (I'd assumed that, much earlier), and a plain locknut added. Don't rely on just a thread for accurate location of the pulley, though; it should be a snug fit on the plain section of the shaft, to ensure it runs true, and have a boss, outboard, which is threaded. This has the added advantage of getting the pulley as close to the tail bearing as possible.

Clive's comments above, about (partially) isolating the pulley from the shaft, is good stuff, and also applies to drilling machine quills, of course. But we're chasing simplicity, aren't we?

Edited By Kiwi Bloke on 25/07/2023 12:23:31

Further to my post, I found the article I was looking for !....

There was a construction article in the Model Engineer three parts to it dated 19th June, 17th July and 21st August 1992 by a Mr. D. Broadley called "A Grinding Spindle".

He described a heavier duty version of it in the last part, for those who have access to older copies of the ME it is worth a look.

Steve

The elegance of the tube system is you make the tubes and spacers all the same length at first and then shave a thou or two off one to get the correct pre-load. The thin spacers between the front two bearings are a bit more tricky to do but you can super glue them to a face plate and skim. Final gapping can be done by "stoning" on say a diamond lap. The pre-load is determined by the difference in length rather than the absolute length so much easier to measure accurately with feeler gauges. The tubes can have fairly thick walls which improves the cartridge and spindle rigidity. Also their inner and outer diameters don't have to be that accurate. You can make one long tube and then part off the thinner spacers and only the face that the parting tool created needs to be trued up.

I'm not sure that given the ER 8 collets and the small cutter/drill you are expecting to use whether you need two bearings at the front.

I've just upgrade a sensitive vertical drill by using a bought spindle with ER11 collets. The spindle is a straight 12mm diameter X 150 mm long and in my case runs in oilite bushes. I don't know if similar spindle are available for ER8 but it would save a lot of work.

The Leinen system used special bearings which had to be assembled in the correct orientation so on reflection the spacing tubes were probably equal length.

CS

Here is the complete shorter spindle for my lathe cross slide.-

The pulley is an arbitrary diameter for now but will use a 3mm drive belt. I forgot to dimension it but the pulley inner/boss outer diameter is 8mm.

The longer spindle will be acting as a quill in a sensitive drill so I'm going to devise an isolated pulley system. I've not even started on the design of all that, I wanted to make sure I could make an effective spindle first.

Steve