Spindle bearing replacement

March 2017

Paying some attention to the spindle bearings came about after a very good friend (and also my inspiration for all this machining malarkey) needed a simple alignment tool to aid assembly of a powered bifold door. I was keen to show off the engineering skills I had collected, however the 90mm-long piece came out tapered…

Before messing with the screws clamping the way to the head, I decided to at least have a look at the bearings.

The existing bearings in the lathe were marked with ‘Poland’ and ‘USSR’, so I think they were the originals (from the lathe’s manufacture in 1989). I could see very little sign of wear in the bearings, so I could have just cleaned / re-greased and re-installed them rather than installing new ones.

The shim washers however were in less good shape. The front one was clearly dished, and the rear one was a bit wavy, rather than flat.

Pulling the spindle is not a difficult or time-consuming job. Details of part numbers & sizes for the bearings and shim washers are below.

When I figure out how to adjust the bearings for pre-load, then I will add details in a separate page…

C-spanners

The spindle assembly is held in place by two collars: one to apply pre-load to the bearings, and the second one as a locking nut.

The holes in the collars are 4.5mm diameter, about 2 to 3mm deep. The collars are about 32mm in diameter.

I made up a pair of c-spanners by turning the rounded part (32mm ID x 50mm OD x 10mm deep), then adding a reamed 5mm hole for the pin. I then added a flat where the handle would be brazed on, and cut the round into two pieces (to make the two c-spanners).

I made the pins out of 6mm silver steel. I turned a 4.5mm diameter section to fit the collars, then a 5mm section to fit the reamed hole in the spanner, and finally I left the last 1mm at 6mm diameter. I did not harden the pin. I glued it into the spanner body using Loctite 603.

The handles are rather heavy for this application, but they were what I had in stock. I rounded the corners slightly to make them more comfortable to use.

My brazing skills need some work… I didn’t bother to tidy them up too much.

Spindle removal

Removing the spindle is straightforward, and is described in the manual. I have reproduced the instructions and exploded diagram below.

My lathe did not come with the part labelled number 4.

With the locking collars removed, ready to tap the spindle out. It is not necessary to remove the chuck back plate – in fact in the manual they advise against it due to the risk of misalignment on assembly. Because I was trying to fix a known problem with my lathe, I took it off (if you just want to check/replace the bearings, then don’t take it off).

You can see the bearing poking out. I was surprised to see that there is no seal to prevent ingress of swarf. When re-assembling, I’d suggest to go heavy on the grease to make sure a lip of grease surrounds the bearing, to catch any swarf. Having said that, the bearings were not worn and looked original (1989 vintage).

Spindle and front bearing as removed with a couple of light taps with a copper-faced hammer. The rear bearing is a sliding fit onto the shaft. The front bearing (in the pic) is pressed onto the shaft. I got it off the shaft by tapping around the edge of the bearing with a suitable punch/drift. To replace the bearing I used the old bearing shell to tap it back into place.

The bearing shells are a close sliding fit in the bore. They are supported on the inside by a circlip (visible in the pic above) and a 1mm shim washer.

The bearings were in great condition, with barely any sign of wear. Pretty good going for 30-year old bearings.

The 1mm shim washers were in less good condition. The rear one was wavy, while the front one was clearly dished (very difficult to get a photo, have done my best above of the dished front washer).

Bearing and shim markings and specifications

  • Front:
    • Markings: “USSR”, “7506”, some Cyrillic text
    • Size: OD 62mm, ID 30mm, width 21.25mm (assembled)
    • Standard bearing: 32206
    • SKF part number: 32206J2Q
  • Rear:
    • Markings: “Poland”, “30305A”, “FLT-8”
    • Size: OD 62mm, ID 25mm, width 18.3mm (assembled)
    • Standard bearing: 30305
    • SKF part number: 30305J2

I bought SKF brand bearings from simplybearings.co.uk. Part numbers are above.

The spacers are 62mm x 50mm x 1mm shim washers. I got mine from www.thebearingcompany.co.uk. They are also available on eBay but I was not convinced as to the quality.

I used Castrol LM Grease to grease the bearings.

Refitting

As the manual says, refitting is a reverse of removal steps. I tapped the front bearing back on to the shaft using the old bearing shell and a drift/punch.

I took great care to clean out the circlip grooves as these are critical to alignment.

I also used some Loctite 641 (medium strength) to positively locate the bearing shells inside the lathe head. This has a 20 minute cure time, so you should have plenty of time to get the spindle back in, tighten the collars and get the lathe running.

I tightened the collar by holding the chuck backplate in one hand and the c-spanner in the other. I got it as tight as I could, then ran the lathe of 15 minutes, then re-tightened. I would like to use a more scientific method, however torque settings for these bearings are application-specific, and the manual is unhelpful in this area…