Setting bearing preload.

This may sound like a naive question but I've never seen it spelt out. I've recently changed the angular contact bearings in my Myford Super7 for taper roller bearings. What is the correct procedure for setting the right preload? I've tightened everything up to remove all trace of movement but not tightened it beyond that. Is this correct?

If you are using taper roller bearing then apart from removing play then thats all you need to do, if they are angular contact ball bearings then you need to tighten them to a required torque based on the differential dimension between the inner and outer spacers, run your spindle to allow it to warm up and make sure the bearings are well oiled after a while stop the lathe and check for end float and adjust accordingly, treat taper roller bearings like you would on the front wheels of a car, no play but well oiled, use for a while and then check for play.

Martin P

Just follow the instructions in the Myford hand book. If you do not have one make it a top priority to get one.

I have the Myford manual but obviously it doesn't cover taper roller bearings.

Neither can I find a concise set of instructions for preload setting. I suppose it's not surprising, given the huge range of different geometries and applications.

The ages-old (official) way of setting automotive front wheel taper roller sets is to apply considerable preload whilst rotating the wheel, by tightening up the stub-axle nut to a given torque (by which time it takes a bit of effort to turn the wheel), and then back off the nut one or two flats. This results in a few thou axial play. So that's not what we want, is it?

Land Rover transmissions were full of opposed taper roller bearings. You set preload by shimming the outer tracks and measuring the torque required to rotate the supported gear-shaft (by wrapping string around the gear and pulling it with a spring balance). It could take several trial assemblies to discover which shims to install, so it wasn't a quick job. I got to know the L R parts man quite well. He had large boxes of shims, all untouched. "Oh no, we never open 'boxes here, we just put in reconditioned ones". Having had my fill of L R unreliability and rebuilding, I now have a Toyota...

The vertical spindle of my Maho milling machine has opposed taper roller bearings, the rollers bearing directly on the hardened spindle. I assume this is for reasons of compactness and to minimise possible sources of eccentricity and 'slack'. The manual instructs one to grease-pack the bearings and assemble with a bit more preload than necessary to just remove any axial play. Then run at high speed and monitor the temperature of the housing. >50C = too hot = too tight. Sorry this is a bit vague - manual is packed away: I'm between workshops...

Seems a good stratagem, in the absence of other info. However, for a small bearing set, and oil-lubricated, perhaps raising the temperature of a relatively massive housing to this sort of temperature would only result from massive over-tightness. So I suppose the message is to tighten up until end-play is certainly abolished, and then a bit more, but if the spindle feels tight or runs hot, slacken off a bit. If you look at the manufacturers' data sheets, the permitted axial loading is surprisingly high, so we can afford to experiment.

Sorry, not much help. Might be comforting to know that you're not alone though...

I have been told temperature rise test is the best one for taper rollers.

The other big issue is that most people put in far too much grease, manufacturers recommend filling about 25% of the empty space

Neil

Hi Chris

This is what I was told by Myford. It's how they used to set things up at the works.

"The gospel according to Malcolm"

1. Power Down

2. Remove all belt tension.

3. Remove Chuck

4. Rotate RH collar one complete turn. (top towards you).

5. Rotate LH collar one complete turn. (top towards you).

Spindle should be completely free of front bush.

6.Loosen allen screw on collar at end of spindle.

7. Using the Allen key tighten collar as tight at it will go by hand.

Inner tapered roller races are now locked together with correct pre-load.

8. Back off LH collar one complete turn + a bit (top away from you)

9. Rocking the spindle by holding the spindle nose tighten the RH collar by hand (top away from you).

10. When you feel resistance to movement stop.

11. Collar should be just tight enough that spindle can just be moved by hand holding the nose.

12. Do up the LH collar by hand (top towards you).

You should still feel resistance when turning the spindle by the nose.

13. With the crescent wrench on the LH collar tap the end smartly with a 12oz hammer.

The spindle should move forwards by a couple of tenths and be completely free running.

14. With the lathe running slowly (lowest direct speed) put the oil gun in the front oil cup and pump until oil issues from the front bearing. (It comes out of the back and is sprayed around by the bull wheel at least it did on mine)

15. All should be sweetness and light with great rejoicing in heaven and on the earth.

regards Martin

OK so I just fully read the post and you have tapered rollers. However I don't see why you should not just adopt the same proceedure. It's the front bearing clearence that's critical.

regards Martin

When I wrote my original article on fitting taper roller bearings to a Myford Super 7 that appeared in MEW I did not comment upon any preload.

When setting the Super Gamet roller bearings for my M300 there is a specified preload to be applied to a warm bearing.

On my Myford Super 7, I tightened the the ring by hand, locked the clamp cap head screw,. ran the spindle for a short while to warm it up. I marked the position with marker pen, loosening the clamp I applied a nominal extra load by loosening the ring and then tightened the ring to its original interim marked position + a small amount (ie a small a movement beyond the marker pen as I could see). Locked the clamp with the cap head screw. Bear in mind that the ring should be just free when being applied as in "theory" a very loose ring can rise up the thread angle!.

Further running of the spindle showed a smooth running bearing which can just be felt by hand. End movement of the spindle as a whole to adjust the front bearing clearance as per Myford practice.

Neil. Lubrication remains by NUTO32 oil NOT grease

Edited By KWIL on 26/02/2015 12:21:38

> Neil. Lubrication remains by NUTO32 oil NOT grease

Fair enough if you want, but I can't see why, taper rollers run perfectly well on grease and it's a lot less hassle!

Neil

Anyone here understand why taper rollers need preloading? In the mean time I'll head for Wiki and see if anyone has a definitive answer as I'm quite intrigued.

Jesse,

I posted this on the "making a new lathe spindle" thread ...

MichaelG

Connected with the running condition of the rolling elements (the tapered bearing pins) - ensures they roll rather than skew or slip - both bad things in a roller bearing!

Mark

I had to adjust the bearings many (30) years ago on my Boxford lathe. I followed the instructions that were in the manual for the lathe (that I no longer have so cannot refer back to it). If I remember correctly, it consisted of wrapping a thread around the spindle and attaching a spring balance and then pulling on the balance to cause the thread to unwind. The spindle was adjusted until the required reading showed on the balance.

On my Myford, I set the taper roller bearings in relationship to each other at the point where you could just detect that all end float had gone with just a suggestion of tightness (clear of the front bronze bearing). I then adjusted the whole paired assembly back to a point just prior to where heavy pressure from the tailstock on the spindle would be picking up a little drag from the tapered bronze front bearing at the front. I then ran the lathe for a while to check if the front bearing was getting warm or the motor labouring when the tailstock was applying some pressure (with a rotating centre obviously) fine tuning it as necessary. There is definitely a more solid feel to things with the taper roller bearings and I've been running the lathe at this setting ever since with no problems.

Hi Norman, I think you are refering to this one for the Boxford.

Regards Nick.

Bogstandard : Thank you as that's exactly the conclusion I came to after reading various write ups on the subject.

Maybe some of my marbles are still in the bag.

Thanks also to Michael Gilligan for your artical and keeping me up past my bed time today.

Does not pre-load exist to accomodate machining forces? with no preload as soon as an axial force is applied to the spindle the bearing pair will only be working on one race and not be fully constrained. Pre-load will vary with expansion but the bearings are back to back not far apart so I don't see that is much of an issue.

regards Martin

Chris,

Pleased you came to the same conclusion that the use of roller bearings is an advantage as regards the rigidity of the over spindle set up

John,

Precisely put as usual.

Edited By KWIL on 27/02/2015 10:29:08

This is how I perceive it too.

My understanding is the preload should be slightly greater than the maximum load or force that the machine is designed to operate under. The theory being something along the lines that (as Martin points out), there shouldn't then be any disengagement of the unloaded bearing from its race, thus ensuring stability of the setup.

The subject of thermal expansion is a bit of a minefield in my opinion.
If the bearing/housing/shaft are made from materials which expand at broadly similar rates, then given they are heated in a uniform way, bearing preload should remain fairly constant. It's only when differential expansion of the assembly occurs that preload alters, but when the temperature stabilises, preload should return close to its initial setting.

Rolling bearing need little lubrication - only the elements which have rubbing contact really need a touch of oil or grease, so it's mainly needed for the cage and also the ends of taper-rollers where they rub the thrust face of the inner race.

Martin.