Thread: Lathe spindle taper bearing end playadjusment

Afternoon all,

I have just removed and replaced my lathe spindle for a belt and oil seal change, and am now re-assembling it.

Question is, can anyone give me a clue on how tight the spindle bearing retaining nut should be to eliminate end play.

It is a bit too heavy to just tighten it up until tight then releasing some pressure like a wheel bearing on a car.

I have seen a thread somewhere, (can't find it now when I need it), where a demo was given using a dial indicator on the chuck side of the spindle and it was tightened until no more play and the a certain number of flats more to achieve the correct pre-load.

The manual I have doesn't go that deep into maintenance.

Lathe is a HAFCO AL900a 12 x 37" made in Taiwan. The spindle is 50mm on the outer end and 75mm on the chuck end and the bearings are Nachi taper bearings

Any words of wisdom and guidance would be greatly appreciated.

I am assuming your lathe has taper roller bearings - else please disregard my advise.

Taper roller (TR) bearings in a lathe do need CONSIDERABLY more preload than TR bearings in a car wheel. Definitely no end float - positive preload is required! Insufficient preload in a lathe spindle leads to poor surface finish and reduced accuracy. However, excessive preload will very quickly overheat TR bearings and destroy them. There is a very fine line between insufficient preload and excessive preload. A little too loose and the lathe performance is greatly degraded, a little too tight and the bearing will overheat in a matter of minutes. The two extremes are often only a few degrees in the adjuster nut apart, say about 1/8 of a turn.

To make things harder, you can sometimes not finely adjust preload, because the left bearing inner race is too tight a fit onto the spindle. This is often so with low cost Chinese lathes, whose spindles bearing registers are only turned not ground. Sometimes the registers are ground all right, but not to a fine enough surface finish to permit a proper fit to the bearing race. So your first step must be to ensure the inner race of the left bearing is a snug but free to slide fit onto the spindle journal. It should require quite firm hand pressure to slide the race onto the clean and oiled journal. If it will not go on and seems to need a hammer, it is too tight. If it slides on without much resistance it is too loose. In both cases you need to rectify the problem before proceeding with assembling. If the fit is too tight, any preload adjustment will happen in "jerks", it will not be possible to finely set the preload. Also remember, if the bearing that was on originally needs a hammer to go onto the spindle, it has already been destroyed - the inner race has been over-expanded and it lost its accuracy. It may not matter much if the bearing in question is only automotive grade precision. But if you bought nice expensive new P5 or better precision bearings, you would reduce them instantly to "junk precision" if pressing onto an oversize spindle.

That problem out of the way you can proceed to preload setting. If the headstock is not of the oil filled type, make sure the bearings are freshly oiled with proper spindle oil (Velocite or the like). If the lathe uses greased bearings, make sure these were packed to not more than 50% of the available free volume with a suitable grease. Over-packed bearings behave totally differently in their first hours of running, making a correct preload adjustment simply impossible.

Some better lathes with good manuals give some information about how much torqure to apply to the adjuster nut as a coarse preliminary setting. The fine tuning is then done by monitoring temperature rise. Some other lathes give empiric information, like how may turns a given chuck attached to the spindle shall turn when spun as hard possible by hand and the gear in neutral (or belt off). Fine tuning is again by monitoring temperature rise.

If no info is available for your lathe, preload the adjuster nut (often a castellated nut requiring a hook spanner) until you start feeling all play is gone and you begin to feel a light resistance when turning. Then fine tune by monitoring temperature rise.

Fine tuning: if you installed new bearings, let them spin some 5 minutes at low speed, then gradually increase speep and let run a few minutes at each speed step for a total of maybe 15 minutes. That will let the bearings gently settle in their seats and run-in. Check the above coarse preload setting again, it may have changed if the bearings had to settle in the seats. Now let the spindle run at its maximum rpm. Feel the headstock casting close to the bearings by hand. Depending on lathe size, the headstock should feel warm after about 15 to 30 minutes. If not, increase preload slightly, spin again 15-30 minutes. Repeat as often as necessary. You want the headstock to feel fairly warm, but not hot, and settle at that temperature regardless how many hours the spindle runs at full rpm. The preload is optimum just slightly below the point where the heat generated in the bearings is radiated away by the surfaces of the headsock. The preload is excessive when the headstock temperature runs away, eg keeps rising all the time as the spindle is turning.

If you do not like the "feeling" of the temperature part, you can use a more scientific digital thermometer. Place it as close as possible to the bearings, using some grease to make a good contact between thermocouple tip and headstock casting. Some 60-70C bearing temperature after 30 minutes running is normal for greased or total loss oiled bearings, less if it is an oil filled geared headstock where the spindle bearing are in the oil bath. Chris

CBA,

Thanks for a very informative, and fast reply. The bearings are taper roller bearings running in an oil bath on either end of the spindle for each bearing, using 20w50 engine oil.
I think at this stage, having checked all the seats, and inner race fit on the spindle which seem to be a pretty good fit, I will creep up on the pre-load by tightening it up until just past the point of no movement (about 1/16th of a turn) and see how the temperature behaves.

Obviously the first adjustment will be made cold, but should the fine tuning be done once everything has warmed up. i.e. if adjusted when warm, will the resulting shrinkage when it cools down cause it to be too tight? It may sound like a dumb question, but every little bit of info helps.

Once again, I appreciate your input.

Yes, final adjustment when warm as then things will be in an expanded state.
Can I also suggest that you get rid of the engine oil and use a good quality plain oil (non multigrade without the detergents). A number of people here use hydraulic oil.

Michael

I think Phil (machtool) mentioned something somewhere that if you can see the rollers, do them up until they all turn and they are not slipping...

And go from there... Excessive heat being a sign of too much preload.... My Chin Hung lathe has three tapered roller bearings in the spindle and as they are set now, from the factory, there would be two settings.... First is the torque required to start the spindle moving, and then the torque required to keep it moving.... If I grab the spindle it resists turning, then once it starts is easy to keep going... it is quite noticeable...

I remember when repairing a grader once the specs called for setting the bearing preload by a weight on a string at a distance from the axle, which is exactly how torque is measured, A newton metre just means a force of 1 newton applied at the end of a weightless (or balanced) bar that extends 1 m from the centre of the axis...

Thanks Michael,

Yes I wondered about the oil type. I re-checked the rather skimpy manual and it says use SAE 20 oil. Not sure how that differs to 20W50, but assume it is a straight mineral oil rather than synthetic. Correct assumption??

I should add that the lathe has been very accurate and is quite sturdily built, which seems to back up the opinions that Taiwanese are superior to the newer Chinese built machines. Mine was manufactured in 1985.

Cheers,

Ned

oil.jpg

This is from an engineering site. SAE 20 is equivalent to ISO 46. The engine oil thing has been discussed before but basically a multi-grade oil for an engine has additives that don't do anything for you (and could harm seals and things) and because it has been designed for an engine does not get hot enough to do anything in a machine tool. Apparently super cheap will sell small quantities of hydraulic oil so you may find some there.

Michael

Originally Posted by iron bark

..........Obviously the first adjustment will be made cold, but should the fine tuning be done once everything has warmed up. i.e. if adjusted when warm, will the resulting shrinkage when it cools down cause it to be too tight? ...............

Lathe headstocks are designed with those heat expansion issues in mind. Meaning that the distance between the two fixed bearing races in the headstock is expanding/contracting with temperature at a similar rate to the spindle. Also, the headstock walls that are carrying the two fixed bearing races are usually designed with some degree of freedom to flex. These two walls bow slightly inwards as you apply preload to the spindle bearings. Only a few hundreds of a mm, but enough to act as a spring to buffer preload and take up any remaining difference in expansion. Think about it, some flex is necessary because the spindle (having a smaller mass) heats up quicker than the headstock. If the headstock designer did not want these walls to bow inwards at all, he/she would simply have put a steel pipe around the spindle to brace the two bearing seats.

As for the oil to be used in total loss lubrication bearings, the most important issue is to regularly top up the cups. The type of oil used begins to matter at higher spindle speeds. Thick oil contributes by churning to heat buildup. If your lathe is fitted with a VFD it may be capable of 5,000rpm at top speed. That is great for small diameter tasks in a collet chuck. But with the wrong oil combined with the necessary pretty high bearing preload, that may result in excessive heat. The ideal oil for this application is a dedicated high speed spindle oil such as "Mobil Velocite" - you can buy this in small quantities from eBay USA. If your lathe is not exceeding some 2-3,000rpm top speed, you should be fine with plain hydraulic oil as used for tractors etc. Or use air compressor oil, that is plain oil, without all the additives to suspend particles or clean engines.

Mobil Velocite™ Oil Numbered Series