Needle thrust bearings

I have a pair of needle thrust bearings which I intend to install on the cross slide feedscrew of my Myford, as described George Thomas’s book ‘The Model Engineers Workshop Manual’. I now realise I don’t understand how these bearings work. In a taper roller bearing, the inner end of the roller is smaller than the outer to allow for the smaller circumference per revolution. In the needle thrust bearing, the rollers are parallel. Won’t that mean that the rollers are in a state of sliding or scrubbing rather than rolling? How do they work without friction? Am I missing something?

Andrew

Shh! Don't let on, or there are millions of thrust bearings across the planet that will cease to work...

They are best for slow, high load applications. thrust ball races are best for high speed.

In practice they just wear faster than ball thrust bearings. Not a problem for cross slides!

Neil

Long time since I read the GHT write-up, but I suspect that he saw the greater longitudinal stiffness of roller bearings as a good reason for fitting them to his Myford feed-screws, as much as the friction benefit perhaps.

I'm not totally familiar with the ML7, but it appears that there is some confusion in terms here. Thrust bearings are used to resist thrust on a shaft axially, whereas needle bearings act on rotational forces in the same way as ball and roller bearings do. See the arcticle on Arc Euro Trades site for clarification and explanation of the ML7 moification.

**LINK**

John

Edit: spelling error

Edited By John Hinkley on 16/10/2018 12:22:04

Think again John. **LINK**

Murray

I think they are termen Needle thrust bearings so you don't get confused with ball thrust bearings

Edited By JasonB on 16/10/2018 12:42:40

The needle thrust bearings are ideally suited for this application, one of the main things for them is the low profile over a ball thrust race.

Ian S C

The thrust bearings called up are NTA 411 which are indeed needle roller type. https://www.motionindustries.com/productDetail.jsp?sku=00094460

To answer the original question, yes there will be some skidding in this type of bearing, so a drop of oil is a good idea, but it is obviously far less than the friction between two flat surfaces. The NTA 411 is rated to 26,000 RPM at 522 kg force so I think it will be OK on a cross-slide

Robert.

Needle thrust bearings are often used where a lower profile or small diameter are required when compared to ball or roller thrusts. One thing to bear in mind when mounting is that the housing washer is better fitted into a sized counterbore as there is no radial location as in a ball type which has a grooved track. I would normally fit the housing washer into a recess and the shaft washer onto the shaft of all thrust bearings to prevent the washers moving, but I accept it is not important on hand rotation.

Murray/Muzzer and Jason,

I've had a rethink and re-read the OP's post. I have misunderstood what he was getting at when he compared his bearings with taper roller bearings. I had ( wrongly, it appears ) to have thought he was referring to what I understand as taper roller bearings - i.e. like those used in lathe headstocks to support the spindle. Ignore my previous post - it's muddying the water.

John

You can also use accurately machined surfaces instead of hardened thrust washers to save space. This approach is endorsed by SKF.

Neil

It is ‘orses for courses’ and a variety of solutions are available to be used, as and when deemed sufficient or appropriate. One might consider when a simple spacer becomes a thrust washer - simply maintaining a position or resisting a directional force.

I don’t think tapered roller bearings are generally designed for excessive continuous thrust - they are only preloaded to a suitable amount so the bearing elements roll rather than skid.

Apart form the conditional "excessive" (nothing is designed for excessive anything by definition), this is incorrect. One of the principle reasons for using TRBs is their ability to accept axial and mixed loads. The only conition is that a single row can only accept load in one direction.
It's the taper on the rollers that allows them to roll without skidding.

OK, sorry. Insert heavy for excessive, maybe?

Maybe you can enlighten why taper roller bearings always need careful adjustment for preload? Or am I misled on this operation?

This needle thrust conundrum is something that I have often thought about. If the needles were perfectly parallel and the race surfaces dead flat then scuffing of some sort must take place so it would not be a 100% rolling bearing. Today though I found that some manufacturers of this type of bearing 'relieve' the ends of the needles to reduce stress points.

Maybe this relief effectively make the needles microscopically barrel shaped so that at light loads its nearer to point (rather than line) contact. In conventional ball races, I thought that the inner and outer race race ball grooves would always be exactly a true part of a circle, however I now know that a lot of technology is used to shape the groove to give the best performance and that parabolic and other profiles are used.

If the whole point of needle thrusts is to minimise the overall axial length of a thrust bearing then another approach might be to use ball thrust bearings with very small balls. It seems to me that in the same radial distance that a needle occupies there could be several concentric ball thrust races.

Ian P

Pre-load is required to ensure stiffness of the bearing assembly under load, esp. important with machine-tool spindles, gear shafts etc. Not so important with, say, vehicle wheel axles.

Bearing life is an inverse function of load, roughly a cube law I think. So there's a trade-off which needs careful control. Slight over-tightening can put large loads on the bearings.

Preload is required to minimise end play - as close to zero as possible. Wheel bearings should be tightened until they drag and then released by a fraction (or number) of flats of the securing nut (dependent on thread pitch and nunber of flats, of course). Running tight would overheat the bearing.

I have engines with steel crank and cast iron crankcase and others with sreel crank and aluminium crankcase. Obviously each require a different preload, to take account of thermal expansion at running temperature, to achieve the permitted end-float (always positive, never zero - which might be ideal - because the bearings might be axially loaded).

Consequently, I am not sure how much end thrust most of our spindle bearings would take continuously. My lathe has ball thrusts either side of the drive pulleys and they have survived for about 50 years of hobby use. Never yet seen any that have failed (given sensible treatment - lube and setting up an cleanliness).

Crankshaft in old Velo motorcycles ran in taper rollers. When rebuilding you had to shim one of the outer races so that the bearing was preloaded by deforming the crankcase when you bolted it together. No doubt this preload reduced when the while thing warmed up, but that would take several minutes.

When using ball races it is a good idea to spring load one of the outer racesaxially. This ensures that there is a lways some force between ball and race so that the balls roll rather than skidding,at least that's the theory

I'll link this again, it's a good read:

www.model-engineer.co.uk/news/article/lathe-bearings/19136

Neil

I just get a blank screen