Plain bearing speeds

[zytooohParticipant @zytoooh]

Hi folks

Does any one have any thoughts on the safe maximum rpm on plain bearings?

I realise the answer depends on bearing size, side loading, lubrication and materials of construction.

My scenario is:- Zyto lathe head bearings, steel mandrel running in cast iron bearings with oil cup lubrication, lubricated with castrol gtx. I would like to run a bit faster than the maximum available with the current belt set up for machining small parts <10mm to give a better cutting speed. I can get or make different motor and counter shaft belt pulleys and the resulting reduction in torque should not be a limiting factor. The counter shaft will obviously run faster, but that has been fitted with wide phosphor bronze bearings and is fairly free running. Maximum rpm without load a the moment is just over 900 rpm.

Any thoughts on what could be safely achieved without wrecking the headstock bearings would be appreciated, or am I at the safe maximum already?

Thanks in advance

Peter

[Howard LewisParticipant @howardlewis46836]

With a 10 mm diameter workpiece, for a surface speed of 30 metres /minute, you would need to run at 954 rpm.

So your present 900 rpm would suffice, although with steel running in cast iron, I would suggest for limited periods only, and with copious oil supply (As much for cooling, as lubrication)

The action of cutting will generate heat which will eventually reach the Headstock bearings, adding to the heat generated by churning the oil within the bearings.

To find just how much heat is generated by cutting, try to hold a piece that has just been parted off, even at low speeds, with oil applied by brush.

You are unlikely to hold it for long!

Howard

[Lee RogersParticipant @leerogers95060]

The general rule for Drummond lathes ( close enough to a Zyto) is 700 RPM max . Users report that 900 to 1000 with light cuts and keep the bearings constantly oiled can be OK. I say it can be OK because at 100 years old who knows?HSS tooling will keep the loads down  and before you push your luck ,check out the price of Bronze bar.

[not done it yetParticipant @notdoneityet]

Quite a lot of users would like to run at higher rpms than recommended.  Folly, IMO –  the manufacturers were not wrong when they set the maximum revs.  Anyone exceeding that is wrong!

[Roderick JenkinsParticipant @roderickjenkins93242]

My Myford S7 with its 3 phase inverter tops out at 750 rpm in the lower ratio.  The only time I use the higher ration is for drilling really tiny holes for fuel jets, otherwise I’m very happy with the finish I get from CCGT carbide tips at the lower speeds.  Interestingly, my Zyto had thin bronze shell bearings in the headstock.

Rod

[zytooohParticipant @zytoooh]

Thanks to you all, I think I have a definitive answer to leave well alone, and now I don’t need to find or make new belt pulleys. As has been pointed out the manufacturers knew what they were doing and its lasted this long as designed. I think it’s older than me but not by much and some of my original parts still work as designed without modification.

Thanks again

Peter

[IanTParticipant @iant]

Peter doesn’t say what 10mm material he is turning.

As a regular user of a small lathe with plain bearings (an EW) I can turn both brass & free cutting steel (leaded) somewhat below the ‘recommended’ RPMs and still get good results.  Tubal Cain (the UK one) is my usual mentor in these things. However, if Tubal says you need 1,425 RPM for 10mm FCS (but your lathe can only do 900) it doesn’t exactly help.

A few things come to mind. First make sure that your ‘set-up’ is as rigid as possible – minimum ‘stick-out’ of both tool and material. Lock any unused slides. Second, use only ‘sharp’ HSS. Insert tooling may work sometimes but is best avoided in my view on older machinery. By sharp, I mean honed. Grind the tool with enough clearance on the important faces but don’t fuss too much over angles. I use zero top rake for brass and 12 degrees for steel with a 5 degree front clearance.  For a good finish (as opposed to roughing) I hone the cutting edge and slightly round-off the tool nose.

As an aside, i also use carbon steel tooling on both my EW and hand-shaper. I’m generally taking light cuts and whilst the work may get warm, it normally doesn’t get hot enough to damage carbon tooling in my experience. You can also get a good edge on carbon and it has its uses.

So my advice would be not to worry so much about your max spindle speed but to make sure that your set-up is as rigid as possible and to use sharp tools, correctly set.

Regards,

 

IanT

[SillyOldDufferModerator @sillyoldduffer]

There isn’t a safe maximum as such, it’s more to do with how long the owner expects the bearing to last.

Plain bearings made of suitable materials are long life provided they’re well lubricated.  Unfortunately they are only well lubricated at relatively low RPM, usually under 1000rpm.   Speeding causes the oil-film to break down resulting in rapid wear due to metal to metal contact and is why manufacturers suggest keeping rpm low.

Though they get hot, over-sped bearings don’t fail obviously in a shower of sparks.  Instead bearing life is quietly reduced, perhaps dramatically.  A bearing that would have lasted 50 years at the recommended speed fails after 5 years, or sooner.

Much more damage is caused by prolonged high-speed running.  Occasionally running a plain bearing a bit too fast shouldn’t do much harm.

Using an oil of the correct viscosity would help, but I don’t know how to work out what it should be.  Thin oils flow through the bearing too quickly, and thick oils don’t flow fast enough.  Grease is far too thick.  Professionals measure how much metal is in used oil and adjust from there.

Not sure about the Zyto, but inexpensive lathes of that era often came with a cheap plain bearing simply made by boring and polishing a hole in a cast-iron head-stock.  Works well, but…  Not replaceable, so treat with consideration.  The hole can often be clamped to compensate for wear but taking up slack eventually cracks the headstock, a common fault in older lathes.

Plain bearings are good for lathes because they minimise vibration.  Unfortunately their short life, fussy lubrication and inability to cope with high speed is a problem, even with HSS.  Early attempts to fit roller bearings didn’t go well until technology came up with ways of making perfectly round balls of exactly the same size – took decades.  Now that problem is fixed a Far Eastern hobby lathe fitted with ordinary ball-bearings will happily spin at 2000 or 3000rpm. And ball bearings are replaceable if the owner beats them up.

The Zyto isn’t a good choice if high rpm is needed.   Though owners are allowed to thrash their tools to get the job done, it feels wrong to me to force a Zyto – it’s an antique.  Up to you though – if the lathe was bought to do hard work and longevity isn’t an issue, then go for it.  But keep under the speed limit if the lathe is expected to become an heirloom.

Dave

 

[Robert Atkinson 2Participant @robertatkinson2]

Saying you use Castrol GTX is not enough information. There are a wide range of oils under that name. Unless you are using the 0W-16 grade it is likely too thick.
These headstock bearings are generally run with a SAE32 oil. That has a 32 mm2/s viscosity at 40 deg C. GTX 0W-16 is 36 mm2/s. GTX 10W/40 is 95mms/s.

Robert.

[old martParticipant @oldmart]

The Smart & Brown model A has plain bearings, made of bronze and runs at up to 1430 rpm. The later models with exactly the same bearings run up to 1800 rpm. Ours runs exclusively on 5W30 fully synthetic motor oil with a total loss system, a world better than the primitive oils available 75 years ago.

You could probably make a set of bronze bearings for your lathe if you are worried about the cast iron ones.

[mark costello 1Participant @markcostello1]

Would bronze last longer than cast iron, everything else being equal?

[Roderick JenkinsParticipant @roderickjenkins93242]

A few years ago I bought an old Flexispeed lathe that had been used without a countershaft i.e. it had a direct drive from the 1400rpm motor to the lathe pulleys.

This is what the unhardened spindle looked like:

Severe wear and scoring at the chuck end.  The headstock itself though was un-worn so I made a new spindle.  The Flexispeed manual stresses a maximum speed of 1000 rpm, and that’s for a spindle of only 1/2″ diameter.

Rod

 

 

[Nigel Graham 2Participant @nigelgraham2]

In any question like this, I question phrases like “need” x fpm or y rpm cutting speeds.

They are often maxima based on trade practice – or quoting professional engineers’ hand-books – that has to balance productivity, tool and machine life and product quality.

Metals CAN be machined well at low speeds, especially with HSS tools but even with carbide ones if selected appropriately. It just takes longer. That a metal can be cut at great speed, or the cutting tip is capable of it, does not mean it has to be.

.

OldMart –

I have something of the sort in mind for restoring my EW lathe; but with a task like this there must be a fine limit to the over-boring of the original headstock (two separate castings on the EW).

.

Mark –

Not sure about the bearings but you need consider the shaft as well. If the couple is not appropriate the steel shaft can wear more than the bronze lining. I think phosphor-bronze is particularly bad in that way, if the shaft is not hardened and ground. It would be something to investigate before starting any such repair (or building new).

[zytooohParticipant @zytoooh]

Rod is correct, my Zyto does indeed have bronze shell bearings in the head stock I’d forgotten that, the last owner even provided a packet with brand new spare ones. I’ve just obtained a spare tail stock with a modified hand wheel with steel thread insert as opposed to the cast iron ones which slowly break their teeth over the years. The original tail stock was mt1 same as the head stock therefore all of my tooling is mt1, I then found the newly obtained one looks to be mt2, I’ll have to get a reducing sleeve for now.

Which is the most common with other Zyto owners mt1 or 2?

[duncan webster 1Participant @duncanwebster1]

Leaded bronze is the stuff for bearings

[vintagengineerParticipant @vintagengineer]

White metal bearings will run up to 4000rpm+

[bernard towersParticipant @bernardtowers37738]

So what about Main and big end bearings running at 10,000 rpm?

[zytooohParticipant @zytoooh]

I had thought about mains and big ends, but generally they’re pressure fed, temperature controlled, filtered and originally designed to do that. I don’t think Mr Tyzack and son considered we would reach those sorts of speeds with the original treadle power.

[Howard LewisParticipant @howardlewis46836]

Bearings in high speed internal combustion engines are very carefully developed “white metals” with carefully controlled clearances and carefully controlled shaft surface finishes (Less than 16 micro inch centre line average), fed with filtered oil under pressure. The oil lubricates but the pressure ensures a high flow through the bearing to keep it cool at high surface speeds.

An unhardened shaft, running in a plain bearing, even a low friction one, is limited in speed, when lubrication is intermittent. Literally, boundary lubrication.

If high speeds and minimal friction are required, rolling element or air bearings start to appear. But  they introduce other complications.

The plain bearings used in the sort of turbochargers, with which we may be familiar, reduce rubbing speeds by the shaft running in a sleeve bearing, which runs in the body of the unit. So the sleeve bearing rotates reducing the surface speed at each surface, and again fed with large volumes of filtered oil under pressure.

A steel shaft, direct in a cast iron housing, is very limited in speed.

Howard

[Author]

Posts