Plain bearing spindle speeds

[magneticanomaly]

I am looking for some education.

It is a truism that plain bearing machine spindles do and must turn slower than rolling-element bearing spindles.

However, IC engines almost all have plain bearings supporting the crankshaft, and these run at many thousands of RPM. I have read the articles in Machinery's Handbook and Marks Handbook on plain bearings, and I understand that the faster a spindle turns for a given oil viscosity, the more load it can carrry., and conversely the slower it turns, the heavier oil you need to carry the same load,,,to a point.. It's the hydrodynamic oil wedge, same idea as a motorboat getting up on plane.. As a spindle turns faster, the oil also must be thiner so that too much energy will not be wasted in viscous drag, and also so that it will not overheat because of the same lost energy turning up in the bearing as heat.

I also know that engine bearings have pressure lube, or at least ample lube from splash or directed dippers, whereas slow plain machine-tool bearings usually have only wick or maybe drop-feed lube.

At last the question: Could you not run a plain machine-tool spindle bearing at some arbitrarily high speed, if you gave it enough thin oil to keep it cool?

 

[johansen]

High pressure oil feed is only necessary to push oil through the bearing for the purpose of cooling it. In an engine for example, the oil in the bearing stays in the bearing for the duration of the combustion stroke. The ecentricity of the bearing picks up more oil and friction pushes the oil into the load bearing wedge. The oil film pressure might be a few thousand psi, far exceeding your oil pressure. As such the location of the cross drilled crankshaft hole matters a great deal.

I am quite confident I could run my south bend 9 at 3 to 5k rpm with a force fed oil delivery system. It runs fine and heats up uniformly warm at 1500 rpm with 6cst oil@100C, wick fed.


A typical small engine turbo has a journal bearing 1cm diameter 1 cm wide and runs at 100k rpm. The oil is fed into the middle of the bearing so most of it sprays out as it only has to travel 3mm to reach the end of the bearing. This provides cooling.


The journal bearing books also show you how to calculate how much oil the bearing will consume. You can then do the math on how much cooling the oil provides.

A 6inch long 4 inch diameter drip fed slow speed journal bearing might work for a lathe no problem. A few dozen psi of oil would make it last a lot longer. A few thousand psi might push enough oil through it such that the same 4" diameter shaft can run at 3 to 10K rpm and put 20,000 hp out of a turbine.... Running similar clearances

 

[enginebill]

Yes, you can. There were plenty of flat belt grinders out there that ran steel on cast iron bearings at 2 to 3 thousand rpm with nothing more than oil cups for oiling. Just make sure everything is balanced for the speed.

 

[Conrad Hoffman]

I've seen plain grinder spindles that run very fast. The problem with an old lathe like a Southbend is where will it sling the excess (water thin) oil at that speed? My guess is right up the front of my shirt!

 

[dundeeshopnut]

I've seen plain grinder spindles that run very fast. The problem with an old lathe like a Southbend is where will it sling the excess (water thin) oil at that speed? My guess is right up the front of my shirt!

Shirt's not too bad, [I get far worse on my farm clothes], but my bet's on your glasses.

 

[Gary Main]

it is my understanding that the bearings are the least of the issues, most older lathes are thread on iron chucks which tend to fly apart at high speeds;.