Too Tight Bearing Preload?

[cdhknives]

I am building a spider (replacing the preload collar with a long collar that can be threaded for 4 set secrews) to fit the left side of my spindle on my 10F, and in the course of setup I checked the threads on the existing threaded preload/retainer collar. Upon reinstall I snugged the collar down and tightened an additional rotation of about 2 teeth of the drive gear that drives the leadscrew gear train. I was going from memory but I recalled that was the preload recommended in the assembly manual.

My question: Previously the spindle was loose enough that even with the drive belt lever pulled forward (disengaged) belt friction would (slowly) spin up an unloaded spindle. This no longer happens when the belt is set for higher speeds. The idler pulley is spinning under a stationary spindle drive belt. Once the tension lever is started back to lock in the drive belt the spindle quickly spins up and works fine.

Did I set the bearing preload collar too tight? After running for about an hour at 684 RPM turning down a block of wood the right bearing area (near the oiler, for example) of the headstock casting was slightly warmer than ambient...it was noticeable but far from uncomfortable...maybe 10-15 degree rise. This is new. Previously there was no detectable difference in temperature form the bearing area to any other part of the headstock casting.

I am running Mobil (Vactra?) Spindle Oil...a squirt before startup and a half dozen drops every 5-10 minutes of operation because the oil just flows right through.

 

[wa5cab]

You are probably OK. The book does say 2 teeth. If anything, it may still be a tad loose, as the 2 teeth are starting from zero end float.

You may actually be over oiling. I would recommend installing the felt reservoir plugs that the factory added in the 1960's.

 

[randyc]

You could actually tighten it a bit more, in my opinion, 10-15 degrees C bearing rise is mice nuts.

If you do an online search for bearing manufacturers, many applications recommend/allow temperature rise far in excess of that. If you can comfortably rest your hand on the front bearing cap after running the lathe for 15 minutes at maximum spindle RPM, you likely have a safe amount of pre-load ! And if the spindle bearing cap temperature rise is very slight then likely you can reduce chatter by increasing the pre-load.

(Incidentally, here's a rule of thumb regarding heat: holding an object at 60 degrees C is the threshold of pain for most people.)

Another rule of thumb, and one that several lathe manufacturers recommend (the Austrian company Emco is one) although it is highly subjective, is hand-rotating the spindle. With the motor drive disconnected so that there is no external drag, give the chuck a "brisk" pull. According to the rule of thumb, the spindle should rotate one or two revolutions before stopping.

I've always mistrusted this and there's no need for it. Infra-red non-contacting thermometers are incredibly cheap these days so it's easy to monitor the bearing temperature under running conditions. The device shown below came from HF and was less than $25.



PS "Vactra" isn't spindle oil and shouldn't be used for anything except way and gear lube.

 

[randyc]

Snagged this from a bearing maintenance site. Note the comment about desirable bearing temperature being less than 100 degrees C -

 

[pdentrem]

Spindle oil for the Atlas should be SAE 20 non detergent. It is not common but is available.
Pierre

 

[wa5cab]

For the benefit of those who have no use for the poorly designed RMKS system, 60 C is 140 F. Outside of special circumstances, there is no excuse for operating bearings anywhere near 212 F.

 

[cdhknives]

My spindle oil is Mobil spindle oil. My way oil is mobil Way oil. I know one of those blue gallon bottles is Vactra...I guess it is the Way Oil.

 

[randyc]

For the benefit of those who have no use for the poorly designed RMKS system, 60 C is 140 F. Outside of special circumstances, there is no excuse for operating bearings anywhere near 212 F.

And yet, the majority of bearings - without any excuse - DO operate at those temperatures - scores of them in every internal combustion engine. Billions of them working hard every day at nearly 100 degrees C for years and years

Most lathes, especially older ones, represent a very benign environment for bearings - that's why there are tens of thousands of operational original lathes made in bygone days and almost no operational original automobiles made at the same time

I suppose if someone was really ham-handed, the pre-load thing could be overdone. But, unless the bearings were already shot, I've never heard of anyone damaging a lathe bearing from too much pre-load. I'd be willing to bet that over 90% of lathes found in a hobby environment have too little pre-load, which can be an even worse condition !

Pay attention to the bearing cap temperature and if it passes the "test" previously noted, it's highly unlikely that there will be a problem: "If you can comfortably rest your hand on the front bearing cap after running the lathe for 15 minutes at maximum spindle RPM, you likely have a safe amount of pre-load !"

 

[wa5cab]

IC engine bearings are the "special circumstances" that I was referring to. And they aren't roller bearings, anyway. There is no excuse for operating tapered roller bearings on a lathe spindle anywhere near 212 F.

 

[cdhknives]

If nothing else, 160+ degrees F is a contact hazard I'd rather not have in my workplace. I am prone to resting my hand on top of the headstock as I watch tool feed progress...and the last thing I want is a blister from touching the bearing cap!