Machine tool spindle lubrication experiment

[NC Cams]

I have contributed a number of times to the discussion threads that involve "what type of lubricant can I use for my whatever?".

One particular thread became rather intense (Tormach 4th axis) even though it did result in some rather informative discussions.exchanges along the way.

Recently, however, we ran into a real PITA situation on our cam grinder. The grinding spindle is sleeve bushed and oil drip fed from a sump tank. Lube was recommended as Velocite spindle oil but we had problems with that and switche do Dexron some time ago.

Things were going along uneventfully until one day the grinding wheel started to "bounce" while dressing. The spindle runs with about 0.0008" vertical clearance and it would iterally "bounce" around (orbit actually) in the clearance space. You could shut the machine down, it would stop, but a short time later, the "bounce" would reappear.

We were using a generic grade of ATF as it was supplied private label to the store by a noted "brand" packager so we didn't think much would/could go wrong. This problem, since it only occured while the machine was warm, led us to do some experimenting.

The ATF we were using was fairly air free even though aeration is an issue - they have a slinger that catches oil and drags it up to a trough and it drip feeds the spindle from the drip fed trough. Again, some "bubbles" but no frothing or what you''d call "aeration". Even so, indications were that temp and cavitation were occurring which would lead to the "bouncing".

We figured that a lube experiment would cost less than $450 worth of new bearings to remove a tenth or so of wear so off we went to the parts store to see what they had in the way of "experimental lubes". We found 4 possible candidates.

Valvoline "high mile" Dexron/Mercon ATF, Mobil 1 Syntetic and Royal purple and a Mobil Blend of base and sythetics.

Ok, time to do some research so, armed with the names, I checked out the internet and read up on the "benefits" of each.

Not wanting to have to disassemble and clean the spindle sump, I elected to try the valvoline "High Miler" stuff. It had 'special additives' for worn boxes and some other additives to help maintain viscosity and pressure under what may be sloppier clearances of used/worn/high mile boxes. It was a bit more costly than the generic stuff but way less costly compared to synthetic or the other options. Again, for less than $10, it was cheaper than new bearings.

We poured it in and fired up the spindle. IMMEDIATELY we noticed a difference. There was NO frothing or bubbling whatsoever in the oil bath sight window. The machine had a different "sound". The proof would be in the grinding. Right from the start, the wheel ground smoother. Interesting....

The proof would come over time when the machine came up to temp. So we let it run to warm up. It did get noticeably warmer (viscuous drag in the fluid perhaps) but no "bouncing". We put the machine back to work and back to grinding some irritatingly difficult profiles - they wanted to chatter, espeically when we do our finish grind where we try for mirror-like finishes.

The difference was noticieabel and just shy of miraculous. The "bounce" was gone and so was the wheel chatter. Valvoline's "High Miler" ATF made all the difference in the WORLD in the grinding smoothness of our spindle. From this point on, the generic ATF we previously used for lubricant we now mix with WD-40 and use as rust inhibitor for finish ground parts.....

In our case, ATF is/was a proven viable oil that we proved/tested as being suitable for our spindle. Dexron is a suitable gearbox lube for many applications as it now is fortified for use in A/T and many manual transmissions and some transaxles with hypoid gearing. It may serve adequately for your gearbox spindle on your mill or lathe (consult owner manual and test at your own risk).

From personal experience, however, there DEFINITELY is a difference between ATF's. In light of this experiment, Valvoline "high miler" ATF is now our lube of choice for our cam grinder spindle as it definitely and noticeably outperformed the "it don't matter generic stuff" we used previously.

My dear departed father once told me, "buy the best you can barely afford, you won't be disappointed if/when you do....".

Good words to live by.... and I should have listened to them sooner.

[Oldmanandhistoy]

Originally Posted by NC Cams My dear departed father once told me, "buy the best you can barely afford, you won't be disappointed if/when you do....". Good words to live by.... and I should have listened to them sooner.

Hi NC_Cams,

Nice write up, very informative but I would expect no less from you. As for what your father told you, very good advice and something I have always endeavoured to do. What I find hard is knowing my limits and being satisfied, but I often find I need to rob Peter to pay Paul because I always want the best.

[jrrdw]

Couldn't the "bouncing" be coming from air bubbles/frothing getting inbetween the two running surfaces? There for, illimitating the bubbles is the cure for the "bouncing". It takes alot of pressure to create cavitation.

[Geof]

Originally Posted by NC Cams ......My dear departed father once told me, "buy the best you can barely afford, you won't be disappointed if/when you do....". Good words to live by.... and I should have listened to them sooner.

I think these words make a good complement to words from my father: "When someone is explaining something to you, shut up and listen; you can learn a lot that way."

It is funny (or sad) how many people do neither.

[NC Cams]

Cavitation occurs when a couple of things develop.

High frequency vibrations cause cavitation. The phenomenon occurs as follows: the shaft starts to oscillate/vibrate. In doing so, it moves away from the adjacent wall faster than the lube can fill the void. The suction/vacuum of the void raises the vapor pressur of the lube to the point that it boils and then you lose support of the oil film.

When if the film redevelops, problem solved. Should the situation do so repeatedly, and sympathetic oscillation can develop and the shaft vibrates (bounces) within the void/clearance space. Or, if the bubble persist, the bubble doesn't support load well and the shaft drops when the oil film collapses.

A similar deal can occur if the lube can not be replenished fast enough in a drip feed, self purge system. This is what we think has started to happen to ours.

We looked in to the lube passages and found that there are several sharp edged orifices in the path from the drip tray that feeds and the oil feed slot in the top of the bearing. In one particular place, the mid stream oil feed hole is smaller than the draing/feed holes both to the bushing being fed and the replenishment hole above it. Thus, instead of the bearing clearance acting as the final restricting oriiice, something mid stream is acting as a/the lube restrictor.

Sometimes, simply stopping the machine, letting oil to drain into the bearing for a while, restaring and the thing will "fix" itself and party on. Or, it can continute to do it ad nauseum.

We ran today and things ran OK then, POOF, the damn wheel started bouncing. For grins, we drained out the hot/warm oil and refilled with cold oil of exact same type (Valvoline "hi mile" ATF). The problem went away IMMEDIATELY and stayed away till we stopped work for the day.

THis led us to conclude that the problem IS both viscosity sensitive. temp sensitive and perhaps replenishment rate sensitive.

Snce cold oil will result in better/thicker oil films, we seem to want more oil of a higher viscosity getting to the bearings. Since the oil does work fine until/unless a certain temp is reached, we don't think the oil formulationj, per se, is the root cause of the problem - it is clearly viscosity/temp related. This is a factor in ANY cavitation issue as oil temp affects the vapor pressure point of the oil and a vapor pressure (vacuum bubble) is the culprit when it comes to ridding oneself of cavitation.

This idea also led led us to look into the lube feed hole issue - and found some areas that would improve oil feed rate and amount regardless of type of oil. The restriction we found in the drilled passages could be ESPECIALLy critical when the oil is warmer and potentially leaking out faster than you can put/drip it back in.

When we get a chance once this jobe is thru, we'll pull out the bushings (real PITA jobe to do) and drill more/larger oil feed holes. In light of the other "fixes" we've made to the machine and in light of the dated history of the device (it was designed in the late 50's/early 60's) maybe its time for some lubrication system "tune-ups", albeit only nearly 40 years late....

We'll keep the group posted....

[jrrdw]

Be nice to see pics of it. You said this is a "cam grinder" as in automobile camshaft?

[NC Cams]

Yes, as in automotive, truck and various protoype cams for IC engines. We also do some OD grinding from time to time - some pretty big/long stuff will fit into the grinder.

We grind NASCAR and NHRA one-offs and some trick production cams for diesel tractor pulling fuel injector pumps. We offer cam design and TOTAL valvetrain system integration services from the seat to the lifter and all parts inbetween (prototying, design analysis, failure analysis etc). We also can offer master creation services for folks who use/have Berco machines.

We also offer valve spring design and prototyping services. Our valvesprings are being used in an OEM American "GT" road race series. So far, thousands of racing miles with NO spring failures or issues whatsoever.

The cam grinder is a Berco RAC1500 - there have been pictures posted of the machine on the internet. Due to some modifications we've made to it, we aren't able to post pictures - sorry, trade secret issues.