A/C bearing update

[bob1112]

To update from previous posts, we installed A/C bearings in our spindle which was a very straight forward installation and we installed with surgical precision but had a noise in our brand new bearings that could not be found. We took it apart several times and no failured were obvious. The machine holds tolerance, runs quiet for the most part, good part finish, all things to say we had good bearings.

Well we had this obvious intermittant random tick that sounded like maybe ball skidding or vibration and it would not go away. I was scratching my head and decided that my ears may not be wrong and that noise was those balls trying to make room and move that heavy Kluber grease out of the way. Well, I reduced my grease amount my just wiping some out of the bearings and reinstalled.

Noticeably sweeter sound. Now my question is, is ther any problem in what I have done? felt I was on the money with my grease pack but maybe not. Has anyone else heard of this? Seems a rooky problem to me but my mechanical background never lead me down the A/C path until now. They are fun to work with when they work right. I guess I just need air/oil huh?

Brandon

[NC Cams]

I was wondering what had happened to you and your situation.

You have re-proven first handedly a previously known phenomenon called "grease noise".

Grease is basically oil with a thickener - industrial gravy to some extent. Different greases use different thickeners. Depending on the physical propeties of the grease and/or its thickener, it will do different things under different conditions. Grease thickeners include sodium nitrate (nitrite, I can' recall which), clay, PTFE and other agents that slipped from memory long ago.

Some are quieter than others and some are flat a$$ horrible for noise. You may have inadvertently gotten a grease that is not as quite as some other alternatives.

A/c's used in machine tools are often EASILY over lubed. Depending on the speed, these bearings only need 20% to 30% of the cavity grease fills (how much is 20% of a very irregular cavity is VERY hard to determine). For continued high speed, you can go as low as 10% or 15%. When you're using such low/precision fills, you don't simply slather in the grease like you might pack a wheel bearing for a coaster wagon. Not hardly.

In precision fillls, you literally place/insert the grease in the groove and around the balls and in the ball pocket in the cages with a syringe. You then slowly rotate the bearing to distribute the grease evenly. Start ups are slow and pulsed that get longer and longer in duration. If you put in too much grease, ball skidding can occur. If you try to run balls out from the get go, skidding can occur. If a p poor, wrong, unlucky grease choice is made, the damn things can squeal and howl horribly.

Viscous drag can/will cause overheating. Sometimes, simply running the bearings at low speed to channel the grease will alleviate the problem. Sometimes, you have to wash out the grease and repack it with the syringe method. Sometimes, you have to use a different grease because some grease are just plain "noisey" and only a different grease will fix the problem.

I wasn't about to bad mouth your grease choice as the grease you chose to use or were given to use is DEFINITELY a premiums grease. However, as your experience clearly shows, it may not be as quite a grease as you might wan to used.

What would I have used/recommended? I've been out of the industry too long to keep abreast of the latest in grease trends. I recall that grease suppliers in the USA and Europe were NOT as noise sensitive as the Japanese grease makers are/were at the time I was involved with bearings. One company that did a LOT of grease noise work with the bearing company I worked for was Kyodo Yushi.

If they have an agent in the USA, they might be the company I'd call to try to find a quiet machine tool bearing grease. Otherwise, your guess would be as good as mine as to selecting an alternative grease with lower noise potential. As far as fill percentage is concerned, that too is often an empirically derived value.

Keep in mind that the less grease you use, the less tolerant you'll be toward contamination and/or life extension.

If you're bound and determined to run at high speeds, oil air mist might be the preferred option. Or you might simply try going to a "dry sump" fed lube system wherein DEXRON III were jetted or drip fed to the bearings. We went this way on some previously grease lubed dynamometer bearings and NEVER had another problem with them after going the DEXRON route.

[bob1112]

Well I figure we are about 15% fill right now and we actually already decided that we are buying a new machine. We need way more machine than we are asking if this one. I think the speed can be pushed at least to 8K at this point since al bearings are rated for 12K at this point but we decided that with our needs, a Haas with the optional 10K spindle is what we want.

Now one guy has mensioned using Prolong as an additive in the bearings for reduce the viscosity of the grease and reduce noise. I just don't know but sounds like he has first hand experience with this problem.

I kinda wondered about this problem after I started thinking about the "light preload" and that it might not take much to stop a ball. They seem to run great at this point, just hope the skidding did not affect life any.

Brandon

[NC Cams]

Oil additives may or may not help the life of a bearing - it depends what the "additive" adds or improves and if it is compatible with the base oil in the grease. When properly applied and selected, mineral or synthetic greases will generally provide more than adequate service and life as long as the lube does note get contaminated or overheated/burned up when the right amount and type are used.

I know of a carnival test used that oils will readily fail in and some trick additives will survive. What the oil additive maker WON'T tell you that regular old CocaCola will survive the test better than the additive, ESPECIALLY if it has real natural sugar in it instead of corn sweatener. Why? Once you flash off the liquid, the syrup and sugar congeal and melt to form a lubricant that is unbelieveably slippery. Yet, you'd hardly want to use Coke or Coke syrup to pour into your crankcase or use in your oil air mist system would you????

The use of "thick heavy" grease can be pretty much counter productive in a high speed spindle bearing - especially if the shear viscosity is not conducive to high speed operation. Basically, you need enough lube to create a lube film between the balls and raceways and, the faster the bearing goes, you essentially need LESS lube to create a film and/or to have to be pushed out of the way - hence the logic for oil/air mist in high speed spindles.

The oil mist provides enough fluid/lube to generate an elastohydrodynamic oil film for the balls to glide on (sort of like water skiing) and the air jet provides cooling. This is not unlike a dry sump system - use just enough oil to do the lubrication and cooling and once you do that, get it out of the way so it doesn't drag things down by generating viscous drag and/or heat.

When i learned to lube high speed A/C's, I was amazed at how LITTLE the amount of grease was put into the bearing. HOWEVER, the grease that was applied gingerly (surgically actually) placed in the grooves ahead of the balls. It was also oriented so that the balls would get covered when they rolled and yet NOT have to push th stuff around to move/roll thru the raceway afterwards.

When you run preloaded bearings, you are trying to force ALL the balls to carry load. You are also forcing ALL the balls to sit and stay in the raceway grooves. If you don't put the grease there to begin with or it gets pushed out of the way without getting dispersed all over the parts on the contact surfaces, you run the risk of making metal-to-metal contact without a lube film between the parts.

If/when this happens, the metal parts will initially scuff/cold weld to each other and the resultant surface distress accelerates the demise of same. This is why you should slowly pulse start a freshly rebuilt spindle. This is also why you should start a spindle slowly and ramp up the speed until things get warmed up - it is absolutely NO different than warming up a race engine.

Your noise could have either been cage rattle (ticking) or ball skidding (squealing) or a bit of both. Rattle is fairly harmless, squealing due to skidding could mean that you have lit the fuse and the thing will go off down the road - I can't tell from your post which transpired. Once either the balls or raceways get scuffed, they do NOT heal and they generally deteriorate at an accelerated rate.

If/when you crack into the spindle again (God forbid), take a magnifying glass (spark plug magnifier works fine) and look CAREFULLY at the balls and the raceways. Shiney roll marks would be normal. Blueish discoloration or debris dents (happens from some thickeners) or ball cuts or ball "frosting" would NOT be desireable.

Ball guided cages may be a bit noisier than land guided cages and, for high speed use, I'm pretty sure that I recall that outer ring guided cages are the preferred type.

Should the ball preload get unloaded due to spindle runout or overzealous feed rates, the cages will rattle and the balls simultaneously skid due to the balls going into and out of the load zone. Why? The balls coast when unloaded and are moved along/hit by the back side of the cage yet they "drive" the cage when loaded due to the friction/load being applied to the raceway thru the balls) - wahlah, cage rattle.

Your episode is a PERFECT example of what the machine designers go thru during spindle development. This is EXACTLY why high speed spindles are a high priced option.

SOMEBODY has to go thru this sort of iterative development exercise to figure out what the spindle wants/needs in the way of preload, lubrication type and amount as well as to figure out operating methods so that the owner/operator can turn it on and simply run it. This stuff doesn't simply "happen" in a plug and play fashion.

Any DIY'er who's thinking about ramping up the speed of their spindle by installing some new bearings and high $$$ grease should make this thread a mandatory read. The technique may work and it may not.

In this case, you may only have had a learning experience. In another instance, you could have toasted a set of bearings and been none the wiser after doing so. At that point, as it surely has happened, a "defective set of bearings" gets improperly blamed.