Bearing Noise with Krytox Lube

[sdk_imported]

I recently did a back-to-back test on R6 bearings running at 12,000 RPM. In the first test, I used Nye Rheolube 374-C (NLGI=4, lithium soap thickener) and the machine ran very smoothly. I replaced the bearings with new bearing lubed with Krytox 205 (NLGI=2, Teflon thickener) and I now hear a tinty "rattle". I've tested multiple new bearings with similar results. I've tried reducing the fill but it makes no difference.

The only difference between the smooth "quiet" running bearing and the noisy bearing is the lube. Any ideas on what is causing this rattle? What differences in the lube could explain this and what is colliding to make the noise?

 

[rnd2]

What is "colliding" is metal to metal.
Remove all trace of krytox. Replace with lithium grease & try again. If no noise, remove all trace of lithium grease and replace with Krytox. If noise returns you can confirm how but not why.
Also try and measure temp rise from start to finish.

 

[diamondjim]

I wonder if the difference is
because you are using NLGI 2
instead of NLGI 4 grease.
Is there a teflon based 4 grease?

 

[sdk_imported]

Both lubes habe synthetic base oils. The big difference is the type and quantity of thickener. I expected better washout resistance with Krtyox because it has a teflon thickener ... but this noise issue has me concerned. I I have not been able to find "off-the-shelf" NLGI 4 teflon thickened synthetic lubes.

 

[unclesyd]

This may point you in the right direction for a NLGI 4. The Halocarbon is 25-20M

http://www.halocarbon.com/e5.htm

Look under products and check out some of the valve sealers

http://www.sealweld.com/cleaners.htm

 

[Tmoose]

At a customer's request We tried to get a precision spindle with Barden C108 (preloaded angular contact hybrid ceramic) spindle bearings to run 8000 rpm using various hit-temp lubes. With a dN of 320,000 this should be dead easy.

Despite several free samples, and repeated assurances from the (2) major lube manufacturer's tech departments that their grease would work just fine, we never got a spindle to work. All ran hot, and were noisy with buzzes, screeches and squeals popularly called "cage noise" and some developed damaged rough bearings. Our assembly techs assured me that similar tests over the years ended similarly.

Getting the spindles to run cool and quiet with our standard Kluber NB-52 was wicked easy. I have no affiliation with Kluber, except having worked for a spindle company who was a satisfied customer.

The moral of the story for me was grease can make a BIG difference in the way a bearing runs, and even big grease manufacturers have not done testing with spindle bearings.
(preloaded angual contact).

 

[sdk_imported]

Tmoose,
In you experiments, did you try any Braycote products?

Thanks.
Scott

 

[Tmoose]

Nope. 80 year old Company got sold to a competitor 800 miles away. Now the plant's closed and I'm looking for work.

 

[Gerry45]

Greases of the 'Krytox family' are not generally very good lubricants for rolling element type bearings where smooth running is required. The noises you hear can be caused by a combination of two characteristics which this type of grease has.

1) Chanelling - I have found that Krytox type greases dont channel like other greases. They seem to have an afinity continually slump back (or be pulled back by the cage)into the path of the rolling elements. This can cause momenetary disturbances to the speeds of the rotating components.

(This 'instability' of these types of grease was demonstrated to me when I accurately monitored outer-ring temperatures on a test rig. With conventional greases, there is a rise in temp. after start-up followed by a steady decline, before levelling out at the normal running temperaure. Using the same quantity of a 'Krytox type' grease, you get a rise followed by a series of fluctuations with each apmlitude different to the previous.

2) Thickener particle size - The solid teflon (ptfe)material is like millions of small rocks as far as a high quality rolling element brg is concerned. In addition, they have a tendency to become compacted which only makes the problem worse. These types of greases are never recommended for noise critical applications for this reason.

Gerry

 

[sdk_imported]

Gerry,

I have seen comments similar to your in the literature.

Related - there seems to be a debate over the effectiveness of using "channeling" lubes for high speed applications. I've come across proponents for both low and high NLGI lubes for high speed applications.

I am interested in hearing any opinions or experience on this issue.

Scott

 

[Tmoose]

If by high speed you mean over 500,000 dN (brg bore in mm X rpm) I think you will find there are relatively few lube companies participating. Several bearing companies sell Kluber grease re-packaged.

Here is a reference that lists characteristics of several greases that can make it beyond 500,000 dN.
Some are described as chanelling, and some as non-channeling.

http://www.precisionspindle.com/grease.htm