Rotary shaft seal gets very hot

[doctordoctor]

I have a tool that attaches to a CNC mill called a "right angle head".

Its just a 90 degree adapter so you can point the tool sideways.

Made internally just how you'd think..taper bearings on input shaft, angular bearings on output shaft.

And a rubber lip seal on the end of the output shaft that gets exposed to the outside world.

The tool is new to me, and in new condition.

It gets VERY hot after operating for a few minutes. The whole body of the tool probably gets to 150F.

The part of the output shaft right next to the lip seal probably gets to 200F in a matter of SECONDS at 4000 rpm.

I dont know much about lip seals, but from what I can tell, this may be normal?

The internal bearings require grease. so the lip seal gets lubed with grease.

Its the simplest lip seal, one contact point, no spring backup. Probably nitrile.

I tried replacing the seal, no change.

The lubricant is premium spindle bearing grease, Isoflex NBU15.

I isolated the overheating to the lip seal by making a spacer so the output shaft gets pushed out beyond the lip seal so its not touching. Things never got above 115F even after 15 minutes at 6000 rpm.

So whats the deal here? Is this lip seal badly designed, or maybe the grease isnt right? Or maybe I'm supposed to put a TON of grease behind it?

This isnt a cheap tool, they go for $2000 USD new, although the manufacturer of this one seemed to know very little about it, like it was some rarity. Maybe this is why..

I'm thinking of replacing the rubber lip seal with a PTFE seal which I believe would run at a lower temperature?

Help please!!

 

[BrianE22]

I don't quite understand what you did with the spacer experiment so I'll ignore that. It sounds like you believe the seal is producing the heat? Are you sure it's not produced elsewhere and the shaft is getting hot because of that?

 

[doctordoctor]

The spacer test made it so the lip seal was not touching anything and therefore could not generate heat, but all the bearings in the unit were still preloaded and as normal. In that case, nothing got hot. So the lip seal is generating the heat. Plus, the output shaft right next to the seal gets very hot within seconds of turning on.

 

[BrianE22]

What shaft diameter is it? Above about 3000 fpm, the speed is getting high for general seals. Nitrile is good to about 225 Deg F. Other materials can handle much higher temperatures. Make sure you have grease between the shaft and seal (don't want to run dry).

These measurements were done without any load on the spindle? That does sound bad. The attachment is rated for that speed?

 

[BrianE22]

SKF is a good source for seal info. I found the following page on seal friction:

http://www.skf.com/group/products/s...-seals/radial-shaft-seals/friction/index.html

 

[geesamand]

Are you sure it's not coming from the gearing? Small right-angle reducers like that can be lossy and lack appropriate cooling, leading to runaway thermal properties.

I'd suggest a non-contacting bearing isolator but since it's probably filled full with grease that would inevitably leak. A PTFE seal is not a bad idea.

David

 

[Dougt115]

Sounds like the lip seal is running dry.

I can't see from the pictures but if any moving part is in contact with the lip seal I would expect that there is insufficient lubrication. Meaning what ever you are using is getting wiped away. PTFE should work better.

Is there much of a contact force on the seal to surface? You may be able to form the PTFE better to keep a small clearance but not actually touching.