Sliding in housing?

[Hup]

Hello bearing folks,

I am trying to measure axial preloading forces between angular contact ball bearings. It is on a simple two-bearing back-to-back spindle with rigid spacers, <=> . My aim is to measure preloading forces during rotation when everything warms up and the preload is expected to change due to thermal expansion.

Now I use a spacer equipped with strain gauges. It should work well in case the bearings can slide freely in the housing (there is initial clearance). Nevertheless it looks like bearing outer rings expand so fast (compared with housing) that they stick in the housing and the preloading force is no more transfered through the spacer.

Is this behaviour normal? I have seen some relatively high-speed aplications that rely on this OR-housing sliding. They don't use spliting the housing and adding a ball cage (obviously because of cost) at spindle's floating end.

Thanks for any response, H-up

 

[Skogsgurra]

Outer ring should move freely in housing. Does it seem to stick also when there is thermal equilibrium?

Gunnar Englund

http://www.gke.org

 

[Hup]

It looks like the OR is stuck during short period after starting the spindle. And how long is this period depends on speed.

It seems logical: heat source at OR, thermal resistance at the OR-housing clearance, heat sink (and large capacity) at housing. When temperature difference (OR x housing) is larger, the positive clearance could exist no more.

 

[Tmoose]

Is your arrangement like this <<------>> ?

The initial OR expansion can swell and strongly resist sliding, sometimes just resulting in a stick-slip situation (periods of extra heat and vibration, then a pop and a period of relief), and sometimes to the bearings' demise.
I've also had bearings cock and jam.

Good sliding of components relies strongly on a length to diameter (L/D)ratio greater than some number like 2 to 1. With a pair of bearings mounted tandem ( >> ) The L/D is one bearings width/OD, and is rarely better than 0.33 and each bearing is kind of floppy, inviting jamming.

A rigidly preloaded pair <> can behave much better, but with light preload and heavy radial load I beleive they will still cock independently and will potentially jam.
Spring preloaded bearings

Too much iron around a high speed spindle's bearings guarantees slow temp equalization and murderous bearing radial (and axial) restraint. A hand laid upon the spindle feels only coolness and the spindle specialist smiles and thinks all is well. Inside, the bearing is heating and swelling up inside the powerful housing, and feels like this -

http://www.tulipgulch.com/images/DayViseHead.jpg

 

[Hup]

Hello Tmoose,

thank you. Now I can imagine well what the bearing thinks of such spindle experts. :-]

Some kind of stick-slip effect is probably what my bearings experience. Their arrangement is like <--->, there are just two altogether.

Probably a 2-housing design with a ball cage (or a brass sleeve) is needed for measuring undistorted axial load between bearings.

But isn't there similar experience with sleeve bearings between 2 housing components? Heat source and smaller capacity inside ... heat sink and larger capacity outside -> I expect also the clearance to decrease. The question is how much...

 

[Tmoose]

Lets say a sleeve bearing has 0.002 inch diametral clearance. If the warm shaft diameter grows 0.001 inch, and the length grows 0.010 inch, no harm is done.

With spindle bearings 0.001 inch diameter increase is likely to lock the bearing in the bore, even if the internal preload didn;t drive running temps thermonuclear in a matter of a minute.

The axial offset between light and heavy preload spindle bearing with 60 mm bore is less than 0.001 inch. The speed rating (heat limited usually) of a heavy preload bearing set can be less than 1/2 the speed of light preload. The <> arrangment is required to have much transient heat/speed tolerance because initial axial shaft growth tends to relieve preload, not increase it.

I think having an axially flexible mount at one end is the best solution. So sliding friction or tight ODs pose no threat.

For years in order To make spindle bearings go real fast a huge volume of oil was required for cooling. Gallons per minute of oil flow is required to cool the sleeve bearings in you Chevy too. Greased spindle bearings need extremely good heat management since their heat rejection

 

[Hup]

The <> arrangement with relatively large length

L=2*dm/tg(alpha)

is recomended to accomodate expansion without preload increase. Just theoretically since this assumes homogenous temperature difference between inner and outer parts.

Radial expansion of material close to heat sources (shaft under bearings) appears much faster than axial expension of the shaft in all the length -> it probably causes a peak in preload immedialetly after start (together with centrifugal forces). This is shown by our strain gauges in period before the bearings swell and lock themselves in the housing.