Fitting and clamping of bearing rings

[JeanMicheling]

Hello everyone,

I'd like to discuss with you about the bearing and end cap torque for different bearings. For angular contact bearings, I have seen different methods. When I looked in the SNFA book, which is to me the best reference for angular contact bearings, they explain how to calculate the tightening torque in fonction of the preload and other parameters whereas NSK gives a unique torque value which is in fonction of the threads.Which one is the best? I'd say SNFA but I'd like to have your opinion. Moreover, SNFA says that this is a rule of thumb and the preload should be verified during operation. What is the best way to do it? NSK talked about three methods which are starting torque method, thrust static rigidity and natural frequency method but they don't give enough details. Does it really work great?

A second point is the locknut and end cap torque needed for non preloaded bearings such as deep groove or double row angular contact bearings. I haven't found something that could tell more about that. I know, I should call the bearing manufacturer, but honestly, I'd rather have the opinion of users than builders!

Thanks,

 

[Tmoose]

Bearing outer ring distortion is easily caused when cover screws are close to the bearing bore. When the screws are tightened the housing material bulges inward at each screw, causing matching lobed distortion of the outer ring. Some work with a dial bore gage will reveal if this is a problem. It often amounts to several 0.0001s of an inch. Similarly race engine builders use "torque plates" when finishing cylinders to create round cylinders in service. Nowadays some production engines use similar technology when finishing cylinders.

Another way to distort an outer ring is to have extra gap between cover and housing, so the cover pilot diameter pinches inward or "oil cans" at each cover screw and distorts the race through face contact. That would only apply when the cover contacts and clamps the outer race, as is practically universal at one end of electric motors and machine tool spindles.

Limiting the cover gap (which usually requires adjustment at each overhaul) to 0.001 inch is intended to limit the oil can effect to a harmless level.

 

[Tmoose]

The difference between light and heavy preload in a typical 15 degree spindle bearing (B7022.....)with 110 mm bore is about 30 microns (0.0012 inch) of axial motion per bearing and about 900 lbs of axial force.

Torrington catalog 102-10M-998 wants a 75 mm bearing locknut tightened to 173-250 lb-ft, at which point it would exert between 3600 and 5200 pounds of clamping force on the inner races and spacers. Depending on bearing arrangement [ <> or >< } that would increase or decrease preload.

If the nut thread pitch is 1 mm then 0.002 inch of axial motion would require about 0.05 turn or 18 degrees to go from light to heavy preload, and cut the thermally limited speed about 50%.

The 1992 Machine Design article used a 7022 bearing. It says "CAP FORCE EFFECTS -
As the retaining cap clamps the assembly
into the bearing housing, the bearing
outer races, spacers, and joints between
them compress. This tends to reduce the
mounted preload and stiffness of the
bearing assembly." This cap is clamping bearing outer rings against precision spacers.

 

[JeanMicheling]

Diamondjim, you should look further in the SNFA's website cause I found it there and it's in english. Tmoose, I already experimented that problem about the distorsion caused by bolts, it can be controled with FEA analysis easily. I'm more concerned about the gap between the cover end face and the housing. The 0.001'' gap you talked about, is it before of after tightening. Books aren't clear on that point, I always assumed that 0.001'' gap, for instance, meant that you have 0.001'' tigntening load. In other words, before tightening the cover bolts, you ensure you have 0.001'' gap and after tightening the bolts, the cover is fully rested on the housing. Like I said before, I went to a training gived by a repair company and they used to set 0.003'' to 0.005'' gap before tightening the bolts to their full torque.

 

[diamondjim]

JeanMicheling,
please list the url for the article where you found
the technical specs or information from SNFA. I did
find the SNFA website, but did not find any technical
data.

 

[JeanMicheling]

The link is

http://www.snfa.com/index.php?id=23.

you will also find good references for bearing failure analysis.

So, if I understand, methods for preloading the locknut may varie from one user ot another one. If you have ground bearings, one might want to torque the locknut enough to rest the rings together apart from the type of preload and other parameters. That's what NSK seems to believe. But another one might use the SNFA methods which will give you a specific torque for each type of bearing. The only difference, I guess, is the influence on the spacers and rings that could, by the way, affect the preload.

Can someone answer my question on the cover clamping load (Does the gap mentionned above must be set up before of after tightening the bolts?)

 

[Tmoose]

The gap would be set before tightening in order to limit clamping and also allow full screw tightening.

 

[JeanMicheling]

I just read someting interesting on the snfa assembly manual found on the link above in the tightening procedure. It says that you have to tighten the end cap bolts three times the recommended torque, loosen them off and re-tighten them to the specified torque. Then, you measure the gap and reduce the spigot depth by an amount that is equal to this gap and tighten the bolts to their full torque. Can someone explain me what they mean by spigot depth and how it's possible to do that? This method looks really great but seems hard to do. To me, it sounds like you set up the load and once it's done, you secure the end cap by tightening the bolts to their full torque, you put a spacer between the end cap and the housing to make sure the rings won't be overloaded.

 

[diamondjim]

Spigot is an European term for
a pilot diameter.