Angular contact bearing clearance? 1

[Spudie]

I have a couple of questions to ask regarding a double row angular contact bearing that we are intending to use in a flat belt tensioner.

The bearing in question is a NSK 32BD219DUK. For reference the size is – 35mm x 55mm x 20mm and has a nominal internal clearance of 47 microns. Single piece outer & single inner & seals on both sides

The application for the bearing is to be used in a timing belt tensioner set up where a solid shaft will be press fitted into the inner race and a sleeve press fitted onto the outer race to bring the overall diameter up to 60mm. The bearing assembly will have a stationary inner race/ rotational outer that will apply tension to the back of a timing belt.

The question that I need to shed some light on is the clearance of the bearing regarding weather or not the bearing needs to be in a preloaded state or not. This can be adjusted by the amount of interference that the shaft and housing will produce. I have been told that angular contact bearings need to have a certain amount of preload to perform correctly. Is this correct?

The reason why I have to ask this question is that NSK produce a timing belt tensioner that has this bearing installed in a similar layout to the one that we are designing, and as they have no stock of the bearing at present, I obtained a number of these tensioner units to use the bearings as a stop gap. When checking the clearance of the NSK tensioners it was found that several units have a clearance of up to 20 microns and several others having zero. I can only presume that the tolerances of the bearing diameters and the shaft & housings that have been made are accumulating the manufacturing tolerances so that certain units had clearance and others had none – therefore putting the bearing into preload, hence my question regarding preload!

The other thing that will probably be a deciding factor is that the operating temperature will be approx 120 degrees C

Sorry for the long (1st) post! but would greatly appreciate any input on the above.

Thanks

 

[diamondjim]

It depends on the application, load
information, and speed as to whether it
should be preloaded or have clearance.
Unless rigity is the main concern, often
clearance is allowed to reduce the turning
torque and reduce the heat build up in the
bearing. I assume the bearing manufacturer
specified the bearing and clearance for
your application.

 

[quadtracker]

You probably already know this info but I will include it in the post to help anyway. The angular contact bearings are sold 3 or 4 classes such as c2,normal,c3,and c4, each with different radial clearances(non-mounted).C4 bearings have the greatest internal clearances. Skf bearing maintenance handbook has tables to select the bearing for the specific application.(including a belt tensioner) Given the bore size of the bearing you want to use and the application, there is another table of recommended tolerances in the housing and on the shaft with press fits (interference fit) and push fits (size on size). With that info you can select the bearing with the right initial radial clearance to meet the final operating radial clearances, when the tensioner is in normal operation. Skf recommends operational radial internal clearances should be greater than zero; while a slight preload for ball bearings usually has no detrimental effect. How much is slight, the bearing manufacturer did not say, but I would try for no preload. Another source of info, reads; the usual bearing installation has one ring (race) with an interference fit and another ring with a push fit. The general rule in mounting a straight bore bearing is that the rotating ring is a press fit and the non-rotating ring is a push fit. Some installations use a press fit on both rings. I hope the info is useful, I recommend you obtain a copy of the Skf maintenance handbook to help you make your choice.

 

[electricpete]

As you know for single-row angle contact bearing, axial preload is critical.

For double-row angle contact bearing, axial preload is already established when the bearing is manufactured.

Fit is also an important parameter.

At this page you will see NSK's recommendation for shaft fit for thrust bearings such as double-angle contact:

http://www.tec.nsk.com/Handbook.asp...ternalClearances/ThrustBearingsAndShafts.html

To convert the letter into recommended shaft/bearing bore interference go here:

http://www.tec.nsk.com/Handbook.asp...ndInternalClearances/ShaftsAndInnerRings.html

At this page you will see NSK's recommendation for housing fit for thrust bearings such as double-angle contact:

http://www.tec.nsk.com/Handbook.asp...Clearances/ThrustBearingsAndHousingBores.html

To convert the letter into recommended outer ring/housiong clearance go here:

http://www.tec.nsk.com/Handbook.asp...dInternalClearances/HousingAndOuterRings.html

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[Mendit]

You never mention about the speeds the bearing will see, any thing over 1000 rpm you need clearance. Preloading is only used when precision bearings are assembled on spindles, or slow moving parts with heavy alternating forces. At 120 C how do you keep it cool ?