Endplay settings of spherical roller bearings

[geesamand]

I've seen a number of gearboxes with vertical shafts and spherical roller bearing on each end of the gear shaft. These are not mounted on sleeves, so they have essentially the normal factory internal clearance. As you know, that translates to a larger clearance axially - .010 to .060 in many cases.

Sometimes I've seen the end float adjusted to eliminate excess axial slop (allowing the outer race to center on the rollers). Other times I see the end float set to a few thousandths just like a taper roller bearing. I can see how excess end float is risky, but it's also risky to remove so much float that the inboard rollers will always coast and be at risk of skidding.

I do not have context on why these were chosen, so I'd like to hear your thoughts on the matter.

David

 

[Cierva]

Double-row spherical roller bearings can carry thrust (as allowed by the equivalent load formula). The clearance (IRC) stated in the catalog is for an unmounted bearing. When the bearing is mounted with an interference fit on the shaft (or in the housing) some of that clearance is removed.
When mounted, a certain minimum IRC as per bearing tables needs to remain. These cold clearances will reduce further when machine reaches its normal operating temperature. On a given shaft one bearing should be fixed and the second bearing left free to float axially. All these conditions would have been considered by the engineer who designed the machine. The clearance in spherical roller bearings is not intended for any external adjustments.

Cierva

 

[Tmoose]

"Sometimes I've seen the end float adjusted to eliminate excess axial slop (allowing the outer race to center on the rollers)..."

Was that adjustment OEM recommended? How much was the "excess slop" ?

Generally speaking the second bearing on the shaft must not be restricted from moving axially lest thermal expansion preload the bearings against each other, with potentially disastrous consequences.
The pillow block housings that use Spherical roller bearings often have spacers over 1/8 inch thick on each side of the bearing to create the "fixed" bearing. When used as a floating bearing the spacers are removed, and the the bearing should be installed close to centered axially.

Some machines leave both bearings "floating" and at assembly set the housings to create some nominal, fairly large initial axial clearance, proven empirically to be sufficient to be "neutral" at operating temperatures. I'm thinking this may be what you were describing.

http://www.babcockpower.com/pdf/RPI-TO-0011.pdf

 

[geesamand]

In fact, this adjustment practice came from a major gearbox OEM. It is the only place I've seen it so far. After discussion with SKF they did not know much about it, but they did not deny it might work.

"Excess slop" = what Tmoose describes with both end bearings floating but housings set to an axial endplay that correlates to axial play resulting from internal clearance of the bearings themselves plus a bit for thermal growth of the shaft. Said another way, imagine installing a set of these bearings with zero internal clearance - and shimming the housings to match the length exactly. That's the length we shim to, except that the bearings do have internal clearance and that's what you see in terms of total end play.