Turn the block over

February 2016

Feb 2016_Tip of the month

Problem:

Bearing housings are cracking, or the bearings are not lasting as long as they should, so you initiate a Root Cause Analysis investigation to determine the reason for these failures. You find that the bearings and housings are the right size and type, there doesn’t seem to be anything wrong with what products you are using. Now it’s time to investigate the assembly and mounting procedures. You start to notice that the applications with early failure problems all use shim stock under the base to assure the proper base to shaft centerline height. Could there be a link?

Solution: 

It’s time to turn the housing over and look at the machined support surface on the bottom of the housing. If the machined surface goes all the way across the bottom, you need to shim the whole section, not just under the bolting feet.
Standard shims are made to be easy to use and come pre-cut with a slot for the bolt and a handle for you to use when inserting the stock, as shown below.
These shims are fine for most ball bearing pillow blocks, because they have a relief under the bearing to avoid stress risers in the cast iron, and only contact the pedestal at the two housing feet. But roller bearing housings tend to be machined all the way across the bottom due to the heavier loads these housings typically carry. The heavier load carrying capacity of roller bearings requires full support of the housing base. Consequently the base is fully machined and needs to be supported all the way across with either full-sized shims or ground spacer plates.
If a base like the one pictured on the right is only supported under the bolting feet with pre-cut shims, there will be excessive stress at the 6 o’clock position on both the bearing and the housing. Over time, the housing will react to this by cracking along the base, usually perpendicular to the boltto-bolt line. But this condition can affect the bearing immediately. The bearing may lose internal radial clearance, which can cause it to run hot. It can also experience two load zones, one at the bottom where it should be, and another at the top, where the bearing should run unloaded. All three conditions can compromise lubricant effectiveness, reducing bearing service life, even if you have the right lubricant.
Pay close attention to the bearing pedestal surface. The acceptable flatness tolerance is 0.002" per foot and 0.001" is preferred. Any gaps under the housing should be identified only after the complete housing has been fully and properly torqued to the mounting surface. Use a piece of 0.002" shim stock or a feeler blade to check for gaps and then shim and re-torque if necessary.
Never check a bearing housing for flatness on a flat inspection plate. The housing is clamped rigidly in a fixture for machining at the factory and when removed, internal stresses may cause the housing to distort slightly. When installed on a properly prepared flat surface, the housing will regain its machined dimensions. If shims are used to correct this potential distortion, the final shape and dimensions of the bore may not be correct and the bearing will not be fully supported.

Moral:

Something as simple and often overlooked as a piece of shim stock can cause premature housing and bearing failure, resulting in lost production, increased downtime and unnecessary maintenance expenses. So always remember, when mounting bearing housings and using shim stock: Turn the base over and look for the machined surface so you can shim appropriately. Prior to installation, check the bearing pedestal surface for any irregularities and for flatness. Machine as necessary to provide the best possible match between the pedestal and housing.

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