Reliability Engineering Snapshot TM

The Illustrated Newsletter For Failure Analysis, Non Destructive Evaluation, and Predictive Maintenance

Machine Design

Case Study No. 42:

When installing bearings, taper lock bearings offer advantages over plain bearings. However, when installing these bearings, if the reduction in running clearance is not accurately measured then the bearing can go on too loose and cause it to fail prematurely.

Both of these bearings were installed at the same time and came off of the same shaft . The picture to the upper left shows a taper lock bearing that was properly installed. The picture to the upper right shows a taper lock bearing that was improperly installed. Their respective tapered sleeves are shown below. The engineering principle and installation is quite simple. The inner diameter of the bearing's inner race has a taper on it; so does the outer surface on the tapered sleeve (shown below). The inner surface of the tapered sleeve is a straight bore, and it fits on the shaft. During installation, the bearing and taper sleeve are slid onto the shaft loosely. After they are properly located, the mechanic will then begin to tighten up the lock nut on the taper sleeve. Tightening up the sleeve causes the inner diameter of the inner race to ride up on an ever increasing diameter on the sleeve. This action expands the inner race while at the same time the taper sleeve is radially contracting (the cut slot in the taper sleeve allows this to happen). The compressive forces generated by the inner race locks the taper sleeve onto the shaft in much the same manner as if it had been heated up and allowed to cool, and shrink, onto the shaft. This is where the problem begins. In order to install this bearing properly, the mechanic needs to have feeler gages so that he can measure the reduction in the running clearance between the rolling elements and the outer raceway. The reduction of clearance occurs as the taper sleeve expands the inner race, while it is being tightened. If the mechanic is afraid that he will tighten the bearing too tight he will leave it loose and there will be insufficient compressive force to keep the inner race and taper sleeve from coming loose. The picture to the lower right shows fret marks. This is what happens when the taper sleeve is not properly tightened on the shaft. There is not enough force to hold everything together tightly. Instead, the components will vibrate.


Clicking on the upper right picture, you can see the fret marks along the inner diameter of the taper sleeve. The inner diameter of the inner race is shown in the picture to the right. The white line indicates the centerline location of the taper sleeve slot. Notice the fret marks on the inner race I.D. When the bearing is on the shaft too loose it will tend to vibrate a lot more. This in turn disrupts the oil film thickness and the bearing will tend to run hotter. The vicious cycle creates greater running clearances which in turn creates more vibration. Loss of metal due to fretting, caused by the vibration, again, loosens everything up.