Drive Related Failures and Broken Shafts

Many OEM airend manufacturers include disclaimers in their warranty statement for failures due to torsional or drive problems. Our experience would indicate that this includes a fairly large percentage of premature failures.

We have included shaft breakage with drive related failures as a broken shaft is usually the result of another drive problem, rather than the cause. 

Perhaps the most common failure of this type that we see is shown in the photo below. In this very common scenario, the drive end bearing on the male rotor has spun on the shaft, causing a catastrophic failure. In looking at this shaft one might question the term “catastrophic”. But the real damage has occurred inside the airend as the inner race of this bearing continued to spin, cutting into the softer shaft material a mere .015” as it runs.

Because the rotor to cylinder radial clearance on this machine is normally .0025 to .003 inches, we would find that the sealing strips would have been scrubbed off from rubbing against the cylinder for half the length of the rotor, requiring their replacement before reassembly.   

But, what lead up to this failure? What caused this bearing to spin while the thrust bearings still look new?  Perhaps most directly, HEAT!  When this bearing was originally mounted on this shaft it was very likely heated up to 250 degrees F to expand the inner race to overcome the light .0003” to .0007” interference fit commonly used to hold it in place. (We have also seen some manufacturers use line on line or even slight clearance fits to speed their assembly). The combination of high belt loads, high rotational speeds, and perhaps marginal lubrication can combine to expand the inner race sufficiently to break its grip on the shaft. Once the bearing begins to loosen, it is doomed to fail. Occasionally, debris in the lubricant will become trapped in the rolling elements of a bearing, causing it to “skid”. The friction from this skidding roller quickly heats up the inner race with much the same result. If not caught early in the failure process, the combination of weakened shaft and dramatically increased drag on the rotor as it scuffs the cylinder may eventually cause the shaft to snap as seen on the next photograph.  

This 127mm belt driven rotor displays burnt spots on the inlet end of all four lobes indicating considerable thermal growth. (There is normally quite a lot of space between the drive end of the rotor and inlet bearing housing.) The relative lack of debris damage to the rotor profile points to a sudden rather than protracted failure. The consistent sealing strip height would seem to rule out a gradual bearing failure on either end of the rotor. The dry varnish stains point to oil starvation as a possible culprit. 

Although perhaps not the cause of this failure, the inlet bearing surface of this rotor had apparently been welded during a previous rebuild.  

We no longer weld most drive end bearing surfaces on belt driven airends, finding that installation of a replacement heat treated shaft offers much greater reliability under harsh conditions.  

Are belt drives a problem? NO, but overly tight belts are. To quote a well known German manufacturer: 

“Belt tension: Excessive tension will significantly reduce bearing life and void any warranty coverage.”