Steve Skenzick
HPS Electrical Apparatus Sales & Service

At my service center, we have seen problems with previously repaired shafts that were metal sprayed. In these cases we received motors for overhaul. Upon inspection and measuring the bearing shaft fits, we found something that just didn’t “feel” right. We could tell from the appearance that the shafts had been repaired prior to the current overhaul.

In most cases, the bearing fit area had the common appearance of a frosted look, indicating that the bearing fit had been metal sprayed in the past. The appearance wasn’t the real problem. It was the fact that when we measured the bearing fit shaft size, the micrometer didn’t feel right; mushy, not solid, although the journal was very close to the specified size. 

After experimenting with one of these frosted fits and using two micrometers (not our good calibrated set of micrometers), here is what we found. We tightened down the first micrometer and then placed the second one 90 degrees away and measured the second location (see Figure 1). The sprayed portion changed just a little bit, about 0.001” to 0.002” (0.025 to 0.051 mm) larger than the first location. We could loosen up the first micrometer, and then tighten the second micrometer, thereby changing the readings in the other direction.

Upon checking with the customer we found that the 150 hp (112 kW), 3600 rpm motor connected to a refrigeration compressor had been repaired by others about three years earlier. Also, some time after being returned to service the customer installed a variable frequency drive (VFD) in the application. The motor came in to our service center for repair because it is in a critical application and the customer had a preventive maintenance schedule that called for it to be overhauled.  

Fortunately for the customer, their scheduled maintenance program resulted in catching this impending failure before any damage had occurred. Further, this made it possible to see and deal with a potential problem before a very expensive failure of possibly not only the motor, but the compressor which could have spoiled the refrigerated product in a very large cold storage warehouse.

The shaft journal was repaired by machining out the metal spray area, welding the shaft and machining to correct size. Because the motor was now supplied by a VFD, we discussed the option of installing a shaft grounding system. The customer agreed, making it a win-win situation.

Balancing machine incident

A similar problem that a balancing service company had to deal with was on a rotating piece in a balance stand. Unbeknownst to the balance technician doing the job, the bearing journal fits on both ends of the shaft of an armature brought in for balancing had been metal sprayed. The technician was not familiar with the appearance of metal spray repaired shafts and therefore didn’t know to ask about that possibility. Further, there was no information provided in the balanc-ing request that mentioned the metal sprayed journals. The technician placed the armature in the balance stand and started spin-ning the unit, a large heavy armature of about 4000 lbs (1800 kg). After a short time, as the armature was gain-ing speed on the balance stand, the metal sprayed areas on both bearing journals started to crack, crumble and peel (see Figure 2) because of the weight and pressure exerted by the two wide bearings on each cradle of the balance stand. The technician was able to get the rotating piece stopped safely, but not before damage was done to both shaft bearing journal fits and also to all four cradle bearings.

Note: A contributing factor in this failure was that the metal sprayed surface was not properly prepared and would have resulted in the journal failing even if the balance stand had not degraded it.

The end result was that the armature bearing journal fits both needed to be redone; the four cradle bearings of the balance machine needed to be replaced because spray metal material was pressed onto and into the bearing outer races (see Figure 3). 

Some possible solutions when dealing with a repaired shaft that needs to be put onto a balancing stand include balancing the rotor with the bearings installed. If there are inner bearing caps, these can be tied with string so they don’t rotate, with a little oil under them on the shaft. Another alternative is to machine steel sleeves to use on the bearing journal areas and have them run on the balance stand cradle bearings.

Conclusion

BE CAUTIOUS WITH METAL SPRAYED JOURNALS. When a rotor has bearing journals that have been metalized, they should never be run on balancing machine roller supports. The journals will be damaged and can create a significant safety hazard.

AVAILABLE IN SPANISH

Categories: Repair procedures & tips, Safety, Service center equipment, Shafts

Tags: Bearings Shafts Metal spraying Safety Bearing journals Bearing fit Armatures

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