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While generally regarded as a heavy duty design, gear driven airends can bring their own set of concerns to the compressor package. Gear driven airends are designed to maintain set clearances between the drive and pinion (driven) gear. As any of several drive end bearings wear, these clearances open up, and can cause the gear to shell off teeth on startup.
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In the SSR 3000 diesel airend shown above, it would appear that the cause of the gear failure might have been a loss of the inboard thrust bearing on the discharge end of the male rotor. Notice the burnt marks on the near face of the gear from contact with the gearcase. As the thrust bearings wore, the male rotor pushed away from the discharge end housing, bringing the gear with it. The discoloration on the inner race of the cylindrical roller bearing and the wear pattern on the driven gear also indicate that the rotor had traveled some 1/8 inch in a very expensive direction before failure. This so-called “sudden catastrophic failure” was probably screaming for someone to shut it off for weeks before seizing. Usually an airend approaching a failure of this sort would be louder than normal. Discharge air temperatures would increase due to excessive rotor clearances and increasing friction. The gears would rumble considerably as the misalignment and wear patterns on the gear teeth degraded. Listen to your equipment! It could save you money.
Many airends scavenge oil through large ports at the bottom of the gearcase and suck it back through the rotors. You can see the added challenges that feature might provide for the restoration of this airend. At 52-56 Rockwell hardness, the gear teeth are the hardest materials in the airend, and pass through the airend like kidney stones. Ouch!
One
of the more common problems we see is incorrect fit of the input shaft key. If
the gear is allowed to rock on the shaft at all, it will eventually come loose.
When replacing or installing drive gears, special attention should be paid to
the key and keyway. A worn key should be replaced without question. Clean the
keyway thoroughly to assure that the key is properly seated and that the keyway
is not tapered or worn.
Some
rotary screw compressors have been known to crack or chip gear teeth simply from
switching the compressor from manual to auto while the machine is running.
Consult your owner’s manual or your
servicing distributor before ever attempting it.
In
certain compressors, such as some Ingersoll-Rand packages, one gear is mounted
on the male rotor, and the other is mounted directly on the electric motor
shaft. It is most
important to care for the drive end motor bearings on this type of compressor,
as they directly influence the longevity of the drive gears, and, in turn, the
airend.
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Following a motor overhaul it is important that the motor flange is correctly installed and the proper clearance or gear lash is attained between the teeth of the drive and driven gears. Too little clearance and the gears will run hot and wear prematurely, too loose and they may crack.
And
don’t forget those all important gear spacers! If the spacers are left out or
installed in the wrong positions, gross gear misalignment can occur. As you can
see from the photo at right. The result can be disastrous.
Are
gear driven machines prone to failure? No!
But if you ignore their needs, they can get expensive. Hoping not to repeat myself…. Listen to your equipment.
