Centrifugal blower lubrication problems may be caused by failure of the oil pump to prime, low lube oil level, clogged oil passages or oil filter, or a defective relief valve, which is designed to maintain proper lube oil pressure.

If an unbalanced rotor is the cause of shaft or bearing trouble, there will be excessive vibration. Unbalance may be caused by a damaged turbine wheel blading or by a damaged blower impeller.

Operating a blower when the exhaust temperature is above the specified maximum safe temperature generally causes severe damage to turbocharger bearings and other parts. Make every effort to find and eliminate causes of excessive exhaust temperature before the turbocharger is damaged.

Turbine blading damage may be caused by operating with an excessive exhaust temperature, operating at excessive speeds, bearing failures, failure to drain the turbine casing, the entrance of foreign bodies, or by turbine blades that break loose. Damage to an impeller may be caused by thrust or shaft bearing failure, entrance of foreign bodies, or loosening of the impeller on the shaft.

Since blowers are high-speed units and operate with a very small clearance between parts, minor damage to a part could cause extensive blower damage and failure. Although there is considerable difference in operating principle and construction between the positive-displacement blower (Roots) and the axial-flow positive-displacement blower (Hamilton-Whitfield), the problems of operation and maintenance are similar.

Some of the troubles in a positive-displacement blower are similar to those already mentioned in our discussion of the centrifugal-type blowers. However, the source of some troubles may be different because of construction differences.

Positive-displacement blowers have a set of gears to drive and synchronize the rotation of the rotors. Many of these blowers are driven by a serrated shaft. Regardless of construction differences, the basic problem in both types of blowers is in maintaining the necessary small clearances. If these clearances are not maintained, the rotors and the case will be damaged and the blower will fail to perform its function.

Worn gears are one source of trouble in positive-displacement blowers. A certain amount of gear wear is expected, but damage caused by excessively worn gears indicates improper maintenance procedures. Whenever you inspect a positive-displacement blower, record the backlash values, according to PMS. You can use this record to establish the rate of increase in wear, to estimate the life of the gears, and to determine when it will be necessary to replace the gears.

Scored rotor lobes and casing may cause blower failure. Scoring of blower parts may be caused by worn gears, improper timing, bearing failure, improper end clearance, or by foreign matter. Any of these troubles may be serious enough to cause the rotors to contact and damage the blower extensively.

Timing of blower rotors involves both gear backlash and the clearances between the leading and trailing edges of the rotor lobes and between rotor lobes and the casing. You can measure the clearance between these parts with thickness gauges, as illustrated in figure 3-39. If the clearances are incorrect, check the backlash of the drive gear first. Then retime the rotors according to the method outlined in the manufacturers technical manual.

Failure of serrated blower shafts may be due to failure to inspect the parts or of improper replacement of parts. When you inspect serrated shafts, be sure that they fit snugly and that wear is not excessive. When

Figure 3-39.Checking clearances of positive-displacement blower lobes.

serrations of either the shaft or the hub have failed for any reason, replace both parts.