axial-flowpositive-displacementblower(Hamilton-Whitfield), the problems of operation andmaintenance are similar.Some of the troubles in a positive-displacementblower are similar to those already mentioned in ourdiscussion of the centrifugal-type blowers. However,the source of some troubles may be different because ofconstruction differences.Positive-displacement blowers have a set of gearsto drive and synchronize the rotation of the rotors. Manyof these blowers are driven by a serrated shaft.Regardless of construction differences, the basicproblem in both types of blowers is in maintaining thenecessary small clearances. If these clearances are notmaintained, the rotors and the case will be damaged andthe blower will fail to perform its function.Worn gears are one source of trouble inpositive-displacement blowers. A certain amount of gearwear is expected, but damage caused by excessivelyworn gears indicates improper maintenance procedures.Whenever you inspect a positive-displacement blower,record the backlash values, according to PMS. You canuse this record to establish the rate of increase in wear,to estimate the life of the gears, and to determine whenit will be necessary to replace the gears.Scored rotor lobes and casing may cause blowerfailure. Scoring of blower parts may be caused by worngears, improper timing, bearing failure, improper endclearance, or by foreign matter. Any of these troublesmay be serious enough to cause the rotors to contact anddamage the blower extensively.Timing of blower rotors involves both gear backlashand the clearances between the leading and trailingedges of the rotor lobes and between rotor lobes and thecasing. You can measure the clearance between theseparts with thickness gauges, as illustrated in figure 3-39.If the clearances are incorrect, check the backlash of thedrive gear first. Then retime the rotors according to themethod outlined in the manufacturer’s technicalmanual.Failure of serrated blower shafts may be due tofailure to inspect the parts or of improper replacementof parts. When you inspect serrated shafts, be sure thatthey fit snugly and that wear is not excessive. WhenFigure 3-39.—Checking clearances of positive-displacement blower lobes.3-34
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