One of the most important parts of a root type blower is the set of gears that drive and syn-chronize the two rotors. Satisfactory operation depends on the condition of these gears.

Worn gears are found by measuring the backlash of the gear set. Gears with a greater backlash than specified in the applicable technical manual are considered to be excessively worn and, if not replaced, will eventually cause extensive damage to the entire blower assembly.

A certain amount of gear wear is to be expected, but scored and otherwise damaged rotor lobes resulting from excessively worn gears are inexcusable. It is the duty of the engineering force to inspect the gears and lobes, and to measure the clearance at frequent intervals. During the inspec-tion, it will be possible to measure accurately the values of backlash. These values should be re-corded. By observing the rate of increase of wear, it will be possible to estimate the life of the gears and to determine when it will be necessary to replace them.

Lobe clearance can be found by determining the difference of the maximum and minimum rotor lobe clearance at the same distance from the center. To find the maximum clearance, hold the rotors so that there is maximum clearance between the two rotor lobes. Then, with feeler gages deter-mine the value of the rotor lobe clearance. (See figure 3-25.)

Figure 3-26.Checking the backlash of blower rotor gears.

The minimum clearance is found in a similar manner except that rotor lobes are held in such a position as to take up all slack and backlash. The difference of the two clearance readings is the value of the backlash of the rotor lobes. Since a change in lobe clearance is normally caused by wear of the gears, the gear clearance must be checked. The most direct method for checking gear clearance is by the use of feeler gages. (See figure 3-26.)

Any gear set which has excessive lash or shows any sign of fracture must be replaced with a new set. Since blower drive gears come in matched sets, gears from different sets must not be interchanged.