Chapter 3—ENGINE MAINTENANCEdanger that the piston will strike valves that maybe open and extending into the cylinder. Makecertain that any gears removed are replaced in theoriginal position. Special punch marks, ornumbers (figure 3-24), are usually found on gearteeth that should mate. If they are not present,make identifying marks to facilitate the correctmating of the gears later.Bearing, bushing, and gear clearances must beproperly maintained. If bushing clearances exceedthe allowable value, the bushings must be re-newed. The allowable values for backlash andbushing clearances should be obtained from theinstruction manual for the engine involved.Usually, a broken or chipped gear must bereplaced. Care should be exercised in determin-ing whether a pitted gear should be replaced.BLOWER ROTOR GEARSOne of the most important parts of a root typeblower is the set of gears that drive and syn-chronize the two rotors. Satisfactory operationdepends on the condition of these gears.Worn gears are found by measuring thebacklash of the gear set. Gears with a greaterbacklash than specified in the applicable technicalmanual are considered to be excessively worn and,if not replaced, will eventually cause extensivedamage to the entire blower assembly.A certain amount of gear wear is to beexpected, but scored and otherwise damaged rotorlobes resulting from excessively worn gears areinexcusable. It is the duty of the engineering forceto inspect the gears and lobes, and to measure theclearance at frequent intervals. During the inspec-tion, it will be possible to measure accurately thevalues 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 gearsand to determine when it will be necessary toreplace them.Lobe clearance can be found by determiningthe difference of the maximum and minimumrotor lobe clearance at the same distance from thecenter. To find the maximum clearance, hold therotors so that there is maximum clearance betweenthe two rotor lobes. Then, with feeler gages deter-mine the value of the rotor lobe clearance. (Seefigure 3-25.)Figure 3-26.—Checking the backlash of blower rotor gears.The minimum clearance is found in a similarmanner except that rotor lobes are held in sucha position as to take up all slack and backlash.The difference of the two clearance readings isthe value of the backlash of the rotor lobes. Sincea change in lobe clearance is normally caused bywear of the gears, the gear clearance must bechecked. The most direct method for checkinggear clearance is by the use of feeler gages. (Seefigure 3-26.)Any gear set which has excessive lash or showsany sign of fracture must be replaced with a newset. Since blower drive gears come in matched sets,gears from different sets must not beinterchanged.CHAIN MECHANISMSIn some engines, chains are not only used todrive camshafts and auxiliaries but also to drivesuch parts as rotating supercharger valves. Con-necting links for two types of chains are shownin figure 3-27. Note that the connecting pins inone are secured by cotter pins, while the joint pinsshown in the other are riveted.The principal causes of drive chain failure areimproper chain tension, lack of lubrication,sheared cotter pins or improperly riveted jointpins, and misalignment of parts, especially idlergears.Chain drives should be checked for any symp-toms of such difficulties, in accordance with theinstructions in the appropriate engine manual. Thetension should be adjusted as required during3-39
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