RECONDITIONING BRAKE DRUMS AND SHOESBrake lining in an eccentric or out-of-round drum cannotmake full contact with the drum This will result in rapid oruneven lining wear and could even cause brakes to seizeor chatter. Maximum allowable out-of-round oreccentricity should be 0.25 mm (0.010 inch).If the inspection shows that any of the precedingconditions exist, brake drum should be either turned orreplaced. To assure a balanced braking system, alwaysinstall turned or new brake drums in pairs on each axle.Any time a new brake drum is to be installed on avehicle, the runout should be checked as follows. Placethe new brake drum with hub and wheel assembled inbrake drum lathe making certain drum is centered.Mount Dial Indicator SE-1848 on lathe and check runoutabout 12.7 mm (0.5 inch) in from edge of drum (onbraking surface) as shown in Figure 6. Runout must notexceed 0.25 mm (0.010 inch).Before assembling drum, hub and wheel, all parts mustbe clean and free of foreign matter.Figure 6 - Checking Drum Runout on BrakeDrum LatheREFINISHING BRAKE DRUMSOn brake drums manufactured after January 1,1971, themaximum diameter to which drum can be worn isstamped or cast into the drum Drum should be discardedif worn beyond this limit.Minor scores on brake drum can be removed with fineemery cloth or steel wool, but always clean emery orsteel wool particles from the drum after this operation.More heavily damaged or out-of-round drums should beground or turned on brake drum lathe.If depth of scoring, bellmouth or barrel shaping exceeds0.13 mm (0.005 inch), measured with micrometer acrosspart of all of brake surface, drum should be refinished.Reboring limits (see drum) must not be exceeded and noheat checks, cracks or bluing should be evident. Use amicrometer also to check for an out-ofround drum. Makecheck by measuring drum brake surface diameter atvarious points 450apart around circumference.Eccentricity (out-of-round) should not exceed 0.25 mm(6.010 Inch) on diameterRemember that each time brake drums are turned, lessmetal remains to absorb the heat developed by brakingaction. Brake drums containing less metal will operate ata higher temperature As a result, brake fade, slowrecovery and erratic wear will be more noticeable. Also,extremely high temperatures shorten lining life andcause heat checks and cracks (Figure 7) to form oninner surface of drums These conditions will becomeprogressively worse until drums fail.Figure 7 - Cracked DrumREMACHINING DRUMSBrake drums that are otherwise in good condition can beturned in a lathe However, It must be remembered thatthe recommended remachining or rebore limit for brakedrums with a diameter over 355 mm (14 inches) may notbe increased more than 2 03 mm (0.080 inches)diameter (total cut) and discard at 3 05 mm (O 120inches) over normal diameterCTS-4082B - Page 4
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