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Brake Drum Micrometer |
No. 908500 |
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Operating Instructions and |
90850050 |
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Parts Identification |
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AMMCO Tools, Inc. • Hennessy Industries • 1601 J.P. Hennessy Drive, LaVergne, TN 37086-3565 |
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(800) 688-6359 • (615) 641-7533 • (615) 641-5104 FAX |
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Lock Screw |
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Lock Screw |
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Graduated Shaft
Anvil |
Dial |
Figure 1 - Basic Parts Identification |
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8500 |
8500-50 |
Unit of Measure |
Inch |
Metric |
Drum Diameter |
6" to 16.125" 15 cm to 41 cm |
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Scale Graduations |
.001" |
.1 mm |
(Increments) |
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Graduated Shaft 1" and .125" 1 cm and 2 mm
Optional Shaft 16" – 26" 39 cm – 66 cm
Measuring the Drum (Figure 2)
1.Loosen 2 lock screws and move both dial and anvil along the shaft until the “whole” number of the drum diameter is visible at each arrow.
NOTE: Metric shafts have even numbers on one side, odd on the other. Inch shafts have identical scales.
2.Place micrometer inside the drum across the greatest diameter to be measured. Hold anvil steady and move dial back and forth against the braking surface to obtain the highest reading.
Calibration (Figure 3)
The micrometer is calibrated at the factory. Check the calibration using a No. 909582 Checking Gauge or a standard outside micrometer. To recalibrate, loosen the jam nut (#22) and adjust set screw (#21) until the correct reading is obtained. Tighten jam nut while holding set screw stationary.
Figure 3 -
Calibration
No. 909582
29 28 Gauge
11.000" Standard
27.94 cm Metric Equiv.
Figure 2 - Measuring the Drum
Arrows for
Reading Scale
Dial
Lock Screw
Scale
Arrow
Drum Condition
Drums should always be carefully inspected for cracks, worn or loose bearing races, scoring, heat checks taper, bell-mouth, and out-of-roundness. Drum out-of-round conditions are checked by taking micrometer readings at 2 or more positions around the drum. Scoring, out-of-roundness, bell-mouth, and taper should be corrected by machining.
Correcting most of the above conditions will require cutting material from the drum wall. If the drum diameter after machining is larger than the manufacturer’s specified rebore limit it must be replaced.
Drums that are too thin are apt to be weak and springy, and will not dissipate heat well, resulting in a loss of braking power. Defective drums should be scrapped.
INCH (Examples For Reading The Inch Micrometer)
METRIC
11.000 in. Diameter
11.000 in.
11.375 in. Diameter
11.000 in. + 0.375 in. = 11.375 in.
11.390 in. Diameter
11.000 in. + 0.375 in. + 0.015 in. = 11.390 in.
27.00 cm Diameter
27.00 cm
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