With digital readouts instead of the veinier scale, gear tooth calipers can be read more easier, However, metric gear tooth calipers are available but have little value, because most bridge machinery in the future will be made to the metric system, until most U.S industry converts to metric.
You must be extremely careful when place a gear tooth caliper on a gear, because it must be straight and square to measure accurately. The gear must be properly prepared before using the gear tooth caliper, and the gear must be absolutely clean to prevent from the affecting of grease, dirt, and rust of the readings, both on the faces of the teeth and on the tip of the gear the differential make only small fractions of a revolution about their axes. The bevel gear and bevel pinion teeth are in contact when the unit is being inspected are likely to be the only teeth that are even in contact. They should thus be the only teeth on these gears that worn. If other teeth, that are in the clear so that a tooth caliper can be placed on them, have noticeable wear, it means that the differential gears have been turning. Unless the mechanism has been dismantled, this implies a strong like-lihood that there is excessive play somewhere in the drive train. This condition should be investigated carefully. If the differential is acting normally, there will be no wear on the accessible bevel teeth. Backlash measurements should be used to obtain an approximation of the gear wear. The mesh of the bevel gears and pinions should be checked, as any increase of center distance will increase the backlash, every with no wear at the thrust bearings hoking the bevel gears in mesh with the bevel pinions. The recorded backlash value can be adjusted for increased center distance by measuring how far out of mesh the bevel gears and pinions are, and computing or estimating the effect of this displacemen on backlash. It is quite unusual for the bevel gears in enclosed differential units to have excessive center distances, particular if these gears are supported in antifriction bearings. Clutch differential gears on vertical lift bridges can have several worn teeth, because the differential bevel pinions can romate when the bridge is being seated, and may make considerable rotation if the bridge tilts or skews and this tilt or skew is corrected in seating the bridge.