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Using dial indicators for axial gear measurements


When "dialing-in" the gear, axially, and the kiln has been adjusted to run continuously against the thrust roller, can it be done using only one dial indicator?

No. Axial run-out measurement on a kiln gear can never be done satisfactorily using only one dial indicator.

The use of two dial indicators for measuring axial run-out is a means to eliminate the effects of axial "end-float" that is inherent in all rotating, roller supported equipment. The kiln shell, due to design features that accommodate thermal expansion and other factors such as thrust control and simply economics of construction has the ability to move in the axial direction. This movement, measured at the gear, should normally not exceed the difference between gear face and pinion face dimensions, nominally about ± ½" [12mm].

The thrust tire, the one which is constrained from axial movement by thrust rollers, is normally the closest tire to the gear. This is the fixed point on the shell from which thermal expansion causes the shell to grow in either direction away from it. The gear, therefore, being as close as possible to this point, has its position closely fixed to line up with the pinion, almost independently of thermal expansion conditions of the shell.

If then, with the thrust tire running continuously on its thrust roller, is the axial "end-float" of the shell not eliminated permitting the use of only one dial indicator? This presumes that the thrust tire is running true to the shell axis of rotation and that the shell axis is perfectly straight. In other words the tire has no wobble. This is virtually never the case. All tires, thrust tires included, wobble. The only thing we can discuss is how much it wobbles. Axial run-out requirements are fairly tight, in the neighborhood of 0.030 - 0.060 inches total [0.8 - 1.5 mm]. Each gear will have its specific allowable maximum. We can see therefore that the slightest amount of shell movement or the slightest surface imperfections on the thrust faces can introduce sufficient axial "end-play" to produce erroneous axial run-out readings on the gear.

Any wobble of the thrust tire would be seen at the gear as axial run-out of the gear when only one dial indicator is used. There is no way to tell the difference. Ironically, the gear will then unwittingly be set with an axial run-out to exactly (or as close as the practitioner can manage) oppose the run-out of the tire. The single indicator will then show acceptable axial run-out. At best all that has been accomplished is to make the gear run-out mirror the tire run-out. Its magnitude will be the same as the tire while its direction will be opposite. Irregularities in the surfaces of the thrust faces may introduce sufficient "noise" into the readings making an otherwise straight forward task very confusing.

The real problem is, when only one dial indicator has been used, that the axial run-out is still unknown. It simply has still not been measured. There is so little extra effort involved in making these measurements correctly, by using two indicators, that nothing else should be acceptable. The use of only one indicator either shows a lack of understanding of kiln mechanics or a blatant disregard for them. In this illustration the shell has been partially removed from view to show the dial indicator set-up positions required for axial run-out measurement.



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