Measuring Lateral Motions In A Rotating System With The Strobolux


Type 648-A Strobolux has recently turned up in connection with the measurement of small lateral motions in a rotating shaft.

An investigation of these minute irregularities in the moving parts of their line of tachometers was being carried out by the Barbour Stock well Company of Cambridge, Massachusetts. These tachometers are of the conventional fly-ball type, having three weights spaced at 120° around the spindle, each connected by one link to a sliding collar near the other end of the spindle. A helical compression spring surrounding the spindle between the fixed and sliding collars tends to separate these collars. Centrifugal force moves the weights outward from the shaft, thus causing the links to swing outward from their sta

tionary position parallel to the shaft. This moves the sliding collar along the shaft toward the fixed collar in opposition to the restoring force furnished by the helical spring. The position of the sliding collar is translated into dial reading by means of a spring-loaded follower, bearing against the face of the sliding collar and connected to the dial mechanism.

In the course of the investigation, a contour comparator of the familiar shadow-projection type was used. This consists of a light source with a suitable lens and mirror combination so arranged that a sharply defined and greatly enlarged shadow of the sample is projected upon a ground glass screen. For measurement purposes, the screen had vertical and horizontal reference lines. Graduated micrometer screws were provided to move the stage in a plane normal to the light beam so any point on the image could be set to the fixed reference lines.

It then occurred to Mr. Frank P. Wilkins, one of the engineers working on the project, that the dynamic conditions might be rather different from the sLatic conditions. These could be readily observed if the light source of the comparator were replaced with a strobo-scopic light. The arrangement shown in Figure 1 was set up with the lamp housing of a Type 648-A Strobolux replacing the incandescent lamp. The convex glass was removed from the Strobolux lamp housing and a piece of heavy paper substituted. This had a 23^" hole cut in the center to admit light

Figure 1. View of the tachometer mounted in the contour comparator with Strobolux lamp above.

to the condensing lens, while shielding the ground glass screen from direct light. It was recognized that this arrangement would pass only about 10% of the available light into the system.

The tachometer sample was then rotated at a speed of 2500 rpm, and the Type 631-B Strobotac, which controlled the Strobolux, was set to flash at a speed only a few rpm less than this. The picture shown in Figure 2 illustrates the image which appeared, rotating slowly, on the ground glass screen. A cyclic motion of the spring-loaded follower bearing against the face of the sliding collar was immediately seen. This appeared to be caused by a very slight wobble on the face of the collar, although the condition had not been noticed on a static check when the spring surrounding the shaft was extended. Only a little less obvious was a small erratic lateral motion of the shaft which appeared to take place slowly under the effect of the light from the Strobolux. It w as also possible to observe and to measure the very small variations in the radial displacement of the three weights.

Two separate improvements in the tachometers were made possible or greatly .facilitated by the use of the Strobolux with this arrangement, j The first was the elimination of a small amount of bouncing of the spring-loaded follower against the face of the rotating sliding collar. Before the investigation this had been manifested only as a very small and erratic motion of the pointer on the dial at medium high speeds.

The second improvement was effected by experimenting with the ball bearings on the spindle and observing the results. This enabled the engineers to fix tolerance standards for these bearings that will eliminate the small lateral motion of the spindle.

The engineers on this project felt that the results were quite satisfactory but decided to reduce the loss in the strobo-scopic illumination by constructing a small holder and reflector for the Strobolux lamp which could concentrate a greater part of the available light into the small condensing lens of the contour comparator. This will produce an even sharper and brighter image of the sample permitting greater accuracy in the measurements.

Figure 2. View of the image appearing in the ground glass screen of the comparator.

Figure 2. View of the image appearing in the ground glass screen of the comparator.

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