712,481. Testing balance of rotary bodies. CHURCHILL & CO., Ltd., V. L. Sept. 12, 1951 [Sept. 12, 1950], No. 22388/50. Class 106 (2) A balance testing apparatus for rotary bodies comprises means for furnishing an electrical pick-up wave form in response to oscillations of the body or its supporting means, a mounting for said means adapted to permit selective stimulation thereof by oscillations occurring in a particular direction or mode, an electronic amplifier converting the pick-up wave into a wave form having a recurrent and substantially vertical wave front in constant phase relationship to a series of points on the pick-up wave form each occurring at the same time stage in respective cycles of the pick-up wave form, and an illumination device controlled by said wave fronts to cause intermittent illumination in the light of which a reference mark on or rotating with the rotary body may be viewed stroboscopically. The pick-up means, Fig. 2, comprises a horse-shoe magnet 26 on a series of telescoping tubes 27, 28 and 29. The tube 27 is secured by two resilient metal strips 23, 24 to a housing 10, these strips permitting the tubes 27, 28 and 29 and the magnet 26 to vibrate in a direction along the axis of the tubes. Secured to the tube 27 are two permanent magnets 20, 21 disposed one on each side of a stationary armature 18 surrounded by a winding 19. Any vibration of the tube 27 will thus induce an alternating voltage in the winding 19 and this voltage is applied, through amplifying means, to control the flashing of a discharge lamp. In alternate constructions the pick-up device may be of the piezoelectric or capacitance type. The housing 10 is pivoted on a series of telescoping tubes 14, 15 secured to a base member 13. The pick-up device can be used in connection with a part of the supporting structure of a rotary body under test which is vibrating horizontally, and in this case the assembly 25 extends laterally as shown and is connected to the vibrating body by the magnet 26. For co-acting with vertically moving parts of the supporting structure, a clamping nut 12 is temporarily slackened and the housing 10 is turned until the assembly 25 extends vertically. The telescoping tubes are then adjusted until the magnet 26 can engage beneath the vertically moving part. The wave form from the pick-up device will be of a form similar to that illustrated by the curve V1G, Fig. 1. The amplifier is arranged to convert this wave form into positive- and negative-going spikes as shown by the wave form V3G, Fig. 1. The latter wave form, when applied to the triggering grid of a gas containing discharge lamp, initiates discharges between the main electrodes. The instant of each discharge occurs at a precisely defined instant in relation to the cycle of oscillation set up by the unbalanced rotary body. Correcting static out-of-balance conditions.- The method is described with reference to a flat disc having a single sector of higher specific gravity than the remainder. The disc is supported in a manner permitting it to oscillate in its own plane. In stage (a), Fig. 4, a reference mark R is noted to be in the position shown when observed stroboscopically whilst rotating. The overweight sector is, for example, S (the position of which is not known). The disc is then stopped and a balance weight B is secured to it at any position. The disc or wheel is then brought up to speed and observed stroboscopically to see if the reference mark R has shifted as is the case in Fig. 4(b). The cause of this displacement in that the addition of the balancing weight B has caused the effective overweight sector S of the wheel to change. The test is now continually repeated, the body B being moved angularly around the disc each time it is stopped until the reference mark R is brought back to its original position, Fig. 4(c). The balance weight B will then coincide with the overweighted sector S. Finally, by varying the magnitude of the weight B or by moving it diagonally across the disc a balance can be obtained, such a balance being indicated when the flashing lamp fails to flash at regular intervals and the mark R thereby fails to appear to be fixed in space. Correcting dynamic out-of-balance conditions. -This type of balance is exemplified by a flat wheel of uniform thickness and specific gravity having secured thereto on opposite faces and at opposite ends of a diameter a pair of equal weights. The wheel, when in motion, thus tends to wobble. To correct such wheel for balance it is rotated in a structure permitting it to oscillate about an axis through and parallel to its own plane. At resonant frequency let the reference mark R as viewed stroboscopically, appear as shown in Fig. 5(a). S1 represents the unknown position of an out of balance mass on the rear face of the wheel and S2 represents the corresponding out of balance mass on the front face of the wheel. The wheel is then stopped and two equal masses B1, B2 are arranged on the wheel, one on each face and diagonally opposite to one another. The wheel is then brought up to speed and again observed stroboscopically. If the reference mark R has shifted to a different position such as shown in Fig. 5(b) the test is continually repeated with the weights, rotated angularly to different positions with respect to the wheel until the reference mark R is brought back to its original position, Fig. 5(c). When the condition illustrated in Fig. 5(c) is attained the balance weights will coincide with the sectors S1 and S2. To achieve a final dynamic balance the value of both balance weights should be varied by the same amount and if it is found that neither reduction nor increase results in attainment of a dynamic balance, both balance weights should be transferred to opposite faces of the wheel or body, in which case it will be generally found that the reference mark R has shifted to its diametrically opposed position, Fig. 5(d). A progressive reduction in value of both weights is then made until a balance is obtained, such a balance being indicated when the flashing lamp fails to flash at regular intervals, and the mark R thereby fails to appear to be fixed in space.