Rotating Steel Mirrors—Failure and SuccessBerlyn Brixner Citation: Review of Scientific Instruments 36, 1297 (1965); doi: 10.1063/1.1719881 View online: http://dx.doi.org/10.1063/1.1719881 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/36/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in HIFU treatment of liver cancer—Successes and failures J. Acoust. Soc. Am. 113, 2280 (2003); 10.1121/1.4780571 Successive reflections of a light beam on a pair of plane mirrors Am. J. Phys. 58, 565 (1990); 10.1119/1.16450 The rotating mirror Phys. Teach. 19, 253 (1981); 10.1119/1.2340771 Tandemmirror success leads to expanded MFTF Phys. Today 33, 17 (1980); 10.1063/1.2913784 A Theory of Successive Rotational Transitions in Crystals J. Chem. Phys. 21, 176 (1953); 10.1063/1.1698593

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

137.189.170.231 On: Fri, 19 Dec 2014 17:14:23

THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 36, NUMBER 9 SEPTEMBER 1965

Rotating Steel Mirrors-Failure and Success*

BERLYN BRIXNER

University of California, Los Alamos Scientific Labaratory, Los Alamos, New Mexico

(Received 18 May 1965; and in final form, 9 June 1965)

Four main causes of rotating steel mirror failure-excessive vibration, excessive speed, hydrogen embrittlement, and defective steel-are discussed and illustrated. Performance characteristics are given for a successful, 10 000 rps, 25X38 mm mirror drive.

A SUCCESSFUL turbine drive for the 25X38 mm mirror used to obtain full frame size at the highest

possible speed in a rotating-mirror framing cameral is shown in Fig. 1. This successful drive was achieved only after a series of failures caused by two then unknown condi­tions: (1) the safe-speed limit for a 25 mm sq mirror face, the largest size we had built for maximum performance, and (2) a mirror-rotor shape which vibrated so violently that we could not restrain it within tolerable limits. The mirror shown in Fig. 1 has now been run a total of 185 times at 10 000 rps without evidence of deterioration. The graphs of Fig. 2 show the performance characteristics. The mirror speed, gas consumption, and time to reach 95% of maxi­mum speed are each given as a function of air pressure and helium pressure. Since much of the art of building such drives is empirical, the purpose of this paper is to pass on to others the lessons learned from our failures.

Until we had gone too far in certain directions, we did not know how far we could go. Over the years we have made a number of successful drives for a variety of sizes of high-speed turbine-driven steel rotating mirrors2 for fast framing and sweeping-image cameras. The drive which caused trouble was intermediate between two successful designs, one for lOX 17.5 mm (24000 rps) and the other for 40X60 mm (4500 rps). In these designs, as in the faulty ones, the mirror was the face of a prism with a triangular cross section; the axis of rotation was parallel to the longer

FIG. 1. The successful 10 000 rps turbine drive for the 25X38 mm mirror.

* Work done under the auspices of the U. S. Atomic Energy Com­mission.

1 B. Brixner in Proceedings of the Sixth International Congress on High-Speed Photography, edited by J. G. A. de Graff and P. Tegalaar (Tjeenk Willink, Haarlem, The Netherlands, 1964), pp. 93-100.

2 B. Brixner, Rev. Sci. Instr. 30, 1041 (1959).

1297

face dimension. The rotating mirrors are all made of a hardened low-alloy silicon steel which has consistently demonstrated its reliability. At a hardness of Rockwell C-56 to C-57 the steel has an estimated tensile strength of 240000 psi at the elastic limit and perhaps 300000 psi ultimate strength, with appreciable ductility remaining. The mirrors are the polished faces of dynamically balanced steel prisms that are rotated by turbines powerful enough to drive the mirrors to destruction by excessive speed.

We attribute our failures in making this particular mirror to four main causes (not counting human errors): excessive vibration, excessive speed, hydrogen embrittlement, and defective steel. Each of these four conditions causes the elastic tensile strength of the steel to be exceeded repeatedly somewhere in the rotor.

EXCESSIVE VIBRATION

Vibration was a major cause of difficulty. The trouble­some vibration is normal to the axis of rotation and is at

014r--'--~~r.--'--r--'--'---r-, [:J .\ 0.. II / 'J) 13

>i \ SPEED --CON~'K -­TI~lE -----~ 12 V

/\ AIR

~

"' 11 '" o ~10

.. ::: E U 6

Z o ;::: c..

§ 4

'" z 8 3

Jl t.l

HF,LIUM

/ \ I \

\

\ \~

\ \

\

\ \ ,

, / x .... J

.,

/"<.....-/" "'--

,/ /

c.. O~~--~~~~ __ L--L __ ~~L-~ 1'1) 0 10 20 30 40 50 60 70 80 90

PRESSURE, psi

FIG. 2. Performance characteristics of the 10 000 rps turbine drive.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

137.189.170.231 On: Fri, 19 Dec 2014 17:14:23

1298 BERLY~ BRIXNER

FIG. 3. Rotating-mirror fracture caused by excessive vibration from runs at 8500 rps.

an amplitude maximum when the speed of rotation is the same as one of the mirror vibration frequencies. Figure 3 shows a chrome-plated and polished 25X25 mm mirror which fractured from excessive restrained vibration after 9 runs at maximum speeds of 8000 to 8500 rps. Two of the three edges of this mirror were fractured when the spinning mirror struck the drive case after breaking loose from the shafts. There is no fracture through the central section of the mirror body. Both hubs and one bearing shaft are fractured through. Although the other shaft was strained much beyond the elastic limit, it shows no obvious fracture. In addition to the fracturing, there was excessive bearing wear. This wear had been observed even during tests at lower speeds. It was presumed to result from the small amount (0.13 mm) of radial clearance allowed for vibration of the bearing journals, which were supported on small neoprene O-rings of 1.78 mm diametral cross section. After the mirror failures the journal bearings were found to be greatly enlarged. (The amount of clearance, which was insufficient for the 25 X 25 rom mirror, had been sa tisfactory in many earlier drives.) To decrease the bearing wear we first doubled the radial clearance, and later we also doubled the O-ring diametral cross section. But although we thus decreased the bearing wear, we at the same time increased the amplitude of vibration to a point which would impair the optical functioning of the camera and reduce the effi­ciency of the turbine drive. The calculated vibration fre­quencies for this mirror are 2150, 8250, and 39900 cps. At the time of this failure the calculation program and the vibration measurement technique described below had not yet been developed. Therefore we do not have the meas­ured frequencies or estimates of the relative amplitudes of vibration for this destructive test.

The vibration problem arose from the shape of the mirror we were trying to use and from the high speed at which we had hoped to operate it. In the beginning we had hoped to rotate a 25X25 mm mirror at 12000 rps. We knew from earlier experience that rotating-mirror vibration needed to be controlled by supporting the bearings on neoprene O-rings. But the problem of controlling vibration had always been a minor one.2 The range of vibration had al­ways been controlled by adjusting the size of the O-ring,

which damps vibration as well as sealing the pressurized oil. Fortuitously the vibration amplitude was relatively small in the speed ranges of most of the mirrors we used. Our methods of control were mainly empirical. We knew that the stress caused by vibration of the rotating mirror is greatest at the bearing shaft and minimal near the rotat­ing-mirror center and that the maximum stress is usually at the junction of the hub and shaft. The floating but damped journal bearings restrain the shaft from vibrating. We estimated the vibration forces from the bending-stress limits of the shafts and held vibration within tolerable limits by adjusting empirically the sizes and shapes of the mirror body, the hubs, and the bearing shafts. The rotors are subjected to maximum vibration stress for approxi­mately 5 sec per run.

The initial drives had 25 X 25 mm mirror faces because this was the maximum size needed for the camera. But after three drives exploded we admitted defeat. To change the vibration characteristics we increased the mirror size to 25X38 mm to obtain a rotating-prism shape similar to the shapes used in the successful designs mentioned in the introduction. The 25 X 25 mm mirror had a persistent tend­ency to vibrate at great amplitude (0.38 mm or more). This amount of vibration would impair the optical functioning of the camera and would reduce the efficiency of the turbine drive. The 25X38mm shape had a much smaller amplitude of vibration.

Because of the difficulties we had in constructing the present drive, we realized that we needed a technique which would give precise measurements of the frequencies and the relative amplitudes of mirror vibration. To facili­tate the choice of a favorable design at an early stage, we also needed a program for predicting the vibration fre­quencies from the rotating-mirror design parameters. In the past it had been postulated that the mirror vibrated at the fundamental or at a harmonic frequency and exhib­ited a maximum amplitude at the equivalent speed. These frequencies were measured by observing the speeds at which excessive bearing wear occurred or atwhich excessive driving gas was used. However, this crude measurement could be made only if the vibration amplitude was relatively large. A satisfactory measurement technique has been developed by Max A. Winkler3 and a calculation program by Burton Wendroff.4 The calculation program gave data which pre­dicted approximately the measured vibration characteris­tics of all the designs we have tested.

EXCESSIVE SPEED

Figure 4 shows the fragments of a 25X25 mm rotating mirror which exploded when the rotor's safe maximum

3 M. A. Winkler, Rev. Sci. Instr. 35, 790 (1964). 4 B. Wendroff, "Computing the Critical Frequencies of Stepped

Shafts," Los Alamos Scientific Laboratory Report LAMS-2954 (1963).

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

137.189.170.231 On: Fri, 19 Dec 2014 17:14:23

ROTATING MIRRORS 1299

speed was repeatedly exceeded. In theory we knew that a mirror would explode if the centrifugal force repeatedly exceeded the elastic tensile strength of the steel. 5 In prac­tice we could only get an accurate estimate of the steel's elastic limit and fatigue characteristics through trial and error. The effect on rotating-mirror life of fatigue caused by stresses near but below the elastic limit is virtually unknown. Framing camera users often want to operate the rotating mirrors at the highest safe speed. The definition of the highest safe speed is determined empirically and changes with experience.

The maximum safe speed is one which creates stresses well below the elastic limit of the steel we use so that fatigue will not be a cause of failure. The stress caused by centrifu­gal force is maximum at the rotating-mirror center and minimum at the periphery. To evaluate mirror stress, we assume that the centrifugal force on the center of mass is restrained by the tensile strength of the central cross sec­tion. If the steel is stressed beyond the elastic limit, the mirror surface is permanently distorted and its utility in the camera optical system lowered, especially in sweeping­image cameras. After the rotating mirror has been re­peatedly overstressed, it is likely to explode, presumably because minute fractures are formed, and these reduce the strength of the steel. Loss of magnetism is a sensitive in­dicator of overstressing. (We magnetize the steel mirrors to obtain a rotating magnetic field for speed measurement.)

In short, we did not know how much stress was enough until we had applied too much. The mirror prism fractured at an estimated 10 000 rps while slowing after the 16th run at maximum speeds ranging from 11 800 to 12 200 rps. The highest speed (12200), we now estimate, causes stresses well above the elastic limit of this steel. This mirror had an interior hardness of Rockwell C-S6, which gives it opti­mum strength and at the same time considerable duc­tility.6 Since this mirror had not been chrome-plated,

"

FIG. 4. Rotating-mirror fracture caused by repeated stress beyond the elastic limit from runs at 12200 rps.

5 T. Lyman, editor, Metals Handbook (American Society for Metals Metals Park, Ohio, 1964), 8th ed., Vol. 1, pp. 217-224. '

6 F. B. Lounsberry, "Ludlum 609 Blue Sheet," Allegheny Ludlum Steel Corporation, Pittsburgh (1948).

FIG. 5. Rotating-mirror fracture caused by repeated stress beyond the elastic limit from runs at 10 500 rps.

hydrogen embrittlement could not have caused the failure. The mirror body fractured into three approximately equal pieces. (The middle piece was sectioned.) The hubs and shafts were all fractured, which suggests that vibrational stresses were also present. (As mentioned in describing the vibration failure, this mirror size exhibited a resonant vi­bration peak of great amplitude at ~8000 rps.) The ap­pearance of the bearings and associated turbine parts (not shown) suggests momentary violent eccentric rotation. The sharp mirror edges were probably eroded before the pieces embedded themselves in the steel case. This probability suggests that the shafts and hubs fractured before the mirror body fractured and that the mirror edges were eroded by being rubbed against the inside of the steel case. The main cause of the explosion, then, seems to have been repeated operation at speeds which caused stresses con­siderably exceeding the steel's elastic limit. The next failure was caused by repeatedly exceeding the elastic limit by a small amount.

Another example of too much stress appears in Fig. S. Here we have the remains of a 25X38 mm mirror which exploded at about 10 500 rps on the 57th run at that speed. Before that the mirror had been run 60 times at about 10 200 rps. Presumably operation at the higher speed pro­duced stresses slightly greater than the elastic limit of the steel. When the stress was repeated many times, a minute fatigue fracture was produced. This minute fracture in the central region of the body spread with explosive violence through the adjacent highly stressed region. When the fracturing reached regions of less stress, the effect was less violent. The figure shows the fragments as they appeared after the fracture. The smaller pieces come from the center and the larger from the periphery. The condition of the hubs and shafts suggests that this time the effect of vibra­tion was at a minimum, though some is always present. One hub and shaft is intact. The other has been split longi­tudinally, presumably because of the explosive violence of

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

137.189.170.231 On: Fri, 19 Dec 2014 17:14:23

1300 BERLYN BRIXNER

FIG. 6. Rotating-mirror fracture caused by hydrogen embrittlement from runs at 10 800 rps.

the central fractures combined with some vibrational force. Since the steel had not yet been chrome-plated, hydrogen embrittlement was not a factor. The brittle fracture was caused by the interior hardness of the mirror (RC-60). At this hardness the elastic limit is very high, but there is little remaining ductility. The calculated vibration fre­quencies for this mirror are 2330, 7110, and 35 900 cps. Vibration measurements showed a peak at 7300 rps. Vibra­tion corresponding to the 2330 cps mode was too feeble to be measured.

HYDROGEN EMBRITTLEMENT

Figure 6 shows a chrome-plated and polished 20X 20 mm mirror which apparently fractured as a result of hydrogen embrittlement. Hydrogen embrittlement is caused by ad­sorbed hydrogen in highly stressed steel. 7 As a result of this embrittlement, steel will fracture at stresses far below the elastic limit. Our chrome-plated mirrors become hydro­genated because the plating process produces atomic hy­drogen which is readily adsorbed by steel. The adsorbed hydrogen will diffuse out at room temperature at an un­determined slow rate. At 260°C it will diffuse out rapidly, and the chrome-plated mirror blank will be otherwise un­affected by this heating. The chrome plate inhibits diffu­sion, but the amount of tolerable hydrogen residue has not been established. Studies of the hydrogen diffusion prob­lem7 suggest that steel held at 260°C for 5 h will be freed from deleterious amounts of hydrogen. This is the proce­dure we subsequently followed. Probably we had not had trouble with hydrogen embrittlement before because most of our drives have large safety factors and because the plated mirrors had been stored for weeks or months before they were used. The hydrogen may have been spontane­ously removed before the steel was highly stressed.

The drive which exploded had been tested with 125 runs at 10 800 rps prior to chrome plating and polishing. After

7 M. Smi lows' i, Hydrogen in Steel (Pergamon Press Ltd., London, 1 %2), pp. 200-257.

it was plated, it was not dehydrogenated. It fractured on the 9th run at 10 800 rps. This series of 9 runs represents only 9 cycles at maximum centrifugal stress. Accompany­ing these runs were perhaps 100 000 cycles of vibration stress producing large stresses of unknown values. The in­terior hardness was RC-57. Another mirror of the same dimensions was given 92 runs at 10 800 rps before plating. This one was dehydrogenated after plating and was given 338 runs at 10 800 rps as a reliability test before delivery. Since delivery it has been run in the camera hundreds of times at about 10000 rps. Although there has been exces­sive bearing wear in both drives, an indication that vibra­tion is disturbing smooth operation, the drive continues to perform well, which suggests that the vibration is within tolerable limits. There is a vibration peak at 4100 rps, but vibration is thereafter of very low amplitude for the remainder of the speed range. The calculated vibration fre­quencies for this mirror are 3960, 13 800, and 45 800 cps.

DEFECTIVE STEEL

There is no way to prove whether defective steel played a part in the series of explosions with which we were plagued in making the 25X38 mm mirror. Among the five mirrors which exploded during the 13 years preceding the current difficulties, only one was known to be caused by defective steel. In that case a gross defect caused a single fracture which exposed the flaw for macroscopic and micro­scopic examination. In the present case there was minute fragmentation in the central region of the mirrors; co~se­quently there was a possibility of defective steel but httle possibility of analysis. Defective steel is so obvious a cause of rotating mirror failure that we have developed reliable techniques for testing and selecting bar stock, for heat treatment, and for finishing the hardened blank. Primarily we are on guard against a slag inclusion. Since the strength of slag is nil, a slag inclusion acts as a void which may pos­sibly create a hoop-stress condition having only half the strength of a solid piece.

ACKNOWLEDGMENTS

Thanks go to Max A. Winkler for developing the vibra­tion measurement technique and making performance measurements, to William J. Wynne for fabricating and assembling the components, to the Shops Department for precision machine grinding of the steel mirrors, to Charles G. Phelps for mirror balancing and polishing, and to V. J. Stephens and Burton Wendroff for developing the calcula­tion program.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

137.189.170.231 On: Fri, 19 Dec 2014 17:14:23