US3557603A - Shock machine - Google Patents

Abstract

A HIGH IMPACT SHOCK MACHINE FOR IMPARTING SHOCK OF A CONTROLLED AND REPRODUCIBLE MAGNITUDE TO TEST SPECIMENS. EQUIPMENT TO BE TESTED IS MOUNTED EITHER VERTICALLY OR HORIZONTALLY ON A TABLE WHICH ROLLS ON A TRACK WITH SOME CLEAR TRAVEL. A FREE-TRAVEL LIKE PERMITS THE TABLE TO REACH ITS PEAK VELOCITY BEFORE BUFFERS ARE ENGAGED TO SLOW THE TABLE WITH A CONSTANT DECLERATION.

Description

W. CARR Jan. ,26,

SHOCK MACHINE Filed March 26. 1968 3 Sheets-Sheet 1 INVENTOR WILL MM 5. 04m? IGV" HAMMER v2 f a Fla, 2.a

ATTORNEYS W. E. CARR SHOCK MACHINE Jan. 26, 1971 3 Sheets-Sheet 2 Filed March 26. 1968 DECELERATION 0 El A F/GI 4;

50 I00 FREQUENCY IN CYCLES PER SECOND O DEFLECTlON X 5 T N 3 E6 m LG U W QG Em u F R T. E W! 2 F F m M U E B T R A W E TM F m e w P m S T w M 3 P M H M W L 7 H G H G W R P S T c A P W 0 mom-Om O DEFLE CTION x -x O womOm INVENTOR WILLIAM E. CARR FIG. 9.

womom DEFLECTION x 1 &

ATTORNEYS Jan. 2 1971 Filed March 26. 1968 w. E. cARR snocx mcnnm [HAMMER IMPACT VELOCITY MEASURED ouooas use 133.: NI AJJOO13A TIME IN MILLlSECONDS 3 Sheets-Sheet 5 INVENTOR WILL/AM 5. CAR/? ATTOR NEYS United States Patent O 3,557,603 SHOCK MACHINE William E. Carr, Rockville, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Mar. 26, 1968, Ser. No. 716,156 Int. Cl. G01n 3/08 US. CI. 73-12 4 Claims ABSTRACT OF THE DISCLOSURE A high impact shock machine for imparting shock of a controlled and reproducible magnitude to test specimens. Equipment to be tested is mounted either vertically or horizontally on a table which rolls on a track with some clear travel. A free-travel link permits the table to reach its peak velocity before buffers are engaged to slow the table with a constant deceleration.

BACKGROUND OF THE INVENTION This invention relates generally to apparatus for producing a mechanical shock and more particularly to apparatus adapted to administer a mechanical shock to various types of electrical and mechanical equipment to simulate upper deck shock.

Shock tests on modern combatant ships have revealed that electronic equipment often becomes damaged. Since weapon systems depend on many items of complex electronic equipment, weapon delivery capability of the ship is sometimes impaired. In many cases, this impairment is due to failures in target detection and fire control systems, many of .which are located on or above the main deck. Realistic shock testing of these sensitive systems or its subcomponents could have revealed many of their shoc'k deficiencies, thus permitting corrective action to be taken prior to equipment installation on board ship. For optimum reliability against shock, it may be necessary for the test motion to match the in-service shipboard shock for the equipment in certain respects.

Practical considerations require keeping the number of test motions to a minimum. An examination of shipboard data indicates that at least two test motions are required to adequately simulate the shock environment throughout a ship; one, characteristic of positions at and above the main deck; the other, characteristic of positions at lower decks.

Prior art devices come close to simulating characteristics of positions at lower decks, however, modifications are necessary to approximate the motion characteristic of the upper region.

Items of shipboard equipment have failed during light or moderate shock tests even after being tested on prior art machines, undoubtedly because of the differences in the shock. Actually, the motion produced by these standard high-impact shock machines resembles that encountered at the lower levels of a ship. Because of the nature of this motion. it is often necessary to use resilient mounts to enable equipment to pass the shock tests. When the equipment is later installed high up in the ship, the mounts may be unnecessary, oreven harmful and actually aggravate the problem they were intended to solve. One way to' correct this situation is to require equipment to survive a shock test similar to that which it will experience aboard ship;

SUMMARY OF THE INVENTION The general purpose of this invention is to provide a shock tester that has all of the advantages of similarly employed shock testers and has none of the above described disadvantages. To attain this, the present inven- 3,557,603 Patented Jan. 26, 1971 ICC tion provides a pendulum hammer for imparting shock of controlled magnitude to a component test specimen mounted on a table. The table rolls on a track with a clear travel of one foot. A one inch free-travel link allows the table to reach its pea-k velocity before buffers are engaged to slow the table with a constant deceleration. The table has a vertical and a horizontal surface to which equipment may be attached and tested. The design permits a flexibility in features of the generated shock motion by allowing the substitution of impact springs and buffers of different characteristics.

A shock motion suitable for testing equipment for use at main deck level was derived from a comprehensive study of shipboard shock data from various types of ships. The motion of the present invention is simplified from the usual shipboard shock, but in important respects, it causes the same equipment response as measured at upper decks of ships during shock tests.

An object of the present invention is to provide a shock testing device.

Another object is to provide a shock testing device for imparting of controlled characteristics to a test specimen.

A further object of this invention is to provide a mechanical shock apparatus which has accelerations and motions of a character similar to those under which the equipment to be shock tested must operate.

A still further object of this invention is to provide shock apparatus which simulates the motion characteristic of positions at or above the main deck of a ship.

BRIEF DESCRIPTION OF THE DRAWINGS With these and other objects in view, as will hereinafter more fully appear, and which will be more particularly pointed out in the appended claims, reference is now made to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a plan view of a mechanical shock apparatus embodying the fundamental principles of this invention;

FIG. 2 is a side view of the apparatus, partly in section;

FIG. 3 is a longitudinal view of a detail of the apparatus;

'FIG. 4 is a graph showing the effect on shock spectra of changing the deceleration phase of the basic motion;

FIG. 5 is a schematic of the system during the acceleration phase;

FIG. 6 is a schematic of a mathematical model;

FIGS. 7, 8, and 9 show graphs of Force-Deflection relations of impact spring, equipment mounting spring and buffer. 1

FIG. 10 is a graphical comparison of a computed response and a measured response of the apparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will best be understood upon reference to FIGS. 1 and 2, by first considering the main elements of the apparatus which accomplish the functions mentioned above. Numeral 11 indicates the frame on which is carried .both a vertical weight 29 (dotted lines) and a pendulum hammer 12. A weight such as hammer 12, which is pivoted in a bearing 13 on a suitable extension 14 of the hammer 12, swings through an arc of substantially about the axis of bearing 13 from its uppermost horizontal position (phantom outline) to its position of impact against an impact spring 15. The frame 11 may be fabricated in the form of a steel weldment. An impact spring 15, which may be used in an embodiment utilizing the principles of this invention is a type MRL 711 liquid spring programmer, manufactured -by Monterey Research Laboratory, Inc. This particular liquid spring has a spring rate which is variable in four steps from 47,500 lbs/ft. to 52,500 lbs./ ft. depending upon the type of fluid selected. The maximum stroke of the spring is .815 inch. The impact spring 15 takes the impact of the hammer 12, swinging from the right as shown in FIG. 2, and smooths the impulse to produce a half-sine pulse of acceleration at the anvil table 16, upon which impact spring 15 is mounted.

Anvil table 16, the test platform, is mounted substantially horizontally on rollers 17, for horizontal movement within tracks formed in a suitable channel member 18. The anvil table 16 rolls on a track within channel members 18 with a clear travel of a suitable distance. In a particular embodiment the clear travel was one foot. Also mounted on the impact end of anvil table 16 is a freetravel link 19, details of which are shown in FIG. 3. The free-travel link 19 is connected through suitable linkage 21, to one end of a buffer 22 which is mounted at its other end to frame 11. A buffer 22 which may be utilized in an embodiment utilizing the principles of the invention is a type 309,554 manufactured by Houdaille Industries. This buffer is essentially a hydraulic damper. Escape ports for the fluid are arranged in a specific pattern so that, for a certain starting velocity from a specific position, a desired retarding force is attained.

A suitable mechanical lifting apparatus including gearmotor 23, pulleys 24 and 25, cable 26 and latch element 27 is provided to lift hammer 12 to the armed position as indicated in dotted lines in FIG. 2. The vertical slide 28 may be used in an alternative embodiment to guide a free-falling hammer 29 which is used to strike the top of an anvil plate 31. The anvil plate 31 is also loaded by impacting with a pendulum hammer in a manner similar to the swinging of hammer 12. A measure of damage potential was determined for the alternative embodiment and the preferred embodiment and the results are compared as described below. The damage potential of each of the embodiments was compared by subjecting a standard Navy receiver to a series of tests. Tests made on the preferred embodiment were more devastating to this receiver than those on the alternative embodiment using procedures specified for acceptance testing of shipboard equipment.

To test an item 20V of equipment along its vertical axis, the item 20V is attached by its base to the vertical test plate 16V of anvil table 16 (base in vertical plane) if it is intended for installation on the deck of a ship or on a shelf or table, and to the horizontal test plate 16H if it is intended for installation on a bulkhead or a partition.

To test an item 20H along its horizontal axis to correspond to the athwartship motion on shipboard, the procedure is reversed and the item 20H is attached to the horizontal test plate 16H if intended for installation on a deck or on a shelf, and attached to the vertical test plate 16V if intended to be mounted on a bulkhead in a ship. The equipment to be tested is installed on the test plate 16V or 16H by the same method used to install it aboard ship.

Velocity meters 32 and 33 and accelerometer 34 may be installed on both the anvil table 16 and the equipment to be tested. Velocity meters 32 and 33 measure the initial part of the motion whereas the accelerometer 34 measures the motion over the entire travel. Integration routines using a computer may be applied to determine a complete time history of the velocity and displacement. Recording may be carried out by magnetic tape to simplify data reduction.

An example of a velocity meter 32 which may be used in practicing the invention is one which operates on the generator principle and which requires no external power source. Such a velocity meter utilizes a magnet that is seismically suspended within an electrical coil wound over the meter body. A simple control circuit for calibrating and adjusting the output of the meter may be used.

4 A recorder which may be used is a Type CP- Ampex, Magnetic Tape.

An example of an accelerometer which may be used in practicing the invention is a Statham Model ASA-250- 500, which consists of unbonded strain gages suspending a seismic mass, connected to form a Wheatstone bridge, having four active arms. The accelerometer has a natural frequency of 1200 c.p.s.; frequency response of 0-800 c.p.s.; damping of 60% critical (fluid); sensitivity of 100 micro: v./v./g.; and a range of $250 g. The accelerometer may be used in conjunction with a simple control box which provides the necessary excitation voltages, calibration, and gain control.

The design of the present invention permits a flexibility in features of the generated shock motion by allowing the substitution of impact springs and buffers of different characteristics.

Formulas for computing the system parameters are described below:

A hammer weighing w lb. strikes one end of a linear spring of stiffness k lb./in. with a velocity V ft./sec. The other end of the spring is attached to a platform weighing W lb. which is free to move in. the direction of impact. The allowable compression of the spring is limited. The platform is accelerated from rest to maximum velocity V, in a given time I The platform has reached its maximum velocity when the hammer and impact spring separate. A constant retarding force is then applied which brings the platform to rest. The system parameters which will produce a particular motion may be evaluated. The motion is divided into two parts, the acceleration and deceleration phases.

ACCELERATION PHASE If it is desirable to use a single hammer, the system parameters which may be varied are W/w and k.

FIG. 5 is a free-body diagram of the system during the acceleration phase. The equations of motion for the system are:

where F is the spring force. The initial conditions are :I].,=V (depends on hammer drop height), y =0, and a5- =x =0. The force exerted by the spring while the hammer is in contact may be expressed by a half-sine pulse:

1 FnW s1n l) Equation 3 in Equation 1 and solving for y:

mgt t t i! (VA- 15 mg sin i where r is the ratio W/w. Similarly, substituting Equation 3 in Equation 2 yields:

By substituting Equations 4 and 6 in Equation 7, this expression may be written:

Since the hammer and the platform move the same distance at the end of the acceleration pulse, It may be evaluated by letting t equal t in Equations 4 and 6 and setting x equal to y:

V 1r 1 n at Substitute Equation 9 in Equation 8 and solving for t t 1rZ The expression for maximum platform velocity is obtained by substituting Equations 9 and and evaluating at [1.

)t= 1/ From Equations 3, 9, and

EXAMPLE If it is desired to accelerate a table weighing 560 lbs. to ft./sec. in 12 msec. and then arrest it at a constant 4g deceleration, the 560 lb. weight would conrespond to that for the anvil table 16 and a 400 lb. load. The hammer weight w and drop height h are 400 lbs. and 5 ft. respectively. Parameters in the analysis thus have the following values:

This is the maximum hammer velocity available. If more had been required, the table weight W would have had to be reduced.

From Equations 13 and 10 V,,t 1s 12 12 X 10- T 11' I DECELERATION PHASE After the platform has reached its peak velocity 5c it is decelerated by applying a constant force. The magnitude of this force is:

m =0.825 in.

wheer D is the deceleration in multiples of gravity.

The platform displacement at which the retarding force is first to be applied, x(t and the stopping distance are also of interest.

From Equations 6 and 9:

V n o+ The stopping distance x is expressed:

x, i nnx.

If it is desired to decelerate the table at 4g and continuing the previous example: From Equation 15 18X 12X 12X 10" and from Equation 16 (15 X 12 x -10.5 in.

The total platform travel would be about 11.6 in. No retarding force is applied during the first 1.08 in.

In an example of a preferred embodiment utilizing the principles of the present invention, the apparatus may be arranged differently from the free-body diagram in FIG. 5, as diagramatically shown in FIG. 6. The free-body diagram of FIG. 6 is characteristic of the apparatus shown in FIGS. 1 and 2. The free-body diagram in FIG. 6 comprises three rigid weights 35, two nonlinear springs 36 and a buffer 37, thus permitting a more realistic investigation for rigidly attached loads as well as an investigation of the effects of resiliently mounted loads.

The non-linearities in the force-deflection relations, as shown in FIGS. 7, 8, and 9 may be chosen to simulate the static characteristics measured for the impact spring 36, the buffer 37 and a typical resilient mounting 38. Important observed characteristics included a gradual stiffening of the impact spring with increasing compression, the initial slack in the buffer, followed by a constant limiting force, and the stiffening of resilient mounts as they approached a bottoming deflection in either compression or extension.

The condition for equipment which is rigidly mounted may be simulated by making the equipment weight W small (1 1b.), by making the force deflection rate for the equipment spring large (10,000 lbs./in.) and by adding to the weight of the table W an amount equal to the weight of equipment under study. Therefore, the motion which may be calculated for the table is the same as that for the rigidly attached load or equipment. The test may be coded for a computer from which the dynamic responses of the three rigid weights as a function of time may be calculated. The displacements, velocities, and accelerations ofthe weights, representing the hammer, table and equipment may be calculated for specific hammer impact velocities and for weights and spring parameters chosen to fit experimental conditions.

FIG. 10 compares the computed and measured response for an lb. test load resiliently mounted on typical, well known,shock mounts. The agreement, while not exact in detail, is close enough to suggest that the representation of the various springs in terms of their static characteristics and the representation of the important parts of the present invention in terms of rigid masses is valid for the major characteristics of the motion.

Shock spectracorresponding to the motion on the table may also be computed. Shock spectra represent the maximum response of massless oscillators of different frequencies to a particular shock motion. These spectra are computed from accelerations recorded during the different hammer impact velocities. The effect on shock spectra of changing the deceleration phase of the basic motion are shown in FIG. 4. The platform spectra were computed for the proposed test motion, in which the decelerations were 2g, 4g, and 6g, utilizing the free-body diagram shown in FIG. 6.

Therefore, shock apparatus has been described, which will simulate shipboard shock for testing equipment. The present invention includes components which may be easily replaced by others if different characteristics are desired.

Various modifications are contemplated and may obviously be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter defined by the appended claims, as only a preferred embodiment thereof has been disclosed.

What is claimed is:

1. Mechanical shock apparatus comprising:

means for supporting equipment to be tested and having a point of impact;

7 means for impacting and thereby means for supporting equipment; means for smoothing a shock impulse mounted at said point of impact;

means for decelerating said means for supporting equipments; and

means interposed between said supporting means and said decelerating means for enabling said supporting means to reach a peak velocity before said decelerating means is engaged to decelerate said supporting means; whereby said equipment to be tested is subjected to shocks characteristic of actual operational environment.

2. Apparatus of the character set forth in claim 1 in which said means for smoothing a shock impulse comprises:

a liquid spring programmer having an adjustable spring rate.

3. Apparatus of the character set forth in claim 1 in which:

said means for decelerating said equipment means comprises a hydraulic damper having an adjustable retarding force; and

said enabling means comprises a free-travel link.

4. Apparatus for simulating a particular shock environment to determine the response of an object thereto, said apparatus comprising:

a frame having at least two horizontal support tracks and having an upright frame member;

accelerating said a platform adapted to support equipment to be sub- 3 jected to mechanical shock and having a point of impact;

a hammer mounted on said upright frame member for pivotal movement through an arc terminating against the point of impact of said platform;

roller means mounted on said platform and adapted to roll in said horizontal support tracks thereby providing horizontal movement of the platform relative to the frame;

a liquid spring programmer having a variable spring rate mounted on said platform at the point of impact for smoothing a shock impulse created by an impact of said hammer;

a hydraulic damper having a variable retarding force attached to said frame at a first end; and

a free-travel link connected between said platform and a second end of said hydraulic damper;

whereby said platform is permitted to reach its peak velocity before said hydraulic damper is engaged to deceler- 20 ate said platform thereby simulating actual shock conditions.

References Cited- UNITED STATES PATENTS 3,226,974 1/1966 Bresk et al. 73-12 2,412,860 12/1946 Baudry et al 7312 3,200,634 8/1965 Rickards 73-12 3,282,088 11/1966 Joannou 73--12 0 CHARLES A. RUEHL, Primary Examiner

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