US3046789A

US3046789A - Differential pressure transducer

Info

Publication number: US3046789A
Application number: US660111A
Authority: US
Inventors: Harold O Boss
Original assignee: Borg Warner Corp
Current assignee: Borg Warner Corp
Priority date: 1957-05-20
Filing date: 1957-05-20
Publication date: 1962-07-31
Anticipated expiration: 1979-07-31
Also published as: CH367340A

Description

July 31, 1962 H. o. BOSS DIFFERENTIAL PRESSURE TRANSDUCER Filed May 20, 1957 l v i r 5 3,946,789 Patented July 31, 1962 3,046,789 DIFFERENTIAL PRESSURE TRANSDUQER Harold 0. Boss, Newport Beach, Calif., assignor to Borg-Warner Corporation, Chicago, 111., a corporation ofIllinois Filed May 20, 1957, Ser. No. 660,111 4 Claims. (Cl. 73-398) This invention relates to vibrating string transducer devicesfortranslating forms into electrical eticcts, and more particularly the invention relates to string or-wire vibration .and the tension is adjusted on the string or wire in order to change its frequency of vibration in accordance with a forcewhich it is desired to measure. Still more. particularly the invention relates tosuch string transducers which are fluid pressure operated for measuring the magnitudesof the fluid pressures applied thereto.

It has heretofore-been proposedto provide a string transducer utilizing a vibrating string effectively connected in a rigid frame with one end of the string being connected to a piston ordiaphragm that is responsive to fluid pressure so that the tension on the string is changed in accordance with changes in the fluid pressure.

. It is an object of the present invention to provide an improved transducer of the fluid pressure operated type which may be utilized for'rneasuringthe difference in two different fluid pressures in addition to being utilizable, if desired, for measuring the changes in pressure of a single source.

It is a more particular object of the invenn'on to provide an improved transducer having a fluid pressure responsive element connected to a vibrating wire and so arranged that fluid pressure may be applied to opposite sides of the element so that changes in tension on the wire due to displacements of the fluid pressure responsive element will be. in accordance with the differential of fluid pressures applied to the element.

It is another object of the invention to make the above mentioned fluid pressure. responsive element in the form of a diaphragm having a flexible Web portion substantialiyspaced from its center and to provide fluid pressure sealing diaphragms on both sides of the pressure responsive diaphragm which form fluid tight cavities on the opposite sides of the pressure responsive diaphragm for application of fluid pressure to the latter diaphragm. It is. contemplated that preferably the sealing diaphragms together with the pressure responsive diaphragm are rigidly connected to a pin fixed to one end of the vibrating string, so that the center portions of the three diaphragms may move together. It is also an object to provide flexible web portions in both of the sealing diaphragms located more'closely adjacent to. the centers of the diaphragms than the flexible web portion in the pressure responsive diaphragm, so that the sealing diaphragms are substantially inflexible opposite the major portion of the pressure responsive diaphragm located inward with respect to the flexible web portion of the pressure responsive diaphragm but the extreme inner parts of the sealing diaphragms tendto move along-with the pin.

It is another object of the invention to provide a calibrating or adjusting diaphragm also connected to the pin, the calibrating diaphragm likewise having a flexible web portion which may be made to a proper thickness so as to provide a desired overall elastic resistance to movement of the pin.

The invention consists of the novel constructions, arrangements and devices to be hereinafter described and claimed: forcarrying out the above stated objects and such other objectsas will be apparent: from the following description of a preferred form of the invention illustrated with referenceto the accompanying drawings, wherein:

v transducers in which the string or wire is maintained in v and FIG. 3 is an electrical diagram showing various electrical components connected to the transducer illustrated in FIGS. 1 and 2.

Like characters of reference designate like parts in the several views.

Referring now to FIGS. 1 and 2, the illustrated transducer'may be seen to comprise a frame 7, a vibratory wire 3 extending longitudinally of the frame 7 and end assemblies 9 and 10 holding the wire 8 in tension between them. The frame 7 carries a C'shaped permanent magnet 1 having its poles 12 and 13 parallel to and on opposite sides of the wire 8 so that the magnetic lines of force extend transversely across the wire 8.

The end assembly 9 for holding one end of the wire 8 comprises a ring 14 secured on one end of the frame 7, an inner annular support diaphragm 15 having a flexible web portion 16, a pin 17 extending through a central opening of the diaphragm 15 and fixed thereto and having one end of the wire 8 fixed thereto, an outer annular diaphragm 18 having a central opening through which the pin 17 freely extends and abutting against the diaphragm 15 at the outer rims thereof and having a flexible web portion 19, an insulating coating 20 around the peripheries and sides of the diaphragms 15 and 18, a sleeve 21 fixed to the ring 14 so as to hold the parts 15 and 18 with respect to the ring 14, an adjusting screw 22 extending through the diaphragrns 15 and 18, and a nut 23 on the screw 22 and fixed to the diaphragm 18. The adjusting screw 22 is in threaded relation with the diaphragm i5 and passes freely through the diaphragm 18 and has a different number of threads per unit length with respect to the diaphragm 15 as compared to the nut 23, so that a rotation of the screw 22 moves the diaphragms either together or apart. The above described end assembly 9 forms the subject matter of a copending application, Serial Number 662,065, filed May 28, 1957, now Patent No. 2,977,802.

The end assembly 1% comprises a calibrating. diaphragm 2d, a sealing diaphragm 25, a pressure diaphragm 26 and a round base 27 also constituting a sealing diaphragm, all of which are fixed together in a unitary assembly. The base 27 is provided with a circular groove 28 for receiving one end of the frame 7, as shown.

The base '27 comprises a hub 29 spaced from theouter remaining portion of the base 27 by means of a thin web portion 39. The sealing diaphragm 25 is a quite similar construction and has a hub 31 joined by. means of a thin web portion 32 to the outer portion of the sealing diaphragm. The sealing diaphragm 25 is provided with an annular cavity 33 in one face thereof adjacent the pressure diaphragm 26.

The pressure diaphragm 26 is provided with annular cavities 34 and 35 on opposite sides thereof which are closed respectively by adjacent faces of the sealing diaphragm 25 and the base 27. The cavity 35 is defined in its outer portion by means of a thin web 36 in the pressure diaphragm 26.

The calibrating diaphragm 24 is formed with a hub 37 connected by means of a thin web 38 with the outer portion of the diaphragm 24.

A pin 39 extends through coaxial central, openings in the calibrating diaphragm 24, the sealing diaphragm 25., the pressure diaphragm 26 and the base 27v and hasthe other end of the wire 8 fixed thereto. Thepin 39 is fixed particularly with respect to the hubs 37 and 29., and the hub 31 and the inner portion of the pressure diaphragm 26 are thus likewise fixed with respect to the. pin 39.

It will be apparent that the assemblies 9 and 10 rigidly fixed to and connected by means of the frame 7 constitute a framework for holding the Wire 8 in taut or stretched condition.

A fluid passage 48 is provided through the calibrating diaphragm 24 and the sealing diaphragm 25 to the cavities 33 and 34 which are in communication as shown. A fluid passage 41 is provided through the

diaphragms

24, 25 and 26 and the base 27 to the cavity in the diaphragm 26.

Referring to FIG. 3, the wire 8 is connected to an electrical system comprising a feedback amplifier 42. The amplifier 42 is connected to an electrical bridge network 43 having resistors 44 and 45 connected at one end to a lead 46. A static wire 47 is connected at one end to the resistor 44, and the wire 8 is connected at one end to the resistor 45, and the other ends of the

wires

47 and 8 are grounded.

An amplifier tube 48 having a cathode 49, a grid 58 and a plate 51 is controlled by the bridge network 43. The cathode 49 is grounded through a bias resistor 52 to a ground line 53. A transformer 54 couples the grid 56 to the bridge 43 and comprises a primary winding connected to the junction between the static wire 47 and the resistor 44 and to the junction between the vibratory wire 8 and the resistor 45. The transformer has a secondary winding56, one end of which is connected to the grid 59 and the other end of which is connected to the ground line 53 through a capacitor 57.

A positive plate voltage is maintained on the plate 51 from a battery 58 or other source of D.C. voltage, the battery being connected between the ground line 53 and a lead '59. The plate 51 is connected by means of a resistor 60 with the lead 59 and thereby with the positive terminal of the battery 58.

A second amplifier tube (or tube portion) 61 comprising a cathode 62, a grid 63 and a plate 64 is connected to be driven from the tube 48. The grid 63 is connected through a coupling capacitor 65 withthe plate 51. The cathode 62 is connected through a biasresistor 66 with the ground line 53, and the grid is also connected through a resistor 67 with a lead 68. A resistor 69 is in the lead 68 and connects one end of the secondary transformer winding 56 at its junction with the capacitor 57 with the remainder of the lead 68. The plate 64 is connected with the lead 59 and one side of the battery 58 through a resistor 70. The plate 64 is also connected to the lead 46, and a condenser 71 and a resistor 72 are in the line 46 between the bridge network 43 and the plate 64.

A third amplifier tube (or tube portion) 73 is driven from the tube 61 and comprises a cathode 74, a grid 75 and a plate 76. The cathode 74 is connected through a bias resistor 77 with the ground line 53. The grid 75 is connected through a coupling capacitor 78 with the plate 64 and is also connected to the ground line '53 through a resistor 79. A transformer 80 having a primary wind-- ing 81 and a secondary winding 82 is driven from the tube 73, the primary winding 81 being connected at one end to the plate 76 and at its other end to the lead 59. The plate 76 is connected to an output terminal 83 through a capacitor 84; and the terminal 83, together with a ground terminal 85 connected to the ground line 53, constitutes the output terminals of the amplifier 42.

An automatic volume control system is provided for the tubes 48 and 61 and comprises a bridgenetwork 86, the arms of which are formed by

crystal diodes

87, 88, 89 and 98. The transformer secondary 82 is connected to the junction between the

diodes

87 and 89 and also to the junction between the diodes 88 and 98.

The junction between the diodes 87 and 88 is connected to the ground line 53, and the junction between the

diodes

89 and 90 is connected to a resistor 91 and a capacitor 92 and also to the lead 68. The resistor 91 and the capacitor 92 are also connected to the ground line 53. The junction between the diodes 89 and 90' is also connected through the lead 68 to a resistor 93 and to a capacitor 94 connected between the leads 68 and 53.

It will be understood that the

cathodes

49, 62 and 74 7 the same static resistance as the wire 8, and the wire 47 is so'disposed that it is of substantially the same temperature as the wire 8. The static wire 47 may be in the same housing as the vibratory wire 8 but is outside of the magnetic field of the magnet 11. Inthe bridge network 86, each of the

crystal diodes

87, 88, 89 and 90 are so disposed as to conduct in a direction allowing current to flow from the junction between the

diodes

89 and 90 to. the junction between the diodes 87 and 88 so as to establish a negative polarity at the former junction.

A frequency meter 95 is connected to the

terminals

83 and 85 through a transformer 96 having an input winding 97 and an output winding 98.

In operation, the wire 8 is positioned in the magnetic field between the poles 12 and :13 and vibrates in a direction perpendicular to its axis at a natural frequency with a nearly sinusoidal motion, and the wire 8 generates an alternating voltage therein having the same frequency. The tension in the wire 8 is changed by different fluid pressures being applied to the end assembly 18 as will be described and the frequency of vibration of the wire 8 changes accordingly.

Sustained vibration of the wire 8 is obtained byemploying the wire 8 as an impedance element in a selfoscillating electrical circuit, including the bridge 43 and the amplifier 42.

The bridge 43 constitutes a filter which is used in a feedback circuit of the amplifier 42 to render the amplifier operable as an oscillator. one of whose arms contain the vibratory wire 8, constitutes a balanced Wheatstone bridge under static conditions, assuming no vibration in the wire 8. The upper resistors 44 and 45 are equal in value, and the static wire 47 has the same resistance as the vibratory wire 8, the wire 47 preferably being precisely like the vibratory wire 8 except that it is not positioned in a magnetic field.

As the network '43 is balanced when the wire 8 is'not V in motion, no current flows to the transformer 54 when the wire 8 is still. When, however, the wire 8 vibrates in the magnetic field between the poles 12 and 13 it develops therein a counter-electromoti've force in well known manner, thus developing an effective dynamic impedance greater than its static impedance which unbalances the bridge network 43 and allows current of this frequency to flow through the transformer primary "55.

The current from the dynamically unbalanced bridge network 43, flowing through the primary winding 55 of the transformer 54, deyelops a corresponding voltage in the secondary winding 56 which is impressed on the tube 48 which amplifies it producing a corresponding voltage applied to the tube 61 which likewise amplifies it. A portion of the plate energy from the tube 61 in the amplifier is fed back to the network 43 through the condenser 71 and the resistor 72 so as to provide an alternating current through the bridge network 43 and the vibrating wire 8 in the network 43. This alternating current is of the same frequency as the vibrating wire and maintains the wire 8 in sustained vibration. It is essential that the secondary winding 56 be so connected with the grid 50 of the tube 48 as to provide for a proper phase of voltage application to the tube 48 to produce sustained oscillation of the vibrating wire 8.

The output energy from the tube 61 on its plate 64 r The network 43,

of the wire 8 in the network 43. The

various resistors

52, 66 and 77 are used in accordance with standard practice for properly biasing the grids t}, 63 and 75 with

respect to'the cathodes

49, 62 and 74; the resistors 6t and 70' are plate load resistors; and the

resistors

67, 79 and 69 are leak resistors for the grids 50, 63 and 75, respectively.

An automatic volume control system is provided for the purpose of limiting the gain of the amplifier tubes 48 and 61 and thereby limiting the energy that is fed back through the lead 46 to the network 43 for the purpose of limiting the vibration amplitude of the wire 8 in the network 43. The amplitude of vibration of the wire 8 is limited so as to avoid slight changes in the frequency of vibration of the wire 8 which would occur with unduly large amplitudes of vibration. Since the wire 8 is in a constant magnetic field of the magnet 11 and is maintained in vibration by passing through it an alternating current whose amplitude is limited by an automatic volume control, the amplitude of vibration of the wire 8 is limited. The wire 8 has therefore a definite vibration frequency for each adjusted tension of the Wire 8 and insures the maintenance of this frequency after adjustment.

The automatic volume control system includes the network 86 of

diode rectifiers

87, 8, 39 and 96 which are in the form of -a Wheatstone bridge constituting a full wave'rectifier circuit. The output of the transformer 80 energized from the plate 76 of the tube 73 is applied across one diagonal of the bridge network 86, namely, to the junction between the

diodes

87 and 89 and to the junction between the diodes S3 and 9t Rectified output of the bridge network 86 is applied between the lines 68 and 53, with the negative output being applied to the line'68. The rectified voltage output of the network 86 is filtered by the

resistors

93 and 69 and by the

capacitors

94 and 57 which are disposed circuitwise with respect to each other in accordance'with conventional practice. The resistor 91 and the capacitor 92 form the conventional load for the

diodes

87, 88, 39, and 99.

The output signal from the amplifier 42 is taken from the plate 76 of the tube 73 and is'passed through the capacitor 84 to the output terminal 83. The alternating voltage developed between the

terminals

83 and 85 is impressed across the primary 97 of the transformer 96,

and the voltage developed across the secondary 98 is impressed upon the frequency meter 95 which interprets the quantity to be measured in terms of the variation in frequency.

' The wire 8 is at all times under tension, and the assembly 16 regulates and changes this tension in accordance with various fluid pressures applied to the assembly 10, and the frequency of vibration of the wire 8 may be read from the frequency meter 95 so as to thereby determine the values of fluid pressure. The assembly may be under the influence of one or two fluid pressures applied singly indifierent ways or may be under the influence of two difierent fluid pressures applied simultaneously so that in the latter case the frequency read at the meter 95 indicates the diflerential between the two fluid pressures.

A source of fluid pressure may be connected to the assembly '10 through the fluid inlet passage 4% in communication with the cavities 33 and'34, and the fluid pressure in these cavities will move the portion of the pressure diaphragm 26 disposed inwardly of the thin web portion 36 to the right as seen in the drawing. Such movement of the inner portion of the diaphragm 26 is allowed by flexing of the web portion 36 and causes corresponding movement of the hubs 31 and 29 of the diaphragms 25 and 27 as well asof the hub 37 of the calibrating diaphragm 24, inasmuch as all of these hubs as well as the inner portion of the diaphragm 26 are fixed with respect to the pin 39. Such movement of the

hubs

29, 31 and 37 are respectively allowed by the thin flexible web portions 3%, 32 and 33 of the diaphragms 27, 25,

wire 8 so as to lower its frequency of vibration, and

this changed frequency may be read on the frequency meter 95.

If fluid pressure is applied to the passage 41 instead of the passage 4%, fluid pressure is applied instead within the cavity 35 and causes a flexing of all of the

web portions

36, 36, 32 and 3S"'so that the inner portion of the diaphragm 26 and the-hubs 29, 31' and 37, together with the pin 39, are moved in the opposite'direction, namely, to the left as seen in FIG. 1. This movement has the effect of increasing the tension of the wire 8 to raise its frequency of vibration, and this changed frequency may be read on the frequency meter to thereby determine the value of the fluid pressure applied to the passage 41.

It will be noted that the cavity 35 has approximately the same inner and outer diameters as the combined cavities 33 and 34. Therefore, the same fluid pressure as applied to both may be expected to have about the same effect in moving the pin 39.

If two sources of fluid pressure are respectively simultaneously applied to the passages tfl and 41, the fluid pressure in the passage 40 produces a force, as before, on the diaphragm 26 tending to move the pin 39 and connected inner parts of the

diaphragms

27, 25 and 24 to the right, and the fluid pressure applied tothe passage 41 is effective, as before, tending to move the pin 39 and connected parts to the left. The actual movement of the pin 39 in one direction or the other is determined by the difierence between the pressures applied to the

passages

40 and 41, so that the difierential of the pressures is effective to move the pin 39 in one direction or the other to increase or decrease the tension on. the wire 8. The frequency of vibration of the wire 8 is accordingly changed, and the changed frequency may be read on the frequency meter 95 which thus indicates the pressure differential between the fluid pressures applied to the

passages

40 and 41.

7 It will be noted that the flexible web portion 36 of the diaphragm 26 is located considerably farther outward radially than the :web portions 32 and 30' of the diaphragms 25 and-27, so that the fluid pressure in the cavity 35 or in the cavity 34 acts between a substantially rigid portion of the diaphragms 25 and 27 and the movable inside portion of the diaphragm 26, radially inward of its flexible web portion 36; and the portion of the diaphragm 26 within the web portion 36 is caused to move and has a moving force on it tending to move the pin 39 in one direction or another. The web portions Wand 32 of the diaphragms 25 and 27 simply allow corresponding movement of the hubs 29 and 31. The purposes of the diaphragms 25 and 27 are simply to provide a seal with respect to the diaphragm 26 and provide a rigid surface opposite the surfaces of the inside portion of the diaphragm 26 within its flexible web portion 36, the diaphragms 25 and 27 sealing the

cavities

33, 34 and 35 so that fluid under pressure cannot escape from them.

Calibration of the transducer, that is, the adjustment to provide a definite amount of axial movement of the pin 39 for a certain pressure differential applied to opposite sides of the diaphragm 26 is accomplished by thinning, as by lapping, the web portion 38 of the adjusting diaphragm 24. The thinner that the web 38 is made, the greater becomes its flexibility, and the less is its tendency to elastically hold the pin '39 and connected parts against axial movement.

The initial tension of the wire 8 is adjusted by means of the adjusting screw 22 which is in screw threaded engagement with both the inner diaphragm 1'5 and the nut 23 fixed to the outer diaphragm 18. As has been described, the threaded portions in engagement with these two parts differ in the number of threads per unit length,

and this difference may be small, such as 79 threads per inch on one portion and 80 threads per inch on the other portion with the threads of both portions being of the same hand. A rotation of the screw 22 in one direction causes the center portions of the diaphragms and 1% p to be moved-toward each other, and since the web port-ion 19 in the diaphragm 18 is'thinner and more resilient than the web portion 16 in the diaphragm 15, the diaphragm 13 flexes to a greater extent than does the diaphragm 15. The pin 17 is fixed with respectto the diaphragm 15, and the relatively small flexing ofthe diaphragm 15 therefore carries the pin 17 along with it and varies the tension in the wire 8 to a small extent. As will be appreciated, a relatively small movement of the pin 17 is sufiicient for giving a relatively large change in vibration frequency of the wire 8, and hence this fine adjustment for tension has been provided.

My improved transducer advantageously measures the difierence between two applied fluid pressures. The pressure diaphragm 26 is advantageously sealed by means of 1 efiect of the diaphragm web portions provides a high spring rate any influences on the pin 39 and connected parts due to forces applied to the instrument are minimized. Inasmuch as my improved instrument has no metallic bellows, lever arms, or pivots for moving the pin 39 attached to the vibrating wire 8, the transducer is capable of successfully operating under vibration conditions.

I wish it to be understood that my invention is not to be limited to the specific constructions and arrangements shown and described, except only insofar as the claims may be so' limited, as it will be understood to those skilled in the art that changes may be made without departing from the principles of the invention.

I claim: I

1. A pressure sensitive device adapted to be connected to one end of a vibrating wire of a transducer comprising a firstsealing diaphragm having a first sealing diaphragm hub portion, a first sealing diaphragm web portion adjacent said first sealing diaphragm hub portion, a first sealing diaphragm outer portion and an opening therein for receipt of a pin; a second sealing diaphragm having a second sealing diaphragm hub portion, a second sealing diaphragm web portion adjacent said second sealing diaphragm hub portion, a second sealing diaphragm outer sure diaphragm having a pressure diaphragm hub portion, a cavity portion defining a first pressure responsive side of said pressure diaphragm and a second pressure responsive side of said pressure diaphragm adjacent said pressure diaphragm hub portion, a pressure diaphragm web portion adjacent said cavity portion, a pressure diaphragm outer portion, and an opening therein for the receipt of a pin; said pressure diaphragm being interposed intermediate said first sealing diaphragm and said second sealing diaphragm such that said pressure diaphragm hub portion engages said first sealing diaphragm hub portion and said second sealing device hub portion and such that pressure diaphragm 'outer portion engages said first sealing diaphragm outer portion and said second sealing diaphragm outer portion; first means communicating fluid intermediate said first sealing diaphragm and said first pressure responsive side of said pressure diaphragm; second means communicating fluid intermediate said second sealing diaphragm and said second pressure responsive side of said pressure .portion and an opening therein for receipt of a pin; a presjacent said first sealing diaphragm hub portion, a first sealing diaphragm outer portion and an opening therein for receipt of a pin; a second sealing diaphragm having a second sealing diaphragm hub portion, a second sealing diaphragm web portion adjacent said second sealing diaphragm hub portion, a second sealing diaphragm outer portion and an opening therein for receipt of a pin; a

pressure diaphragm having a pressure diaphragm hub portion,.a cavity portion defining a first pressure responsive side of said pressure diaphragm and a second pressure responsive' side of said pressure diaphragm adjacent said pressure diaphragm hub portion, a pressure diaphragm web portion adjacent said cavity portion, a pressure diaphragm outer portion, and an opening therein for the re ceipt of a pin; said pressure diaphragm being interposed intermediate said first sealing diaphragm and said second sealing diaphragm such that said pressure diaphragm hub portion engages said first sealing diaphragm hub portion and said second sealing device hub portion and such that pressure diaphragm outer portion engages said first sealing diaphragm outer portion and said second sealing diaphragm outer portion; first 'means communicating'fluid intermediate said first sealing diaphragm and said first pressure responsive side of said pressure diaphragm; second means communicating fluid intermediate said second sealing diaphragm and said second pressure responsive side of said pressure diaphragm; a pin adapted to be connected to said vibrating wire extending through and connected to said first sealing diaphragm hub portion, said pressure diaphragm hub portion and said second sealing diaphragm hub portion; and a calibrating diaphragm having a calibrating diaphragm hub portion in engagement with said second sealing diaphragm hub portion, a cali-' brating diaphragm outer portion in engagement withsaid second sealing diaphragm outer portion,.and an opening for receipt of said pin. V

3. A device in accordance with claim 1 wherein said first sealing diaphragm web portion and said second sealing diaphragm web portion are located radially outwardly a first distance from said pin and wherein said pressure diaphragm web portion is located radially outwardly a second distance from said pin; said first distance being less than said second distance.

4. A device in accordance with claim 1 wherein said first means communicating fluid passes through said second sealing diaphragm and said pressure diaphragm and said second means communicating fluid passes through said sealing diaphragm.

References Cited in the tile of this patent UNITED STATES PATENTS 1,371,242 Hopwood Mar. 15, 1921 1,624,659 Eynon Apr. 12, 1927 1,992,343 Ahnstrom Feb. 26, 1935 2,050,674 Stover Aug. 11, 1936. 2,079,069 Johnson May 4, 1937 2,265,011 Siegel' 'Dec. 2, 1941 2,306,137 Pabst et a1. Dec. 22, 1942 2,319,011 Meredith 'May 11, 1943 2,689,943 Rieber Sept. 21, 1954 FOREIGN PATENTS 729,894 Germany Dec. 19, 1942