US2820342A - Hydraulic mechanism - Google Patents

Description

Jan.- 21, 1958 R. M. HEINTZ HYDRAULIC MECHANISM 3 Sheets-Sheet 1 Filed Oct. 18, 1954 FEHGUDUEI lllllllll! Q INVENTOR. Pam M fli/A/rz Armin 5Y5 3 Sheets-Sheet 3 IN V EN TOR. 4294; M; HEM/772 Jan. 21, 1958 R. M. HEINTZ HYDRAULIC MECHANISM Filed Oct. 18. 1954 Hi /I/ muw w I nited States atent HYDRAULIC MECHANISM Ralph M. Heintz, Los Gatos, Calif., assignor, by mesne assignments, to Textron Inc., Providence, R. I., a corporation of Rhode Island Application October 18, 1954, Serial No. 462,684

10 Claims. (Cl. 60-52) This invention relates to a hydraulic system, and more particularly to a hydraulic system for oscillating a device about a fixed axis.

A hydraulic system of the type disclosed herein is particularly useful in controlling the motion of a radar reflector wherein it is desired to oscillate the reflector back and forth on either side of a central axis of oscillation and at the same time be able to change the direction of said central axis. The invention is not limited to such use. however, and is equally desirable for producing oscillating movement of many other devices.

The principal object of this invention is to provide a hydraulic system for producing oscillatory movement of a device connected thereto.

A further object of this invention is to provide a hydraulic system for producing oscillatory movement of a device connected thereto wherein the axis of said oscillatory movement may be changed.

A still further object of this invention is to provide a hydraulic system for producing oscillatory movement of a device connected thereto wherein either the length of are through which the device oscillates or the central axis about which the device oscillates, or both, may be changed.

Another object of this invention is to provide a novel form of variable flow pump.

Still another object of this invention is to provide a novel form of variable flow pump wherein two opposed pistons may be varied in phase relation with each other from to 180 to vary the output of the pump from maximum output to zero output.

Various other objects of the present invention will be apparent from the following description taken in connection with the accompanying drawings, wherein:

Fig. 1 is a schematic view of a hydraulic system embodying the principles of my invention.

Fig. 2 is a composite view showing, in a schematic view, the positions of two of the pump pistons 135 out of phase, and a graph illustrating the effective displace ment of the pump in this phase relationship.

Fig. 3 is a view similar to that of Fig. 2 showing the pistons 45 out of phase.

Fig. 4 is a schematic view of the pump portion of the system shown in Fig. 1, and shows the pump arranged as a variable flow pump.

Referring now to the drawings, there is disclosed an oscillating vane-type motor generally indicated at 11). The motor comprises a housing 11, shown in phantom lines, a movable vane 12 and a solid section 13. Hydraulic power for oscillating the vane 12 is supplied by two hydraulic lines 14 and 15 which connect through the solid section 13 to the areas on each side of the vane 12, it being obvious that when hydraulic fluid is forced into the motor through one of the lines, it is exhausted through the other.

A shaft 16 is connected to the vane 12 and extends upwardly out of the housing 11. A dish-shaped radar reflector 17 is fastened. to the upper end of the shaft 16 2 by a bracket 18. A similar shaft 19 extends downwardly from the lower end of the housing 11 and is connected to a limit control mechanism to be described presently.

The two hydraulic lines 14 and 15 connect the motor 10 with a double-acting pump mechanism, generally indicated at 20, line 14 being connected to a pump head 21 and line 15 being connected to the opposite pump head 22. The pump heads 21 and 22 are connected to opposite sides of a pump housing 23. Four pistons numbered 24 to 27, inclusive, are reciprocally mounted within the housing 23. Pistons 24 and 26 are connected together to reciprocate along the same axis but face in opposite directions. Similarly, pistons 25 and 27 are connected together and reciprocate along the same axis, said axis being spaced from but parallel to the axis of reciprocation of pistons 24 and 26. The pistons 24 and 25 are adapted to change the volume of the chamber 28 enclosed by the pump head 21, and the pistons 26 and 27 are adapted to change the volume of the chamber 29 enclosed by the pump head 22. The pistons 25 and 27 are driven through a scotch yoke assembly, generally indicated at 311, and the pistons 24 and 26 are driven by a scotch yoke assembly indicated at 31. The details of this type of mechanism are well known and no further description thereof is deemed necessary herein. Sufficieth to say that such mechanism will cause the pistons to reciprocate with pure simple harmonic motion.

The drive mechanism for the scotch yoke assemblies 31) and 31 comprises an electrical motor 32 having a drive shaft 33. A gear train, schematically illustrated by spur gears 34 and 35, connects the shaft 33 to a shaft 36 which in turn is the drive shaft of the scotch yoke assembly 3%) driving pistons 25 and 27. A right-hand helical gear 37 is fixed to the shaft 36 and meshes with a companion gear 38 fixedly mounted adjacent one end of a countershaft 39. A left-hand helical gear 40 is fixedly mounted adjacent the other end of the countershaft 39 and meshes with a companion gear 41 fixedly mounted on the drive shaft 42 of the scotch yoke assembly 31. From the above description it is seen that the pistons 25 and 27 are driven directly from the motor 32, and that the pistons 24 and 26 are driven through the two sets of helical gears 37, 38, 40 and 41 by means of the countershaft 39.

The countershaft 39 is suitably mounted for both rotational and axial movement in journals 43 and 44, one adjacent each end of the shaft. Two spaced flanges 45 and 46 are formed adjacent one end of the countershaft 39. A yoke assembly 47 embraces the shaft and is confined between the flanges 45 and 46. The yoke 47 is rigidly fastened to one end of a lead screw 48. The lead screw 48 is slidably mounted in two spaced webs 49 and 50 suitably fastened to the frame of the device. An internally threaded nut 51 engages the threads on the lead screw 48 and is confined between the spaced webs 49 and 50. The outer surface of the nut 51 is provided with gear teeth 52. A step down gear train comprising gears 53 to 56, inclusive, operatively connects the nut 51 with a reversible electric motor 57.

When the countershaft 39 is shifted axially by actuating the motor 57, the oppositely disposed helical gear sets cause the shaft 42 to rotate relative to the shaft 36 and change the phase relation between the two pairs of opposed pistons. In other words, pistons 24 and 25 can be made to reciprocate in phase wherein their effects will be combined, or 180 out of phase wherein their effects will cancel each other, or at any point in between wherein their effects will be partially combined.

The effect of this change of phase relation may best be appreciated in connection with Figs. 2 and 3. In Fig. 2 the relative positions of pistons 24 and 25 are shown at the start of a cycle with the pistons out of phase.

For convenience, the start of a cycle has been chosen when piston 24 is at bottom dead center, or in relation to the position shown in Fig. 1, when piston 24 is all the way to the right. The right portion of Fig. 2 shows a graph plotted with displacement against the angle of rotation of the drive shaft. The curve 59 represents the displacement of the piston 24, and the curve 60 represents the simultaneous displacement of the piston 25. The solid line curve 61 shows the net combined displacement of curves 52 and 60. Since the pistons reciprocate with simple harmonic motion, the displacement curves 59 and 60 are sine curves, and the combined displacement curve 61, being the addition of two sine curves, is also a sine curve. The elfective pump displacement, with the pistons 135 out of phase, is equal to the distance between the high and low points of the curve 61 and is shown by the double-ended arrow 62.

Referring now to Fig. 3, the pistons 24 and 25 are shown only 45 out of phase and the displacement curves for pistons 24 and 25 are shown at 63 and 64, respectively. The combined curves give an efiective displacement curve 65. As explained above, the effective displacement of the pump is represented by the distance between the high and low points of the curve 65 and is represented by the double-ended arrow 66. From a comparison of Figs. 2 and 3, it is evident that as the phase angle between the pistons 24 and 25 is increased, the effective pump displacement is decreased until zero effective displacement is reached when the pistons are exactly 180 out of phase. Conversely, maximum pump displacement is reached when the pistons are exactly in phase. Since, as explained above, pistons 24- and 25 move with simple harmonic motion and their individual displacements are sinusoidal, the effective displacement is sinusoidal throughout the complete range of phase relationship therebetween.

While only the left side of the pump 20 has been included in the above discussion of the phase relationship, it is obvious that the same relationship exists between the pistons 26 and 27 since the piston 26 is attached to the piston 24 and, similarly the piston 27 is attached to the piston 25. It is obvious, therefore, that pistons 26 and 27 act in exactly the same manner as pistons 24- and 25 but are 180 out of phase therewith.

Considering now the operation of the device thus far described and assuming the pistons are reciprocating more or less in phase, when the pistons move to the left, fluid is forced outwardly from the pump chamber 28 through the conduit 14 and into the motor on one side of the vane 12, causing the vane to move counterclockwise. Fluid from the other side of the vane is permitted to return through the conduit to the pump chamber 29. When the pistons move to the right, the converse is true and the vane 12 is rotated clockwise. A change in the phase angle between the pistons in the pumps will result in a change in the effective displacement of the pumps, in the manner described above, and hence will result in a change in the amplitude of oscillation of the vane 12 and the radar reflector 17, or other device attached thereto, without changing either the period of oscillation or the central axis of oscillation.

Since a system of this type requires that the entire system be full of fluid at all times, and since there is always the possibility of leakage at various points in the system, a provision is made to provide make-up fluid to the system. The make-up fluid is supplied from a suitable high pressure fluid supply 67 through two branch conduits 68 and 69 connected to ports 79 and 71, respectively, in the pump housing 23. Port 70 is so located as to be momentarily exposed each time piston 24 reaches bottom dead center and port 71 is similarly disposed with respect to piston 27.

It is evident that in a system of this type fluid might, over a period of time, leak past vane 12 from one side of the system to the other and cause the central axis of oscillation to drift to one side or the other. A pair of double-ported solenoid operated valves 72; and 73 are provided to correct for such drift. A conduit '74- connects the high pressure fluid source 67 with the valve 72 through a branch line 75' and with the valve 73 through a branch line '76. Valve '72, when actuated, allows fluid to flow from the conduit 75 through a conduit 7'7 into the main operating conduit 14. Simultaneously, the valve 72 provides communication with a bleed conduit '73 connected to the main operating conduit 15 and a return conduit 79 which carries the fluid back to the sump (not shown). Similarly, the valve 73 is adapted, when actu ated, to permit fluid to be forced from the conduit 76 into the main conduit 15 through a conduit 3 3 and simultaneously bleed the main operating conduit 14 through conduits 81 and 82. It may be seen that when the 1e '72 is actuated, the central axis of oscillation of the vane 12 will be shifted counterclockwise, and when the valve 73 is actuated, the central axis of oscillation will be shifted clockwise.

Automatic means are provided for actuating the valves '72 and '73 in response to a shift in the central ..-:i:; of oscillation including a pulley 33 connected to the shaft 19 extending downwardly from the motor 10. A drive cable 84 connects the pulley S3 with a similar pulley fixedly mounted on a control shaft 36. A first control disc 87 is fixedly attached to the control shaft 36 and oscillates therewith. The first control disc 87 is provided with an electrical contact 38 which projects outwardly from the periphery thereof. An orienting switch bracket 89 is rotatably mounted on the shaft 86 adjacent the first control disc 87, and is provided with a handle by means of which its orientation about the axis of the shaft 36 may be changed. The orientation bracket is adapted to be locked in various adjusted positions by any suitable means (not shown). A pair of stationary electrical contacts 91 and 92 are adjustably mounted on the orienting bracket 89. In operation, the contacts 91 and are adjusted so that their included angle is slightly larger than the angle through which the motor 10 is oscillating.

A second control disc 93 is fixedly attached to the shaft 86. An outwardly projecting electrical contact 94 is mounted on the periphery thereof. Two stationary contacts 95 and 96 are fixed to the frame of the device and are adapted to be contacted by the contac 2 The contacts )5 and 96 represent a maximum travel limit switch, and their included angle is slightly greater than 170", since it is considered that 170 is the maximum extent of travel of the device disclosed herein.

A source of electrical power, indicated at 97 as a battery, is connected on one side by a wire 93 and branch wires 99 and 100 to the control discs 87 and by sliding contacts 101 and 102, respectively. The other side of the battery 97 is connected by a wire 1G3 and. a branch wire til t to one terminal of the solenoid valve 7?. d, similarly, by a branch wire 105 to one terminal of the solenoid valve 73. The other terminal of the scold valve '72 is connected by a Wire 1% and br' 1 wires 1W7 and 188 to the stationary terminals 91 and respectively. Similarly, the other terminal of the solen= va ve '73 is connected by a wire 109 and branch \Wes ill) and ill to the stationary terminals 92 and 96, respectively.

From the above described circuits, it can be that if the vane 12 drifts in a clockwise direction, the contact 83 will touch the contact 91 and complete the circuit to the solenoid valve 72 and cause actuation thereof. When the valve 72 is actuated, fluid is forced into the conduit M and simultaneously bled from the conduit 15 to shift the vane 12 counterclockwise and correct the drift. The operation of the limit control disc Q3 is similar to the action described above, said disc 93 being effective as an ultimate stop, if the orienting bracket is shifted too far or if the contacts 91 and 92 should fail. It is to be understood that should the drift occur in a counterclockwise direction, the solenoid valve 73 will be actuated. If the'phaseangle between the pairs of pistons is changed to change the amplitude of oscillation, the drift control contacts 91 and 92 must be adjusted accordingly so that their included angle again is just slightly larger than the amplitude of oscillation.

Referring now to Fig. 4, the pump 20 is the same in all respects with that shown in Fig. 1 and the parts thereof are given the same reference numerals. The changes shown therein, which are necessary to adapt the pump structure for use as a double-acting variable flow pump instead of as a surge pump, are as follows: The make-up ports 70 and 71 are omitted or closed by suitable plugs (not shown). Surge conduits 14 and 1.5 are replaced by output conduits 112 and 113 communicating with pump chambers 28 and 29, respectively, which join to form an outlet conduit 114. Conduits 112 and 113 are provided with outwardly opening check valves 115 and 116, respectively. Inlet conduits 117 and 113 are each connected with a conduit 119 connected to a suitable sump or source of fluid 120. Conduits 117 and 118 are provided with inwardly opening check valves 121 and 122, respectively, and communicate with pump chambers 28 and 2?, respectively. It is obvious that one piston of each pair could be eliminated, i. e. pistons 26 and 27, without changing the function of the invention but merely changing the pump to a single-acting instead of doubleacting pump.

Fig. 4 also illustrates a modified form of means for shifting the countershaft 39 axially to change the phase relation between the two pairs of pistons. In the modification, a yoke lever 123 is pivoted adjacent its mid-point to the frame of the pump, as indicated at 124. The yoke 126 on one end of the lever 123 is confined between the flanges 45 and 46 on the shaft 39 and a handle 125 is provided at the other. It should be understood that the means shown for shifting the countershaft 39 are illustrative only and that the electrical means shown in Fig. 1, the manual means shown in Fig. 4, or other types of shifting devices may be used with either of the disclosed embodiments. It should further be understood that suitable locking means (not shown) is provided to hold the countershaft 39 in any adjusted position.

From the foregoing description, it may be seen that the present invention provides a relatively simple, inexpensive and reliable hydraulic system for causing oscillation of a device. The present invention further provides means whereby the amplitude of oscillation, or the axis of oscillation, or both, may be changed at will by relatively simple manipulations which are adapted to be made manually or are capable of being remotely controlled.

The invention further provides a novel, relatively inexpensive and reliable variable output pump for which there is a multitude of uses outside of the specific embodiment disclosed herein.

While I have shown the preferred form of my invention, it is to be understood that various changes may be made in its construction by those skilled in the art without departing from the spirit of the invention as defined in the appended claims.

Having thus described my invention, what I claim and desire to secure by Letters Patent is:

1. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor, means connecting the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pairs of pumps being adjustable, and means to vary the phase relation between the pairs of pumps to vary the combined gutput of said pumps connected to one side of said motor and the pumps connected to the other side of said motor and hence the extent-of the path of travel of said motor and the device connected thereto, and means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of said path of travel.

2. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor, means connecting the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pairs of pumps being adjustable, and means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of said path of travel.

3. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor, and the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between said pairs being adjustable, means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of said path of travel, means to correct for drift of said central axis of oscillation comprising a first electrical contact operatively connected to said device to oscillate through an arc equal to the are described by said device, a pair of stationary contacts located one just beyond each end of the normal arc of said first contact, and electrical circuits adapted to be selectively energized when said first contact touches either of said stationary contacts due to overtravel of said device, said electrical circuits being adapted when energized to activate said means for increasing the fluid volume between one side of said pumps and said motor and simultaneously decreasing the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation away from the stationary contact thus touched.

4. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor and the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between said pairs being adjustable, and means to vary the phase relationship between the pairs of pumps to vary the combined output of said pumps connected to one side of said motor and the pumps connected to the other side of said motor and hence the extent of the path of travel of said motor and the device connected thereto, means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto Without varying the extent of said path of travel, means to correct for drift of said central axis of oscillation comprising a first electrical contact operatively connected to said device to oscillate through an arc equal to the are described by said device, a pair of stationary contacts located one just beyond each end of the normal arc of said first contact, and electrical circuits adapted to be selectively energized when said first contact touches either of said stationary contacts due to overtravel of said device, said electrical circuits being adapted when energized to activate said means for increasing the fluid volume between one side of said pumps and said motor and simultaneously decreasing the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation away from the stationary contact thus touched.

5. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor, means connecting the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pairs of pumps being adjustable, means to vary the phase relation between the pairs of pumps to vary the combined output of the pumps connected to one side of said motor and the combined output of the pumps connected to the other side of said motor and hence the extent of the path of travel of said motor and the device connected thereto, means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of said path of travel, said means for varying the phase relation between the pairs of pumps comprising motor means drivingly connected to one pair of pumps, a countershaft, a pair of helical gears connecting said one pair of pumps with said countersh'aft, a pair of helical gears of opposite hand connecting said countershaft to the other pair of pumps, and means to shift said countershaft axially.

6. A mechanism such as set forth in claim wherein each of said pairs of pumps is driven with simple harmonic displacement whereby the effective combined displacements of said one pump of each pair and said other pump of each pair will each vary sinusoidally and wherein said effective displacements will be opposed in phase.

7. A mechanism of the class described comprising a device mounted for oscillator movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor, means connecting the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pumps of each pair being adjustable, means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of said path of travel, said means for varying the phase relation between the pairs of pumps comprising motor means drivingly connected to one pair of pumps, a countershaft, a pair of helical gears connecting said one pair of pumps with said countershaft, a pair of helical gears of opposite hand connecting said countershaft to the other pair of pumps, and means to shift said countershaft axially.

8. A mechanism such as set forth in claim 7 wherein each of said pairs of pumps is driven with simple harmonic displacement whereby the effective combined displacements of said one pump of each pair and said other pump of each pair will each vary sinusoidally and wherein said effective displacements will be opposed in phase.

9. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor and the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pairs of pumps being adjustable, means to increase the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto Without varying the extent of said path of travel, means to correct for drift of said central axis of oscillation comprising a first electrical contact operatively connected to said device to oscillate through an are equal to the arc described by said device, a pair of stationary contacts located just beyond the normal arc of said first contact, and electrical circuits adapted to be selectively energized when said first contact touches either of said stationary contacts due to overtravel of said device, said electrical circuits being adapted when energized to activate said means for increasing the fluid volume between one side of said pumps and said motor and simultaneously decreasing the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation away from the stationary contact thus touched, said means for varying the phase relation between the pumps of each pair comprising motor means drivingly connected to one pair of pumps, a countershaft, a pair of helical gears connecting said one pair of pumps with said countershaft, a pair of helical gears of opposite hand connecting said countershaft to the other pair of pumps, and means to shift said countershaft axially.

10. A mechanism of the class described comprising a device mounted for oscillatory movement, a reversible hydraulic motor connected to the device for oscillating the same to either side of a central axis of oscillation, means for causing periodic reversing operation of the motor comprising a hydraulic circuit including the motor and two pairs of single-acting pumps, means connecting one pump of each pair to one side of said motor and the other pump of each pair to the other side of said motor, said pairs of pumps being opposed in phase, the phase relationship between the pairs of pumps being adjustable, means to vary the phase relation between the pairs of pumps to vary the combined output of the pumps connected to one side of said motor and the combined output of the pumps connected to the other side of said motor and hence the extent of the path of travel of said motor and the device connected the eto, means to inc ease the volume of fluid between one side of said pumps and said motor and simultaneously decrease the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation of said motor and the device connected thereto without varying the extent of the path of travel, means to correct for drift of said central axis of oscillation comprising a first electrical contact operatively connected to said device to oscillate through an are equal to the are described by said device, a pair of stationary contacts located just beyond the normal arc of said first contact, and electrical circuits adapted to be selectively energized when said first contact touches either of said stationary contacts due to overtravel of said device, said electrical circuits being adapted when energized to activate said means for increasing the fluid volume between one side of said pumps and said motor and simultaneously decreasing the volume of fluid between the other side of said pumps and said motor to shift the central axis of oscillation away from the stationary contact thus touched, said means for varying the phase relation between the pairs of pumps comprising motor means drivingly connected to one pair of pumps, a countershaft, a

pair of helical gears connecting said one pair of pumps with said countershaft, a pair of helical gears of opposite hand connecting said countershaft to the other pair of pumps, and means to shift said countershaft axially.

References Cited in the file of this patent UNITED STATES PATENTS 971,358 Clawson Sept. 27, 1910 1,418,616 Boisset June 6, 1922 1,452,501 Gasser Apr. 24, 1923 1,897,075 Sampson Feb. 14, 1933 1,939,886 Ferris Dec. 19, 1933 2,172,103 Kotaki Sept. 5, 1939 2,327,787 Heintz Aug. 24, 1943 2,369,867 Sprake Feb. 20, 1945

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