US2243603A - Pump servomotor with rotary control and torque motor - Google Patents

Description

May 27, 1941.

. H. F.1MacMlLLlN `a1-m..r l2,243,603

- PUMP SERVOMOTR WISTH 4l'iO'JJARY CONTROL'AND TORQUE MOTOR F'filedmaron 15. 193s s sheetssheet 1 May 27, '1941.

H, F.l MacMlLLlN TAL 2,243,603

PUMP SERVOMOTOR WITH ROTARY CONTROL AND TORQUE MOTOR.

File'd March 15. 1939 5 Sheets-Sheet 2 RA E! A www 5y May 27, 1941"- H.F. MacMlLLl'N Erm. 2,243,503 PUMP SERVOMOTOR WITH ROTARYl CONTROL'AND AIORQUE MOTOR Filed March 15, `1939 '5 Sheets-Sheet 3 A roR/vfr Brugg Patented May 27, 1941 UNITED STATES PATENT OFFICE PUMP SERVOMTOB WITH 'ROTARY CON- i TROL AND TORQUE MOTOR `Howard F. MacMillin and Ira B. Lawyer, Mount Gilead, Ohio, assignors to The Hydraulic Development Corp. Inc., Wilmington, Del., a corporation of Delaware Application March 15, 1939, Serial No. 262,004

15 Claims.

This invention relates to hydraulic machinery, and in particular, to control vdevices for hydraulic pumps.

One object of this invention is to provide means for controlling the stroke and therefore the delivery of a reversible variable delivery pump including a pilot-valve member movable inra rotary direction within the servomotor piston to control the direction of motion of this piston.

Another object is to provide a servomotor of the type described above, whereinthe pilot valve is rotated by ya reversible torque motor, thereby providing a smooth rotary action without the jerking motion arising from the use of the solenoids commonly .employed to actuate servomotor controls.

Another object is to provide a servomotor with V 5- 5 1n Figure 2.

a rotary pilot valve for controlling' the direcy tionofmotionof the servomotor so that the servomotor is power-actuated in both directions of its motion rather than by power in one direction, and a compressed spring in the other direction.

Another object is to provide a pump flow-control shifting servomotor for a reversible variable delivery pump with a rotary pilot valve actuated by a reversible torque, motor and a safety centering device for automatically shifting the pump flow-control member or shift ring to its neutral position, in response to a failure of power for actuating the pump.

Another object is to provide a hydraulic pump and press control circuit, including a rotary pilot valve responsive to the motion of the press platen to a predetermined position to actuate the pump servomotor so as to shift the pump flow-control member or shift ring to its neutral position, yet

to put the pump slightly upon stroke 'again in Figure 1 is a diagrammatic view. partly in section, showing a hydraulic press and circuit including a variable delivery pumphaving the rotary controlled vservomotor ofV this invention for shifting the pump. flow-control member, to-

gether with a safety centering devicefo'r placing f ,nected toy this flow-,control member.-

Figure `6 is a cross section taken along the line 5-6 in Figure 2.

Figure 'i is a left-hand end elevation of the pilot valve operating linkage, looking in the direction of the line'1-1 in Figure 1.

Figure 8 is an enlarged longitudinal ksection through the tonnage control valve for regulating the pressing force of the press.

Figure 9 is a wiring diagram showing the circuit control'motor |04 and switch |50.

GENERAL ARRANGEMENT In general, the variable delivery pump control arrangement of this invention consists of a reversible variable delivery pump having a flowcontrol member and a servomotor piston con- Within a bore in the servomotor piston is located a sleeve with ports and a rotary pilot valve member having a rotary valve head with oblique ends cooperating with these ports. The' rotary valve member is connected to a reversible torque electricmotor, the shaft of which is also connected to a linkage leading to a rock arm. The latter is engaged by the control rod of a hydraulic press so as to be rocked when the press platen reaches a collar on the control rod and lifts the control rod.

The flow-control member of the variable de- Y livery pump is also provided with an enlargement engaged on opposite sides by sliding collars which .are urged against this enlargement by springs.

In the event that a failure of power for Iche pump occurs, one of these springs will force the pump flow-control member to its neutral position, thereby acting as a safety arrangement. The weight of the press platen and plunger is therefore prevented from compressing the fluid beneath the plunger and operating the pump in a reversedireotion as a motor, thereby eliminating the danger of rapid coasting down of the press indr release and by-pass valve form no part of tuate the servomotor in one `direction, itfhas i been found necessary to applyaforce to actu' ate the servomotor in the other direction suf- I ficient to compress the spring .and Ithusfstore up the required energy. Where such a spring has been used it ha's also been necessary to make the. spring force considerably` greater than `the required force in order toinsure operation under all conditions, thuswasting powerv in overcoming this excess force. Hitherto, also, lwhere reciprocating pilot valve members have been employed in association with servomotors for shifting' pumpnow-control members, it has been necessary tot such valve members closely within their cylinders. As oil of rather high viscosityeis necessarily used kin such servomotors, it has been found diflicult to push the pilot valve member in an endwise direction without the employment of considerable force.

In the present invention, however, the use ofA a rotary pilot valve member enables the servomotor spring to be eliminated since the servomotor is shifted in both directions by pressure fluid under the control of the rotary pilot valve member. Much less forceis required to Operate the pilot valve member since it has been found to be much easier to rotate Ithe pilot valve member than to .reciprocate it in. an endwise direction, as in prior servomotors. This reduction of the force necessary to shift the pump ow-control member has alsoresulted in a 'greater ease of operation of the safety centering springs which n are provided to shift the pump flowcontrol' member to its neutral position, in the event of a power failure of the motor operating the pump. The rotary pilot valve member of the invention is also easily rotated bythe ,linkage with which it is connected to the press control rod in such a manner as 4to shift the pump flow-control rod to put the pump on just sufcient stroke to take care of leakage and maintain the press platen in its retracted position. The reversible electric .torque motor is also suitable for continuous stalled service as it is of a high torque, low amperage type. The pilot valve, however, can be rotated independently of the torque motor,l either manually or lby the press platen, since the press control rod is ordinarily provided with a manually operated lever for such manuaif control,

Hydraulic 'circuit Referring to the drawings in detail, Figure 1 shows a hydraulic press clrcuitcontaining a var iable delivery pump I, arranged to supply pressure fluid to operate a press 2 Ihavin'ga surge the present invention, and are disclosed in the Hubbert Patent No. 2,061,658, issued November A surge'valve' 5 in the top of .themain cylinder of the press 2 serves to admit fiuidto the 'upper part thereof to facilitate the gravitational descent of the main plunger until the platen of the press encounters resistance. The details of the surge valve 5 likewise form no part of the present invention, and are disclosed in the Ernst Patent No.

1,892,568, issuedDecember 27, 1932. The surge valve 5, as disclosed in the above-mentioned patent, 'is adapted to be forcibly opened upon the return stroke of the press so as to release uid from theupper'part of the main cylinder directly into the surge tank 3.

The circuit shown in Figure 1 likewise contains a pilot pump 6 for supplying pressure fluid to .the

servomotor. of the variable delivery pump I, and

preferably driven from the same shaft. The details of .the pilot pump 6 and its driving connection with the variable delivery pump I- form no part ofthe present invention, and are disclosed in the copendingapplication of Walter Ernst, Ser. No. 41,281, filed September19, 193,5.

The circuit of Figure 1 also contains arelief valve 1, the details of which form no part of the presentinvention, and are disclosed in the Ernst Patent No. 2,086,295, issued July s, 1937. Within the surge tank 3 are pressure relief valves 8 and 9, connected on opposite sides of the main cylinder and by-pass 4,. A check valve III is associated with the relief valve 9. The pressing force which the press exerts is controlled by a tonnage control valve I I, shown in longitudinal section in Figure 8.

. The press 2 contains a double-acting main plunger I2 attached to a platen I3, which has an arm I4 extending laterally therefrom. Collars I5 and I6 are mounted upon a control rod I 1, the collar I5 being engageable. with the platen arm I4 when the platen I3 reaches its retracted position. The control rod I1 is guided in its reciprocation. by brackets I8, one only beingy shown. The bracket I6- projects from. the main cylinder I9 of the press 2. The main cylinder I9 contains a bore 2li for the reciprocation of the main plunger I2, and at its opposite end is prolvided with ports 2| and 22, connected respectively to the lines 23 and 24 leading to the forward and return pressure connections 25 and 26 of the variable delivery pump I. A branch line 21 runs from the pipe 24 to the check valve I8, the

, pressure relief valve 9 and the main cylinder release and by-pass valve 4; likewise to the surge valve 5. From the main cylinder release and by-pass. valve 4 .the branch line 28 leads to the port 29 in the main cylinder I9, and to this branch line 28 is connected the pressure relief valve 8. 'I'he upper end of the' main cylinder bore 20 also, contains a port 30, from which the line 8| runs to the lower end of the tonnage control valve II. f From theA latter the line 32 leads to the surge tank 3.

The pilot circuit, including the pilot pump 6 and the pilot pressure relief valve 1, includes the line 83 running from the surge tank3 to the pilot pump 6. The line 34 leads from thev relief valve 1back to the surge tank 3, and the line 35 leads from the'pressure connection of the pilot pump l to the, casing 36 of the servomotor 31 of the variable delivery pump II The discharge or exhaust line 38 leads from the servomotor 31 back to the surge tank 3.

The reversible variable delivery pump I includes a pump casing 40 having a chamber 4| containing bearing pads 42, which slidably support and guide the pump shift ring or flow-control member 43. The latter is movable in opposite directions from its neutral position to alter the stroke of the pistons within the variable delivery pump, and thereby to alter or reverse the delivery of the reversible variable delivery pump I, as is well known to those skilled in the art.

Servomotor construction The casing 36 of the servomotor 31 (Figure 2) contains cylindrical bores 52, 53, 54 and 55 of de creasing diameters, together with annular recesses 56 and 51. At one end the casing 36 is fianged, as at 58, for attachment to the lefthand side of the variable delivery pump casing 40, and at its opposite end is anged as -at 59 to receive a head 60 bolted thereto. Within the servomotor casing 36 is a stepped servomotor piston 6| having two principal diameters 62 and 63, and continuing in the guide portion 64 which terminates in a piston rod 65, threaded as at 66 to screw into a pump flow-control member or shift ring 43 of the variable delivery pump I. The guide portion 64 contains a keyway 61, within which is fastened a key 68, as by screws 69. the bore 54. Piston rings 1|, 12 and 'I3 reduce leakage past the pistons 62 and 63 and piston rod 65.

'Ihe servomotor piston 6I is bored to receive'a pilot valve sleeve 14 containing ports 15, 16, 11, 18 and 19, and a uniform internal .cylindrical bore 80. Running between the annular recess 56 and the drilled holes or ports. 15 and 16 are drilled transverse holes or ports 8| and 82, passing through the piston 6|. A longitudinal passageway 83 runs from the left-hand end of the piston head 62, through the body of the piston 6| and into a transverse hole or port 84. The piston 6| is likewise provided with transverse holes or ports 85 and 86, leading from the annular recess 51 to the sleeve holes or ports 18 and 19. A pipe connection 81 serves to receive4 the-end of the pipe 35, supplying pressure uid thereto from the auxiliary pump 6 to the annular recess 56.` Similarly, a pipe connection 88 serves for receiving the end of the exhaust line 36 leading to the surge tank 3.

Mounted Within the cylindrical bore 80y of the valve sleeve 14 and accurately tted thereto, is a rotatable pilot valve head 89 having obliquely disposed ends 90 and 9|. The oblique ends 90 and 9| are spaced apart a sufficient distance great enough to insure`their covering the ports 15 and 11. 'I'he pilot valve head 89 is connected to or integral with a pilot valve stem 92. which continues in an enlarged portion 93 accurately A corresponding keyway 10 is provided in.

bored to receive the motor shaft |03 of the torque electric motor |04, and keyed thereto. The portion |02 is squared, as at |05, to receive a circular disc |06 mounted thereon. The disc |06 is cut away in an arcuate path, as at' |01 (Figure 6), to limit the rotary motion of the valve head 89. This motion is limited by the stop pin |08, which is screwed into the head 60. The, torque motor |04 is secured to the head 60 `and flange 59 of the casing 36 by the cap screws |09.

The torque motor shaft |03 is provided with an extension ||0 at the opposite end, and on'this extension is mounted the arm III. The arm I I (Figures 1 and 7) carries a pin I2 which engages a slot ||3 in the arm I|4 of the bent lever II5. The latter is pivotally mounted, as at I I6, and its opposite arm I I1 is pivotally connected; as at ||8, to the link ||9, which in turn, is pivotally connected as at |20 to the rocker arm |2I. The latter is pivotally mounted, as at |22, and at its opposite end is provided with a bifurcated portion |23 withv rounded ends, engaged by the collar I6 fitted within the bore 94 in the casing head 60.

Beyondthe enlarged portion 93 is a further enlargedv portion 95' fitting into correspondingly The enlarged portion 95 is restrained from movement in one direction by the nut 99, which is bored to receive a packing |00, and threaded to receive agland IOI about the hollow portion |02 of the valve stem 92. The portion |02'is on the press control rod |1. I

Safety centering device" Threaded into the opposite side of the pump shift ring or flow-control member 43 from the connection 66 of the piston rod 65, is a centering rod |24 having thereon an enlargement |25, beyond which is an extension |26. Slidably mounted on opposite sides of the enlargement |25 are collars |21 and |28, engaging an internal flange |29 on the centering spring casing |30, which is secured to the pump casing 40 on the opposite side thereof from the servomotor casing 36. Se-

cured to the spring casing |30 is an `end casing I3I, having a port |32 therein, normally closed by a plug and permitting checking the contents of the casing |3I. The casings |30 and |3| contain coil springs |33 and |34, urging the collars |21 and |28 against either the centering rod enlargement I 25 or against the internal ange |29 surrounding the latter.

Tonnage control valve construction The tonnage control valve (Figure 8) consists' of a block |36 containing a longitudinal bore |31, reciprocably receiving a plunger |38; the upper end of which carries a head |39 urged downwardly by the coil spring |40. The upper end of the latter is engaged by the end enlargement |4| of the threadedshaft |42, which is threaded into the bore |43 in the block ||6. A hand wheel |44 serves to adjust the position of the threaded shaft |42, thereby varying the force exerted by the coil spring |40 upon the plunger |38. The latter carriesan annular portion |45, reciprocating in the guide bore |46, leakage being prevented by the washer |41, the packing |48 and the gland |49. A limit switch |50 is positioned with its switch arm 5| adapted to be engaged and shifted by the head |39 as it moves upwardly when the plunger |38 is shifted by a predeterf- -until the plunger |38 of the tonnageeontrol valve II overcomes the thrust or" the coil spring I 40 and rises and opens the switch |50.

` Electric circuit Referring to Figure 9, LI, L2, L3, represent power lines. 'Ihe pressure switch |50 with switch arm |5I is normally closed. Inl the circuit there is a push button switch |53. The reversing switch we have designated as |51. This has switch blades |58 to |63 inclusive. We have shown two solenoids as |55 and |56 for holding the push but- -ton switch |53 closed, and for holding the reversing switch |51 in one extreme position.

It will be noted that when the push button switch |53 which is normally .held open by spring |54 is closed by the operator the circuit is completed through solenoid |55 and |56. The energizing of solenoid |55 holds the push button switch closed after the operator has removed his finger. The energizing of solenoid |56 closes the switch blades |58, |58 and |60 as shown in the sketch. This closes' the circuit to the torque motor, running it in the direction corresponding to a servomotor movement to put the pump on stroke in the proper direction to., close thepress on the work. p

When pressure builds up in the circuit sufhcient to move the plunger |38 (Figure 8) against the action of springs |40 to trip the limit switch |50 this opens the circuit to the .two solenoids, allow'- ing the push button to return to its open position and allowing the reversing switch |51 to be actuatedA by the spring |64, closing switch blades |6I to |63 inclusive and opening blades |58 to |60 inclusive. This reverses the direction of rotation ofthe torque motor producing the reverse movement of the servomotor pilot valve, and sluiting the stroke of the pump to deliver pres- CFI sure iiuid to the press in the proper direction to cause the press to' open..

When thel press platen approaches its open position the arm I4 contacting the collar I5 and operatingthrough the rod I1, the linkage turns the torque motor shaft III against its normal Y Ormrron In the .operation of the hydraulic circuit the lvariable delivery pump I and auxiliary pump 6 are started in operation by the driving motor which drives-both o! them in common. Let it be assumed that the valve head 88 occupies its extreme right-hand position, shown in Figure 2.

and that the auxiliary pump 6 is discharging pressure uid through the line 85, into the port 81 and recess 58. Let it also be assumed that the bore 52 is full of fluid. If now the valve head 88 is rotated in either direction, it will uncover a portion of the port 11 and permit oil to be exhausted' from the chamber 52, through the longitudinal passageway 88 and the ports 84, 11, 18, 85 and 88, to the exhaust line 38. The pressure-of the iluid in the recess 58 will then cause the servomotor piston 8| to move to the left, likewise shifting the pump flow-control member. 48 to the lei't and placing the variable delivery pump I upon stroke.

If the pilot valve'head 88 is rotated only a small pai-tbl' a revolution, the servomotor piston 8| will come to rest as soon as the sleeve 14 and its port 11 have moved suiiiciently far to the left to become again covered by the pilot valve head 88. If, however, the latter is rotated about onethird of a revolution, the servomotor piston 6I will move to the left until its portion 62 strikes the head 60, without having fully covered the exhaust port 11. As previously stated, the stop pin |08 limits the rotation of the valve head 88 t0 the desired amount in either direction.

If it is desired to reverse the pump by shifting the pump flow-control member 43 'in the opposite direction, the pilot piston head 88 is rotated in the reverse direction so as'to cover the port 11 and uncover the port 15. When this occurs pressure uid is admitted from the line 35, through the recess 56 and the ports 8| and 15, to the chamber or bore 52. As the exhaustport 11 is covered in this position of the valve head 88, the pressure in the chamber 52 will move the servomotor piston 6| to the right until the port 15 is covered, or until the servomotor piston` 8| reaches the limit oi its stroke. The rotation of the valve head 88 is accomplished, as previously stated, by the high torque low amperage electric moto.;- |04, which is suitable for continuous stalled service and also is reversible for operation under power in both directions. The torque motor |04 is connected to the same electrical circuit as the tonnagecontrol II so as to be responsive to reverse the variable delivery pump in response to the operation of the tonnage control valve I| when the pressing force reaches a predetermined maximum.

Thus,I the rotation of the valve head 88 in one direction causes pressure iluidto be admitted through the une 2i and port 2|, to the space above the main plunger head I2 in the hydraulic press 2.- The rotation of the pilot valve head 88 in the opposite direction, however, reverses the pump I and causes the pressure fluid to be discharged through the line 24 and port 22, into the space beneath the main plunger head I2. When the main pressing plunger is reversed in response to the control exerted by the tonnage controlv v alve `II, the platen I3 moves upward until its arm I4 lifts the collar I5 and control rod I1 and rocks the rocker yarm.|20. Theactuation of the latter then serves to rotate the torque motor shaft I I0, through the linkage previously described, until the valve head 88 is rotated into a position where it halts the servomotor piston 6| at the point where the pump flow-control member 43 is in its neutral position. This position is just-slightly away from the neutral position so as to compensate for the leakage past the main plunger head I2. By this arrangement, the pilot valve head 88 is auto- .matically rotated until the platen I3 reaches its retracted position. This retracted position can'be varied by moving the collar I6 up or down the press control rod I1.

.From an examination of Figure 2 it will be seen thatthe diameter of the valve stem 82 at 88 is the same as the diameter of the pilot valve head 88, hence, the pilot Vvalve is balanced hydraulically in a longitudinal direction. The port 16 is never uncovered by the pilot valve head 88, but being opposite the port 15, it serves for balancing the valve head 88 in a transverse direction. In order to obtain a perfect balance thereof, the stem 82 may be made the same diameter as the valve head 88 so that the face 8| is replaced by a diagonal slot in the valve head 88, the latter extending to the left as farr as the end'of the sleeve 14 when the parts are in the positions shown in Figure v2.

In the event of a failure of power for driving the pumps I and 6, the main plunger head I2, by reason of the weight of itself and the platen I3, will tend to compress the fluid beneath the main plunger head I2. This pressure fluid will back up in the line 24 and will tend to cause the variable delivery pump I to operate as av motor, thereby accelerating the descent ofthe platen I3. When this occurs, however, the auxiliary pump 6, whichis driven from the same shaft as the variable delivery pump I, immediately also rotates in a reverse direction so that it sucks uid out of the servomotor easing recess 56 (Figure 2). One of the springs I33 or |34 (Figure 1) will then be free to shift thecentering rod |24 and the pump flow-control member 43 to its neutral position, halting the rotation of the pump I as a motor, and immediately halting the downward travel of the platen I3. This feature gives a I safety control in the event of a power failure or breakdown of the pump operating motor.

It will be understood that we desire to comprehend within our invention such modications'as come Within the scope of the claims and the invention.

Having thus fully described our invention, what we claim as new and desire to secure by Letters Patent, is:

1. In a servomotor control mechanism, a casing having iluid inlet and outlet openings, a fluid operable piston reciprocablymounted in said casing and having a bore with ports therein adapted to cooperate with said openings, and a rotatable valve member arranged in said bore adjacent said K ports and having a valve head with a pairf oblique transverse end surfaces thereon for controlling said ports.

2. In a servomotor control mechanism, a casing having an inlet and an outlet opening adapted respectively to receive and discharge pressure fluid, a piston having a bore with ports therein adapted selectively to communicate with said inlet or said outlet opening, and a rotatable valve member in said bore adjacent said ports for controlling the latter, said valve member having a valve head with an oblique end surface thereon.

3. In a servomotor control mechanism, a casing having an inlet andan outletopening, adapted respectively to receive and discharge pressure fluid, a: piston having a bore with ports therein adapted selectively to communicate with said lnlet or said outlet opening, and a rotatable valve member in said bore adjacent said ports for controlling the latter, said valve member having a cylindrical valve head with spaced oblique end surfaces thereon.

4. In a'servomotor control mechanism, a cashaving a valve head with spaced oblique end surfaces.

6. In a servomotor control mechanism, a casing, a piston having a bore with axially spaced radially arranged ports therein and a longitudinal passageway connecting one side of said piston withone of said ports, and a rotatable valve member with oblique end faces in said bore adjacentsaid ports, said casing having a pressure iiuid intake port arranged on that piston side which is remote from the piston side connected with said longitudinal bore, and also comprising an outlet bore for releasing fluid from the opposite piston side.

7. In a servomotor control mechanism, a piston having a bore with ports therein and also "including a longitudinal passageway connecting one side of said piston with one of said` ports, a rotatable valve member in said bore adiacent said ports for controlling the latter, and a casing for slidably receiving said piston, said casing having a pressure iiuid intake opening arranged adjacent one piston side and adapted to communicate with one of said ports, and including a discharge opening adapted to communicate with another of said ports and arranged adjacent another piston side opposite said first-mentioned piston side, and said valve member having a valve head with spaced oblique end surfaces.

8. In a servomotor control-mechanism, a cas'- ing with uid inlet. and outlet openings, a piston having' a bore with axially spaced Yradially arranged ports therein and a longitudinal passageway connecting one side of said piston with one of said ports, said ports being adapted respectively to communicate with said inlet and outlet openings, a rotatable valve member in said bore adjacent said ports, and an electric torque motor adapted for continuously stalled service and having its shaft connected to said valve member, said valve member having a valve head with spaced oblique end surfaces.

9. In a servomotor control mechanism, a casing having a chamber and an inlet and outlet opening adapted respectively to receive and discharge pressure fluid, a piston reciprocably mounted in said casing and having a longitudinal bore 'with a. transverse port adapted to connect ing with fluid inlet and outlet openings, a piston having a bore with axially spaced radially arranged ports therein and a longitudinal passage- Way connecting one side of said piston with one of said ports, said ports being adapted respectively to communicate with said inlet and outlet openings and a-rotatable valve member arranged in said bore adjacent said ports and having an obliquely disposed-end surface thereon.

5. In a servomotor control mechanism, a casing having an inlet and an outlet opening adapted respectively to receive and discharge pressure uid, a piston having a bore with ports therein adapted selectively to communicate with said inlet -or said outlet opening and also including a longitudinal passageway connecting one side of said piston with one of said ports, and a rotatable .valve member in said bore adjacent said ports for controlling the latter, said valve member said outlet opening with said chamber, and a rotatable valve member having a valve head with inclined end surfaces operable to open and close said port in response to the rotation of said valve member and also adapted selectively to establish communication between said inlet opening and said chamber.

10. In a servomotor control mechanism, a casing having a cylinder bore communicating with an inlet and outletopening adapted respectively to receive and discharge pressure fluid, a fluid operable piston reciprocably mounted in said cylinder bore and having a longitudinal bore with transverse ports adapted to connect said longitudinal bore with said cylinder bore, and a rotatable valve member having a valve head with inclined end surfaces operable selectively to open and close at least one of said ports in response to the rotation of said valve member for selectively establishing or interrupting communication between said inlet and one of two oppositely acting piston sides, said piston having a longitudinal passageway connecting one side of said piston with one of said ports. f

11. In 'a servomotor control mechanism, a casing having a chamber with inlet and outlet open-v charging pressure uid, a uid operable piston reciprocably mounted in said chamber nd hav- I ing a longitudinal bore with transver ports adapted to connect said bore with said chamber,

a rotatable valve member having a valve head continuously stalled service connected to saidv valve member.

12. In a servomotor control mechanism, a casing having a cylinder bore with inlet and outlet openings for respectively receiving and discharging pressure iluid, a. iluid operable piston reciprocably mounted in said cylinder bore and having a longitudinal bore with transverse ports adapted to connect said longitudinal bore with said cylinder bore, a rotatable valve member having a valve head with inclined end surfaces operablev selectively to open and close .at least one of said ports in response to the rotation of said valvel member, said piston having a longitudinal passageway connecting one side of said piston with one of said ports, and an electric motor adapted for continuously stalled service and arranged in alignment with and being connected to said valve member for rotating the latter. 'y

13. I n a uid operated servomotor, a casing having a longitudinal bore with a fluid inlet and iluid outlet, a hollow piston reciprocably mounted in said bore and comprising axiallyspaced ports adapted respectively to communicate with said inlet and outlet for actuation of said piston, and a rotatable valve member with a skew head` `head rotatably mounted in said vreciprocable member and cooperating with said ports to selectively cover and uncover at least one of said ports in response to the rotation of said`head, said head having end surfaces disposed 'o'bliqucly to the axis of rotation of said valve member.

15. In a fluid operated servomotor, av casing having a stepped bore with end chambers and spaced inlet and outlet openings disposed intermediate said end chambers, a hollow piston reciprocably mounted in said bore and comprising spaced radially arranged ports adapted respectively to communicate with said inlet and outlet,

openings, and also comprising a passageway adapted to effect communication between one of said end chambers and said outlet openings, and a valve member rotatably mounted in sa'id piston and for controlling said ports in response to the rotation of said'head.

' HOWARD F. MACMILLIN.

IRA B. LAWYER.

having a head with obliquely disposed ends

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