US2969688A - Hand control for a servo system - Google Patents

Description

Jan. 31, 1961 Filed May 12, 1949 mil ill"

D. E. HACKELMAN 2,969,688

HAND CONTROL. FOR A SERVO SYSTEM 5 Sheets-Sheet l v INVENTOR DENVER E. HAGKELMAN BY 'WT'TORNEYS Jan. 31, 1961 E, HACKELMAN 2,969,688

HAND CONTROL FOR A SERVO SYSTEM Filed May 12, 1949 5 Sheets-Sheet 2 INVENTOR DENVER E. HAOKELMAN /%M 49%; BY (Q BQ'WRNEYS United States Patent HAND CONTROL FOR A SERVO SYSTEM Denver Eldin Hackelman, 1747 N. Cambrian, Bremerton, Wash.

Filed May 12, 1949, Ser. No. 92,935 4 Claims. (Cl. 74-388) (Granted under Title 35, US. Code (1952), see. 266) This invention relates to gun control devices and more particularly to means for enabling power gun drives to be changed over from automatic control to manual control with a minimum loss of control during changeover.

Power operated drives for guns are generally provided with a control means for automatic operation by a remotely situated gun director and are also ordinarily provided with manual means which may be used to control the gun movement in the event of a failure of the remote control. In the use of such devices there has been experienced a lapse of control during the change-over period from automatic to manual control. This lapse or period when the gun may move in train without responding to the control member may permit undesired and dangerous rotation of the turret with attendant damage to the equipment and possible danger to life.

In the manual control for gun drives, a differential follow-up consisting of the usual three elements can be used to control the gun position. One element of the differential will be moved by hand while another component constituting a follow-up element will be driven by the power gun drive. During the automatic control of the gun drive, the hand operated element of the manual control will be declutched while the follow-up element will remain connected to the gun drive. Any inertia or friction remaining on the hand operated element of the manual control will cause movement of a third differential element during automatic control. This movement is highly undesirable since the third element of the manual control, which is the effective one to cause movement of the gun, will be moved from neutral position. Thus, during change-over to manual control, an unwanted force will be applied to the gun drive due to the fact that the third element of the differential has drifted from neutral position.

It is therefore an object of the present invention to provide a gun training device and control means therefor which will permit a change-over from remote to local control with a minimum loss of control during the change-over.

It is a further object of this invention to prevent any control movement of the manual control means for the gun while the automatic control means is functioning to move the gun in accordance with signals from the gun director. By preventing control movement of the manual control means during this time, it is posible to maintain the manual control means in its neutral position so that on change-over to manual control, no movement of the gun mount will result until the manual control is hand actuated.

It is a still further object of this invention to provide means whereby the hand actuated differential element of manual control will be positively driven at the same rate as the follow-up differential element, so that no control movement of the third differential element can result during the time time when the gun drive is automatically controlled.

A still further object of the invention is to provide in power operated gun drives of the described type, a means to prevent operation of the manual control unless the third control element of the differential is substantially in neutral position at the time of change-over from automatic to manual control.

Other objects and advantages of the invention will hereinafter become more fully apparent from the following description of the drawings in which:

Fig. 1 is a side elevation of a gun turret employing the present invention, portions of the turret being broken Fig. 2 is a schematic view, showing the arrangement of the various mechanical and hydraulic parts; and

Fig. 3 is a detail view of the manual follow-up differential, portions being broken away to further reveal enclosed parts.

Referring to Fig. 1, a gun 1 is mounted in a turret 2 which is journalled for rotation about its vertical axis on roller path bearings 3. The turret is rotated by pinion 4 which co-acts with internal ring gear 6 rigidly secured to a fixed portion 7 of the structure of the vessel in which the gun is mounted. The pinion 4 is supported in gear box 5 which is fixed to turret 2 and is rotated either by automatic or manual controls to be later described in order to train the gun. An elevation screw 8 is provided to elevate and depress the gun and is actuated by power means (not shown) which form no part of the present invention.

Referring to Fig. 2, the shaft 9 driven by hydraulic motor 10 carries a worm 11 which drives pinion 4 through gear 12 and shaft 13. The hydraulic motor 10 is supplied with hydraulic fluid under pressure through conduits 14 which are in fluid communication with hydraulic pump 15 driven by electric motor 16. The pump 15 and motor 10 form the parts of a conventional Waterbury Gear which is well known in the gun power drive art and which has accordingly been illustrated in diagrammatic fashion. A lever arm 17 is connected to the conventional tilting plate (not shown) of the Waterbury Gear in order to control the quantity and direction of the flow of hydraulic fluid from pump 15 to by.- draulic motor 10 in order to form a variable speed drive interposed between electric motor 16 and drive pinion 4 as is well known in the art.

It is customary in the art to position the above-mentioned tilting plate by automatic means controlled by a remote gun director in order to control the drive characteristic of the Waterbury Gear. Also provision is made to operate the tilting plate by means of a handwheel in the event that the director should fail for any reason.

In the present invention, the automatic control device comprises a conventional synchro or Selsyn motor which is positioned by an electric signal from a similar synchro generator in the gun director, in such manner that angular movement of the synchro generator causes an equal angular movement of the synchro motor as is well known in the art. Rotation of synhcro motor 18 in response to a signal from the gun director, moves an arm 19 secured to the rotor of synchro motor 18, which in turn moves link 20 and follow-up lever 21. e

A booster cylinder 22 having a piston 23 and piston rod 24 is provided to move the tilting plate arm 17 by means of a link 25 pivoted at its respective ends to said piston rod 24 and arm 17. A valve spool 26, slidably moved by an elongated slot conection to follow-up lever 21, is provided to control the position of booster piston 23 and tilting plate arm 17. The stator of synchro motor 18 is rotatably journalled and is rotated in response to movement of the gun turret by means of a gear train consisting of shafts 29, 32, 35 and 38 and gears 28, 30, 31, 33, 34, 36, 37, 39 and 40. l

Displacement of spool valve 26 by rotation of the synchro rotor allows servo oil under pressure in line 85a to enter cylinder 22 to move piston 23 in the opposite direction from the original direction of movement of ,valve 26. At this'time, the transfer valve 93 will be positioned to the'left to allow passage of fluid, as will later be explained. The piston 23 will continue to move until the follow-up lever 21, connected to the piston rod .24, moves va'lve spool 26 to neutral position again. In .the'new position of piston 23 and piston rod 24, the tilting plate arm 17is displaced from its central, neutral position and the hydraulic drive is operative to rotate the turret. As the turret moves into synchronism with the director, the stator of servo motor 18 will be moved by the gearing driven by shaft 9 in a direction to move spool valve 26 in the opposite direction from its original movement. Thus, the piston 23 and piston rod 24 will be moved back towards its central position, in which, condition tilting plate arm 17 is also in its central neutral position and the turret comes to rest.

The turret may be controlled by manual actuation of handwheels 41, 41 if the gun director is inoperative or if for any reason it is desired to control the drive locally. Movement of handwheels 41, 41 controls spool valve 26a which in turn controls the position of piston 23 and Waterbury Gear tilting plate 17 in the same manner as does valve 26 for the automatic drive. The manual control consists of a difierential control screw 42 which is free to move axially and which is moved axially by the combined action of a nut gear 43 and a screw rotating gear 44. The nut gear 43 is internally threaded to mesh with threads on the exterior of control screw 42, while screw rotating gear 44 has elongated internal teeth 44:: meshing with a splined shoulder 45 integral with and near one end of control screw 42. Nut gear 43 and screw rotating gear 44 are supported for free rotation but are restrained from axial movement by bearings 46, 46 and bearings 47, 47 respectively (Fig. 3). When the nut gear is held from rotation and the control screw 42 is rotated by gear 44, the control screw is moved axially. Similarly, when gear 44 is held against rotation, rotation of nut gear 43 will result in axial movement of screw 42. It also follows from the above, that movement of both nut gear 43 and gear 44 at the same speed and in the same direction, will result in control screw 42 being restrained from any axial movement since it then rotates at the same speed as the nut gear into which it is threaded. Screw rotating gear 44 is rotated in response to movement of the turret by a drive connected with shaft Handwheels 41, 41 are secured to a shaft 48 which is journalled in a stanchion 49. A gear 50, secured to shaft 48, rotates a shaft 58 by means of an interposed gear train consisting of gears 51, 53, 54, 56 and 57 and shafts 52 and 55. A gear 59, secured on shaft 60, meshes with nut gear 43. A clutch, indicated generally at 61 having a clutch disc 62 secured to shaft 60 and a clutch disc 62a splined to shaft 58, is adapted to interrupt the drive from handwheels 41, 41 to gears 59 and 43 in its disengaged position. When clutch 60 is in its engaged position, rotation of handwheels 41, 41 will result in an input to the differential screw in the form of rotation of nut gear 43.

Gear 44 and control screw 42 which is splined thereto, are rotated by the output shaft 9 of hydraulic motor during rotation of the turret, by means of an interposed gear train consisting of gears 39, 40, 63, 64, 66, 67 and 69 and shafts 38, 65 and 68.

When control screw 42 is moved axially by diflerential movement of nut gear 43 with respect to gear 44, the spool valve 261: is temporarily moved by means of a bell crank 70 having an arm 71 urged into engagement with one end of the control screw 42 by a spring 72. An arm 73 of crank 70 is connected by means of a pivotally connected link 74 to a follow-up lever 21a which has an end remote from link 74 pivotally connected to piston rod 24. The connection between control screw 42 and bell crank lever 71 is effected by an integral extension 42a on screw 42 which has a rounded end adapted to rotate in socket 71a of lever 71.

Displacement of the spool valve 26a by rotation of the handwheels 41, 41 causes servo oil to flow from passage 8512 into cylinder 22 to move piston 23 in the opposite direction from the original direction of movement of valve 26a. During this time, transfer valve 93 remains in the position shown in Fig. 2. Piston 23 will move until the supply of servo oil is cut oil? by reason of the fact that movement of the piston rod 24 returns valve 26a to its closed position by means of the follow-up lever 21a. In the new position of piston 23 and piston rod 24, the tilting plate arm 17 is displaced from its central, neutral position and the hydraulic drive is operative to rotate the turret.

As the turret rotates, spool valve 26a is moved in the opposite direction from that originally imparted by the handwheels since gear 44 is likewise rotated by shaft 9 while nut gear 43 remains stationary. This movement of gear 44 imparts axial movement to screw 42 which in turn actuates bell lever 70 to return piston 23 to its neutral position. The turret will thus move to a new position and stop when the handwheels 41, 41 are moved to a new position, or the turret will move at a uniform velocity as long as the handwheels are moved at a uniform velocity.

In order to change from local or hand control to automatic control, a selector valve 75 is provided which has a valve spool 76'having a valve stem 77 with a rack gear 78 formed thereon. A pinion 79 is journalled for rotation in mesh with rack 78 and is adapted to be rotated by a handle 80 to move selector valve 75 into the desired position. An index plate 81 is provided to identify the two positions of the valve. When the selector valve is in the position shown in solid lines in Fig. 2, the fluid duct 82 is in fluid communication with a duct 83 which connects with-sump 84. In addition, a fluid duct 83a exhausts spool valve 26 and 26a to sump 84.

When the handle 80 is moved to the automatic position, duct 82 is placed in fluid communication with duct 85 since spool valve 76 is then in the dotted line position of Fig. 2. Duct 86 is usually filled with liquid under pressure delivered from sump 84 through ducts 89 and 90 by pump 87 driven by motor 88. The valve 91 generally permits communication between ducts 90 and 86 as will hereinafter be described. With the valve 75 in the automatic position, duct 82 ports into duct 92 to actuate the transfer valve 93, and into duct 92a to disengage handwheel clutch 61 and to engage a clutch 115 presently to be described.

Clutch '61'is declutched by a shifter lever 61a, pivotally mounted on a bracket 61b and having one end thereof operatively engaging clutch member 62a. Another end of lever 61a engages a piston 61c slidably movable in a cylindrical recess bored in bracket 6112, the recess being in fluid communication with duct 92a. When ducts 92a and 82 are vented to duct 83 and sump 84 by moving selector handle 80 and valve spool 76 to hand control position, a spring 61d abutting against clutch disc 62a and a collar 612 on sh'a ft 58 places clutch 61 in its engaged position to connect handwheels 41, 41 to the differential control mechanism. When fluid under pressure is admitted to duct 92a the force of piston 61c overcomes spring 61d whereby the clutch elements are disengaged.

The transfer valve 93, shown in 2, has grooves 94 and 95 at the right end thereof, which in the position shown connect passages 94a, 94b and 95a, 95b respectively, to render valve spool 26a operative by the local band control. A spring96 is provided to normally urge valve 93 to the right into its local hand control position. When the selector handle 80 is in the automatieposition, pressure fluid passing through ducts 82. and 92 urges transfer valve 93 to the left against the resiliency of spring 96. In this position grooves 97 and 98 in valve 93 are aligned with ports 97a, 97b and 98a, 98b respectively, to render valve spool 26 and the automatic control operative and to render inoperative the local hand control.

In using the above prior art structure with the selector handle 80 in the automatic position, the hydraulic motor in rotating the gun, feeds back a response signal to gears 69 and 44 as has been previously described, and the signal tends to cause differential control screw 42 to drift axially in one direction or the other depending on the direction of motion of the gun, since gears 43 and 59 offer frictional resistance to rotary motion although they are disconnected from handwheels 41, 41

by reason of handwheel clutch 61 being in the disengaging position.

The axial drift of screw 42 causes valve spool 26a to drift away from its neutral position but this drift will not cause trouble in the automatic position because ducts 94a and 95a are closed since valve 93 will be to the left in Fig. 2. However, when the control selector 80 is shifted to its hand control position, an unexpected and violent movement of the gun will occur due to movement of valve 93 to the right by spring 96 when duct 92 is vented after movement of valve 76 by the selector. The drift of the spool valve 26a away from neutral position will allow pressure fluid to immediately enter chamber 22 to actuate the gun drive. This movement persists until response from the hydraulic motor 10 rotates gears 69 and 44 to axially move differential control screw 42 and valve spool 26a to a point where the latter is in its neutral position. During this time, motor 10 is required to accelerate the gun at rates far in excess of acceleration rates encountered in normal operation and the entire drive system may be subjected to damage because of this overload.

In order to overcome this difliculty in accordance with the objects of the invention, an arrangement is provided which causes gears 43 and 44 to rotate at the same speed while the drive is under automatic control in order to prevent any drift of control screw 42 and valve spool 26a from their neutral positions. To accomplish this end, a counter-shaft 99 is provided which is journalled for rotary movement in bearings 100 and 101, about an axis which in the specific modification shown in Fig. 3, is parallel to the axis of control screw 42. Gear 102 is fixed on one end of counter-shaft 99 by a spline 103 and geared to rotate with gear 43 by means of interposed idler gears 104 and 105 which mesh with each other and with gears 102 and 43 respectively. The idler gears 104 and 105 are supported for rotation by pintle 106 and bearing 107 and by pintle 108 and bearing 109 respectively. A gear 110 journalled for free rotation on counter-shaft 99 is geared to gear 44 by means of an interposed idler gear 111 meshing with gear 110 and gear 69. Idler gear 111 is supported for rotation by means of a bearing 112 and a pintle 113, which are mounted along with pintles 106 and 108, in a fixed bracket 114.

In order that gear 110 may selectively rotate shaft 99, a clutch is provided which is generally indicated at 115 and which consists of a clutch plate 116 having friction lining 117 adapted to engage gear 110. Clutch plate 116 is constrained to rotate with shaft 99 by a drive cotter 118 secured at its ends in plate 116 and slidable axially of shaft 99 in a slot 119 in the shaft. A stub shaft 120 is slidably supported in a cylindrical recess 122 co-axially bored in counter-shaft 99 and is rigidly pinned to clutch plate 116 by cotter 118. A compression spring 121 located in recess 122 bears against stub shaft 120 and the top of recess 122 to normally urge clutch plate 116 away from gear 110 to disengage clutch 115.

In order to engage clutch 115 when required, a Sylphon bellows 123 in fluid communication with duct 124 and connecting duct 92a is adapted to be expanded by hydraulic fluid to axially move stub shaft 120: by means of interposed thrust bearing 125. r The head of the bellows 123 and the lower end of stub shaft 120 both project into the bearing 125 to provide for a rotatable connection. In order to restrain gear 110 against undesired axial motion, a sleeve 126 is disposed loosely about counter-shaft 99 and abuts gears 102 and 110 at its respective ends. When the selector handle is in the automatic position hydraulic fluid will pass through ducts 82, 92a and 124 into bellows 123 when valve 91 permits, as will be hereinafter described. The fluid in duct 124 will expand bellows 123 to engage clutch 115 and thus prevent control screw 42 from drifting from its neutral position since the clutch 115 will cause gears 43 and 44 to rotate at the same speed duringautomatic control.

In order to prevent operation in either automatic control or hand control in the event that the diflerential control screw 42 is displaced from its neutral position by more than a predetermined amount, an interlock valve 91 is provided which is interposed between hydraulic fluid supply duct and duct 86 and which consists of a valve spool 127 slidably mounted in a bored valve casing 128. Valve spool 127 is normally held in the position shown in full lines in Fig. 2 by a cam of revolution 129, which is disposed on an extension 130 of control screw 42 and which is abreast of the valve spool 127 when the control screw is in its central, neutral position. Since cam 129 is a cam of revolution, it may be rotated without affecting the position of valve spool 127 providing cam 129 is not also moved axially. In the above mentioned full lined position of valve spool 127, with control screw 42 approximately in its neutral position, the ducts 90 and 86 are in fluid communication and hydraulic fluid is ported into duct 86 to selectively actuate the piston 23 and hydraulic pump 15 by means of valve 26 in automatic control and by means of valve 26a in hand control. In the event that control screw 42 is moved a predetermined distance axially from its central, neutral position, cam 129 will move axially to permit valve spool 127 to be moved to the dotted position shown in Fig. 2 by spring 131. In this latter position, delivery of fluid under pressure from duct 90 to duct 86 is stopped and duct 86 is vented to a vent duct 132 in fluid communication with sump 84. Venting of duct 86 and connecting ducts 85 and 85a through vent duct 132 precludes movement of transfer valve 93 to the left in Fig. 2 since ducts 82, 92 and 92a are then vented either through duct vent 83 when the valve 76 is in the hand control position or through ducts 85, 86 and vent duct 132 when the valve 76 is in the automatic control position.

Since duct 86 and connecting duct 85a are vented, both sides and the center of hand control spool valve 26a will be vented through either duct 83a or ducts 85a, 86 and 132. In the event that piston 23 is dis-' placed from its central position at the time valve 127 is in its dotted line position, valve spool 26a will also be displaced from its central position and both ducts 94a and a will be vented, permitting hydraulic fluid in cylinder 22 to flow from one side or the other of piston 23 as the piston is moved to its central position by one or the other of relatively heavy springs 133 or 134. The control lever arm 17 of the hydraulic pump 15 is thus returned to its central inoperative position and the gun cannot be moved in either automatic or hand control. In the event that this condition occurs, the control screw 42 and cam 129 may be returned to their neutral positions by rotating gears 43 and 59 using handwheels 41, 41, since clutch 62 will then be engaged due to the action of spring 61d and duct 92a will be vented under this condition.

With selector handle 80 in the hand control position, valve 76 will normally vent ducts 92, 92a and 124 respectively, permitting transfer valve93 to be moved by 'a'gseaes's "7 spring 96 to hand control position. Also, Tspring61d will u'rge'handwheel clutch 61 to its closed position and bellows 123 will be collapsed to permit spring 121 to declutch backgear clutch 115. The handwheels 41, 41 are then operative to control the drive through clutch 61 andthe differential follow- up 42, 43, 44 and valve 26a, without interference from the backgear 102, 104, 105, 110 and 111. Upon movement of handwheels 41, 41 the gun hydraulic drive units 15 and immediately cause the gun to respond by rotating and the response is fed back into the differential follow-up by the gearing interposed between gear 39 on hydraulic motor output shaft 9, and gear 69 which meshes with'gear 44 of the diiferential follow-up. This follow-up response causes the control screw 42 to remain very close'to its neutral 7 position.

If for any reason control screw 42 should be moved from its neutral position by an appreciable amount, cam 129 would permit valve 127 to be moved by spring 131 to a position in which cylinder 22 would be vented at both sides of piston 23, thus allowing springs 133 and 134 to center piston 23 to stop the drive, as previously explained.

In automatic control, valve 76 will normally supply hydraulic fluid under pressure to ducts 92, 92a and 124 respectively causing transfer valve 93 to move to a position in which valve 26 is operative to control the drive in response to movement of synchro motor 18. The fluid in duct 92a will cause piston 610 to disengage clutch 61 to eliminate the drag of the handwheel gearing and to make the handwheels inoperative. With'fluid pressure in duct 124, bellows 123will cause clutch 115 to engage to render the backgears 102, 104, 105, 110 and 111 operative to lock gears 43 and 44 for rotationtogether in order to prevent any axial movement of control screw 42.

If control screw 42 should for any reason move an appreciable amount from its neutral position during automatic control or be in such a displaced position when automatic operation was attempted, cam 129 would permit valve 127 to be moved by spring 131 to a position in which ducts 32 and 92 are vented. Spring 96 would then return transfer valve 93 to the hand control position and the cylinder 22 would then be vented at both sides of piston 23, allowing springs 133 and 134 to center the piston 23 to stop the drive as explained above.

Since backgear clutch 115 is operated by bellows 123 supplied with hydraulic fluid through the ducts 90, 86, 85, 82, 92a and 124 under the control of valves 127 and 76, the backgear clutch 115 can only be engaged when the control screw 42 is approximately at its neutral central position with the selector valve 76 in its automatic drive position. The combined effect of the backgearing and the interlock valve 91 is to prevent'the differential control screw 42 from drifting from its neutral position during automatic control so that subsequent shifting to hand control cannot cause any appreciable movement of the gun.

It is to be understood that the form of my invention, herewith shown and described, is to be taken as a preferred example of the same and that various changes in the shape, size and arrangement of parts may be resorted to, without departing from the spirit of my invention. More specifically, various other specific means to couple gears 43 and 44 together for simultaneousrotation when required, would readily occur to one skilled in the art. The specific booster cylinder, transfer valve,

selector valve and follow-up difierential disclosed herein are illustrative only since their specific aspects form no part of the present invention and they may accordingly be replaced by any equivalent element known to the art without departing from the scope of this invention.

The invention herein described maybe manufactured and' used by or fortheGovernment of the UnitedStates 8 of America for governmental purposeswithout the'pay ment of any royalties .thereon or therefor.

What is claimed is:

'1. In a gun assembly, the combination'including gun positioning means, manual means and automatic means selectively controlling said gun positioning meansfsaid manual means comprising a differential unit having two input members and a differential output member, linkage connected between the output member and said gunpositioning means acting upon selection of said manual means to control said gun positioning means responsive to movement of said output member, a manually movable control element for driving one of said input members, motion transmitting means driven by said gun positioning means and driving the other of said input members to provide follow-up action, a drive means interposed between said two input members including a clutch device releasably engageable to connect said one'input member to said other input member to prevent differential movement of said output member during automatic control of said positioning means when said manually movable element is inoperative.

2. The combination defined in claim 1 wherein said manual means and said automatic means are hydraulically operated, a fluid supply line for said manual means and said automatic means, an interlock valve located in said fluid supply line, a cam carried at one end of said output member, said cam normally retaining said interlock valve in open position to permit passage of fluid when said output member is in its normal, central position, said interlock valve being movable by said cam to close said fluid supply line to prevent overload of said gun positioning means during manual control or during change-over from automatic to manual control in the event said output member movesa given amount from said'central position.

3. In a gun assembly having a power driven meansto position the gun, manual control means to selectively actuate said power driven means, said manual control means comprising a first differential unit having two input members and an output'member, one of said input members being connected to a manually movable control element and said other input member being connected to said power driven means to provide for follow-up action, said output member being operative to actuate said manual control means in response to relative movement of said two input members, automatic control means to selectively actuate said power driven means from a remote point, said automatic control means comprising a second differential element, selector means movable from one position to another to render either said automatic control means or said manual control means efiective to actuate said power driven means,'a first clutch controlled by said selector means to disconnect said one input member from said manually movable element when said selector is in the automatic control position, a drive means comprising a second clutch interposed between said two input members, said second clutch controlled by said selector means and operative when said selector is in its automatic control position to connect said one input member to said other input member so that said two input members rotate at the same speed and no movement of said output member results during automatic control.

4. In a selective control system for a power drive,selectable manual control equipment including a differential comprising two input members'and a differential output member, a manually movable control element, linkage connected between said output member and said power drive and acting upon selection of said manual control equipment to control said power drive responsive to actuation of said output member, a first clutch engageable to connect said manually movable control element to one of said input members, means connecting the other'ofsaid inputmembersto said power 'drive'to provide follow-up action, motion transmitting means interposed between said two input members including a second clutch acting upon engagement to connect said one input member to said other input member to prevent differential movement of said output member when said power drive is controlled by means other than said manual control unit, and means acting upon selection of said other control means to disengage said first clutch and engage said second clutch.

References Cited in the file of this patent UNITED STATES PATENTS Ernst July 16, 1946 Wright et a1. Nov. 9. 1948

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