US1978346A - Delayed trip for hydraulic circuits - Google Patents

Description

Oct. 23, 1934. y H. ERNST ET Ax.

DELAYED TRIP FOR HYDRAULIOCIRCUITS Filed sept. 15. 1930 s sheets-sheet 1 Oct. 23, 1934. H. ERNST Er Al. Y 1,978,346

DELAYED TRIP FOR HYDRAULIOCIRCUITS Filed Sept. l5. 1930 3 Sheets--SheeI 2 #fw-JU L/Pwm/o Zo El fa jlfeeaze ammi,

Oct. 23, 1934. H. ERNST 'E T AL 1,978,346

` `IEL'AYED TRIP FOR HYDRAULICYCIRCUITS Filed sept. 15. 1930 s sheets-sheet s 85 'g/@p dmv/l j y A .86 u

Patented a. 23, 1934 DELAYED TRIP FOR HYDRAULIC CIRCUITS Hans'Ernst and Charles W. Goodrich, Cincinnati,

Ohio, assignors to The Cincinnati Milling Ma' l chine Company, Cincinnati, Ohio, a corporation of Ohio Application September 15, 1930, Serial No. 481,939

OFFICE 16 Claims.

' This invention relates to hydraulically actuated machine tools and more `particularly to an improved mechanism for effecting a predetermined pause in the operative cycle thereof.

In thev construction of hydraulically actuated machine tools and more particularly in machines having a translatable part, such as a work support, suitable valve mechanism is employed for eiecting a reversal in the direction of flow of the hydraulic medium to eiect a change in the direction of movement of the translatable part. Such valve mechanisms have been automatically controlled through mechanical means, as well as hydraulic means. Since it is often desirable in machining some types of Work to effect a pause in the movement of the work with respect to the cutter whereby the surface of the work may be properly finished before it recedes from the cutter, it is an object of this invention to provide a new and improved mechanism that may be adapted to hydraulic circuits for effecting a delayed action thereof and thereby a pause in the move- 'ment of the translatable partof the machine.

Another object of this invention is to provide a delay unit for the reversing mechanism of an hydraulically operated machine tool which is actuated directly by the feeding pressure thereby insuring positive actuation thereof and making possible increased accuracy in setting the timeof .reversaL A further object of this invention is to provide a delay unit for the control mechanism of an hydraulic circuit which may be adjusted to vary the time interval that the action of the control mechanism is postponed.,

Other objects and advantages of the present invention should be readily apparent by reference to the following specication considered in conjunction with the accompanying drawings illustrative' of one embodiment thereof but it will be understood that any modifications may be made in the specic structural detailsihereinafter disclosed within the scope of the appended claims without departing from or exceeding the spirit of the invention.

Referring to the drawings in which like reference numerals indicate like parts:

Figure 1 is a diagrammatic view of a complete hydraulic system showing the application of the delay unit thereto in conjunction with the parts operated and controlled thereby.

Figure 2 is a detail view of the trip mechanism for actuating the'pilot valve.

Figure 3 is a view of the pilot valve in reversed position.

Figure 4 is a detail section on the line 4 4 of Figure 2. l

Figure 5 is a diagrammatic view of a delay unit applied to an hydraulic reciprocating cycle circuit to effect a delayin each reversal of the table.

Figure 6 is a longitudinal section through the selector valve.

Figure 7 is a longitudinal section through pilot valve.

Figure 8 is a section on the line 8 8 of Figure 6.

Figure 9 is a section on the line 9-9 of Figure 6.

Figure 10 is a section on the line 10--10 of Figure 7.

In the drawings the reference numeral l0 indicates a reciprocating support of a machine tool, such as a milling machine, which forms an excellent example for the illustration of this invention. Such machines have a cutting tool'11 for acting upon Work, such as 12, moved by the table 10 into contact therewith for effecting a machining operation thereupon. Hydraulic means have heretofore been utilized for effecting the reciprocation of the table and usually comprises an hydraulic motor of the piston and cylinder type in which the cylinder, such as 13, is usually fixed while the piston 14 relatively movable therein is attached to the table for imparting movement thereto through a piston rod, such as 15.

Hydraulic pressure is alternately supplied to opposite ends of the cylinder to effect movement of the piston and for this purpose channels, such as 16 and 17, are connected respectively to opposite ends of the cylinder from which they lead ,to a rate and direction determining means, such 'as a selector valve 18. A stop valve 19 is usually interposed between the selector valve and the cylinder to close or lockthe fluid in opposite ends of the cylinder thereby stopping movement of the table and holding it in such position by means of the fluid trapped in the opposite ends thereof.

In the hydraulic actuation of milling machines it is highly desirable and necessary to give a uniform feed or movement to the Work with respect to the cutter to insure proper finish as well as an economical cutting action of the tool. 'I'his result is accomplished by providing a closed system for the hydraulic medium inl which the medium is forced into one end of the cylinder While being withdrawn from the opposite end in measured'amounts at such rate as to effect the desired feed rate. One method of accomplishing this is to provide a variable displacement pump, as shown at 20, having an inlet.21 and an outlet 22 which are connected to the selector Yvalve by channels 23 and'24. A rapid traverse pump, suchas 25, having an inlet 26 connected with the reservoir 27 and an outlet 28 leading to the selector valve, is provided for effecting quick traverse movements of the table and in such cases 5 the return side of the cylinder is connected to the discharge line 29 leading directly to the reservoir located in the base ofthe machine. To maintain the feeding circuit under constant pressure and replace fluid losses, due to leakage or other reasons, a booster pump, such as 30, is

provided for injecting replacement fluid in the high pressure side of the feed circuit.

The selector valve is rotatable to connec either the feed circuit or the rapid traverse circuit with the hydraulic motor to change the rate of movement of the table from feed to rapid traverse and it is longitudinally shiftable to reverse the ilow in the channels 16 and 17 thereby changing the direction of movement of the table.

20 Due to the pressure of the hydraulic medium flowing through the selector valve and the danger of the valve thus becoming slightly cramped it is desirable, in the interest of positive actuation, to power operate the valve. To this end a bracket, such as 31, Figure 8, is provided on one end 80 a double end lever 66 secured to the end of the valve plunger 67 for effecting rotation and/or reciprocation thereof. Channels 32 and 33 connected respectively with cylinders 62 and 63 are provided for introducing pressure fluid to the 35 cylinders to eifect rotation of the value plunger 67 in a clockwise `or counter clockwise direction as desired. Longitudinal shifting of the valve to effect reversal of the table is effected by introducing pressure\ into the channel 34 which leads to cylinder 68 formed in bracket 69 attached to one end of the valve body 18 having piston 70 therein engaging one end of valve plunger 67 and by coupling the channel 35 connected to the cylinder 71 formed in the bracket 31 having piston 72 engaging the other end of valve plunger 67 with the reservoir or vice versa depending upon the direction of movement desired. To better control the ow of uid in the channels 32 to 35 inclusive, a pilot valve, such as 36 shown in section in Figure 7, is provided which is also capable of rotary as well as reciprocatory movement. These movements are effected by a control plunger 37 adapted to be moved vertically byh dogs, such as 38 and 39, aswell as rotated by dogs 40.or 41 carried on the front edge of the table. Vertical reciprocation of the plunger 37 by the dogs 38 or 39 will effect rotation of the pilot valve shaft 42 thereby chariging the rate of movement of the table and rotation of the plunger by either the dogs 40 or 41 will cause reciprocation of the pilot valve and a resultant change of direction in the movement of the table. This is accomplished in the following manner. Fluid pressure is conducted to 05 the pilot valve from the pressure line 24 through branch line 53. 'I'he valve casing 36'is shown in Figure 7 as' having an inner port connection as at 73 anda contained distributor bushing 74 having cannelure or groove 80 circumferentially 7o communicating with the port 'z3 and a.y port 'z5' throughwhich the uid is conducted to the annularv groove 91 formed in the valve plunger and thence to the longitudinally extending high pressure bores 78 and 79. Distributor passages 77 75 conduct the n uid to ports 76 formed in annular grooves 75 which communicate with the pipes 45, 33, 32- and 34. Other passages 82 similar to 77 are formed at the valve plunger which communicate with a pair of longitudinal low pressure bores 81 angularly displaced 90 with respect to the high pressure bores and discharging into chamber 83 the latter being-connected to reservoir by channel 56. 'Thus by proper positioning of the passages 77 and 82, uid pressure may be conducted to selected channels while other channels are connected to reservoir.

The selector valve member 67 is of general cylindrical form having the various grooves or slots as at 84 for coupling the ports of valve casing 18 and intermediate bushing 85 having cannelures 86 and ports 87 aligned with the ports 88 of casing 18 serving to transmit the power actuating medium. Valve `18 may, if desired, be formed with the central passage 89 communicating as through outlet 90 with the grooves 84.

It will now be understood that the rotating movement of the selector valve will `line up different grooves 84 with different inlet and outlet ports and that similarly longitudinal sliding of the valve member will also couple different series of ports through thegrooves according to whether actuating medium is to be directed/into cylinder 13 for urging the piston in one direction or the other and for actuating the piston in given directions at either a feed ora quick traverse rate.

From the foregoing it should now be apparent that the position of the pilot valve determines the position of the selector valve which in turn determines the coupling of the pump lines to the table cylinder and thereby the-rate and direction of movement of the table.

Mention has been made that it is desirable at times to cause a pause in the movement of the work table and usually at a time immediately prior to a reverse movement taking place, whereby the cutter may cut itself free and thereby properly nish the surface in-contact therewith before separation therefrom. In the present instance the mechanism for accomplishing this object includes a permanent stop, such as 43, having adiustably mounted therein av stop` bolt 44 against which a portion of the movable table may abut at the end of its forward or feeding stroke. This limits the forward feeding movement ofthe table and since it is desired to' properly finish the workv before the table returns, means have been provided for postponing the shifting of the selector valve. f To cause the selector valve to move toward the right, as shown in the drawings, and thereby effect movement of the table to the right, pressure must be admitted to the channel 35. The delay unit is interposed between the channel 35 and the line 45 leading from the pilot valve.

This unit comprises a cylinder 46 having reciprocably mounted therein the slldable valve 47 having spools 48 and 49 thereon. It will be noted that when the valve is in the position shown, that the spool 48 disconnects the pressure line 45 from the channel 35 thereby preventing the admission of fluid to effect shifting of the valve to the right. A reduced portion 50 on the valve connects the channel 35 with the returnline 51 leading to the reservoir. To insure that the valve 47 is maintained in this 'lowered position pressure is supplied to the upper nd of the cylinder 46 through a channel 52 which is .directly connected to the channel 34 and adapted to receive pressure from the feed line 24 through the branch line 53 leading therefrom `to the pilot valve. Pressure in line 52 will flow through the check valve 53 directly to the top of the unit.

With the parts in the position shown, Athe table is feeding toward the left and fluid is flowing 6 in the respective channels as indicated by the arrows in full lines. In other words, pressure is being admitted to the right end of the cylinder and fluid is being Withdrawn from the lopposite end by the variable displacement pump 20. At the same time pressure is flowing. from the channel 24 through the branch line 53 of the pilot valve which in turn directs it to the channeli34 to the right end of the selector valve thereby maintaining it shifted toward the left while the channel 35 is connected to reservoir permitting this movement. and the delay unit 46 is maintained in a downward position through the flow of pressure from the channel 52 through the check valve 53'. Simultaneously pressure is being admitted through the channel 32 and withdrawn through the channel 33 to maintain the selector valve rotated in the proper position for a feeding movement. Just prior to the engagement of the table with the stop 44 the pilot valve will be dog actuated to the position shown in Figure 3. The channel 45 will now be connected with the channel 53. Due to the valve 47 being in its lowermost position channel 45 will not be connected to channel 35 which will prevent immediate shifting of the selector valve to the right. Since the selector valve is unmoved by this operation the table will continue to move to the left and abut the stop 44. However, during' the small amount of movement of the table which will take place between the time of the dog actuation of the pilot valve and the abutting of the stop, the direction of flow in the channels 45 and 52 has been reversed and pressure is being admitted to the'bottom of the valve 46 tending to raise the valve member 47. Since the channel 45 is now connected with the forward pressure line 24 or in other words with the table propelling pressure lline, the pressure in channel 45 will increase upon abutment of the table. with a fixed stop. The check valve 53' will close preventing fiow of fluid therethrough and this will cause the fluid in the chamber 54 of the delay unit to be forced through the needle valve 55.

This valve may comprise a casing having a tapered thread formed therein for the reception of a plug having a pipe thread formed on the end lthereof. The flats on the pipe thread permit a.

slight flow of fluid spirally around the plug. It will be apparent that upon complete tightening of the valve all flow will be prevented while a slight turn of the valve in the opposite direction will permit flow that may be varied according to the amount of movement thereof. Fluid flowing through the valve will escape to the reservoir through channel 52, the pilot valve and channel 56.

It should now be apparent that the needle valve acts as a throttle to delay the upward movement of the valve member 47 and thus delay the establishment of pressure connection between the channel 45 and the channel 35. The time of this delay depends upon the adjustment of the valve 55. The channel 45 is directly connected with the channel 24 which insures positive raising of the valve spool 48. Upon the establishment of connection between the channel 45 and channel 35, the selector valve will be immediately shifted to the right, reversing the direction of flow in the channels 16 and 17 causing return movement of the table. The valve 47 will remain 7 in an upward position due to the constant pres- "effect reversal is delayed in a manner similar sure in the channel 4 5 until the next reversal of the pilot valve. 'Ihe circuit shown in Figure l is arranged to produce a one-way cycle and therefore, upon completion of its return movement, the table will stop. The table may be started, again by manually shifting the pilot valve which will resultin the valve 47 being moved down again to its present position.

In reciprocating milling, a work piece is placed upon each end of the table, and each subjected alternately to the action of the cutter. If the Work were of the type shown in Figure 5, a delay 'in the reversal of the table at the end ofv each stroke would be necessary in order to properly finish the machined surfaces. To this end the circuit shown in Figure l may be modified slightly to effect such a cycle.

In the modification, the admittance of fluid. pressure to the right end of the selector valve to to that previously described. The channel 34 is connected to the delay valve 46 which now becomes a double acting valve. The channel 45 leading from the pilot valve is divided into two parallel branches 57 and 58, the branch 57 having the check valve 59 which permits flow to the delay unit but prevents return flow and the branch 58 having the needle valve 60 through which the return fluid is forced when the check valve 59 closes. i

Upon movement of the table to the right, the pilot valve 36`will be dog actuated to the position shown in Figure 5 connecting the channel 52 with pressure thereby urging the valve 47 downward. This downward movement is delayed by the clos- 110 ing of the check valve 59 causing the fluid in the chamber 61 to be forced through the needle valve which in turn delays shifting of the selector valve.

Completion of the valve movement will establish connection between channels 52 and 34', admit- 115 ting pressure to the right end ofthe selector valve. This will shift the selector valve and thus reverse the movement of the table. It will be noted that the valve has now moved to its lower position which is similar to that shown in Figure'l and 120 the delayed action in shifting the selector valve to theright will be similar to that described for that figure.

Thus a delay unit has been provided for a reciprocating cycle circuit whereby a delayed action is effected at the end of each stroke of the table. Attention is invited to the fact that the use of separate needle valves makes it possible to adjust the time of delay independently for each of the strokes.

That which is claimed is:

1. A machine tool having a reciprocating slide, a fixed stop, hydraulic means including a source of pressure for feeding the slide against the stop and returning it to starting position, a reversing valve for said means, hydraulically' actuated means for shifting the valve to effect a change in direction of slide movement, an auxiliary control valve for coupling said pressure to said hydraulically actuated means, dog actuated means effective upon approach of the slide to the stop to shift the control valve to effect a coupling of the pressure for movement of the reversing valve, delay means to postpone movement of the reversing value, and dog actuated means effective upon return movement of the slide to shift said control valve to directly effect movement of the reversing valve. l

2. A machine tool having 'a reciprocating slide, trip dogs carried thereby, a fixed stop, hydraulic means including a source of pressure for feeding the slide against the stop, a reversing valve,- hydraulically actuated means for shifting the valve, a pilot valve actuable by said dogs for determining the direction of iiow to said hydraulically actuated means, means to delay the ow in one direction to postpone reversing of the slide, and means to by-pass the last named means upon flow in the opposite direction to directly effect reversal of the slide.

3. A machine tool having a reciprocating slide, trip dogs carried thereby, a fixed stop, hydraulic mean including a source of pressure for feeding the sli e against said stop, reversing valve shiftable to`cause retraction of t e slide from said stop, a fluid operable device for shifting the reversing valve, a pilot valve actuable by said dogs for de'- livering fluid to said iiuid operable device and thereby cause retraction of the slide, a shiftable stopvalve between the pilot valve and said fluid operable device for postponing the pressure ow to said last named means, said stop valve having means responsive to the pressure released by the pilot valve to shift the stop valve and effect coupling after a predetermined interval of time of the pilot valve with said iiuid operable device.

4. A machine tool having a reciprocating slide, hydraulic means for reciprocating the same, a reverser for changing the ow and thereby thc direction of movement of the slide, hydraulically actuated means for shifting the reverser, a source of fluid pressure, control means for conducting uid pressure to said hydraulically actuated means to eect movement of the reverser in one direction, additional means for conducting pressure to effect movement of the reverser in the other direction, and a shiftable delay valve associated with said last named means to stop ow therein, and means in the valve automatically responsive upon admission of pressure thereto to shift the valve slowly andeffect after a. predetermined interval ofV time the admittance of pressure to said reverser.

5. A machine tool having a reciprocating slide, hydraulically actuated means for eifecting said reciprocation, a stop, a conduit for admitting pressure to said means to effect movement of the slide against the stop, a pressure shiftable reversing valve in the conduit, a pilot valve therefor, means effective prior to engagement of the slide with the stop to shift the pilot valve and cause a pressure shifting flow to the reversing valve, a shiftable delay valve positioned to stop said shifting iiow, means responsive to pressure to shift the delay valve and effect a delayed opening thereof, a common source of pressure for the slide actuating means and the reversing valve shifting means whereby the sainel pressure utilized for feeding the slide into the stop will actuate the delay valve to shift the reversing valve and thereby cause movement ofthe slide away from the stop.

6. A machine tool having a reciprocating slide, an hydraulic motor for effecting said reciprocation including a piston and cylinder, one of which is connected to said slide, a source of hydraulic pressure, a reversing valve interposed between said source and said cylinder for determining the direction of movement of the slide, hydraulic actuated means for shifting said valve, a pilot control valve for said means, and dog control means effective upon movement of the slide in one direction to shift said pilot valve to cause movement of the reversing valve, a delay valve interposed between the pilot valve and the reversing valve to postpone, the admissionbf pressure to the reversing valve, pressure responsive means in the delay valve for shifting the same to an open position and means to maintain the delay valve in its shifted open position upon establishment of pressure connection therethrough to the reversing valve.

7. A machine tool having a reciprocating slide, an hydraulic motor for effecting said reciprocation, a source of hydraulic pressure, valve control means for determining the effect of the pressure on the motor as to rate and direction, power means for actuating said valve means, trip dogs carried by the slide for controlling the operation of said power means and a shiftable delay unit interposed between the power means and valve means for retarding the actuation thereof, pressure responsive means for shifting the delay unit to an inoperative position, said delay means being actuated by the forward pressure in the circuit.

8. A machine tool having an hydraulically actuated table, means to reverse the direction of movement of the table including a reversing valve, power means for shifting said valve, means to delay the shifting of the valve by the power means including a fluid shiftable valve having ports alternately subjectable to pressure interposed between the power means and reversing valve, and a needle valve effective to retard exhaust from one port during admission of pressure to the other of said ports whereby actuation of the shiftable valve and thereby the establishment of connection between the power means and reversing valve will be delayed.

9. A milling machine having a support, a table reciprocably mounted thereon-for movement between xed abutments, fluid operable means for reciprocating the table, table control means eifective to change the direction of movement thereof upon'engagement with each abutment including 115 a fluid operable reversing device, a trip operable pilot valve shiftable to direct a ilow of actuating uid for power operation of the reversing device, spaced means carried by the table for tripping said valve as .the table approaches each abutment, means eective to establish a timed interval delay between shifting of the pilot valve and the flow from said pilot valve to said fluid operable reversing device, and independent means for adjusting the action of said delay means and thereby the length of the delay at each end of the stroke.

10. A milling machine having a reciprocating table, hydraulic means for eifecting said reciprocation, independent hydraulically actuated means for effecting reversal of the table at the end of each stroke, a source of pressure, control means for directing said pressure alternately through `independent channels to each of the reversing means, means to interrupt the flow in either channel and thereby delay reversal of the table including a shuttle valve shiftable by pressure in either channel to effect opening of that channel and closing of the other, and hydraulic means to retard the shifting movement.

11. A machine tool having a. reciprocating support, power operated means for effecting said reciprocation including a reversible motor coupled to the support, a source of power therefor, reversing means selectively positlonable for determining the directional effect of the power on themotor, 145 power actuable mechanism for positioning said reversing means including a control member trip actuable by a control dog carried by the reciprocating support for potentially coupling the mechanism to the source of power, a shiftable motion 150 interrupter disconnectingv the control member from the power actuable mechanism, said interrupter having means responsive to the power applied thereto upon said potential coupling to effect shifting of the interrupter to a power coupling position, the time of said shifting movement being sufficient to cause a predetermineddelay between the trip actuation of said control member by the support and the actual movement of said reversing means 12. A machine tool having a reciprocating support, power operated means for effecting said reciprocation including a reversible driving member operatively 'coupled to the support, a source of power therefor, reversing means selectively positionable for determining the directional effect of the power on'said driving member, power actuable mechanism for positioning said reversing means including a control member trip actuable by a control dog carried by the support for potentially coupling the mechanism to the source of power, a shiftable motion interrupter disconnecting the control member from the power actuable mechanism, means in the interrupter responsive to the application of power resulting from said potential coupling to effect shifting of the interrupter to' a power coupling position, the time of said shifting movement being suilicient to cause a predetermined delay between the trip actuation of the control member by the support and the shifting of said reversing means.

13. A machine tool having a reciprocating support, power operated means for effecting said reciprocation including a reversible motor coupled to the support, a source of power therefor, reversing means selectively positionable for determining the directional effect of the power on the l motor, power actuable lmechanism for positioning said reversing means including a control member trip actuable by the support during movement thereof, a pair of branch lines connecting the coni trol member to said mechanism, said control member having a rst position for coupling the source of power to one of said branch lines to effect direct actuation of said mechanism, a second po- .sition for potentially coupling the source of power to said mechanism, to eifect reverse movement of said support, an interrupter in said last named branch line, means in the interrupter responsive to the application of power resulting fromsaid potential'coupling to shift the same to a power coupling position, the time of said shifting movement being -suicient to cause a predetermined delay between the trip actuation of saidcontrol member by the support and the actual movement of said reversing means.

14. In a machine tool having a rotatable cutter and a work support, .the combination of meansl either position to the other, a pilot valve constantly connected to said pressure port, delivery channels extending from the pilot valve to the reversing valve actuating pistons, a shiftable valve having a rst port connected to one of said delivery channels, a second port connected to one of said actuating pistons whereby coupling of said rst port by the pilot valve to the pump will operate said shiftable valve to couple said first port to the second port and thereby a shifting of the reversing valve, said shiftable valve having a third port adapted to be connected to reservoir during operation thereof, and means to restrict the return flow from said `third port and thereby retard operation of the shiftable valvev and simultaneously a delay in the shifting of the reversing valve.

15. A delay unitfor an hydraulic control circuit for a machine tool in which a reversing valve having a pair of operating pistons determines the direction of movement of a movable slide thereof and a dog actuated pilot valve determines the coupling of pressure to said pistons and thereby power 'shifting of the reversing valve, including a casing having a reciprocable plunger therein, a first and second port at opposite ends of the casing, a third port connected to one piston ofthe reversing valve and an exhaust port, a rst channel for connecting one of said end ports to the pilot valve whereby upon movement of the pilot valve to a position to couple pressure to said channel the plunger will be shifted to a iirst position coupling the third and fourth ports to effect simultaneous shifting of the reversing valve, and a needle valve operative to restrict return flow from said fourth port upon admission of pressure to the first port to eifect shifting `of the plunger in the opposite direction thereby delaying connection of the vfirst port to the third port and thus a predetermined delay in the shifting of the reversing valve after dog actuation of the pilot valve.

16. A delay unit for an hydraulic control circuit of a machine tool in which a reversing valve having a pair of v.operating pistons is employed in the circuit for determining the direction of movement of a movable slide and a dog actuated pilot valve is employed for alternately coupling pressure to the reversing valve actuating pistons, including a casing having a shiftable plunger therein, ports at opposite ends of the casing, a third and fourth port connected to the respective operating pistons for the reversing valve, an exhaust port intermediate said third and fourth ports, a llrst and second channel means connecting the respective end ports to the pilot valve, each of said channels having a check valve to block return ilow from the ports, and a needle valve in parallel with said check valve operative to retard the return iow whereby upon coupling of pressure to either of said channel means bythe pilot valve, the shiftable valve will be `operated in a delayed manner to couple pressure to a selected one of said operating pistons to effect shifting of the reversing valve, a predetermined time after dogl operation of the pilot valve.

HANS ERNST. CHARLES W. GOODRICH.

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