US2322063A

US2322063A - Booster-assisted fluid pressure compounding systems

Info

Publication number: US2322063A
Application number: US436634A
Authority: US
Inventors: Schnell Steve
Original assignee: Wagner Electric Corp
Current assignee: Wagner Electric Corp
Priority date: 1942-03-28
Filing date: 1942-03-28
Publication date: 1943-06-15
Anticipated expiration: 1960-06-15

Description

June 15,1943. I s. SCHNELL 2,322,063

BQOSTER-ASSISTED FLUID PRESSURECOMPOUNDING SYSTEMS Filed March 28, 1942 2 Sheets-Sheet 1 3|

C H63

70 ,63/ 62 60 v 7| 13 INVENTOR STEVESCHNELL ATTORNEY 2 Sheets-Shae 2 S. SCHNELL BOOSTER-ASSISTED FLUID PRESSURE COMPOUNDING SYSTEMS June 15, 1943.

. INVENTOR s E ESCHNELL k BY ' ATTORNEY Patented June 15, 1943 BOOSTER-ASSISTED FLUID PRESSURE COMPOUNDING SYSTEMS Steve Schnell, Kirkwood, Mm, assignor to Wagner Electric Our-notation, St. Louis, Mo., a corporation of Delaware Application March 28, 1942, Serial No. 636,634

(Cl. Gib-54.5)

18 Claims.

My invention relates to fluid pressure actuating systems and more particularly to a system embodying improved means whereby a large volume of fluid can be displaced during the initial increments of movement of an actuated member and a smaller volume of fluid can be displaced during later increments of movement of said member.

One of the objects of my invention is to provide power-operated means for assisting in the displacement of the large volume of fluid in a fluid pressure actuating system and thereby relieving the operator of some manual eifort.

Another object of my invention is to so construct said large volume fluid displacing means and associate therewith the power-operated assisting means that said power means and the large volume fluid displacing means will automatically become inoperative when a predetermined fluid pressure is reached and subsequently assume their normally inoperative conditions without the necessity of a release of the fluid pressure below said predetermined value.

Other objects of my invention are to produce an eflicient low cost fluid pressure compounding system embodying means having a power-operated member for assisting in the displacement of a large volume of fluid and particularly such means that it can be associated with the present standard parts of a fluid pressure actuating system.

Other objects of my invention will become apparent from the following description taken in connection with the accompanying drawings in which Figure 1 is a schematic view of a fluid pressure actuating system embodying my invention; Figure 2 is a sectional view showing details of the power-operated means; and Figure 3 is a sectional view of the pressure-operated control valve means.

Referring to the drawings in detail and flrst to Figure 1, my improved fluid pressure actuating system comprises, as essential parts, a master cylinder device A, a power-operated device 13 for displacing a large volume of fluid, and a pressure-operated control valve means C, said system being shown as employed for actuating brakes D although other devices may be actuated if desired.

.The master cylinder A is of standard construction and comprises a cylinder I having reciprocable therein a piston 2, said piston being actuated by a piston rod 3 and pedal 4. Above the cylinder is a reservoir 5 which is in free twoway communication with cylinder l by means of a compensating port a only when the piston is in its retracted position where it is returned by a spring l.

A conduit 8. leads from the outlet of the master cylinder to branch conduits a and iii, the former leading to the power-operated device B and the latter leading to the pressure-operated control valve means C. A conduit ii connects the power-operated device B with the fluid motors I2 for actuatingthe brakes D. Another conduit 83 places the pressure-operated valve means C in communication with the fluid motors.

Referring to Figure 2, the power-operated fluid pressure displacing means'will now be described. This device comprises a casing member M provided with a large bore i5 and a smaller bore 16, bore it being connected to conduit 9 leading from the master cylinder and the large bore l5 being connected to conduit ll leading to the fluid motors of the brakes. Within the large bore i5 is a piston ll to which is attached a forwardly extending hollow piston rod l8 havin bearing in a member i9 threaded into the end of the large bore and acting as an end wall thereof. The piston ii is of annular constructionln order to provide a bore 20 which is of the same diameter as the small bore I6, said bore 20 forming a continuation of bore l6 when the piston is in its normally inoperative position as shown in Figure 2. A packing cup 2i is carried by the piston H for sealing it against passage of fluid when the piston is moved forwardly,

said packing cup, however, not preventing fluid from flowing in the opposite direction. In bores i6 and 20 there is mounted a double-headed piston 22, head 23 thereof being positioned in bore l6 and head 25 being positioned in bore 20. Sealing cups 25 and 26 are associated with heads 23 and 24. The bore 20 communicates with bore l Sahead of piston IT by way of a passage 21.

The piston rod l8 for piston H is guided by a guide sleeve 28' integral with member l9 and associated with the piston rod is a packing cup 29 to prevent any leakage of fluid from bore l5. The outer end of the piston rod is connected to a piston 30 which is adapted to reciprocate in a cylinder 3| mounted on member IS. 'The piston and cylinder form a vacuum fluid motor which is the power device employedfor actuating piston H. A spring 32 acts on piston 30 and normally biases said piston to its inoperative position, as shown in Figure 2, which inoperaitve p sition will correspond to'the inoperative posi- ,communicates with the atmosphere.

tion of piston I! as determined by its abutment with the end of large bore I5. The piston carries a sleeve 33 which surrounds the guide sleeve 29 and a packing 34fis associated with the sleeves in order to make the compartment 35 between the piston and member l9 fluid-tight.

The piston 30 carries a cup-shaped member 36 providing a valve compartment 37, said compartment being in communication with compartment 35 at the rear of piston 30 by an opening 38 through the wall of the piston. The compartment also communicates with a compartment 39 ahead of the piston by way of a central opening 40 in the wall of member 36. Positioned within the valve compartment is a valve element 4| having a sealing ring 42 for engaging the wall of member 36 which surrounds opening 40 to thus prevent communication between compartments 31- and 39. One side of the valve member 4| carries an annular projection 43 which slidably fits into the end of the hollow piston rod l8, said end being enlarged to receive a coil spring 44 for biasing the wave element 4| toward a seated position. The other side of the valve element also carries an annular projection 45 and secured thereto is a tubular element 46 which extends forwardly through the forward end wall of cylinder 3|. Packing means 41 seals this tube with the end wall of the cylinder. The end of the projection 45 is provided with a sealing ring 48 with which cooperates a valve element 49 formed as a head on the rod 50 which extends completely through the hollow piston rod I8. The rear'end of this rod is secured to the double-headed piston 22. Packing 5| seals the rod. The end of rod 50 adjacent the valve element 49 is fluted in order that air may flow along the rod and through a slot 52 in the valve element 4| when the valve element 49 is unseated. It is thus seen that compartment 31 can be placed in communication with tube 46 which Valve element 49 is normally held seated by a spring 53 interposed between the double-headed piston 22 and piston IT. This spring also holds element 4| unseated. against the bias of spring 43 when piston IT in the large bore and piston 36 are in their retracted positions. It is to be noted that when piston abuts the end of its bore, piston 22 is still some distance from the end of bore l6 and, therefore, spring 53 is free to expand and pull rod 50 rearwardly.

The end of cylinder 3| through which tube 46 extends has associated therewith an air cleaner 54 for cleaning the air before it passes into tube 46. The end wall of the cylinder also has attached thereto a tube 55 which is connected to a suitable vacuum supply such as the manifold 56 (see Figure l) of the engine of the vehicle. The compartment 39 in cylinder 3| ahead of piston 30 is thus in constant communication with this source of vacuum. When the valve element 4| is unseated, compartment 35 at the rear of piston 30 will also be in communication with the source of suction.

Referring now to Figure 3, the pressure-operated control valve means C will be described in detail. The valve means is enclosed within a casing 57 provided with communicating

bores

58 and 59, the former being slightly larger than the latter. The end of the large bore is closed by a plug 66, which also serves as a connecting fitting for branch conduit l6 leading from the master cylinder. The end of the smaller bore is connected with conduit l3 leading t9 $.11? fill-K motors of the brakes. The plug 60 has a central passage 6| and a cross-passage 62. A packing element 63 is carried by the fitting between the cross-passage 62 and its inner end in order to prevent fluid from flowing from said cross-passage to bore 59 and between the periphery of the fitting and the wall of the bore. The packing cup, however, is so constructed as to collapse and permit fluid under pressure to flow from the bore to the cross-passage 62.

Within bores-58 and 59 are

pistons

64 and 65, respectively, formed integrally with. each other. Piston 64 is provided with a projecting portion 66 which carries a valve element 61 for engage ment with the wall at the end of passage 6| of fitting 50. A spring 68 of predetermined strength is interposed between the small piston and the end of bore 59 in order to bias the valve element El seated and thereby close passage 60. A passage 69 extends through the pistons so that when the valve element 67 is unseated, fluid can flow from conduit Hi to conduit 3 through the valve mechanism.

Pistons

64 and 65 carry suitable sealing elements 10 and H. The pistons are capable of having only slight movement in their bores, which movement is sufiicient to permit the unseating of the valve element 6'! against the predetermined force exerted by spring 68. The amount of movement of the pistons is determined by the distance between piston 64 and the end of bore 58 when the valve element 61 is seated. I i 5 Since piston H in device B will move forwardly and increase the volume of chamber 12 at the rear thereof, provision must be made to keep this chamber continuously filled with liquid. This is accomplished by providing a conduit 13 for connecting reservoir 5 of the master cylinder device "A with chamber 12. Associated with this conduit is a check valve 14 biased to closed position by a spring 15. This check valve prevents fluid from flowing from chamber 12 to the reservoir but will not prevent flow in the opposite direction in order to keep the chamber filled. Inorder to assist flow of fluid from chamber 12 past piston l1 and packing cup 2| as the piston is retracted, holes 16 are provided in the wall of the piston.

Referring now to the operation of the fluid pressure actuating system, all the parts thereof will be in the positions shown in the various figures when the system is inoperative. The valve element 61in the pressure-operated con trol valve means C will be closed by the action of spring 68. Pistons I1, 22, and 36 will be in their rearmost positions as shown in Figure 2 and spring 53 will be effective to maintain the valve element 49 seated and valve element 4| unseated. Under these conditions compartments 39 and 35 on opposite sides of piston 30 will both be in communication with the source of suction and due to this, there will be no force acting to move piston 36.

When it is desired to apply the brakes, the master cylinder A will'be operated by actuating pedal 4. As soon as the compensating port 6 is closed, pressure will be developed by piston 2 and this pressure will be efiective on piston head 25 of piston .22. No fluid can flow directly through the valve means C since passage 6| is closed. As soon as sufficient pressure is developed to cause movement of piston 22 and compress spring 53, rod 50 will be pushed forwardly. As this rod moves forwardly, spring 44 will be eifective to move valve element 4| therewith and cause seating thereof, thus closing off communication beassaoes tween compartments and .on opposite sides of piston 30. Continued movement 'of piston rod will now cause valve element 49 on the end thereof to be unseated, thereby permitting air under atmospheric pressure to enter the valve v compartment 31 and compartment 35 at the rear of piston 30. This will now cause a dinerential fluid pressure to act on piston 30, therebymoving it to the left as viewed in Figure 2. As

the valve open will be that necessary to move piston 22 against the light force of spring 53 and the fluid pressure which is being developed by piston l'l since head 24 of piston 22 is sub jected to the pressure in bore l5 due to opening 21 through piston i1.

The volume of fluid being moved by piston li, however, will be larger than the volume of fluid being displaced by the master cylinder since the area of piston I1 is larger than the area of piston head 23 being acted upon by the fluid pressure developed by the master cylinder. If piston l'l should be twice as large as the heads of the double-headed piston 22, the volume being displaced by piston It will be twice that being displaced by the master cylinder. However, the pressure necessary to obtain this displacement will only be that necessary to move piston 22 against the action of spring 53 and the fluid pressure acting on piston head 2d. The work necessary to move piston ill will be done by the vacuum motor. If the head of piston H is twice as large in area as the piston heads 23 and 26, as assumed, then the work necessary to accomplish the displacement of a large volume of fluid by piston ill will be equally divided between the vacuum motor and the master cylinder since each causes the movement of a piston, the areas of which being eflective' in acting on the fluid displaced are equal due to the arrangement including passage 21.

If the depressing movement oi the pedal should be discontinued, then the vacuum motor will also discontinue its operation. This is caused by the follow-up type of valve means controlling the suction motor. As soon as rod 50 ceases to move, piston 36 will continue to move only enough to seat valve element d9, thus resulting in the pressures in

compartments

39 and 35 bearing such relation to each other that there will be a differential force just sufilcient to hold pistons 36 and it against any return movement by the fluid pressure already developed in bore it. As piston ii moves forwardly, fluid will constantly enter the expanding chamber iii from reservoir 5 and maintain it filled.

When the fluid pressure developed by the pistons i1 and it reaches a predetermined value it will cause valve elements? of valve means C to be unseated and maintained unseated against spring 53.

Xhe unseating .of the valve element 61? is brought about by the action of fluid pressure on the larger area of piston 85, it being noted that this same pressure also acts on the small area of piston 8 due to through passage 69. When the force acting to move piston 5A to the left becomes greater than the force holding the valve 6? seated (spring force plus force from fluid action on small area) the unseating will take place.

with valve element 61 unseated fluid will be capable of flowing directly from the master cylinder to the fluid motors I! of the brakes. When this condition occurs, spring 53 can no longer be compressed since the piston heads 22 and 24 at the opposite ends of piston 22 will both be in communication with the master cylinder and, therefore, subject to the same pressure. Spring 53 will now expand and the relative positions of pistons I1 and 22 will be as shown'in Figure 2.

This will cause the valve element 49 to become seated and the valve element 4| to become unseated. Both compartments 35 and 39 on'opposite sides of piston 30 will now be subject to the same sub-atmospheric pressure and there will be no force-tending to move piston 30 except that of spring 32. This spring 32, which is fairly strong.

will now cause piston 30 to be moved rearwardly and will push piston I! also rearwardly. Piston i! will not be prevented from moving rearwardly by the'fluid in compartment 12 at the rear thereof since this fluid can be forced past the packing cupil by the action of spring 32 and into the portion of the bore ahead of the piston. Pistons I1 and 22 and thacuum motor piston 30 and the valve elements will finally assume the positions shown in Figure 2 where they will be ready for subsequent operation. The return of these pistons to their normally inoperative positions will not have any effect whatsoever on the con-' cylinder through the valve means C. Fluid under pressure can pass the valve element 67 notwithstanding that it may be closed as return flow is permitted by the passage $3 the collapse of the packing cup83 and the cross-passage d2.

If, during the retractile movement of the piston of the master cylinder, it is desiredto re-apply the brakes, the power-operated fluid pressure displacing means B will come into operation only when the fluid under pressure in the system is permitted to drop below the predetermined value at-which the valve means C is opened. If the pressure is below this predetermined value, then the suction motor will become operativeuntil the pressure exerted by the master cylinder piston is again the predetermined value at which time the valve 61 will be opened. The vacuum motor will again become inoperativeand pistons l1 and 22 automatically returned to their inoperative positions in the manner already described.

It is thus seen from the described fluid pressure actuating system that there is provided means for displacing a large volume of fluid during the initial movement of the p ston of a master cylinder. The displacing of this large volume of fluid does not result in any additional effort on the operator since the vacuum motor does the work necessary to move the extra volume as determined by the difierence between the areas of the pistons 22 and H. After the large body of fluid has been displaced, the master cylinder is automatically directly connected to the fluid motors of the brakes and. the system acts as though the large volume displacing device were not incorporated in the system. All the parts of said device automatically return to their normally inoperative positions and without the necessity oi any release of fluid under pressure by release of the master cylinder piston. Because of the large volume displacing device being incorporated in the systom, the size of the master cylinder piston may be reduced since it is not necessary for it to displace a large volume of fluid in order to get the brake shoes initially applied and the slack taken up in the system. With a smaller master cylinder piston, higher fluid pressures can be developed with the same pedal ratio since a considerable amount of the pedal travel will be saved due to the fact that it is not required in displacing fluid to take up slack in the braking system. It is also to be noted that in the use of the power-operated large volume fluid displacing device there will be no noticeable eflect on the operator's foot at the time that the displacing device becomes inoperative and valve 6? is opened to directly connect the master cylinder with the fluid motors of the brakes. When the power-operated device is being operated, the fluid pressure in the fluid motors of the brakes will be substantially the same (slightly lower) as that being develomd by the master cylinder. It is also to be noted in the system disclosed that there is no necessity for a specially designed master cylinder as the poweroperated large volume displacing device B and the valve means 6' may be interposed in the fluid pressure lines wherever it is convenient. Of course, the device B and valve C may be specially designed into the master cylinder to make a single unit if such is desired. It is very easy to install the units in any existing braking system and the cost thereof will be low since it will not be necessary to replace the master cylinder already being employed in the braking system.

Being aware of the possibility of modifications in the particular structure herein described without departing from the fundamental principles of my invention, I do not intend that its scope be limited except as set forth by the appended claims.

Having fully described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device connected to the fluid motor, a poweractuated fluid pressure displacing device connected to the fluid motor and having a source of power independent of the master cylinder device, means for preventing the fluid under pressure developed by the master cylinder from being transferred to the motor until a predetermined fluid pressure is attained in the fluid motor, and means for controlling the operation of the poweractuated device by fluid pressure from the master cylinder prior to said master cylinder being connected to the motor.

2. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device connected to the fluid motor, means for preventing the fluid pressure developed by the master cylinder from beingtransferred to the motor until after a predetermined pressure is reached, means comprising a power-actuated fluid displacing device having a source of power independent of the master cylinder device and controlled by the fluid pressure developed by the master cylinder prior to said predetermined pressure for placing fluid under pressure in the fluid motor, and meansfor causing the power-actuated fluid pressure displacing device to become inoperative whenever the pressure developed by the master cylinder device is above the predetermined pressure.

3. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device, conduit means for placing the master cylinder device in communication with the fluid motor, valve means associated with the conduit for preventing said communication until a predetermined pressure is developed by the master cylinder device, a fluid displacing device connected to communicate with the motor and including a movable member, power means independent of the master cylinder device for operating the movable member, and means controlled by the fluid,

pressure developed by the master cylinder when said pressure is below the predetermined value l for causing said power device to be operable.

4. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device, conduit means for placing the master cylinder device in communication with the fluid motor, valve means associated with the conduit for preventing said communication until a predetermined pressure is developed in the fluid motor, a fluid displacing device connected to communicate with the motor and including a movable member, a second fluid displacing device also connected with the motor and including a movable member, power means having a source of power independent of the master cylinder device for operating the movable member of the first fluid displacing device, and means controlled by the fluid pressure developed by the master cylinder when said pressure is below the predetermined value for causing said power device to be operable and also the movable member of the second fluid displacing device to be moved to displace fluid.

5. In a fluid pressure actuating system, a fluid motor for actuating a device, a cylinder connected to communicate with the motor, a piston in said cylinder. a source of power and power-operated means for actuating the piston, control means for the power means, a source of fluid pressure, means for operating the control means by pressure from the source, and means for placing the source in direct communication with the motor when the pressure in the motor reaches a predetermined value to thereby operate the fluid motor independently of the power means and its source of power.

6. In a fluid pressure actuating system, a fluid motor for actuating a device, a cylinder connected to communicate with the motor, a piston in said cylinder, a source of power and poweroperated means for actuating the piston, control means for the power means, a source of fluid pressure, means for operating the control means by pressure from the source, means for placing the source in direct communication with the motor when the fluid pressure in the motor reaches a predetermined value to thereby operate the fluid motor independently of the power means and its source of power, and means for causing said power means to be inoperative and the piston to return to its normally inoperative position after said direct communication is established.

7. In a fluid pressure actuating system, a fluid motor for actuating a. device, means comprising two movable members for developing fluid pressure to actuate the motor, a source of power and power operated means for actuating one of the members, means for controlling the operation of tially simultaneous movement, a source of fluid pressure, means for moving said other member by pressure from the source, and means for placing the source in direct communication with the fluid motor when a predetermined fluid pressure is eiiectivein the fluid motor to thereby operate, the motor independently of the power means and its source. I

8. In a fluid pressure actuating system, a fluid motor for actuating a device, means comprising two movable pistons for developing fluid pressure to actuate the motor, a source of power and power operated means for actuating one of the pistons, means for controlling the power means by the movement of the other piston so that said pistons will have substantially simultaneous movement, a source of fluid pressure independently of the source of power for the power means, means for moving said other piston by pressure from the source, and comprising a fluid motor connected to the source and having a piston the same diameter as said other piston, and means for placing the source in direct communication with the fluid motor when a predetermined fluid pressure is efiective in the fluid motor. 9. In a. fluid pressure actuating system, a fluid motor for actuating a device, means comprising two movable membersfor developing fluid-pressure to actuatethe motor, power means for actuating one of the members, means for controlling the power means by the movement of the other member so that said members will have substantially simultaneous movement, a master cylinder device, means for. moving said other member by pressure developed by the master cylinder device, means for placing the master cylinder device in direct communication with the fluid motor only when the pressure being developed by the master cylinder device is above a predetermined value, and means for causing the power means to be inoperative when the pressure developed by the master cylinder device is abovethe predetermined value.

10. In a fluid pressure actuating system, a fluid motor for actuating a device, a fluid pressure developing chamber connected to the motor, two movable members acting on the fluid in the chamber, power means for moving one member, control means for the power means, means for controlling the control means by the movement of the other movable member so that the power means moves the first movable member substantially simultaneously with said other movablemember, a source of fluid pressure independently of the source of power for the power means, means for moving said other movable member by pressure from the source, means for connecting the source to the motor when the pressure from the source is above a predetermined value, and means for preventing the power means from functioning when the source is connected to the motor.

11. In a fluid pressure actuating system, a fluid motor for actuating a device, a fluid pressure developing chamber connected to the motor, two movable members acting on the fluid in the chamber, a suction motor for moving one member, a follow-up valve mechanism for controlling the suction motor, means for controlling the valve mechanism by the movement of the other movable member so that the suction motor moves the first movable member substantially simultaneously with said other movable member, a

source of fluid pressure, means for moving said other movable member by pressure from the source, and means for connecting the source to the motor when the pressure from the source is above a predetermined value.

12. In a fluid pressure actuating system, a fluid motor for actuating a device, a fluid pressure developing chamber connected to the motor, two movable members acting on the fluid in the chamber, a suction motor for moving one member, a follow-up valv mechanism for controlling the suction motor, means for controlling the valve mechanism by the movement of the other movable member so that the suction motor moves 13. In a fluid pressure actuating system, a

fluid motor for actuating a device, a chamber connected to the fluid motor, first and second pistons for placing fluid in the chamber under pressure, a suction motor for moving the first piston, a follow-up valve mechanism for control-, ling the suction motor, means for controlling the valve mechanism by movement of the second piston, a master cylinder device, means for moving the second piston and comprising a. third piston subject to the fluid pressuredeveloped by the master cylinder device, said second and third pistons being of equal diameters, and means for placing the master cylinder device in communication, with the motor when the pressure developed thereby is a predetermined value.

14. In a fluid pressure actuating system, a fluid motor for actuating a device, a cylinder in communication with said motor, a piston movablein said cylinder for displacing fluid under pressure to the fluid motor, means comprising a second piston for acting on the fluid in said cylinder, power means for moving the first piston, control means for the power means, a master cylinder device, means for moving the second piston by fluid pressure developed by the master cylinder device, means for controlling the control means of the power means by the movement of the second piston so that the power means moves the first piston substantially simultaneously with the second piston, means for directly placing the master cylinder device in communication with the motor when the pressure in the fluid motor is greater than a predetermined value, a reservoir, means for establishing a chamber at the rear of the first piston,- means for connecting the reservoir to the chamber soas to maintain said chamber filled as the piston moves forwardly, and

means for preventing fluid from said cylinder to Y return to the reservoir.

15. In a fluid pressure actuating system, a fluid motor for actuating a device, a cylinder in commumcation with said motor, a piston movable in said cylinder for displacing fluid under pressure to the fluid motor, means comprising a second piston for actuating on the fluid in said cylinder, power means for moving the first piston, control means for the power means, a master cylinder device, means for moving the second piston by fluid pressure developed by the master cylinder device, means for controlling the control means of the power means by the movement 01. the second'piston so that the power means moves the first piston substantially simultaneously with the second piston, means for directly placing the motor, a piston in said cylinder for displacing fluid .and provided with a bore in communication with the cylinder, a second piston in the bore, a

fluid motor connected to move the first piston, a

master cylinder device in communication with the motor when the pressure in the fluid motor is greater than 'a predetermined value, a reservoir, means for establishing a chamber at the rear of the first piston, means for connecting the reservoir to the chamber so as to maintain said chamber filled as the piston moves forwardly, means for preventing fluid from said cylinder to returnto the reservoir, means for permitting fluid to flow fromthe chamber tothe cylinder ahead of the piston but not in the opposite direction, and spring means for returningthe first piston to its normally inoperative position when the master cylinder is in direct communication with the motor.

16. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device, conduit means for placing the master cylinder device in communication with the fluid motor, valve means associated with the conduit for preventing said communication until a predetermined pressure is developed by the master cylinder device, a cylinder connected with the motor, a piston in said cylinder for displacing fluid and provided with a bore in communication with the cylinder, a second piston in the bore, a fluid motor connected to move the first piston, a follow-up valve mechanism for controlling the operation of the last named fluid motor, means for operating the follow-up valve mechanism in accordance with the movement of the second piston, and means for moving the second piston by the fluid pressure developed by the master cylinder when said pressure is below the predetermined value.

17. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device, conduit means for placing the master cylinder device incommunication with the fluid motor, valve means associated with the conduit for preventing said communication until a predetermined pressure is developed by the master cylinder device, a cylinder connected with the follow-up valve mechanism for controlling the operation oi the latest named fluid motor, means for operating the accordance with the movement of the second piston, means for moving the second piston by the fluid pressure developed by the master cylinder when said pressure is below thepredetermined value, means for establishing a chamber at the rear of the first piston, a reservoir connected ,to said chamber, a check valve between the reservoir and chamber, means for permitting fluid to flow from the chamber past the first piston to the cylinder ahead thereof but not in the opposite direction, and a spring acting on the first piston tending to return it to its normally retracted position.

18. In a fluid pressure actuating system, a fluid motor for actuating a device, a master cylinder device, conduit means for placing the master cylinder device in'communicationwith the fluid motor, valve means associated with the conduit for preventing said communication until a predetermined pressure is developed by the master cylinder device, a cylinder connected withthe motor, a piston in said cylinder for displacing fluid and provided with a bore in communication with the cylinder, a second piston in the bore, a fluid motor connected to move the first piston a follow-up valve mechanism for controlling the operation of the last named fluid controlling the operation of the last named fluid motor, means for so controlling the follow-up valve mechanism by the movement of the second piston that the first piston will be moved by the fluid motor substantially simultaneously with.

said second piston, and means for moving the second piston by the fluid pressure developed by the master cylinder when said pressure is below the predetermined value, said last named means comprising a fluid motor connected to the master cylinder and having a piston the diameter of which is the same as the diameter of the second piston.

STEVE SCHNELL.

follow-up valve mechanism in