US3823558A - Hydrostatic transmission - Google Patents

Abstract

A driven main pump drives a hydraulic motor through high pressure and low pressure conduits, while a feed pump supplies pressure fluid through a check valve to the low pressure conduit. A control conduit connects the feed pump with a pressure maintaining valve opening into a low pressure container, and includes a shiftable control valve operated by fluid from the high pressure conduit, but connecting in all positions the feed pump with the pressure maintaining valve. In the control conduit portion between the control valve and the feed pump, a throttle is provided whose pressure differential is greater than the pressure differential of the check valve between the feed pump and the low pressure conduit.

Description

United States Patent [191 Tittmann et al.

HYDROSTATIC TRANSMISSION Inventors: Egon Tittmann, Leonberg; Werner Romeisch, Gerlingen, both of Germany Robert Bosch GmbI-I, Stuttgart, Germany Filed: Mar. 15, 1973 Appl. No.: 341,609

Assignee:

Foreign Application Priority Data Mar. 17, 1972 Germany 2218472 References Cited UNITED STATES PATENTS 9/1941 Kendrick 60/329 3/1965 Firth et al. 60/DIG. 5

[ July 16, 1974 3,230,699 l/l966 l'lann et al. 60/448 X 3,360,934 1/1968 Moyer et a1. 60/329 3,429,123 2/1969 Burroughs 60/451 X 3,650,108 3/1972 lsaac 60/451 X Primary ExaminerEdgar W. Geoghegan Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT Adriven main pump drives a hydraulic motor through high pressure and low pressure conduits, while a feed pump supplies pressure fluid through a check valve to the low pressure conduit. A control conduit connects the feed pump'with a pressure maintaining valve opening into a low pressure container, and includes a shiftable control valve operated by fluid from the high pressure conduit, but connecting in all positions the feed pump with the pressure maintaining valve. In the control conduit portion between the control valve and the feed pump, a throttle is provided whose pressure differential is greater than the pressure differential of the check valve between the feed pump and the low pressure conduit.

10 Claims, 3 Drawing Figures HYDROSTATIC TRANSMISSION BACKGROUND OF THE INVENTION The German published application 2,223,120 discloses a hydrostatic transmission comprising a hydraulic main pump, a hydraulic motor connected by high pressure and low pressure conduits with the hydraulic main pump, and a feed pump which pumps pressure fluid through check valves into the respective low pressure conduit, and which is also connected with a flush ing control valve through which heated pressure fluid from the low pressure conduit is discharged by a pressure maintaining valve downstream of the flushing control valve.

The transmission of the prior art has the disadvantage that for the flushing control valve and also for the feed pump, separate pressure maintaining and feeding pressure safety valves are required, so that the transmission is complicated.

SUMMARY OF THE INVENTION It is the object of the invention to provide a hydrostatic transmission of the above-described type, which is of simple construction, and can be manufactured at a lower cost than the apparatus of the prior art.

With this object in view, the present invention provides a throttle between the feed pump and the control valve, while the control valve is constructed in such a manner that in all positions of the control valve, the

feed pump communicates with the pressure maintaining valve, and through the same with a low pressure discharge area.

Due to this novel arrangement of the invention, no pressure limiting valve is required for the feed pump. The throttle between the feed pump and the flushing control valve, assures the function of the flushing circult.

A further advantage of the invention resides in that the throttle obtains a speed-depending increase of the feeding pressure during idling of the hydrostatic transmission, permitting an adjustment of the main pump in this condition even at high rotary speeds.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram schematically illustrating a hydrostatic transmission according to an embodiment of the invention;

FIG. 2 is a sectional view illustrating a first control valve construction which may be used in the embodiment of FIG. 1; and

FIG. 3 is a sectional view illustrating a second control valve construction which may be used in the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An adjustable and reversible hydraulic main pump is driven by prime mover 11, which also drives a feed pump 12 in synchronism with the main pump. The main pump 10 is connected by pressure conduits 13 and 14 with a hydraulic motor 15, which may drive the wheels of a vehicle.

The feed pump 12 pumps through an outlet conduit 12a, in which a filter 24 is provided, into a control conduit 16 which is connected with a pressure maintaining valve 18 biassed by spring 29 and being subjected to the pressure of the control conduit by a connecting conduit 28 so that the pressure maintaining valve 18 discharges at a certain pressure into the open container 19. Two outlet conduits 20 and 21 branch off the outlet conduit 12a of the feed pump and are connected with inlet and outlet conduits of the main pump 41. Check valves 22 and 23, opening toward the main pump 41, are provided in the outlet conduits 20 and 21. The fluid pumped by the feed pump 12 through an inlet conduit 44 out of an open container 45, is pumped through filter 24 and the one of the outlet conduits 20, 22 which communicates with the low pressure conduit of the two pressure conduits 13, 14. Assuming that 14 is the high pressure conduit connected with the outlet of the main pump 41, the check valve 23 is held closed, and the check valve 22 opens so that fluid is pumped from the feed pump 12 into the low pressure conduit 13.

The outlet conduit 12a communicates with a control conduit 16 in which a flushing control valve 17 is provided. As schematically shown in FIG. I, and as also shown in FIGS. 2 and 3, the control valve 17 has three positions, and connects in all three positions, control conduit 16 with the pressure maintaining valve 18 which has a spring 29 and is operated by pressure fluid in the shifting conduit 28. In the illustrated position I], only the control conduit 16 is connected with the pressure maintaining valve 18, while in the control position I the low pressure conduit 32 is also connected with the pressure maintaining valve 18, and in the position III, the high pressure conduit 30 is also connected with the pressure maintaining valve 18. Shifting conduits 42 and 43, respectively connected with pressure conduits 30 and I3, and 32, 14, shift the control valve 17 against the action of springs 26 and 27 which bias control valve 17 toward the normal position II when there is low pressure in the fluid circulated in the transmission.

Low pressure conduit 13 is connected with a low pressure conduit 30 which is connected with control valve 17, and a corresponding high pressure conduit 32 is also connected with control valve 17. A portion of low pressure conduit 30 is connected with a shifting valve 31, and a corresponding portion of high pressure conduit 32 is also connected with the shifting valve 31, which has two positions I and II. Shifting conduits 37 and 38 connect low pressure conduit 30 and high pressure conduit 32 with the ends of the shifting valve for operating the same depending upon which of the conduits 30 and 32 is functioning as high pressure conduit, and which is functioning as low pressure conduit.

A connecting conduit 33 between conduits 30 and 32 has a pressure limiting, spring-biassed valve 35 operated by fluid pressure in a shifting conduit connected with connecting conduit 34, which also connects conduits 30 and 32 and includes a pressure limiting springbiassed valve 36 controlled by fluid pressure in a conduit connected with connecting conduit 33. Pressure limiting valves 35 and 36 protect the respective highpressure conduit, for example 32, 14 from overloads. When the main pump 41 is reversed, and conduits I3 and 30 become the high pressure conduits while conduits 14 and 32 are the low pressure conduits, pressure limiting valve 36 protects the high pressure conduit 13.

The shifting valve 31 always connects the respective high pressure conduit, for example, conduit 14, 32, with a conduit 39 controlling a hydraulic motor 40 operating the adjusting means of the main pump 41 for varying the amount of fluid pumped by the main pump 41.

When the pressure in the high pressure conduits 14, 32, 38, 43 is sufficiently high, shifting valve 31 is shifted to control position II and control valve 17 is shifted to position III.

Assuming that conduits 14, 32 are the high pressure conduits connected with the outlet of main pump 41, and conduits 13, 30 are the low pressure conduits connected with the inlet of main pump 41, feed pump 12 pumps fluid from container 45 through conduits 12a, 20 and check valve 22 into the low pressure conduits 30, 13. Check valve 23 is closed since the high pressure from the high pressure conduits 14, 32 acts to close check valve 23 against the lower pressure in outlet conduit 41. When a predetermined pressure is reached in the shifting conduit 43, control valve 17 is shifted against the action of spring 26 to the control position lll.

Due to the fact that a throttle is provided in the control conduit 16 between the check valves 20, 23 and the control valve 17, and produces a greater pressure differential than the check valve 22, the fluid pumped by the feed pump 12 from the open container 45 flows through the check valve 20 into the low pressure conduit and into the inlet of the main pump 41. The same amount of fluid, which was heated during operation of the transmission, flows at a point spaced from the main pump 10 through conduit 30 to the control valve 17 and through the same and the pressure maintaining valve 18 to the open container 19 where the fluid is cooled. The pressure maintaining valve 18 downstream of the control valve 17 determines the pressure in the respective low pressure conduit, assumed to be conduits 13, 30.

When the vehicle rolls, and the hydraulic motor 15 is operated as a pump, and the pressure is equalized at the control valve 17, the throttle 25 will cause an increase of the pressure, which is required for adjusting the main pump 10 at high rotary speeds. This is due to the fact that the pressure at the throttle 25 is transmitted also into the pressure conduit 30. Due to the thereby caused high pressure in pressure conduit 14 and conduit 32, the shifting valve 31 is shifted to position I, whereupon pressure fluid from conduit 30 flows into the conduit 39 and operates the hydraulic adjusting means 40, 10 of the main pump which is set to an increased amount of pumped fluid.

When the pressure in the hydrostatic transmission varies, for example when pressure conduit 13 is the high pressure conduit, and pressure conduit 14 is the low pressure conduit, the operation of the transmission takes place in a corresponding manner due to the symmetrical arrangement of the fluid lines.

FIG. 2 illustrates a control valve 17 which may be used in the embodiment of FIG. 1, and has a longitudinal bore 47 in which a control slide 46 is slidably mounted. Control slide 46 has piston portions 55, 48 at the ends, each of which closes annular ports 49, 90 in the neutral position of the control slide 46. Chambers 51, 52 are provided at the ends of the bore 47, containing springs 53, 54 which hold the control slide in the neutral position. Pressure conduits 30 and 32 are connected with shifting conduits 42, 43, as described with reference to FIG. 1, which supply pressure fluid to chambers 51 and 52 for shifting the slide 46 to the two outer control positions I and III out of the normal position 1. Filter 24 is omitted in the schematic portion of FIG. 2, for the sake of simplicity.

When in the shifting conduit 43 a greater pressure than in the shifting conduit 42 prevails, the slide 46 is shifted against the force of spring 53 to the left as viewed in FIG. 2, so that pressure fluid can flow out of conduit 30 through the annular port 49, the longitudinal bore 47, and the pressure maintaining valve 18 into the open container 19. A corresponding operation of control valve 17 takes place when the pressure in shifting conduit 42 is greater than in shifting conduit 43, due to the fact that main pump 41 is operated in such a manner as to make conduits 13 and 30 high pressure conduits.

FIG. 3 illustrates the modified control valve 17 in which the slide 56 is guided in a longitudinal bore 57 having three annular grooves 58, 59, 60. Chambers 61, 62 are provided at the ends of the longitudinal bore 57, and contain springs 63, 64 urging the slide 56 to the center normal position. In contrast to the embodiment of FIG. 2, the pressure conduits 30 and 32 directly open into the chambers 61 and 62, respectively, so that shifting conduits 42,- 43- of the embodiment of FIG. 2 are not required. The slide 56 has three piston portions 65, 66, 67.

When in conduit 32 the pressure is higher than in conduit 30, slide 56 is shifted against the action of spring 63 to the left, and pressure fluid can flow through annular groove 58 and pressure maintaining valve 18 into the open container 19.

From the above description of the drawing it will become apparent that the hydrostatic transmission comprises a driven main pump 41, a hydraulic motor 15, high pressure conduit means 14, 32, and low pressure conduit means 13, 30 connecting the main pump 41 with the motor 15; a driven feed pump 12 having outlet conduit means 20, 21 with check valve means 22, 23 connecting the feed pump 12 with the low pressure conduit means 13, 30, and inlet conduit means 44 connected with a low pressure area 45; pressure maintaining valve means 18 opening into a low pressure area 19 for cooling a pressure fluid; a control conduit means 16 connecting the outlet conduit means 20, 21, with the pressure maintaining valve means 18; control valve means 17 in the control conduit means 16 having a plurality of positions, and connecting said outlet conduit means 20, 21, 12a with said pressure maintaining valve means 18, and connecting in one of the positions "I also the low pressure conduit 30, 13 with the pressure maintaining valve means 18 so that heated pressure fluid from the low pressure conduit means 30, 13 is cooled; and throttle means 25 in the control conduit means 16 between the outlet conduit means 20, 21, 12a and the control valve means 17, producing in the control conduit means 16 a pressure differential greater than the pressure differential of the check valve means 22.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of hydrostatic transmissions differing from the types described above.

While the invention has been illustrated and described as embodied in a hydrostatic transmission provided with a throttle, and with the control valve connecting in all positions thereof the throttle with a pressure maintaining valve, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. Hydrostatic transmission comprising a driven main pump, a hydraulic motor, high pressure conduit means, and low pressure conduit means connecting said main pump with said motor so that said motor is driven by said main pump; a driven feed pump having outlet conduit means with check valve means connecting said feed pump with said low pressure conduit means, and inlet conduit means connected with a low-pressure area; pressure maintaining valve means opening into a low pressure area for cooling a pressure fluid; a control conduit means connecting said outlet conduit means with said pressure maintaining valve means; control valve means in said control conduit means having a plurality of positions, and connecting said outlet conduit means with said pressure maintaining valve means, and connecting in one of said positions also said low pressure conduit means with said pressure maintaining valve means so that heated pressure fluid from said low pressure conduit means is cooled; and throttle means in said control conduit means between said outlet conduit means and said control valve means, producing in I said control conduit means a pressure differential greater than the pressure differential of said check valve means.

2. Hydrostatic transmission as claimed in claim 1 wherein said control valve means connects in all said positions thereof said outlet conduit means with said pressure maintaining valve means.

3. Hydrostatic transmission as claimed in claim I wherein said main pump includes means for reversing the flow therethrough so that said high pressure conduit means becomes a low pressure conduit means, and said low pressure conduit means becomes a high pressure conduit means; wherein said check valve means connect said outlet conduit means of said feed pump with the respective low pressure conduit; and wherein said control valve means has a normal position connecting only said outlet conduit means with said pressure maintaining valve means, and two control positions for connecting also the respective high pressure conduit with said pressure maintaining valve means; and comprising shifting conduits connected with said low pressure and high pressure conduit means for shifting said control valve means between said normal position and the control position associated with respective high pressure conduit means.

4. Hydrostatic transmission as claimed in claim 3 wherein said control valve means includes a valve slide movable between said normal position and two outer control positions; springs at the ends of said valve slide urging said valve slide to said normal position; and wherein said valve slide closes one of said shifting conduits before opening the other.

5. Hydrostatic transmission as claimed in claim 3 wherein said control valve means connects in said normal position and in said two control positions, said outlet conduit means with said pressure maintaining valve means.

6. Hydrostatic transmission as claimed in claim 1 comprising a filter in said outlet conduit means of said feed pump upstream of said throttle means.

7. Hydrostatic transmission as claimed in claim 1 wherein said outlet conduit means of said feed pump include a first outlet conduit connecting said feed pump with said control conduit, and second and third outlet conduits connecting a point of said first'outlet conduit upstream of said throttle means with said high pressure conduit means and said low pressure conduit means, respectively, and wherein said second and third outlet conduits contain check valves, respectively, opening toward said main pump.

8. Hydrostatic transmission as claimed in claim 7 wherein said main pump includes hydraulic adjusting means; comprising a shifting valve means operated by the pressure fluid in said low pressure and high pressure conduit means to connect said high pressure conduit means with said hydraulic adjusting means for varying the amount of fluid pumped by said main pump.

9. Hydrostatic transmission as claimed in claim 1, wherein said outlet conduit means of said feed pump include a first outlet conduit connecting said feed pump with said control conduit, and second and third outlet conduits connecting a point of said first outlet conduit upstream of said throttle means with said high pressure conduit means and said low pressure conduit means, respectively, and wherein said second and third outlet conduits contain check valves, respectively, opening toward said main pump; and comprising a filter in said first outlet conduit.

10. Hydrostatic transmission as claimed in claim 9 wherein said control valve means connects in all said positions thereof said throttling means with said pressure maintaining valve means.

Claims (10)

1. Hydrostatic transmission comprising a driven main pump, a hydraulic motor, high pressure conduit means, and low pressure conduit means connecting said main pump with said motOr so that said motor is driven by said main pump; a driven feed pump having outlet conduit means with check valve means connecting said feed pump with said low pressure conduit means, and inlet conduit means connected with a low-pressure area; pressure maintaining valve means opening into a low pressure area for cooling a pressure fluid; a control conduit means connecting said outlet conduit means with said pressure maintaining valve means; control valve means in said control conduit means having a plurality of positions, and connecting said outlet conduit means with said pressure maintaining valve means, and connecting in one of said positions also said low pressure conduit means with said pressure maintaining valve means so that heated pressure fluid from said low pressure conduit means is cooled; and throttle means in said control conduit means between said outlet conduit means and said control valve means, producing in said control conduit means a pressure differential greater than the pressure differential of said check valve means.

2. Hydrostatic transmission as claimed in claim 1 wherein said control valve means connects in all said positions thereof said outlet conduit means with said pressure maintaining valve means.

3. Hydrostatic transmission as claimed in claim 1 wherein said main pump includes means for reversing the flow therethrough so that said high pressure conduit means becomes a low pressure conduit means, and said low pressure conduit means becomes a high pressure conduit means; wherein said check valve means connect said outlet conduit means of said feed pump with the respective low pressure conduit; and wherein said control valve means has a normal position connecting only said outlet conduit means with said pressure maintaining valve means, and two control positions for connecting also the respective high pressure conduit with said pressure maintaining valve means; and comprising shifting conduits connected with said low pressure and high pressure conduit means for shifting said control valve means between said normal position and the control position associated with respective high pressure conduit means.

4. Hydrostatic transmission as claimed in claim 3 wherein said control valve means includes a valve slide movable between said normal position and two outer control positions; springs at the ends of said valve slide urging said valve slide to said normal position; and wherein said valve slide closes one of said shifting conduits before opening the other.

5. Hydrostatic transmission as claimed in claim 3 wherein said control valve means connects in said normal position and in said two control positions, said outlet conduit means with said pressure maintaining valve means.

6. Hydrostatic transmission as claimed in claim 1 comprising a filter in said outlet conduit means of said feed pump upstream of said throttle means.

7. Hydrostatic transmission as claimed in claim 1 wherein said outlet conduit means of said feed pump include a first outlet conduit connecting said feed pump with said control conduit, and second and third outlet conduits connecting a point of said first outlet conduit upstream of said throttle means with said high pressure conduit means and said low pressure conduit means, respectively, and wherein said second and third outlet conduits contain check valves, respectively, opening toward said main pump.

8. Hydrostatic transmission as claimed in claim 7 wherein said main pump includes hydraulic adjusting means; comprising a shifting valve means operated by the pressure fluid in said low pressure and high pressure conduit means to connect said high pressure conduit means with said hydraulic adjusting means for varying the amount of fluid pumped by said main pump.

9. Hydrostatic transmission as claimed in claim 1, wherein said outlet conduit means of said feed pump include a first outlet conduit connecting said feed pump with said control conduit, and second and third outlet conduits connecting a poinT of said first outlet conduit upstream of said throttle means with said high pressure conduit means and said low pressure conduit means, respectively, and wherein said second and third outlet conduits contain check valves, respectively, opening toward said main pump; and comprising a filter in said first outlet conduit.

10. Hydrostatic transmission as claimed in claim 9 wherein said control valve means connects in all said positions thereof said throttling means with said pressure maintaining valve means.

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