GB2112092A - Hydraulic actuating system - Google Patents

Abstract

An agricultural or industrial tractor (100) has a transmission casing (T) and/or back axle housing (16, B) which acts as a sump for fluid used to lubricate the transmission and/or back axle and to provide a reservoir of actuating fluid for hydraulically actuated functions. The tractor also includes an hydraulic actuating system, such as a braking system, having a slave cylinder (10, 11) with a bleed facility (41, 42, 43) with a return passage (44) which communicates with an opening (45) in the transmission and/or back axle housing (16), so that on opening of the bleed facility (41, 42, 43) the actuating fluid of the actuating system can be bled back into the housing (16) via the return passage (45). A master cylinder (21, 22) of the hydraulic actuating system may be charged from a hydraulic power system of the tractor which uses the same actuating fluid, for example the power steering system (30, 32, 33, 35, 36), at a pressure insufficient to operate the slave cylinder (10, 11). This will ensure that when the bleed facility (41, 42, 43) is opened a purging flow of fluid through the return passage (45) is ensured from the pump (30) of the power system. <IMAGE>

Description

SPECIFICATION Hydraulic actuating system This invention relates to agricultural or industrial tractors and in particular to hydraulic actuating systems, such as brake or clutch actuating systems, for use on such machines.

One aspect of the invention is concerned with bleeding such actuating systems and a further aspect of the invention is concerned with a particular form of actuating cylinder for use in such a system.

At present the bleeding of such actuating systems is normally a messy operation involving the installation of temporary tubing on the slave cylinder(s) and the bleeding of actuating fluid into a temporary external reservoir with the attendant risk of spillage. Also frequently such bleeding is a two-man operation.

It is an object of the present invention to provide an improved form of slave cylinder and actuating system which overcome the above disadvantages.

According to the present invention there is provided an agricultural or industrial tractor having a transmission casing and/or back axle housing which acts as a sump for fluid used to lubricate the transmission and/or back axle and to provide a reservoir of actuating fluid for hydraulically actuated functions, the tractor also including an hydraulic actuating system having a slave cylinder, the slave cylinder having a bleed facility with a return passage which communicates with an opening in the transmission and/or back axle housing, so that on opening of the bleed facility, the actuating fluid of the actuating system can be bled back into the housing via the return passage.

As will be appreciated, by bleeding the actuating fluid back into the housing, the bleeding operation is made clean and the installation of temporary piping and reservoirs is removed.

The invention is particularly, though not exclusively, concerned with hydraulic actuating systems for brakes and clutches.

In a tractor which includes a pump-driven hydraulic power system which uses the same actuating fluid as the actuating system, the actuating system can advantageously be charged from the hydraulic power system, at a pressure insufficient to operate the slave cylinder, so that when the bleed facility is opened a purging flow of fluid through the actuating system bleed return flow path is ensured from the charging pump of the power system.

This arrangement enables the actuating system to be quickly and easily bled by simply opening the bleed facility until sufficient flow of fluid has passed through the bleed return flow path to purge the actuating system of air or other contaminant. Thus bleeding of the system becomes a simple one man operation.

Where the above charging from a pump-driven power system is included a filter may be provided to filter all fluid entering the actuating system from the power system to prevent contamination.

In accordance with a further aspect of the present invention there is provided a brake actuator comprising a fixed casing and a hollow piston in sealed and slidable engagement with the casing to form an actuating chamber, a brake applying rod which passes through the piston with substantial clearance, a thrust coupling which surrounds the rod and is interposed between the piston and an abutment on the rod, so that pressurisation of the actuating chamber results in movement of the piston and hence movement of the rod via the thrust coupling, first sealing means between the thrust coupling and the casing, second sealing means between the thrust coupling and the pull-rod, and bias means arranged to maintain the thrust coupling in contact with the abutment at all times thus minimising relative movement between the thrust coupling and the rod and hence minimising rubbing of the second sealing means.

Preferably the rod can be mechanically moved relative to the piston to provide a hand brake function.

The thrust coupling is preferably of a two-part construction comprising a first part, one end of which is acted upon by the piston and the other end of which has a first part-spherical thrust surface, and a second part which has, at one end, a second complementary part-spherical surface and which acts at its other end on the rod abutment. Such a thrust coupling allows rocking of the pull rod within the piston during brake operation.

In the above arrangement the first sealing means will act between the casing and the second coupling part.

Preferably the bias means also acts on the first coupling part to maintain the position of the first coupling part relative to the piston at all times.

Several embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a tractor fitted with a brake actuating system embodying the present invention: Figure 2 is a more detailed schematic view of the brake actuating system of Figure 1; Figure 3 is a detailed view on a larger scale of the actuating cylinder used in the system of Figure 2, and Figure 4 is a detailed view on a larger scale of an alternative form of actuating cylinder embodying the various aspects of the present invention.

Referring to Figure 1 this shows an agricultural tractor 100 having an engine E, a transmission T and a back axle B secured together in series. The tractor is fitted with a brake actuating system shown diagrammatically in more detail in Figure 2.

The actuating system comprises a pair of slave actuating cylinders 10 and 11 which are secured to part of the back axle 1 6 of the tractor and which are arranged to apply multi-plate disc brakes 1 7 and 1 8 respectively via lever 1 2 and 13 and toggle linkages 14 and 1 5. Since the brake applying mechanical linkages and the brake construction itself forms no part of the present invention they will not be described further here.

An example of the type multi-plate disc brake which is suitable for application by toggle linkages 14 and 1 5 is shown in UK Patent 583108.

The cylinder 10 and 11 are connected via conduits 1 9 and 20 with proprietary twin compensated master cylinder 21 and 22 respectively. These master cylinder are actuated by pedals 24 and 25 respectively and are interconnected by conduit 23 so that when both master cylinders are simultaneously actuated the braking pressure in conduits 1 9 and 20 is equalised. The master cylinders also include valving to isolate each half of the system when only one pedal is operated.

Master cylinders 21 and 22 are fed from a filter 26 via conduit 27. The filter 26 is in turn fed from she return line 28 of the power steering system of the tractor via a T-piece 29. The power steering system comprises a pump 39 which may be, for example, of the gear type which draws mineral b0S'Gd hydraulic fluid from a sump (diagrammatically shown in Figure 2 under the r efsrence numeral 31) which is constituted by the iOwer portion of the interior of the transmission T and/or back axle casing B of the tractor.

Pressurised hydraulic fluid is fed by conduit 32 to a iotary power steering control valve 33 which is connected with the steering sheel 34 of the tractor so that turning of the steering wheel (as indicated by the arrow X in Figure 2) directs pressurised fluid to one or other of the single acting steering rams 35 and 36 in the known manner. The excess fluid from the steering valve 33 enters conduit 28 and is thus returned to the sump 31 as shown in Figure 2. The pressure in the return conduit 28 is arranged to be at a relatively low level of say 5 Ibs per square inch at the -piece 29 so that filter 26 is pressure fed at this low level.

Preferably the filter 26 includes a micronic filter element diagrammatically shown at 26a which will prevent contamination of the braking circuit by foreign matter which may be present in the power steering circuit and also assist in deaerating the braking circuit fluid.

Figure 3 shows slave cylinder 10 on a larger scale. The cylinder has an actuaging chamber 37, a piston 38 and a push rod 39 which bears against lever 12. A compression spring 40 acts between the end of chamber 37 and piston 38 to limit the travel of piston 38 to the right (as viewed in Figure 3) when the pedal 24 is released and to maintain piston 38 in contact with rod 39.

The slave cylinder is provided with a bleed facility in the form of a bleed screw 41 which has a conical sealing seat 42 which cooperates with a bleed orifice 43 to close the bleed orifice when bleed screw 41 is fully screwed home. A seal 41 a is provided on bleed screw 41. The cylinder 10 also includes a bleed return passage 44 which communicates with a passage 45 in the back axle casing to provide a bleeding return flow path into the casing. A gasket 46 is positioned between the cylinder 10 and the casing 1 6 and the cylinder is held onto the casing by bolts extending through apertures in flanges provided on the cylinder 10 which are not visible in Figures 2 and 3.

The braking system described above can be bled as follows:- The tractor engine is started to drive pump 30 and thus provide the low pressure feed to filter/reservoir 26 via T-piece 29. With both pedals 24 and 25 in the released position the bleed screw 41 on slave cylinder 10 is opened to disengage the conical sealing seat 42 from bleed orifice 43.

This allows pressurised fluid to flow from filter 26 through the master cylinder 21 along conduit 19 and out from slave cylinder 10 via return passages 44 and 45 back into the sump 31 provided by the back axle/transmission casing. Thus the braking circuit associated with pedal 24 can be purged of air or other contaminants by allowing the fluid to flow as described above and return to the sump 31 for a sufficient period. The bleed screw 41 is then fully screwed home to close off slave cylinder 1 0 and the bleeding of that half of the braking circuit is now completed.

The same sequence is now carried out for the bleeding of the other half of the braking circuit by opening the bleed screw 41 a associated with slave cylinder 11 so that hydraulic fluid can purge the other half of the braking circuit returning to the sump 31 via a corresponding return passage 45a illustrated diagrammatically in Figure 2. When the bleeding of the other half of the circuit is completed, bleed screw 41 a is again fully tightened home.

The driver then presses both pedals to subjectively test whether the system has been fully bled and the pedals have the necessary "hard" feel. If not the above procedure is repeated until all air has been bled from the system. As will be appreciated the precise procedure to be followed when bleeding the system will depend on the "plumbing" of the system and the construction of the master cylinders.

Figure 4 shows an alternative form of slave cylinder 50 which has a hollow piston 51 which defines an actuating chamber 52 with the cylinder casing. Chamber 52 has a bleed orifice 53, and a bleed screw 54 with a seal 54a controls flow through the bleed orifice 53 and a return passage 55 which corresponds to the return passage 44 described in relation to Figure 3. Return passage 55 again communicates with a corresponding return passage provided in the back axle casing.

Piston 51 acts against a thrust coupling in the form of a thrust washer 56 which has a partspherical thrust surface 57 which mates with a corresponding part spherical thrust surface 58 on one end of a thrust member 59. Thrust member 59 acts against an abutment in the form of a nut 60 which is threaded onto a pull rod 61 which is connected with the toggle operating linkage of the brakes in the same manner as the pull rods 1 2a and 1 3a shown in Figure 2. A lever member 62 is also arranged to operate on pull rod 61 to provide mechanical operation of the brakes giving a hand brake function. A seal 63 seals the sliding fit between pull rod 61 and thrust member 59 and a rubber boot 64 completes the sealing to prevent egress of hydraulic oil from the back axle casing down the centre of hollow piston 51.A spring 65 retains thrust member 59 in contact with nut 60 so that the minimum relative movement takes place between the seal 63 and pull rod 61 no matter whether the brake is being hydraulically or mechanically operated. Spring 65 (which may act directly on rubber boot 64, as shown, or may act on a thrust component (not shown) which is carried by member 59) also holds thrust washer 56 in position on piston 51 to counteract the natural tendency for washer 56 to drop slightly in direction Z when the brake is mechanically operated and the pressure between surfaces 57 and 58 is relieved.

Slave cylinder 50 operates as follows:- Pressurised fluid is fed to the actuating chamber 52 via, for example, conduit 19 and this displaces piston 51 to the left thus drawing the pull rod 61 to the left via thrust washer 56 thrust member 59 and nut 60. The part-spherical cooperating surfaces 57 and 58 allow the axis of pull rod 61 to rock or tilt within the hollow piston 51 as the brake is applied.

Mechanical operation of the brake is simply achieved by pulling lever 62 to the left which, via screw threaded nut 62a, in turn pulls pull rod 61 to the left to apply the brake. Spring 65 ensures that the thrust member 59 will follow the movements of the rod 61 to the left so that the rubbing on seal 63 is minimised and thrust washer 56 is held against piston 51 ready to re-engage member 59 when lever 62 is released.

The slave cylinder 50 is bled in exactly the same manner as the cylinder 10 described above.

That is, with the tractor engine running, the bleed screw 54 is unscrewed sufficiently to allow a purging flow of fluid through orifice 53 and return passage 55.

Although in the example described above twin compensated master cylinders 21 and 22 are used it will be understood that individual noncompensated master cylinders could be used and that, whatever form of master cylinder is employed, the master cylinder(s) may be associated with valving which provides a trailer braking function.

As will be evident, since in the form of the invention described above the braking system is fed from the main hydraulic sump 31 provided by casing 16, an extremely large volume of fluid would need to lead from the system before brake actuation is impaired. This makes the braking system particularly safe. The integrity of all important seals can be regularly checked by simply applying a heavy foot pressure to the brake actuating pedals in turn and watching for signs of pedal travel indicating loss of fluid from the separate halves of the system.

It will be appreciated that the cylinder bleeding aspect of the present invention can also be applied to power braking systems in general. Also this bleeding concept (with or without a low pressure feed to the reservoir) can be applied to the socalled "annular piston" type of disc brake (currently used for example, on the Massey Ferguson 2000 series tractors) in which a large annular piston of approximately the same diameter as the annular brake disc(s) is used to apply the brake.

Claims (11)

1. An agricultural or industrial tractor having a transmission casing and/or back axle housing which acts as a sump for fluid used to lubricate the transmission and/or back axle and to provide a reservoir of actuating fluid for hydraulically actuated functions, the tractor also including an hydraulic actuating system having a slave cylinder, the salve cylinder having a bleed facility with a return passage which communicates with an opening in the transmission and/or back axle housing, so that on opening of the bleed facility the actuating fluid of the actuating system can be bled back into the housing via the return passage.

2. A tractor according to claim 1 which also includes a pump-driven hydraulic power system which uses the same actuating fluid as the actuating system and in which the actuating system includes a master cylinder connected with the slave cylinder, the master cylinder being charged from the hydraulic power system at a pressure insufficient to operate the slave cylinder so that when the bleed facility is opened a purging flow of fluid through the return passage of the actuating system into the housing is ensured from the pump of the power system.

3. A tractor according to claim 2 including a filter to filter all fluid entering the actuating system from the power system to prevent contamination.

4. A brake actuator comprising a fixed casing and a hollow piston is sealed and slidable engagement with the casing to form an actuating chamber, a brake applying rod which passes through the piston with substantial clearance, a thrust coupling which surrounds the rod and is interposed between the piston and an abutment on the rod, so that pressurisation of the actuating chamber results in movement of the piston and hence movement of the rod via the thrust coupling, first sealing means between the thrust coupling and the casing, second sealing means between the thust coupling and the pull-rod, and bias means arranged to maintain the thrust coupling in contact with the abutment at all times thus minimising relative movement between the thrust coupling and the rod and hence minimising rubbing of the second sealing means.

5. An actuator according to claim 4 in which the rod can be mechanically moved relative to the piston to provide a hand brake function.

6. An actuator according to claim 4 or 5 in which the thrust coupling Is preferably of a twopart construction comprising a first part, one end of which is actuated upon by the piston and the other end of which has a first part-spherical thrust surface, and a second part which has, at one end, a secondary complementary part-spherical thrust surface and which acts at its other end on the rod abutment.

7. An actuator according to any one of claims 4 to 6 in which the first sealing means acts between the casing and the second coupling part.

8. An actuator according to claim 6 in which the bias means also acts on the first coupling part to maintain the position of the first coupling part relative to the piston at all times.

9. An actuator according to any of the claims 4 to 8 with a bleed facility having a return passage adapted for connection with a reservoir, so that on opening of the bleed facility the actuating fluid is bled back into the reservoir via the return passage.

10. An agricultural or industrial tractor having an hydraulic actuating system constructed and arranged substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

11. A brake actuator constructed and arranged substantially as hereinbefore described with reference to and as shown in Figure 3 or Figure 4 of the accompanying drawings.