GB2298240A - Hydraulic actuator - Google Patents

Abstract

A hydraulic actuator comprises a cylinder (2) housing first and second axially spaced pistons (7 and 8). The cylinder (2) is sealed at one end (3) and provided with first and second axially spaced inlet ports (5 and 6) for connection to first and second hydraulic lines respectively. The first inlet port (6) is positioned between the first piston (8) and the sealed end of the cylinder (2) and the second inlet port (5) is positioned between the two pistons (5 and 6). Means are located between the pistons to maintain a minimum spacing therebetween. The arrangement is such that the second piston (7) is moved in a direction away from the sealed end of the cylinder (2) by supply of pressurised fluid to the cylinder via either inlet port.

Description

HYDRAULIC ACTUATOR The present invention relates to a hydraulic actuator.

Hydraulic actuators are widely used in the operation of a variety of mechanisms, such as, for example, vehicle brakes and clutches. For instance, in the case of a vehicle braking system a hydraulic actuator may be used to operate the plunger of a master cylinder, the actuator itself being operated by a supply of pressurised fluid via a hydraulic line connected to appropriate actuation means such as a brake pedal.

In some applications it is desirable to have a single mechanism, such as a brake etc., operable by two or more separate actuation means, e.g. two brake pedals. For instance, dual control cars of the type used to teach learner drivers have two brake pedals mechanically connected to each other. The arrangement is such that if one of the pedals is depressed the other is automatically depressed so that whichever pedal is pressed pressurised fluid will be supplied to the hydraulic actuator along the single pressurised line.

However, there are other applications in which it would be desirable to have a mechanism such as a brake mechanism controlled by two separate actuation means in which it is not practical to simply mechanically link the two (or more) actuation means. For instance, the operator of an agricultural tractor must generally turn towards the rear of the vehicle, i.e. turning away from the main vehicle controls, in order to operate agricultural machinery such as a back hoe mounted on the rear of the tractor. In such circumstances it would be desirable for the tractor operator still to have access to a brake pedal but it may be difficult to design a reliable mechanical linkage between front and rear brake pedals.

Accordingly, it is an object of the present invention to provide a hydraulic actuator which obviates or mitigates the above disadvantage.

According to the present invention there is provided a hydraulic actuator comprising a cylinder housing first and second pistons and means located between the pistons to determine the minimum spacing between the pistons, the cylinder being sealed at one end and provided with first and second axially spaced inlet ports for the supply of pressurised fluid to the cylinder, the first inlet port being positioned between the first piston and the sealed end of the cylinder and the second inlet port being positioned between the two pistons, whereby the second piston is moved in a direction away from the sealed end of the cylinder by supply of pressurised fluid to the cylinder via either inlet port.

The present invention thus provides a single hydraulic actuator (which may, for instance, be used to operate a braking mechanism) which can be operated by supply of pressurised fluid from any one of two (or more) independent hydraulic lines connected to appropriate actuation means (such as, for example, a brake pedal).

The minimum spacing between the pistons may be determined by a pushing member housed within the cylinder between the first and second pistons. For example, the pushing member (which may be an axial rod) may extend from one piston towards the other.

Alternatively, the pushing member could be a separate member, i.e. a member not connected to either piston, which is housed within the cylinder between the two pistons.

In a preferred embodiment of the invention a rod extends axially from one piston and is slidably received within a tubular portion extending from the other piston. With this arrangement either the rod or the tubular member, or both together, may serve as pushing members.

It will be appreciated that in normal use the first piston will never pass either the first or the second inlet ports, and that the second piston will never pass the second inlet port.

A specific embodiment 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 sectional view of a hydraulic actuator in accordance with the present invention; and Figure 2 is a cross-section of Figure 1 taken on the line A-A.

Referring to the drawings, the illustrated hydraulic actuator comprises a block 1 which defines a single cylinder 2. One end of the cylinder 2 is sealed by a plug 3 and the other end of the cylinder is connected to the mechanism to be actuated. For instance, in the illustrated example the latter end of the cylinder has an enlarged portion 4 which may receive the plunger (not shown) of the master cylinder of a brake circuit (also not shown).

The cylinder 2 has two axially spaced entry ports 5 and 6, the port 5 being positioned approximately mid-way along the length of the cylinder and the port 6 being positioned adjacent the plug 3. Each of the ports 5 and 6 is connected to a respective pressurised hydraulic line (not shown) which is in turn connected to respective actuation means, such as a brake pedal (not shown). The arrangement is such that operation of one of the actuating means supplies pressurised fluid to the cylinder 2 via the port 5 whereas operation of the other actuation means supplies pressurised fluid to the cylinder via the port 6.

Housed within the cylinder 2 are two pistons 7 and 8. The piston 7 comprises a cylindrical body 9 which is dimensioned to be a close fit within the cylinder 2. The piston body 9 has an external annular recess 10 defined close to one of its ends which provides a seating for a sealing ring 11 which ensures that the piston is adequately sealed with respect to the wall of the cylinder 2. The piston 7 has a tubular portion 12, of smaller diameter than that of the body, which extends axially from the piston body 9 in the direction of the enlarged end 4 of the cylinder 2. A tubular portion 13, which has the same radial dimensions as the tubular portion 12 but which is approximately twice as long, extends axially from the opposite side of the piston body 9 towards the piston 8.

The axial bores of both tubular portions 12 and 13 are blind.

The bore 14 through the tubular position 12 terminates at the point where the tubular portion 12 meets the piston body 9. However, the bore 15 through the tubular portion 13 extends approximately half way into the piston body 9. The tubular portion 12 is adapted for connection to the plunger of the brake master cylinder. The function of the tubular portion 13 is described below.

The piston 8 comprises a generally cylindrical piston body 16 which is provided with two annular recess 17 adjacent each end, each of which houses a respective sealing ring 18. A conical abutment 19 extends axially from the end of the piston body 16 which faces the plug 3. The dimensions of the conical abutment 19 are such that it prevents the piston body 16 from moving into a position in which it covers the port 6 which is positioned adjacent the plug 3.

Extending from the opposite side of the piston body 16 is a cylindrical rod 20 the end of which is slidably received within the tubular portion 13 of the piston 7, the dimensions of the rod 20 corresponding to the dimensions of the blind bore 15. The rod 20 has an elongate groove 21 formed in its outer surface to allow hydraulic fluid to escape from the blind bore 15 as the rod 20 slides into it.

Further details of the structure will become apparent from the following description of its operation.

Suitable operation of the actuation means connected to the hydraulic line which is connected to the port 5 will supply pressurised hydraulic fluid to the cylinder 2 into the region between the piston bodies 9 and 16. The piston 7 will thus be forced towards the enlarged end 4 of the cylinder 2 and the piston 8 will be forced towards the end of the cylinder 2 sealed by the plug 3. Movement of the cylinder 7 thus operates the mechanism attached to the actuator, i.e. in the present example the movement of the piston 7 will force the brake plunger into the master cylinder.

Once the actuation means is released, the brake plunger will move back towards the cylinder 2 and thus push the piston 7 towards the sealed end of the cylinder 2, the rod 20 of the piston 8 sliding within the blind bore 15 of the piston 7 as it moves. However, the length of the tubular portion 13 of the piston 7 is such that it will abut the body 16 of the piston 8 (and similarly the end of the rod 20 will abut the blind end of the bore 15) before the piston body 9 reaches the entry port 5, even with the piston 8 pressed right up to the plug 3. That is, the piston body 9 cannot pass or block the port 5, so that subsequent reactivation of the actuation means attached to the pressurised line connected to the port 5 will operate the actuator.

If, alternatively, pressurised fluid is supplied to the cylinder 2 via the port 6 (as opposed to the port 5) by appropriate operation of the actuation means connected to the end of the hydraulic line connected to the port 6, the piston 8 will be forced away from the plug 3. In doing so the piston 8 will push the piston 7 towards the enlarged end 4 of the cylinder 2. That is, the rod 20 of the piston 8 will slide to the end of the blind bore 15 whereafter the piston 7 will be pushed in front of the piston 8. Thus, the plunger attached to the piston 7 is once again moved into the master cylinder. The length of the rod 20 (and tubular portion 15) is dimensioned, relative to the length of the cylinder 2, such that the piston body 16 of the piston 8 cannot move past the port 5.This prevents the piston body 16 from blocking the end of the port 5 (which could prevent subsequent operation of the actuator by supply of fluid through the port 5).

Thus it will be seen that the hydraulic actuator can always be operated by supplying pressurised fluid through one or other of the ports 5 and 6 and that the hydraulic lines connected to each of the ports 5 and 6 are at all times kept isolated from one another. This isolation, which is necessary for the function of the actuator as described above, also provides a safety backup whereby should one of the hydraulic lines lose pressure the actuator may be operated by application of pressure via the other line.

It will be appreciated that details of the above described structure could be modified considerably. For example, the rod 20 (and tubular portion 13) may be omitted and other means may be provided for transferring the motion of the piston 8 to the piston 7.

For instance, the piston 7 could have an appropriate rod extending towards the piston 8 or a separate member, i.e. a member not connected to either the piston 7 or the piston 8, could be slidably housed within the cylinder between the two pistons.

It will be understood that although the above described embodiment of the invention is a hydraulic actuator which may be controlled via two independent hydraulic lines, the actuator could be constructed for control via three or more independent lines. That is, the length of the cylinder could be extended to accommodate further axially spaced entry ports and additional pistons configured appropriately, and co-operating in a manner similar to that described above, could be added.

It will be appreciated that actuators according to the present invention could have many applications in addition to those mentioned in the introduction to the specification. That is, the actuator may be used in connection with a wide variety of mechanisms other than brake mechanisms.

Claims (6)

1. A hydraulic actuator comprising a cylinder housing first and second pistons and means located between the pistons to determine the minimum spacing between the pistons, the cylinder being sealed at one end and provided with first and second axially spaced inlet ports for the supply of pressurised fluid to the cylinder, the first inlet port being positioned between the first piston and the sealed end of the cylinder and the second inlet port being positioned between the two pistons, whereby the second piston is moved in a direction away from the sealed end of the cylinder by supply of pressurised fluid to the cylinder via either inlet port.

2. A hydraulic actuator according to claim 1, wherein said means to determine the minimum spacing between the pistons is a pushing member housed within the cylinder between the first and second pistons.

3. A hydraulic actuator according to claim 2, wherein said pushing member extends from one piston towards the other.

4. A hydraulic actuator according to claim 2, wherein said pushing member is a rod extending axially from said one piston.

5. A hydraulic actuator according to claim 3, wherein said rod is slidably received within a tubular portion extending from said other piston.

6. A hydraulic actuator substantially as hereinbefore described, with reference to the accompanying drawings.