Technical Field The present invention relates to a device at a rail vehicle brake unit for preventing concurrent service brake application by compressed air and parking brake application by means of a mechanical parking brake arrangement connected to the brake unit.

Background of the Invention As is well known in the art, a rail vehicle brake unit in principle consists of a brake actuator with a slack adjuster or brake regulator built-into the same housing. The brake actuator is to be supplied with pressurized brake fluid, which in most cases is compressed air but for example can also be hydraulic fluid. The pressurized brake fluid acts - for obtaining service brake application on the rail vehicle on which the brake unit is mounted - in a cylinder of the brake actuator on a piston movable therein. The obtained brake force is transmitted to a brake block or brake pad by any brake rigging suitable for the purpose.

A parking brake application may be obtained by means of a mechanical parking brake arrangement acting with a mechanical force on the piston of the brake unit.

If the parking brake arrangement is applied concurrently with the supply of the pressurized brake fluid for service brake application, an undesired addition of the service brake force and the parking brake force may result.

The main object of the invention is to obviate such an undesired addition of the two brake forces.

The Invention

This object is according to the invention attained in that a valve member in the brake unit is movable under the mechanical control of a member of the parking brake arrangement between a service brake position for allowing compressed air to enter the brake unit and a parking brake position for venting the compressed air from the brake unit into the atmosphere, and in that the parking brake arrangement comprises an externally pivotable shaft, which is journaled in a cover for the brake unit and is provided with a lever for mechanical engagement with a piston of the brake unit, the shaft also being provided with a cam member for engagement with the valve member. The valve member can be axially movable in a bore in the cover. This bore is in connection with an inlet for compressed air to the brake unit and opens into the cylinder of the brake unit. The valve member is preferably biased by a spring into engagement with the cam member. The valve member is preferably hollow and provided with peripheral holes in flow connection with the inlet in a service brake position, in which the parking brake lever is out of engagement with the piston, and in flow connection with a venting channel in a parking brake position, in which the lever is in engagement with the piston.

Brief Description of the Drawings The invention will be described in further detail below under reference to the accompanying drawings, in which

Fig 1 is an enlarged, isometric view, partly in section, corresponding to the area I in Fig 2 and illustrates a device according to the invention,

Fig 2 is an isometric view, partly in section, of a brake unit cover provided with a device according to the invention, and

Fig 3 is a section through the brake unit cover shown in Fig 2 and a piston in the brake unit.

Detailed Description of an Embodiment

Reference is first made to Fig 3. Shown therein are a piston 1 and a cover 2 for a rail vehicle brake unit (not shown), in which the piston 1 is axially movable for service braking under the action of compressed air admitted to the space above the piston in the cover 2. A person skilled in the art is well acquainted with a rail vehicle brake unit and its main parts, and no further description thereof is given here.

The cover 2 is in this case provided with a mechanical parking brake arrangement. A transverse shaft 3 is pivotally journaled in the cover 2. At an end extending out of the cover 2, the shaft 3 is provided with an arm 4 for pivotal parking brake actuation by mechanical means (not shown), such as a rod or a wire.

Inside the cover 2, a lever 5 is attached to the shaft 3. At its free end remote from the shaft 3, the lever 5 is provided with a roller 6 for engagement with the upper surface of the piston 1.

It appears that the piston 1 will be mechanically pushed down by the roller 6 at a counterclockwise movement of the arm 4 for accomplishing a parking brake application.

A return spring 7 is provided around the shaft 3 for returning the arrangement to the shown rest position after terminated parking braking. Service braking by compressed air without interference from the parking brake arrangement in the shown rest position is possible by admitting compressed air to the space above the piston 1 through an inlet 8 therefore.

If the brake is applied by compressed air for service braking and the mechanical parking brake is also applied, the two braking forces are added with an unfavorable result. The main purpose of the device according to the invention is to avoid concurrent application of both the service brake and the parking brake.

The shaft 3 is provided with a cam member 9 for controlling the axial movement of a valve member 10, which is axially movably arranged in a bore in the cover 2. The valve member 10 is biased into contact with the cam member 9 by a compression spring 11, which is supported by a nut 12 at the outer end of the bore.

The valve member 10 is generally tubular but has a closed end towards the spring 11. It has a number if holes 10' around its periphery in a position for allowing compressed air to flow from the inlet 8 through the holes 10' and further through the interior of the valve member 10 to the space above the piston 1 for accomplishing a service brake application. The prerequisite for this is that the parking brake is not applied or in other words that the parking brake arrangement is in the idle position shown in the Figures. In this position, the cam member 9 pushes the valve member 10 to the right in the Figures against the bias of the spring 11. If the parking brake now is applied by a counterclockwise rotation of the shaft

3 with the cam member 9, the valve member 10 will be allowed to move to the left in the Figures under the bias from the spring 11. The valve member holes 10' will first be brought out of flow connection with the inlet 8 and shortly thereafter into flow connection with a venting channel 13 in the cover 2, so that service brake air is vented out from the space above the piston 1 and an addition of service and parking brake forces is prevented.

The term brake unit is used throughout this specification, but the invention is equally applicable to a brake actuator alone. Also, the term compressed air is used for ease of description, but the use of any other pressurized brake fluid is within the scope of the invention.

Modifications are possible within the scope of the appended claims.