Pressure limiting valve

The pressure limiting valve limits the pressure in the brake chambers of the trailer depending on the load status.

The lever of the pressure limiting valve must be set manually, corresponding to the Ioad status of the trailer. Depending on the type, the settings “no-load”, “half load”, “full load”, and in some cases “1/4 load”, “3/4 load” and “release” are provided.

When the service brake system of the trailer is actuated, the compressed air from the relay emergency valve flows into inlet port 1 of the pressure limiting valve and then through the open outlet port 2 and onto the brake chambers. 

At the same time, the pressure in outlet port 2 is also applied via a hole to the control piston. When a pressure corresponding to the lever setting has been reached in the brake cylinders, the control piston is shifted downwards against the force of the spring until both inlets close. The outlet remains closed. Further increase of pressure in the brake cylinders is not possible. A drop in pressure in outlet port 2 (e.g. as a result of a leak between the pressure limiting valve and brake cylinder) is immediately compensated by the control piston moving upwards and opening the upper inlet. 

The spring under the control piston is preloaded corresponding to the lever position by means of the cam arranged on the lever shaft. In the lever position “full load”, the control piston is prevented from moving downwards so that the inlet remains open and during full braking, the brake cylinders receive the full operating pressure.

The lever must be moved into the position “release” (if provided) in order to manoeuvre the uncoupled trailer vehicle with the brakes applied. In this way, the spring under the control piston is completely relieved, the inlet closed, the outlet opened and the compressed air escapes from the brake cylinders into the
atmosphere via the vent holes in the protective cap. 

In the case of versions without the “release” setting, a release valve flange-mounted on the relay emergency valve must be actuated for releasing the brake system for manoeuvring purposes.