Brake cylinder with two pistons

A spring-applied, air-released parking/emergency brake is combined with an air-actuated service brake through a friction clutch between the parking/emergency brake piston and a first nut member that is threaded onto a screw rod of the service piston.

cylinder pistons

The screw rod and service piston are connected through another friction clutch. The screw rod is provided with a second nut member that is integral with a thrust rod to which the brake rigging is connected. A friction lock slack adjuster arrangement limits retraction of the thrust rod during brake release in accordance with the desired brake shoe clearance to be maintained. When the thrust rod movement is inhibited by the slack adjuster, the service piston clutch becomes disengaged to permit rotation of the screw rod during the release stroke of the service piston corresponding to overtravel due to shoe wear. Brake shoe clearance is thus maintained constant. The service piston clutch is engaged during a service application so as to induce rotation of the first nut member, which is accordingly maintained at a given point of axial reference relative to the parking/emergency brake piston, so that the actuating spring force is constant any time the parking/emergency brake is activated.

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