Brake cylinder with two pistons

A brake cylinder device comprising:

• (a) a casing;
• (b) a duty brake piston subject on one side thereof to the force of fluid pressure urging movement of said duty brake piston in a brake application direction;
• (c) a screw-threaded rod;
• (d) a non-rotatable thrust rod having an integral first nut member threadedly connected to said screw rod;
• (e) first clutch means between said duty brake piston and said screw rod for locking said screw rod against rotation upon clutch engagement in response to movement of said duty brake piston in a brake application direction and for unlocking said screw rod upon clutch disengagement in response to movement of said duty brake piston in a brake release direction to thereby free said screw rod to rotate;
• (f) means for urging said screw rod and said duty brake piston in said brake release direction;
• (g) slack adjuster means for limiting axial displacement of said thrust rod in said brake release direction a predetermined distance, whereby continued axial displacement of said screw rod in said brake release direction corresponding to overtravel of said duty piston during said brake application induces rotation of said screw rod relative to said first nut member in a direction to effect lengthening of the axial dimension between said screw rod and said thrust rod;
• (h) a parking/emergency brake piston subject on one side to the force of a spring urging movement thereof in said brake application direction, and subject on the opposite side to the force of fluid pressure normally sufficient to overcome the force of said spring to maintain said parking/emergency brake piston in a brake release position,
• (i) a second nut member threadedly disposed on said screw rod for rotation thereon;
• (j) second clutch means for inhibiting rotation of said second nut member during movement of said parking/emergency brake piston in said brake application direction to lock said second nut member axially in relation to said screw rod, such that the force of said spring acting through said parking/emergency brake piston is transmitted to said thrust rod, said second clutch means including:
• (i) a first sleeve member that is connected to said parking/emergency brake piston in such a manner as to be rotatable relative thereto, said first sleeve member including a force transmitting surface;
• and
• (ii) a force receiving surface formed on said second nut member so as to be engageable with said force transmitting surface upon movement of said parking/emergency brake piston in said brake application direction to effect said locking of said second nut member;
• (k) means for limiting axial movement of said second nut member relative to said parking/emergency brake piston so as to maintain a predetermined spacing between said force transmitting and receiving surfaces irrespective of the stroke of said duty brake piston, whereby the degree of expansion of said parking/emergency brake piston spring and thus the force exerted thereby on said one side of said parking/emergency brake piston is constant for a given reduction of fluid pressure from said opposite side of said parking/emergency brake piston;
• (l) third clutch means normally having engagement for rotatably locking said first sleeve member against rotation;
• and
• (m) means for effecting disengagement of said third clutch means upon movement of said parking/emergency brake piston in said brake application direction a distance exceeding said predetermined spacing between said force transmitting and receiving surfaces, thereby permitting said second nut member and said first sleeve member to rotate relative to said screw rod under the thrust of said spring, the direction of rotation being such as to permit said parking/emergency brake piston to be moved axially into engagement with an end stop on the casing of said brake cylinder device to thereby cage said spring and accordingly relieve the spring force from said thrust rod.