Patent GB-1341656-A (GB)
Title: Mechanical overrunning braking system for a vehicle trailer
IPC: B60T7/20
Filed: 1972-03-13
Abstract: 1341656 Braking systems for trailers; brakes H MERZ 13 March 1972 [11 May 1971] 11658/72 Heading F2E A mechanical over-running brake system for a vehicle trailer comprises over-running brake operating means 4, to operate wheel brakes 18 of the trailer when the trailer over-runs a towing vehicle, each brake having two shoes in a double servo arrangement, auxiliary brake operating means 9, means 6 for limiting the braking force caused by the over-running brake operating means, and brake force transmitting means common to the over-running brake operating means and the auxiliary brake operating means. The system is such that there is a braking force threshold between a shoe of each wheel brake which acts as a primary shoe in the reverse direction of travel of the trailer and brake shoe actuating means which act on the brakes only when the auxiliary brake operating means is actuated and which act on a shoe of each brake which acts as a primary shoe in the forward direction of travel of the trailer, each braking force threshold being insurmountable by the over-running brake operating means due to the means for limiting the braking force caused by the over-running brake operating means. As shown in Fig. 1, the operating means 4 comprises a longitudinally displaceable draw-bar 2 and a spring 7 which is compressed to close the over-running distance s of the trailer with respect to the towing vehicle, the draw-bar acting on a lever 13 whereby the brakes 18 are applied via cables 15. The auxiliary operating means comprises a manually operable brake lever 9 connected by way of a telescopic lostmotion device 10 to the lever 13, and a cable 12, adapted to give way if the trailer breaks away from the towing vehicle, is connected to a lever 11 associated with the lever 9. A vibration damper 8 is included in the over-running means. Different transmission levers 13, 13<SP>1</SP>, Fig. 2, may be provided for the over-running and auxiliary operating means, both levers acting on the cables 15 or a brake rod 16, and the braking power limiting device 6, acting in tension, being located between the lever 13 and rod 16; the lost-motion device 10 is disposed between the lever 9 and the lever 13<SP>1</SP>. The lost motion device 10 may be replaced by a slot (17), Fig. 3 (not shown), and the braking power limiting device is omitted from the over-running operating means. The shoe drum brake according to Fig. 4 comprises shoes 22, 23 coupled by pull-off springs 26 and 37 and 38, and a bracket 24 on which is pivoted a lever 25 operated by the cable 15 or rod 16 to expand the shoes into contact with the drum 20. The bracket is loosely and displaceably disposed against the brake back plate 19 and is connected to a stop bolt 29 carried by a support 21 between which and a washer 31 is arranged a stack of dished spring washers 30. When the brake drum rotates in a forward direction V corresponding to forward motion of the trailer, the shoe 22 acts as the primary shoe and is applied to the drum by the cable 15 or rod 16, the end of the shoe remote from the lever 25 bearing on a slide member 33 which then operates on the shoe 23 which acts as the secondary shoe. Upon rotation of the drum in a direction R corresponding to backward motion of the trailer, the shoe 23, when operated by the over-running operating means, is prevented from acting as a primary shoe and the shoe 22 only is effective. However, with operation of the auxiliary means 9 and compression of the spring 30, the shoe 23 is engaged with the drum and acts as the primary shoe, the bearing pressure of the shoe 22, acting as a secondary shoe, being absorbed by a stop 28 or by the lever 25 and bracket 24. Automatic adjusting means may be located between the stop bolt 29 and washer 31 or at the member 33. In modifications, (a) a lever 47, Fig. 6, connected to the shoe 22 and operated by the cable 15 or rod 16 acts on a pressure member 48 which in turn acts on a displaceable intermediate member 49 and a pressure member 55, there being a clearance a between the members 49 and 55, and the spring 51 being carried by the member 49; adjusting nuts 42 and 43 may be provided, (b) a pivoted lever 57, Fig. 7, has an upper extension 59 which supports a stop bolt 56 between which and the shoe 23 there may be a clearance, the lower end of the lever being engaged by a spring 66, (c) non-biased spring elements are disposed between the bracket 24 and the washer 31 or the extension 59, or the intermediate member 49 encloses a non-biased spring acted upon by a piston engaged by the pressure member 48.