GB1448553A - Internal shoe drum brakes - Google Patents

Abstract

1448553 Internal shoe-drum brakes; overrun braking; brake linkages; automatic control AUTOMOTIVE PRODUCTS Ltd 29 Aug 1973 28 June 1972 [22 Jan 1973] 30339/72 and 3216/73 Headings F2E and F2F The two wheels of a caravan have respective internal shoe-drum brakes 38, 39 which include an expanding device 53 operable by an overrun mechanism when the caravan moves forward relative to its tractor vehicle, as, for example, during service braking, and also operable by a hand operated mechanism for parking purposes, the arrangement being such that during reversing of the caravan by the tractor vehicle the braking force is reduced. Operating mechanism.-A hitch bar 13 of the caravan passes through a hollow housing 15 fixed to the caravan and has one flange 16 within the housing 15 and another flange 19 between the housing 15 and a socket 14. The flange 16 is biased towards end wall 18 of the housing 15 by a coil spring 17, or alternatively by a gas filled hydraulic damper. The hitch bar 13 defines a slot 20 which receives a pin 21 carried by a lever 22 supported by a fixed pivot 23, and the end 24 of the lever 22 remote from the slot 20 is hinged to a link 25. The link 25 is connected to a yoke 35 from which extends flexible tension members 36, 37 (e.g. Bowden cables) connected to the shoe expanding devices 53 of the brakes 38, 39. A link 26 is also connected to lever 22 and has a slot 28 which receives a pin on a handbrake lever 29 having an associated pawl and ratchet mechanism 33, 34. Thus the shoe expanding devices 53 can be operated by leftwards movement of the housing 15 upon forward movement of the caravan relative to the tractor vehicle or by actuation of the handbrake lever 29. First embodiment of internal shoe drum brake.- In the embodiment of Figs. 2-5 the shoe expanding device 53 comprises a housing 55 received between one pair of adjacent ends of shoes 66, 67 slidably mounted on a backplate 40 and enclosing a lever mechanism 54 operable by a rod 50 connected to the respective tension member 36 or 37. The lever mechanism 54 comprises a strut 59 hinged to a bell-crank lever 62 mounted at its elbow on a pivot pin 63 anchored to the housing 55 and hinged to the operating rod 50. The shoe 66 contacts the housing 55 and carries a snail cam wear adjusting device 87. The shoe 67 is mounted for circumferential movement on a carrier 68 comprising two arcuate plates 69, 70 and one end of the web 74 of the shoe 66 is received in a recess 65 in the strut 59 and the adjacent ends of the plates 69, 70 are held in contact with the strut 59. The plates 69, 70 are connected in spaced parallel relationship by rivets 71, 72, 73 and receive the web 74 of shoe 67 between them. A pin 89 which is held in holes in the plates 69, 70 also passes through a pear shaped aperture 91 in the web 74 and the circumferential movement of the brake shoe 67 relative to the carrier 68 is thus limited to that permitted by the pin 89 in aperture 91. An abutment assembly 41 is provided between the other pair of adjacent shoe ends and comprises a fixed cup shaped casing 42 with an axially extending central pillar 44 fixed to its base and a sleeve 45 slidable on the pillar 44 and biased by a spring 47 towards a circlip 46. The web 77 of shoe 66 contacts the base of casing 42 while the ends of plates 69, 70 which extend beyond the adjacent end of web 74 are held in contact with the sleeve 45 so that the end of pillar 44 is between the plates 69, 70 and is spaced from web 74. Pull-off springs 75, 76 act between the webs 74, 77, and the brake shoes 66, 67 and the expanding device 53 are urged towards the backplate 40 by the action of a tongue 52 integral with the backplate 40 on the spring 76. The drum is rotatable anti-clockwise during forward movement of the caravan and when the rod 50 is moved to apply the brakes during forward movement the strut 59 imparts a thrust to plates 69, 70 the reaction to which is transmitted via pin 63 to the housing 55 which is moved to actuate shoe 66. The thrust on plates 69, 70 is transmitted to the brake shoe 67 via the rivet 72 and the inner surface of web 74-thus the thrust imparted to the brake shoe 67 to urge it into contact with the drum is approximately twice the thrust imparted to the carrier 68 due to the lever action of the carrier. When the shoe 67 engages the drum it also rotates anti-clockwise relative to carrier 68 and this movement is transmitted to shoe 66 via the slidable housing 55 so that the braking force exerted by shoe 66 is increased. The reaction to the thrust applied to carrier 68 is provided by the coil spring 47 which yields when this thrust exceeds a predetermined maximum to prevent locking of the wheel. During reversing movement of the caravan, circumferential movement of the shoe 67 relative to the carrier 68 is prevented by the pin 89 and the resultant braking force applied by shoe 66 is less. By movement of the hand brake lever 29 the rod 50 is movable a sufficient distance to overcome spring 47 and move the adjacent end of web 74 into contact with the fixed pillar 44. Second embodiment of internal shoe drum brake. -In the embodiment of Figs. 6-8 the shoe expanding device 112 comprises a lever mechanism 113 with a strut 118, and the abutment assembly 101 comprises a fixed U-shaped structure 102 having a central pillar 103 on which is slidable a flanged sleeve 104 urged towards a pin 105 by a spring 106. The web 130 of arcuate brake shoe 126 is located for sliding circumferential movement between plates 128, 129 of a carrier 127 by pairs of arcuate dimples 131 on plates 128, 129. A central part 136 of the web 130 is held in contact with a roller 137 carried by the carrier 136, which part 136 is defined by an insert 145 of hardened material which is profiled to provide portions 147, 148, 149. Respective springs 154 for plates 128, 129 are held between a pin 153 and a respective recess 152. Ears 156 on the brake shoe 126 locate the carrier 127 against axial movement. Upon braking during forward movement of the caravan, the strut 118 is moved to apply a thrust to the carrier 127 which via roller 137 moves the shoe 126 into contact with the drum. The shoe 126 is rotated with the drum to act via the housing 114 of the expanding device 112 to move shoe 125. Upon braking with reversing movement of the caravan the brake shoe 126 is dragged away from the housing 114 to extend springs 154 and move portion 147 out of contact with roller 137. The portion 149 results in the force applied to the drum by shoe 126 being reduced until the load exerted on the shoe 126 is just balanced by the springs 154. The rod 109 can be moved during handbrake operation to bring web 130 towards pillar 103. The coil spring 106 ensures an adequate braking force when the handbrake is applied and the caravan tends to roll forward. Should it roll backwards, web 130 will contact pillar 103 which acts as a stop.