Patent GB-1187767-A (GB)
Title: Improvements in or relating to Friction Clutches and Brakes
Inventor: OBERMOSER KARL
IPC: F16D65/08
Filed: 1967-04-19
Abstract: 1,187,767. Internal shoe drum brakes; friction Clutches. K. OBERMOSER. 19 April, 1967, No. 55473/66. Headings F2C and F2E. An internal shoe drum brake or clutch having at least one leading shoe is adapted to give a substantially constant torque for a given applying force, regardless of variations in the coefficient of friction between the shoe or shoes and the drum, by an arrangement comprising a shoe 6, which carries friction members 2, 2<SP>1</SP> at two spaced positions along its circumference, and which is pivoted by means of a knife edge 7 within a recess 8 of a shoe-carrying lever 5 to which a brake applying force K is imparted. The pivot 9 of the lever 5 may be either fixed, or carried by a link for transferring force to a secondary shoe 19. The disposition of the friction members 2, 2<SP>1</SP> and the location of the knife edge 7 and pivot 9 are such that, with the drum 1 rotating in the direction of the arrow relative to the shoe, the respective contributions of the friction members 2, 2<SP>1</SP> to the resultant P res  of the contact and frictional forces acting on them vary such that the sum of the friction forces U and U<SP>1</SP> is independent of the coefficient of friction between the friction members and the drum over a range extending between Á = 0À15 and Á = 0À45. This is accomplished by positioning the friction member 2 such that the line of its reaction force P corresponding to Á = 0À15 passes through the knife edge 7 whereas a typical reaction force P 2  corresponding to a higher coefficient of friction is on a line passing to the left of the knife edge. This means that as Á increases a greater moment is exerted about the knife edge 7 causing a greater proportion of the applied force to act on the friction member 2<SP>1</SP> which is positioned such that, under static conditions, the contact force N1 for a given applying force K is much less than the contact force N 2  acting on the member 2, the position being also such that the line of the reaction force P 1  is to the right of the knife edge 7 even for the highest value of Á to be encountered. The effect is thus equivalent to that of a single friction member being adjustable automatically along the periphery of the shoe 6 in dependence upon the value of Á. The lines of action of the forces P, P 1  and P 2  are calculated by reference to the circles centred on the drum centre O to which they are tangential, the radii of which are given by r sin p, where p is the angle of friction between the respective friction member and the normal to the drum surface passing through O and r is the radius of the drum. Since the line of the resultant P res  is inclined to the vertical for all values of Á except the maximum value of 0À45 there is normally a horizontal component of force H in addition to a vertical component of force V acting on the knife edge 13. This may be cancelled out, however, by the use of an additional shoe 10 having a friction member 2<SP>II</SP> positioned such that, with shoe 10 mounted on the knife edge 13 such that the reaction of the latter is opposed to H, there is produced, under the influence of the reaction R H  between its opposite end and a fixed plate 20, a reaction force P H  the horizontal component of which approximately balances H. The member 2<SP>II</SP> may be positioned in any one of the recesses 29 in the shoe 10. Where, as shown, a secondary shoe 19 is used, the knife edge 13 is carried by a member which is both adjustable, by means of a ratchet 14, to compensate for wear, and which has an arcuate portion 15 by which the vertical component of the reaction force P res  is transferred via a link 16 to the shoe 19 which is anchored at 32 on the fixed plate 20. Shoe 19 carries two friction, members 2<SP>III</SP>, 2<SP>IV</SP> which are positioned such that, for an average value of Á = 0À37, say, the lines of the respective reaction forces P I , P II  are inclined equally to the vertical, the friction forces produced being therefore equal for this value of Á and the sum of these forces therefore being substantially independent of variation of Á. Each of the friction members 2, 2,<SP>I</SP> 2<SP>II</SP>, 2<SP>III</SP> and 2<SP>IV</SP> is mounted in a recess in the respective shoe and retained in place by a spring clip (21, Figs. 5 and 6, not shown) which is inserted and removed axially and is arranged to allow limited rocking of the friction member within the recess.