Patent GB-581796-A (GB)
Title: Improvements in systems for controlling the energisation of electro-magnetically operated brakes
IPC: F16D51/02
Filed: 1945-03-27
Abstract: 581,796. Brakes. THORNTON, A. A. (Warner Electric Brake Manufacturing Co.). March 27, 1945, No. 7702. [Class 103 (i)] [Also in Group XXXVII] An electro-magnetically operated brake system includes a variable resistance unit 36 for controlling the energization of the brakes, the said unit comprising at least twelve resistance elements 38 and a contact member 49 which is moved, by the brake pedal, to connect the elements 38 successively in parallel in the energizing circuit, the arrangement being such that the increase in energizing current for a given movement of the pedal is progressively greater as the pedal approaches the position of maximum brake application. This gives a relatively weak deceleration response to pedal movement during light applications, and a corresponding increase in sensitiveness to enable the driver to negotiate, for example, slippery roads with an empty vehicle. Suitable resistances for the elements 38 are mentioned, the resistances varying from element to element and having values between 24.9 ohms and 0.115 ohms. The invention is described as being applied to brakes wherein the braking torque of an electro-magnetically applied disc brake is relayed to expand a friction band arranged with a brake drum. The flexible strips 41 associated each with a resistance 38 are of progressively increasing lengths and, when unflexed, all the ends 46 are in a straight line. The member 49 is a flat blade secured to, but insulated from, an arm 48 which is pivoted at 55 between arms 56 on the shaft 50 which is operated by the link 53 connected to the pedal. Springs 61, bearing on a pin 57 carried by the arm 48, tend to keep the pin against shoulders 60 on the arms 56, but when the pressure on the arm 48 causes the springs to yield, the movement of the pin 57 along the surfaces 58 of the arms 56, is limited by stops 59. A return spring 62 anchored at one end, and acting at the other end on the pivot pin 55, tends to maintain the contact 49 clear of the strips 41. When the pedal is moved to apply the brakes, the contact 49 first engages the longest strip which bends as successive strips are engaged and bent. When about five strips have been bent, the reaction on the contact 49 causes the springs 61 to yield, which allows further contacts to be engaged and, at the same time, the contact to move outwardly to produce a rubbing action between the opposed contact surfaces. This progressive yielding of the springs 61 allows all the strips to be effectively contacted by the member 49 without undue flexing of the longer strips 41. The friction of the bent strips assists in returning the member 49 when the brakes are released, and this results in a lack of symmetry between the strip-engaging and the strip-disengaging relations of the strips 41 and member 49. This produces differences in energizing current for a given pedal position according to whether the pedal is being used for a brake application increase or decrease. The difference in energizing current is advantageous in that it offsets the hysteresis effect in the electro-magnetic brakes.