An Ingenious Brake Design
Page 45
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Parallel Action, Compensation and Progressive Shoe-expansion Force Among Numerous Desirable Features. Few Unlike Parts Simplify Manufacture
THE clever design of internal-expanding brake shown in the accompanying drawings—made from a model—is the invention of Mr. John Day, 4, Alma Road, Canonbury, London, N.1. It is provisionally patented, and the inventor would like to get into touch with persons or manufacturers interested.
One of its chief attractions is that the whole of the frictional area of both brake facings bears against the drum with practically the same pressure at every point. This, of course, is not the case with the type in which each shoe is pivoted at one end. Moreover, owing to the toggle-action of the shoe links, the expanding force rapidly increases as the shoes separate.
Each shoe is supported on two, or more, pegs which fit in slots formed for the purpose. These, whilst permitting the necessary lateral movement, withstand torsional forces. That is to say, they take the rotational thrust of braking.
Application of the shoes is effected by raising (as drawn) the central annular-shaped piece, which, at each end, carries vertical extensions moving in guides. It is operated by the lever at the back of the brake plate, to which the brake rod, cable or hydraulic-cylinder plunger is coupled.
To each extension is pivoted a swinging link, to the lower end of which the free ends of each pair of shoe links are pinned. The mechanism constitutes a parallel-motion gear, the shoe links operate on the toggle principle and the swinging links ensure perfect compensation, so that if one shoe tends to come into operation before the other, or if the facings be not of equal thickness, these irregularities are automatically cOrrected.
Adjustment is effected in the usual manner merely by altering the effective length of the cables or rods. In the case of hydraulic application, an adjust able return stop would have to be incorporated. To remove the shoes for refacing purposes the whole assembly can be lifted from the plate after the retaining washers on the guide pins have been detached. Incorporating only a few different parts, the brake should be cheap to produce. Neither the shoe nor the central piece presents any manufacturing difficulty, whilst all the links are of the same dimensions. For the sake of simplicity it is necessary to offset the two links of each toggle, but, by arranging those of the other toggle in the opposite manner, the pressure is imparted to each shoe symmetrically.
The design is extremely ingenious and appears to have certain advantages over the hinged-shoe type of brake.
For operation without power-assistance, it would be necessary for the toggle arms to approach as near to dead centre as possible to ensure that adequate pressure be Ianparted to the shoes. It occurs to us that this might create a risk of the levers going over centre when the fabric facings become worn.