Lockheed Introduces New Brake with two leading shoes
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interesting Features incorporated in New Braking System by Well-known Maker IT is vitally necessary that braking efficiency should, at least, keep in step with improvements in engines and .ransmission systems, as the ability to stop a machine under any given condition within a reasonable distance is far more important than being able to maintain a high average speed.
Brake designers have .certainly been faced with many difficult problems as vehicles have developed from a modest speed range into the present-day higher power-weight ratio machines possessing a much wider range of engine speeds.
The name Lockheed has always been synonymous with good braking, and the company now announces a new designof brake, incorporating the two-leadingshoe principle.
Advanced Design In a conventional assembly, the two shoes arc so anchored as to give a leading shoe and a trailing shoe. When the brakes are applied, the leading shoe tends to ' cling " to the drum, whilst the trailing shoe maintains its contact with the drum only by virtue of increased pressure on the brake pedal. Appreciation of this shortcoming of the conventional type of brake led to the introduction of the two-leading-shoe type, of which the new Lockheed is an advanced example.
It is, of course, well known that, when the brakes on a vehicle are applied, there is a transference of weight to the front axle, the extent of which is governed by the degree of deceleration Thus, the higher the rate of retardation, thc greater may be the front-brake action up to the point where the wheels will tend to lock.
In the new Lockheed system, twoleading-shoe brakes are used on the front wheels and the more conventional leading-and-trailing-shoe type on the rear wheels.
An outstanding feature of the Lockheed brakes is the use of what is termed flat floating anchorage for the brake shoes. This type of anchorage ensures that the brake facings make full-area contact with the drum under all normal conditions, and when, as happens in all types of brake, the coefficient of friction of the facing varies, the floating anchorage minimizes any tendency for the shoes to " self-lock " or "grab."
All he shoes arc of the floatinganchor type, but because the front and rear brakes operate on different principles, the anchor points, or abutments for the shoe ends, are dissimilar. In the case of the front brakes, the closed ends of the respective actuating cylinders are used, an arrangement which permits of the retention of the features of the existing Lockheed slotted shoe, which include self-centring characteristics and immunity from "grab."
Twice the Force
Thus, the effective drag on each frontbrake shoe on the drum is approximately twice the force applied at its tip by its actuating cylinder, and, owing to weight transference, the front wheels have sufficient' road adhesion to accept this addition t. braking effort.
In the rear brakes the free ends of the shoes abut against a member secured to the back plate, the shoes being held up to the abutment by the pull-off springs. Provision for mechanical operation by the hand lever is in the hydraulic cylinder itself, by making the piston in two portions, of which the outer one is operatA by a lever having its pivot in the cylinder body. The inner half carries the sealing cup and is moved by the hydraulic pressure, carrying the outer half of the piston with it to apply the leading shoe.
The trailing shoe is applied by movement of the cylinder body in each ease. The cylinder body is free to slide a sail cient amount on the back plate, and this it does, due to hydraulic pressure, or to the reaction on the pivot of the mechanically operated lever, as the case may be.
In the past, the provision of mechanical operation, in addition to a hydraulic wheel cylinder, has been attended by numerous difficulties. Some form of actuating lever had to be provided, with linkage to both shoes; and this lever and linkage became something of a problem when the back plates had to be considerably dished, or when the rear-axle end occupied most of the available space between the shoes.
Arrangements are made for maintaining brake-facing clearance, the only tool required being a screwdriver. Regarding the master cylinder, this remains substantially the same as before, retain ing the self-bleeding and self-compensating characteristics.
Simple Adjustment Regarding adjustment, the screwdriver. which is inserted through a hole conveniently placed in the brake drum. turns a duplicated snail cam. The spindle between the cams rests in a slot at the end of the brake shoe. The snail cams have serrated faces, and these engage protrusions, formed in the locating fork on the piston end, by which the outer ends of the snail-cap spindle are kept in alignment.
In addition to keeping the snail cams at the desired setting, the serrations provide a sensitive safeguard against overclose adjustment. In some applications the adjustment is made by an accessible turnbuckle on the back plate.
The maker is Automotive Products Co., Ltd., Tachbrook Road, Leamington Spa.