THE SUCCESS OF AN HYDRAULIC BRAKE.
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• Mechanical v. Hydraulic Means for Brake Operation. Difficulties in Developing a Fluid System and How They Have Been Overcome.
11/rUCH attention is being directed to the problem of imIX1proving brakes and their methods of operation. In many vehicles the ordinary hand or foot-applied brake is not so efficient as it should be, the actual brakes themselves (by which we mean the drums, shoes and operating cams) are often well designed and would, if operated direct, prove quite satisfactory, but the loss in the transmission of power between the pedal and the cam is usually considerable, and where there is a large number of joints the reduction in efficiency may be as great as 60 per cent.
Servo devices have done much to solve the problem, in that they add to the power exercised by the driver, but attention is also being devoted to the possibility of operating mechanical brakes satisfactorily by hydraulic means, for in a well-designed equipment using this method of application tho 'efficiency of power transmission up to the point of frictional application is as high as 90 per cent.
It would appear to be quite a simple matter to produce a satisfactory layout for such a brake, but what looks good on a drawing-board does not always turn out well in practice, and, actually, there are many difficulties to overcome in developing such brake gear.
One of the most successful types now on the market is the Lockheed, marketed by the Automotive Products Co., 3, Berners Street, London, W.1. This brake equipment is of American origin, but in the near, future it will be maimfactured in this country. It is interesting to mention that a well-known bus company in the Midland sI is gradually converting its fleet to the Lockheed system, a large number of buses having already been equipped with Lockheed brakes acting on all four wheels, but the Automotive Products Co. does not yet recommend that the present standard brakes should be fitted to the heavier types of commercial vehicle, and the types which will be manufactured here, to begin with, will be suitable for vehicles of up to 2-tons useful load capacity. The reason why the aforementioned buses are being equipped with the brakes is because the company wants a smooth-acting brake which will not cause excessive discomfort to the passengers and yet will work with efficiency and reliability.
We have already referred to the difficulties which have occurred in the design of hydraulically, operated brakes, and we will proceed by pointing out how these have been overcome in the case of the Lockheed.
One of the secrets of the success of this gear is the special rubber cups used in the master cylinder and operating cylinders, but such cups, although they are made to a special formula, would perish if ordinary oil were to be employed as the power-transmitting medium, and to overcome this difficulty use is made of a 'special fluid cornprised of a mixture.of neutralized castor oil and denatured alcohol, treated with chemical agents. to prevent aetion on
B38 bath the metal and rubber parts of the system. Tbis fluid is obtainable through the trade or direct from the company.
The average maximum pressure iu the system is between 1,000 lb. and 1,200 lb. per sq. in., and herein lies another difficulty. It is simple enough to obtain metal piping to resist such pressures without appreciable expansion, but when it comes to the flexible connections to the brakes this is altogether a different matter, as expansion is bound to occur. This being the casg, the designers of the Lockheed took the bull by the horns and so constructed their tubing that it starts with an initial expansion equal to that which would occur under the pressure.
To effect this, the hose is subjected to an internal pressure of 1,200 lb., and a coiled-wire spring is forced into it to maintain a permanent expansion. This in itself is one of the chief reasons for the success of the equipment. At the same time, the ends of the hose are expanded into the connecting ferrules.
With an hydraulic system, it is essential that there should be no loss of fluid, or, if loss does occur, it must be made up automatically. Now, the alcohol component in the Lockheed system would evaporate if exposed to the atmosphere, so the tank, in which is situated the master cylinder, which is operated direct by the pedal, is sealed. The master cylinder is drowned in the liquid, and any dirt in the latter drops to the bottom of the sump. In the tank there are two valves acting in opposite senses. If expansion. occurs in the fluid due to heat, one valve opens against the spring and allows part of the contents to escape. The other valve permits air to enter if the level of the fluid drops, but there is a spring on the release valve and, when in operation, there is a constant pressure of 11 oz. in the tank.
• An improved feature is that the movement of the pedal is never affected. It always returns to the off position.
Keeping the Cup Washers Expanded.
The master piston is retained by a spring which seats on a valve, the compression of this spring being so adjusted that there is always 8 lb. effective pressure in the system, this serving to keep the cup washers expanded.
If any loss of the fluid occurs in the system, the piston, being forced back by its spring, tends to create a partial vacuum and more oil is drawn past the cup leather. Completely to fill the cylinder there is a tiny hole in the cylinder just at the end of the piston travel.
At present the shoes are adjustable individually by hand for clearance by special eceentric !devices, but d,wvelopraents are likely to occur in this connection, and in such case the adjustment will be entirely automatic. We have seen the device which may be employed. This appears to
be extremely simple and effective, but we are not yet at liberty to disclose its nature. With the two-shoe Lockheed brake the maker is now obtaining practically the same braking power as with the three-shoe servo type, and there is the advantage that there is no fear of grabbing occurring. Naturally, there must be an independent brake as a; stand-by, and this is obtained in an extremely neat manner. The same two shoes which are normally expanded by the hydraulic device can also be acted upon by an expanding cam controlled by a hand lever, and when this is in use the shoes merely run away from the hydraulic pistons. To avoid toeing-in of the shoes they are ground slightly eccentric, the actual amount Of eccentricity being .01 in.
There is nothing diffienit in the actual fitting of Lockheed brakes to a chassis. There are no points at which greasing must be carried out, and compensation of the brakes as between front and rear wheels is obtained automatically.
There are, of course, many points to he considered in the adoption of any particular type of brake equipment, but there certainly appears to be a large field for the employment of such a brake gear as this, which is apparently almost fool-proof, and when the automatic adjustment is brought into use will provide brakes which will require practically no attention cxcept for refacing.