The evolution of machinery for producing motion in its various forms has necessitated the constant improvement of the brake, upon the efficiency of which the safety of millions of workers and travellers depends
MUCH of the inventive genius of the engineer is used in converting various forms of energy into motion. Motion, whether it takes place in a straight line or whether it is rotary, is one of the necessary consequences of a machine, and the constant demand for higher speed in every phase of life has been responsible for the development of more and more efficient machines for producing motion in its various forms.
Mere motion, however, is useless and even dangerous unless it is under control, and the brake — a contrivance for bringing a moving object to a standstill — has had to develop as rapidly as have the various forms of moving machinery.
The origin of the brake is nebulous; it is probably as ancient as that of the wheel, which is popularly supposed to be one of man’s earliest inventions. It is reasonable to suppose that the rotary motion of the first wheel was checked by the application of some stationary object to its rim. If this was the first brake, then the basic principle of the brake has remained the same throughout the centuries.
It is certain that the only means of checking the forward motion of the earliest and most primitive carts and barrows was a crude wooden block roughly applied to the rim of a wheel by means of a wooden rod — the forerunner of the lever which is so important a part of the most modern systems of braking.
Brakes to-day vary greatly in their design and in their applications. The braking system of the small motor car and that of the electric train, for instance, have little in common, except the purpose for which they were designed — to retard motion as quickly, but as smoothly as possible whenever such retardation is desired.
Brakes on cranes, escalators, steam engines, aeroplanes and in ships have to be designed to work in a particular set of conditions of their own. The science of braking is another of those highly specialized subjects which are so frequently met in the broad field of engineering.
The braking of the motor car has been bought to a fine art in recent years. The extraordinary increase in the efficiency of small engines, the tendency towards ever higher driving speeds, and the need for ease of control and safety have, together, been responsible for the development of several braking systems which approach and even attain the theoretical maximum efficiency. Various systems have their own adherents, but all of them are efficient and have features of considerable interest. Motor car braking is a complicated subject, because it necessitates the conversion of the simple movement of a single pedal into a simultaneous retarding action on four wheels. This action must be perfectly balanced if the highest possible efficiency is to be attained.
The speed of a car is controlled and checked by frictional forces developed by the expansion of shoes inside a drum. The nature of the frictional forces is a function of the brake assembly and its component parts. The brake linings, the shoes on which they are fixed and the method of mounting the shoes all play an important part in determining the magnitude and character of the forces applied to the drum and to the wheel.
The difference between the various braking systems now in general use lies in the means adopted to produce the expansion of the shoes — in the connecting link between the brake pedal and the brake assemblies attached to the four wheels of the car.
One popular method that is widely used is the Lockheed hydraulic system. The pressure exerted on the brake pedal is conveyed to the brake shoes by a column of fluid, through the medium of a master cylinder, in which the hydraulic pressure is generated,
and of a line, consisting of copper tubing, flexible hoses and unions; this line is interposed between the master cylinder and the wheel cylinders, which are placed between the pairs of brake shoes on each wheel assembly.
INTERNAL EXPANDING BRAKE for motor cars, as typified by the Girling system. The brake shoes are forced outwards by the movement of a pull-rod which protrudes, from the “ expander,” through the back plate. A detailed sectional diagram of this system is on page 998. The expander mechanism is enclosed in a die-cast housing which retains a supply of lubricant and protects the working parts from mud and water.
The cylinders and the pipe line form one vessel, which is completely filled with fluid. The master cylinder has a single piston which is operated by the brake pedal, and each wheel cylinder has two opposed pistons. When the brake pedal is depressed the master cylinder piston applies a force to the fluid. As a fluid is incompressible, this force causes the wheel cylinder pistons to move apart, until the brake shoes are expanded sufficiently to touch the drums. Further effort applied to the pedal increases the force applied to the brake shoes.
As the pressure generated in the master cylinder is transmitted, with equal and undiminished force, to the pistons of each wheel cylinder, the pressures applied to all the brake shoes are identical. As soon as the pressure on the foot pedal is released, the brake shoe return-springs force the wheel cylinder pistons together, and the fluid flows back to the master cylinder, ready for the next application of the brakes.
One of the most important features of this hydraulic system is the provision made to allow for expansion of the fluid because of change in temperature. The master cylinder is connected to a supply tank by means of a small feed hole which is uncovered whenever the brakes are off. When the brakes are applied this hole is closed, and it is then impossible for the fluid to flow back from the master cylinder to the tank.
A refinement is the tandem master cylinder, which consists in effect of two barrel-type master cylinders in line, without any direct fluid communication between them. Each cylinder is fed by an independent compartment in the supply tank. One cylinder operates the rear brakes; the other the front. Thus there are two separate systems in operation, and in the event of an accidental failure or leakage there still remains an effective brake.
The flexible hoses which transmit the pressure from the pedal to the wheels play an important part in the functioning of a hydraulic braking system. Although the working pressure is about 800 lb. per sq. in., these hoses are tested to pressures of 3,000 lb. per sq. in., and their bursting pressure is claimed to be approximately three times this figure The unions and end attachments stronger, and the piping used is of special design, made from copper to an accurate specification and well annealed. The braking systems of large commercial vehicles have to be specially designed, particularly in one detail. The inside diameter of the tyre rim limits the possible size of the brake drum; so a vehicle weighing, perhaps, ten times as much as a private car has to be equipped with brake drums which may be less than twice as large as those of the smaller and lighter vehicle.
“Telescopic Wheel Cylinder”
Hydraulic brakes for commercial vehicles sometimes make use of a “telescopic wheel cylinder”. Extremely high pressures are required at the tips of the brake shoes, and space does not permit the installation of- a correspondingly large internal cylinder. A telescopic cylinder is therefore mounted outside the brake drum, and it operates the normal type of brake cam through a shaft and lever.
If the cam and lever are designed to meet the particular requirements, the wheel cylinder can be given a long stroke, with a corresponding increase in the energy imparted to the brake shoes. One advantage of this arrangement is that the wheel cylinders, being outside the drums, are in a free air current and unaffected by the heat of the brake drum.
One of the modern tendencies in commercial haulage is to make increasing use of trailers. These trailers must be equipped with brakes of their own, and the efficient braking of trailers involves a number of problems. Control must be entirely automatic; the braking effort must be proportionate not only to the load on the trailer, but also to the degree of braking applied to the towing vehicle. Braking must be equalized on all wheels; the trailer brake must remain on when the vehicle is at rest, and it should work as a sprag on hills.
The trailer brakes must come into action automatically if the trailer becomes accidentally detached, and the brake operating unit must be entirely self-contained on the trailer.
The Lockheed trailer braking system is designed to fill all these requirements. It is self-contained on the trailer, with the exception of an electrical connexion to a switch operated by the brake pedal of the towing vehicle (see diagram below).
AUTOMATIC BRAKING SYSTEM FOR TRAILERS. The upper diagram shows how the brakes are applied when the drawbar (left) connecting the trailer with the towing vehicles is not in tension. This position is automatically taken up when the towing vehicle reduces speed. When the drawbar is in tension (lower diagram) the brakes on the trailer are automatically released. An electrical connexion from the brake pedal of the towing vehicle is the only braking connexion with the trailer.
The tractive effort from the towing vehicle to the trailer is transmitted through a drawbar spring, and the drawbar is connected also to a reservoir containing two piston assemblies. One of these is known as the “dash pot” piston, and the other as the master piston.
By the use of suitable valves in the dash pot, the drawbar spring is maintained in the compressed position. When, however, the brake of the towing vehicle is applied, and the brake pedal switch is actuated, an electrical device opens control valves in the dash pot and allows the drawbar spring to operate the master piston, which applies the brakes on all wheels of the trailer in the same
way as the master piston of the ordinary car braking system applies the four-wheel brakes.
The trailer brakes are operated entirely by the energy stored up in the drawbar spring. In the event of extremely abrupt braking, the overrun of the trailer, augmenting the action of the spring, causes a proportionately greater effort to be applied to the brakes of the trailer. This action is proportionate not only to the braking of the towing vehicle, but also to the load on the trailer, as a heavy load causes a more forcible overrunning effect.
Should the trailer become accidentally disconnected, the drawbar will fall to the ground. Arrangements are made to ensure that this action operates the electrical gear and applies the trailer brakes. When the trailer is being “man-handled”, dropping the drawbar at once applies the brakes.
When the towing vehicle and trailer are brought to rest on a hill, the trailer brakes remain on until they are released by forward movement of the towing vehicle and the consequent tension of the drawbar spring. This feature makes restarting on steep gradients an easy matter, as it works as an efficient sprag.
There is no mechanical brake connexion between the towing vehicle and the trailer; the only control necessary is the electrical connexion which supplies current, from the lighting system of the towing vehicle, to the solenoid which operates the valves of the hydraulic system. This connexion is of the quick release type, and may also carry the current for the rear lamp and the stop light on the trailer.
Ingenious Rod Linkage
In the earlier days of the motor car, one of the disadvantages of the braking systems in use was the prevalence of frictional and torsional losses between the operating pedal or lever and the braking assemblies on the wheels. These disadvantages, however, are not inseparably bound up with mechanically-operated braking systems, and modern brakes of the mechanical type are extremely efficient. The Girling brake, now standard equipment on many makes of car, obtains its high overall efficiency by eliminating torsional and frictional losses and by making it possible to use a high ratio between the pedal and the tip of the brake shoe.
A simple pulling motion on a rod protruding from the brake assembly on the wheel is converted into a steady application of the brake shoes by the mechanism illustrated below. The pull-rod actuates a hardened steel cone; between this cone and the plungers which force the shoes apart are hardened steel rollers which reduce friction to a minimum. This whole expander mechanism is enclosed in a die-cast housing which retains a supply of lubricant and protects the working parts from mud and water. This housing is attached to the back plate of the brake assembly by studs and spring washers, and it does not have to withstand any of the stresses of braking, as it virtually floats between the brake shoes.
EXPANDER MECHANISM of the Girling system. When the pull-rod is moved outwards the steel cone, moving between rollers, forces the plungers apart. The plungers engage directly with the ends of the brake shoes. The whole assembly is attached to the back plate by studs and spring washers only and it virtually floats between the brake shoes. Thus the brake shoes are self-centring.
The steel rollers are freely mounted and roll up grooves in the plunger and down the inclined face of the cone, an arrangement which doubles the overall leverage. The effort of the pull-rods is multiplied by 6.33 by the time it is applied to the brake shoes.
An ingenious rod linkage is used between the operating pedal and the wheel assemblies. Longitudinal rods, operated by the brake pedal, transmit motion to transverse rods through the medium of balance levers mounted on the axles. These balance levers change the direction of the motion from left to right.
By this system of linkage all cross-shafts are eliminated, as are all torsional members (rotating shafts). The pedal effort is divided at the base of the pedal, thus equalizing the loading between the various rods. All rods are in tension and equally loaded, and the load on all rods is low, because of the high step-up ratio that is obtained in the brake assembly — between the pull-rod and the shoe-tips.
The brake-operating mechanism is housed between the upper ends of the brake shoes. Between the lower ends is the compensating device for taking up wear of the brake linings. This device consists of a hardened steel cone, the spindle of which is screwed with a fine thread. The cone engages two plungers with inclined faces, and at their remote ends these plungers have grooves in which the ends of the brake shoes are carried. When the adjusting spindle is turned, the cone is forced, by the screw thread, farther in between the plungers, forcing them apart.
Brakes used on petrol-driven vehicles are almost invariably of the internal expanding type. The two systems which have been described make use of the well-known principle of causing shoes, fitted with linings of some special material, to expand and press against the interior of a drum carried rigidly on the same axle as the wheel.
Brakes of this kind are coming into increasing use on pedal cycles, many of which, however, use brake blocks which are applied to the inner surface of the rim of the wheel.
Transmission brakes, which were once extensively used on motor cars, particularly as hand brakes, are still used for other purposes in industry. The general principle of the transmission brake is the application of frictional force to the shaft which transmits motion to the wheels — the propeller shaft in a motor car. Such brakes are almost invariably of the external-contracting type.
The brakes used on trains and tramway cars involve an entirely different technique from that which is applied to the design of motor car braking systems. The designer of brakes for trains is not confronted with the necessity for converting pressure on a pedal into pressure on four wheels mounted conveniently closely together; braking effort must be continuous throughout the length of the train, and the total effort is of such magnitude that simple mechanical linkages are ruled out of consideration.
WHEEL BRAKE UNIT FOR AIRCRAFT. The annular expansion chamber is connected to a compressed-air cylinder. When compressed air is admitted, the expansion chamber shoes outwards against The the forces the brake drum of the wheel. _________
operating mechanism is linked with the rudder bar in such a way that automatic differentia! control of the braking forces on the two wheels is obtained.
The well-known Westinghouse automatic brake is operated by compressed air stored in reservoirs. On steam trains a steam compressor and governor supply the pressure in the reservoir; on electric trains an electrically driven compressor is used. In other ways the systems are identical.
Compressed-air brakes and vacuum brakes, both of which are extensively used on the railways, are diametrically opposed in the first part of their operation, and represent the development in matters of detail of two completely different schools of thought.
The compressed-air brake relies on pressure to drive pistons out and to apply the brake blocks to the wheels; the vacuum brake operates by virtue of the pressure of the atmosphere on one side of a piston in a brake cylinder, on the other side of which a vacuum is created.
The brakes on all passenger trains must be continuous — that is, attached to every vehicle — and they must be capable of being applied at will by the engine driver or by the guard. They must be instantaneous in action, and they must be self-applying, should the apparatus go out of order or should part of the train become uncoupled.
The compressed-air brake and the vacuum brake lend themselves particularly to these requirements. By the very nature of their operation it is a simple matter to arrange that they may be applied from either end of the train, and that they may apply themselves automatically in an emergency or accident. Railway brakes are of the externally applied type, and the brake blocks are almost invariably of cast iron.
A type of brake frequently used in various branches of industry is the magnetic brake, which consists of a pair of brake blocks acting on a drum or brake wheel secured to a motor shaft or countershaft. These brake blocks are normally applied by powerful springs. When, however, current is supplied to the motor the current also passes through an electro-magnet which withdraws the brake blocks, forcing back the compression springs. When the current is cut off — by accident or design — the brakes are at once applied.
Magnetic brakes, with various modifications, are used also on tramway cars, many of which are equipped with a combined axle, wheel and track brake which is capable of stopping and holding a car on the steepest gradients. Ordinary wheel brakes tend to lock the wheels of a tramway car, causing almost complete loss of control. With the combined brake there is little risk of skidding. The essential parts of this brake system are the ordinary wheel brake blocks, the track brake shoes, carried by light springs and mounted between the wheels, and a system of levers so arranged that, when the brake shoes are drawn into contact with the rails, the ordinary brake blocks are applied to the wheels.
The track brake blocks are magnetized by solenoids when the controller handle is moved to the braking position, and the supply of current to the motors is cut off. The motors continue to revolve, being driven by the forward momentum of the car, and act as a brake on the axles. Moreover, they act as dynamos and generate the current which is used to magnetize the track brake shoes.
Emergency brakes of enormous size are used on escalators. These are almost invariably of the magnetic type and are self-applying when the current fails or is cut off. Cranes, hoists and lifting machinery of all kinds use similar systems, and the emergency brakes on passenger lifts are also of a type that applies itself if the current fails, or if the downward motion of the lift exceeds a predetermined figure (see the chapter “Lifts and Escalators”).
Brake gear for aeroplanes has been greatly improved and developed during recent years. Efficient braking is an absolute necessity, as it not only improves the performance of an aeroplane on the ground, but also enables ground crews to be reduced to the minimum number of men.
One widely used type of aero brake is hand-operated by the pilot and gives automatic differential control of the braking of the wheels according to the movement of the rudder bar.
The brake unit is pneumatic and of an entirely different design from anything found on motor cars. Between the brake shoes and the supporting plate is an annular expansion chamber which, upon inflation, forces the shoes outwards against the brake drum of the wheel. Strong springs return the shoes to the “off” position when the pressure in the airbag is released. Compressed air is supplied from a container which can be used for pressures up to 400 lb. per sq. in. This container is replenished either by a hand pump or by an engine compressor fitted to the aircraft.
When the rudder control is set in the “dead ahead” position the braking effort is equally applied to the wheels. As the rudder is moved to steer the aircraft on the ground, the retarding force is steadily increased on the inside and decreased on the outside wheel. On the instrument board is mounted a triple pressure gauge on which the pressure in the air container is continually shown, the pressures in the individual brakes being shown during operation.
An interesting device recently developed, and one which may be regarded as a highly specialized form of brake for the motor car, is known as the “Hill-Holder”. It is intended primarily to be adopted in the design of new cars which are equipped with hydraulic braking systems. The “Hill-Holder” is not a sprag; it is a gravity-controlled brake which is ready for action when a car is ascending an incline. The mechanism acts as soon as the clutch pedal is depressed. The device consists of a valve interposed in the main hydraulic pipe line between the master cylinder and the wheel cylinders, and it is entirely automatic in its action.
The valve is contained in a cylindrical body about 3 in. long which is mounted not quite horizontally, and lengthwise with reference to the car. At one end a sealing plug is screwed in, and at the other the blank wall is pierced in two places. A central intake orifice is coupled with the pipe from the master cylinder and an outlet connects with the pipe leading off to the four wheel cylinders. Sliding endwise in the cylinder is a light assembly, consisting of two plates coupled by rods forming a cage in which is a steel ball.
The end plate of this cage carries a rubber seating with a small port through its centre; one side can seal the intake, and the other can be sealed by the steel ball.
In normal running the ball is free to “float”. When the car is climbing a slope, however, the ball is caused by gravity to run towards the rubber sealing, and to rest against it. The clutch pedal is coupled to the device in such a way that when the clutch is disengaged and the ball is resting against the sealing, the hydraulic fluid closes the valve completely and is trapped, so that the brakes are automatically held on. Immediately the clutch is engaged (i.e. the pedal released) the intake orifice is unsealed and the brakes are released.
Brakes are used in cable ships for restraining the movement of the cable as it is paid out; huge hydraulic brakes are used on some types of gun to absorb the recoil. Other brakes of specialized design are used in many spheres of engineering. All brakes, whatever their design, have certain features in common, and all are used for stopping, or checking motion, whether the motion be rotary or in a straight line.
HYDRAULIC BRAKING SYSTEM in diagrammatic form. Pressure applied to the operating pedal (top) moves a piston in a master cylinder and causes fluid to flow along the pipe lines to the four wheel cylinders, in which it forces pistons outwards towards the brake shoes against the action of return springs. The fluid in the master cylinder is supplied from a tank which is inspected and filled periodically.