The document discusses the braking system of vehicles. It covers the purpose and design requirements of brakes, including good anti-fade characteristics and brake efficiency. It describes how braking force depends on factors like vehicle speed, road conditions, and friction. It also discusses drum brakes and disc brakes, including their construction, types, and advantages/disadvantages. Finally, it covers brake fluids and their requirements to have a high boiling point and maintain viscosity with temperature changes.

1. Braking System
●1 Service Brakes: Slows or stops the vehicle (foot-pedal operated).
●2 Parking Brakes: Holds the vehicle stationary when applied (foot- or hand-operated).
►Purpose: To stop the vehicle within smallest possible distance.
●1 Brakes must stop the vehicle within a minimum distance during emergency, but still
give the driver proper control over vehicle without skidding.
► Design Requirements:
●2 Good Anti-Fade Characteristics: Their effectiveness should not decrease with
constant prolonged application (descending hills). It demands efficient cooling of brakes.
Caused due to loss of friction at high temperatures. Reversible changes occur in friction-
material, which are restored when the material cools-off.
►Brake Efficiency: Brakes are 100% efficient if they produce vehicle’s deceleration
equal to acceleration due to gravity (g = 9.81 m/s^2).
--100% brake-efficiency is not desirable due to (1) safety of passengers in public
(2) safety of vehicle-body, especially for heavy goods vehicles. It varies 50% - 80%.
Minimum for any vehicle is 50% for foot-brakes, 30% for hand-brakes.
►Stopping distance depends on:
●1 Vehicle-speed ●2 Condition of road-surface, tire-tread ●3 Coefficient of friction
between tire-tread & road-surface, between brake-drum & brake-lining ●5 Braking-force
►Stopping distance depends on time:
●1 driver’s reaction-time ●2 time between driver pressing brake-pedal, and brakes being
actually applied at the wheels ●3 vehicle-deceleration caused by brakes .

2. Friction
--Brakes depend on frictional-force for their operation.
--Friction is the force that opposes relative motion of one body in contact with another.
It is the resistance to motion between 2 objects in contact with each other.
--Friction always acts opposite to the force producing the motion.
--When energy is used to overcome friction, heat is generated.
--Force of friction depends on ●1 roughness, lubrication, material of surfaces in contact
●2 force pressing them together ●3 relative speed (faster motion means lesser friction).
--Coefficient of friction is ratio of frictional-force to force holding 2 bodies in contact.
This is a constant ratio for 2 particular surfaces.
Hydraulics
--3 types of friction: ●1 dry ●2 greasy ●3 viscous. Automotive-brakes use dry-friction.
--2 kinds: ●1 static-friction, or friction of rest ●2 kinetic-friction, or friction of motion.
More force is required to put an object into motion, than is required to keep it moving.
Thus static-friction is greater than kinetic-friction.
--Hydraulics is use of liquid under pressure to transfer force or motion, or to increase
applied force.
--Increasing pressure on gas compresses it into a smaller volume.
Increasing pressure on incompressible liquids, transmits it equally in all directions to
every part of the liquid (Pascal’s Law).
--For the same pressure, output-force is directly proportional to output-area.

3. Braking System
►Weight-Transfer During Braking:
--Inertia-force acts at vehicle-center-of-gravity. Retardation-force due to braking acts at
road-surface. These 2 forces cause an overturning couple, which increases the
perpendicular-force between front-wheels & ground, and decreases at rear-wheels by
same amount. Thus some vehicle-weight is transferred from rear-axle to front-axle.
►Brake Application Hardness:
--If brakes are applied so hard that wheels lock, friction between road & tires is kinetic-
friction. If brakes are applied less hard, wheels continue to rotate, hence in static friction.
Since static-friction is greater than kinetic-friction, vehicle stops in a shorter-distance if
wheels do not lock. Thus brakes must be applied to the point where wheels are almost
ready to lock.
►Wheel Skidding:
--Force of adhesion between wheels & road depends on:
●1 vehicle-weight acting at wheel ●2 tire inflation-pressure ●3 type of tire-tread-pattern
●4 coefficient of friction between tire & road, which further depends on road-condition.
--If braking-force is less than force-of-adhesion, vehicle decelerates gradually till it stops.
If braking-force is more, then wheel stops rotating and skids till vehicle kinetic-energy is
dissipated as kinetic-friction between wheel & road. Vehicle stops in shorter distance if
wheels don’t lock. Skidding causes rapid tire-wear, loss of steering-control (dangerous).
--Due to vehicle-weight-transfer, rear-wheels are easier to skid. This is reduced by a
relief-valve which releases excess brake-fluid pressure in rear-lines in sudden braking.

4. Braking System Classification
►Purpose: ●1 Primary or Service ●2 Secondary or Parking
►Location: ●1 At Wheels ●2 At Transmission
►Construction: ●1 Disc ●2 Drum
►Method of Actuation: ●1 Mechanical ●2 Hydraulic ●3 Electric ●4 Vacuum ●5 Air
►Extra Braking Effort: ●1 Manual ●2 Servo (Power-Assisted) ●3 Power (-Operated)
--Disadvantages of Transmission-brakes:
●1 Entire braking-torque has to be transmitted through universal-joints, propeller-shaft,
Differential, rear-axle; thus their sizes must be increased proportionately.
●2 Wheel-brakes dissipate more heat than transmission-brakes because:
(1) wheel-brakes are located in more air-circulating area.
(2) there are 4 brake-drums which increases area available for heat dissipation.
--Wheel-brakes are used universally.
--Advantages of Transmission-brakes:
●1 Braking-torque is equally divided between automatically by the differential between 2
wheels, hence no special compensation is needed.
●2 Due to reduction at differential they are stronger than wheel-brakes of same capacity.
--For medium-& heavy-weight-vehicles, driver’s braking-effort must be supplemented
with additional power.
●Servo: Driver has to apply some power partly, and remaining comes from vehicle.
●Power: All power comes from vehicle, driver has to apply none of braking-effort.
►Method of Contact: ●1 Internal-Expanding ●2 External-Contracting

5. Drum-Brakes: Construction
Drum: ●1 encloses brake-assembly ●2 keeps out dust & moisture ●3 attaches to wheels
●4 attached to concentric axle-hub.
Backing-Plate: ●1 completes brake enclosure ●2 holds brake-assembly to car-axle
●3 supports brake-shoes, expander, anchor ●4 absorbs complete torque-reaction.
--It is mounted ●rear-axle: on axle-casing ●front-axle: to steering-knuckle.
--It is made of pressed-steel-sheet, and is ribbed to increase rigidity.
Brake-Linings: Friction-material ●1 riveted ●2 cemented (bonded) to metal brake-shoes.
Brake-Assembly: Attaches to steering-knuckle, axle-housing, or strut-spindle assembly.
Brake-Shoes: Bottoms are held apart by attaching to back-plate either as:
●1 both fixed-anchor-pins ●2 one fixed anchor-pin & one floating-adjusting-screw.
--Tops are held apart by either ●1 expander-unit ●2 wheel-cylinder.
Retractor-Springs: Keeps brake-shoes away from drum when brakes are not applied.
Adjuster-Mechanism: To compensate for wear of friction-lining.
►Mechanism:
●1 In Drum-Brakes: Fluid pressure pushes lined brake-shoes against a rotating drum.
●2 In Disk-Brakes: Fluid pressure pushes lined brake-pads against a rotating disk.
--When brakes are applied by pushing down on brake-pedal, brake-fluid flows through
brake-lines to operate brake-mechanisms at wheels, which applies force on rotating
wheels to slow or stop them.
Brake Mechanism

6. Drum-Brakes: Types
●1 Leading-Trailing (or) Non-Servo Type: Action of one-shoe does not affect other-shoe.
●2 2-Leading (or) Duo-Servo Type: Action of one-shoe reinforces action of other-shoe.
--Used in rear-wheels of front-wheel-drive vehicles.
--Used in rear-wheels of rear-wheel-drive vehicles.
--Retractor-springs hold both-shoes against ■top: wheel-cylinder ■bottom: anchor-pins.
--Friction between leading-shoe & drum, causes leading-shoe to try to rotate with drum
(self-energizing action). This forces the bottom of shoe against fixed-anchor-pin.
--When trailing-shoe contacts drum, drum-rotation tries to force shoe away from drum
(no self-energizing action).
--Thus leading-shoe does most of braking, hence wears lot; trailing-shoe wears little.
--When vehicle is braked while moving in reverse, leading & trailing-shoes swaps jobs.
--This type is more dependent on force supplied by wheel-cylinder, than 2-leading type.
--Tops of both shoes rest against a single anchor-pin.
Bottoms of both shoes are linked together by a floating-adjusting-screw.
--When shoes touch rotating-drum, primary-shoe’s top pulls into drum &move downward.
--Primary-shoe’s bottom pushes adjusting-screw rearward.
--This forces secondary-shoe’s bottom against the drum.
--This pushes secondary-shoe’s top upward against anchor-pin.
--Further drum-rotation pulls both shoes more tightly into the drum.
--Primary-shoe: shoe facing front of vehicle; Secondary-shoe: shoe facing rear of vehicle
--Secondary-shoes provide twice braking-force than primary-shoes (so has longer-lining)

7. Drum-Brakes: Types
●3 2-Trailing Type:
Adv: Both shoes have self-energizing action. Thus total braking-force is greater than
force supplied by wheel-cylinder.
Disadv: ■1 When vehicle is braked moving in reverse, both shoes become trailing-shoes
with significantly reduced braking-force. But this may not matter much as vehicles are
driven in reverse at very low-speeds thus requiring very little braking-force.
Disadv: Braking-force is considerably reduced for same force applied at brake-pedal.
Thus generally used with servo-brakes or power-brakes, to reduce driver-fatigue.
Adv: Has better anti-fade, thus provides more consistent braking.
■2 It’s sensitive to coefficient-of-friction changes, thus not best for prolonged application.
Hydraulic-Brake vs. Mechanical-Brake
Adv:
●1 Equal braking-effort at all 4 wheels, as fluid exerts equal pressure everywhere.
●2 Simple in construction, due to absence of brake-rods, joints etc.
Pipelines can be shaped according to underside of body-structure.
●3 Less wear-rate due to absence of joints.
●4 Self-lubricating system.
Disadv:
●1 Even slight leakage of air into braking-system makes it useless.
●2 Suitable only for intermittent-braking.
Needs mechanical-brakes for parking as no pressurized-fluid is available then.

8. Drum-Brakes vs. Disk-Brakes
●1 Disk: Friction-surfaces are directly exposed to cooling-air. Heat dissipation is more.
Drum: Friction occurs on internal surfaces. Heat is dissipated through drum- conduction.
●2 Material: Asbestos-fiber with metal-oxide fillers bonded with organic-compounds.
Disk: Friction-pads are flat, thus uniform-wear on pads.
As they are not subjected to bending, wider range of materials to choose from.
Drum: Friction-pads are curved, thus non-uniform wear.
●3 Disk: Expansion of disk merely changes relative positions on friction-surfaces slightly,
without increasing the clearance. Thus no loss of efficiency due to expansion.
Drum: Expansion of drum moves friction-surfaces apart, thus loss of effective pedal travel.
●4 Disk: Has better anti-fade characteristics. Requires lesser and constant braking-load
during prolonged brake application.
Drum: Requires larger and increasing braking-load.
●5 Brake-Factor is the self-energizing factor.
Disk: Change in brake-factor for unit change in friction-coefficient is much less.
This consistency of braking is due to absence of self-servo action.
Drum: Larger changes in brake-factor with change in friction-coefficient.
●6 Disk: Easy to replace friction-pads.
Drum: More difficult as brake-linings are riveted or fixed with adhesives to brake-shoes.
●7 Disk: Area of pads is less (1/4th), higher pressure-intensity, needs frequent relining
due to higher wear-rate for which (a) thicker-pads (b) wear-resistant materials are used.
Simpler design. Fewer number of parts to wear or not function properly. Weighs 20% less.

9. Brake-Lining: Material
●1 Solid-Woven Type: Asbestos-base, and 2 Types of Linings
●2 Molded Type: Molded from mix containing asbestos-fibers, resin-powders, and fillers.
2. Metallic-Linings, with Zinc-Wire-Inclusion :
Better wear-resistance
Better anti-fade characteristics
Working temperature is reduced as zinc conducts away some of heat.
1.Non-Metallic-Linings:
Coefficient of friction is 0.4 up to a 250oC
Good anti-fade characteristics up to 300oC
Resists up to 350oC
Coefficient of friction is 0.4
Resists up to 400oC – 450oC
Good anti-fade characteristics
Good anti-wear properties
Ex: Ferodo 2629F (asbestos-based), Rane 9011 DC (asbestos-free).
●3 Fiber-Glass or Semi-Metallic Material: that withstands heat-producing braking-action.
Disadv: Asbestos, being danger to human-health, is being phased-out.

10. Brake-Lining: Methods of Fastening
●1 Riveting: Used for commercial-vehicles.
●2 Using Synthetic Resin Adhesives: Used for cars and light-vehicles.
Adv: (1) greater efficiency (2) better heat dissipation (3) free from scoring-action
(4) increased life of lining (5) 20% less cost compared to riveting-method.
Procedure:
(1) Preparing the Surface: Binding-surface of brake-shoe & liner-surface are roughened
by shot-blasting & sanding & degreased in a solvent(tri-choloro-ethylene)-vapor-bath.
(2) Adhesive Application: Adhesive (containing solvents) is applied on liner & shoe, and
solvents are dried-off in a ventilated oven at 80oC.
(3) Curing: After solvent is evaporated, parts are kept dry for few hours, clamped
together, heated under pressure, for adhesive to cure. This is in oven 145oC-200oC.
Time for curing at 150oC is 30 min, at 180oC is only 4 min.
(4) Cooling: After curing, the assembly is removed from oven, cooled off, pressure is
released. Extra adhesive if any is scrapped off, bond-strength is tested.
Brake-Shoe
►Description: Pressed steel constructions of curved shape, which accurately fits on the
inside curved geometry of brake-drum.
►Material: Pressed steel, made of T-type cross-section which provides both bending &
torsional rigidity. Aluminum, though has good thermal conductivity, is not used
because of its low-strength and springiness.

11. Brake-Fluid Composition
●1 Ethyl-Alcohol + Castor-Oil:
Disadv: (1) low boiling point (2) poor low-temperature properties.
●2 Poly-Glycols (Lubricant) + Glycol-Ethers (Dilutent): Mostly used brake-fluid.
●3 Silicone-based:
Adv: (1) wider operating temperature-range (2) doesn’t absorb moisture & attack paint.
Disadv: (1) more costly (2) wears the hydraulic-system more.
●4 50% Butyl-Alcohol + 50% Castor-Oil: Can be used in emergencies.
●5 65% Rectified-Ethyl-Alcohol + 35% Rectified-Glycerine: Can be used in emergencies.
●6 Never use any Petroleum-liquid as brake-fluid.
Air-Brakes
Adv: ●1 More powerful than mechanical- or hydraulic-brakes (so used in heavy-vehicles)
Required braking-effort can be developed by very small driver’s effort.
●2 Components of the system can be located anywhere on the chassis, and connected
by pipe-lines. Hence it simplifies chassis-design.
●3 Apart from braking, compressed air from reservoir can be used for tire-inflation,
windscreen-wipers, horn, steering-gear-booster, door open-close, other accessories.
●4 Most convenient & dependable device for braking full-trailers & semi-trailers.
Disadv: ●1 Involves more parts ●2 Air-compressor uses certain amount of engine-power.

12. Brake-Fluid Requirements
●1 High Boiling Point:
(1) To avoid vapor formation, as considerable heat is generated during braking.
(2) Boiling –point should not decrease with continued operation of the fluid.
(3) Generally it should be between 250oC – 300oC.
●2 Minimum Change in Viscosity with Temperature:
(1) Viscosity that is optimum for flow-conditions should be maintained.
(2) Contamination of brake-fluid with moisture increases its viscosity significantly at low
temperatures.
●3 Good Lubrication Properties: Should lubricate master- & wheel-cylinders.
●4 Should Not Swell or Shrink Rubber:
--Shrinking rubber-seals cause:
(1) Loss of pressure in brake-lines
(2) Leakage of brake-fluid
(3) Entry of air into system, which makes brakes dangerous.
--Swelling rubber-seals cause:
(1) Excessive pedal-pressure is required for brake-application
(2) Higher wear-rate in master-cylinder
(3) Jamming of brakes, if swelling is excessive.
●5 Should Not Corrode Metal: Like pistons of master- & wheel-cylinders.
●6 Long Storage-Life and Stability: Should not spoil for at least 3 years of storage.