Unit 6: Braking systems and Clutches

• Types and construction,

• Mechanical Brakes
• Hydraulic system,
• Diagonal braking system,
• Antilock braking system.
• Numerical Examples.
 Braking systems: Intro
• It absorbs energy from the moving part and slows down the
vehicle with the help of friction.
• The function of the brake system is to stop the vehicle within the
smallest possible distance and hence this is done by converting the
kinetic energy of the vehicle into the heat energy which is dissipated
into the atmosphere.
• Friction Braking: Kinetic Energy is converted to thermal energy and
dissipated to atmosphere
 Braking systems: Intro
Perspective:
• Consider vehicle of mass 1000 kg
0 Km/hr to 70 km/hr in 20 seconds (20 m/sec)
Average Acceleration = (20-0)/ 20 =1 m/sec2
Accelaration power =
Functions
• Brake should be able to decelerate the vehicle and stop it when
necessary
• Should provide sufficient brake force
• Maintaining the speed of vehicle while traveling down hill
• Should be able to dissipate the heat energy effectively
• Hold the vehicle in stationary on a grade
 Evolution of brakes
• Wheels were braked by wedging a shoe against their rim
• External contracting brake [Band Brake]
• The band may rub against the drum due to thermal expansion
• Internal Expanding brake (Drum Brake)
 Evolution of brakes
• Internal Expanding Brake (Drum Brake)
 Functional Grouping of Brakes
• Service Brakes: used for normal brakes
• Secondary/Emergency Brakes: Used during
partial brake failures
• Parking Brake:
Note:
• Stopping distance: It is the sum of distances
covered in the
Driver Reaction time (tdr)
Brake response time (tbr)
Braking time (tb)
Stability of vehicles during Braking
• A good brake must ensure a short stopping while maintain the vehicle
in stable condition
• This requires proper distribution of brake forces at the front and rear
wheels
Reliability
• The transmission ( Mechanical links, hydraulic system, compressed
air) is typically weakest link in brake system
• Hence a redundancy is introduced in transmission, that has led to dual
circuit brake system
 Components of Brakes
1)Source of Energy
Human Effort
Brake assist System Eg: Vacuum booster
Power Brake: Driver input is used to modulate the source of
energy
Ex: Air brake
2)Mechanism for applying the brake
Hand Lever or Foot Pedal
 Components of Brakes
3) Energy transmitting Medium:
Mechanical Brake: Use levers, rod and cables etc to transmit energy
Hydraulic Brake: uses an incompressible fluid
Air Brake: Uses compressor
Combination of these : Air over hydraulic fluid

Split System
Single Circuit System:
Use only one circuit for transmitting the energy required for braking
No braking would be available if this circuit fails
Dual Circuit system:
Uses two circuits to transmit energy
Partial braking is available in the event of failure of one of two circuits
 Components of Brakes
Foundation brakes Drum Brakes
Brakes at the wheels

Disc Brakes
 Drum Brakes
Brake Shoe

Thermal expansion of the drum resulting in increased clearance

Brake Fade: Reduction in brake friction lining coefficient with
increase in temperature
 Disk Brakes

Caliper Assembly
 Advantages of Disk Brakes
• Disc brakes provide better stability since these have uniform pressure
distribution over the pads than that of the brake linings in the case of
drum brakes.
• Increased temperature does not affect the disc pads much compared to
the brake linings of the drum brakes.
• Maintenance and repairs of disc brakes is easy
 Disadvantages of Disk Brakes
• Disc brake assemblies are costlier than drum brakes.
• The pads wear off fast compared to brake shoe linings of drum brakes.
Disc brakes have higher brake pressures.
• Complete protection to the disc from road residue is provided with
great difficulty.
• The high temperature operation of disc brakes causes evaporation of
the brake fluid and weakening of seal.
• Handbrakes can be installed on drum brakes because these have self
energizing effect. Disc brakes offer difficulty in installing hand brakes.
Why do Most Vehicles have Disc Brakes Only at the Front

• When brakes applied leads to the shift of weight in the vehicle. The load that was equally
distributed between the front and rear axles, shifts on the front axle

• When this shift in weight happens, the front end of the car nose down as the suspension
compresses. This ultimately results in increased grip levels at the front of the vehicle and
decreased grip levels at the rear.

• Hence manufacturers naturally choose to install more powerful brakes up front than at the rear.

• Disc brakes are more powerful and much more effective than drum brakes. This is mainly due
to the fact that disc brakes primarily rely on hydraulic pressure for their working, while drum
brakes primarily rely on mechanical strength.
Hydraulic Brakes
 Hydraulic Brakes
• Uses incompressible fluid as the energy transmitting medium
• Vacuum Booster: Augments the drivers brake input force
• Master cylinder is where mechanical force is converted to a fluid
pressure
• Master cylinder has two circuits (Dual circuits), primary and
secondary circuits
• Primary circuit actuates left front disc brake and right rear drum brake
and Secondary circuit actuates right front disc brake and left rear drum
brake. This split is called as Diagonal split or X split
Vacuum Booster
 Master Cylinder

Converts the force output from vacuum booster to fluid pressure

 Wheel Cylinder

Converts the fluid pressure to an actuation force on the brake shoes of drum brake
 Combination Valve
• It is a combination of various valves
• Found in most of the cars with front disc brake and rear drum brakes
• Proportioning Valve: Ensures that the fluid pressure provided to the
front and rear brakes proportioned/distributed appropriately .
More brake force output from the front brake
• Pressure Differential Switch: Used to alert the driver if there is a loss of
pressure in either side
• Metering Valve: Drum brakes take slightly longer to be applied than
the disk brakes. This valve delays providing brake pressure to front disc
brakes thereby providing sufficient time to apply the rear drum brakes
 Brake Fluid
Desirable properties

• High boiling pointing

• Low Compressibility
• Should not corrode metal parts
• Lubricating Property
• Low Toxicity
 Anti Lock Braking System (ABS)
• To prevent lockup of wheels during brake
• A wheel is said to be locked, when wheels stops rotating, but the
vehicle is still in motion.(Skidding)
• Wheel Slip Ratio (Longitudinal Wheel Slip)
λ= (V-rω)/V
 Anti Lock Braking System (ABS)
• An anti-lock braking system (ABS) is an active safety feature designed to
prevent a vehicle’s wheels from locking up after an instance of heavy
braking, thereby reducing the risk of skidding and allowing the driver to
retain steering control.

• On wet or slippery roads, ABS can also reduce the distance required to come
to a stop.

• When drivers slam on the brakes in a vehicle without ABS, the wheels are
prone to stop spinning all together (“locking up”).

• When the wheels lock up, the driver of the vehicle no longer has any steering
control and the vehicle may begin to skid.
 Objectives of ABS
• To reduce stopping distance
• Stability
• A locked-up wheel generates a reduced braking force, smaller than the peak
value of the available adhesion between tires and road. A locked-up wheel will
also lose its capability to sustain any lateral force. This may result in the loss of
vehicle stability.
• The basic purpose of a conventional ABS system is thus to prevent any wheel
from locking and to keep the longitudinal slip in an operational range by cycling
the braking pressure.
• Steerability
• If an obstacle appears without warning, emergency braking may not be
sufficient. When the wheels are locked, car no longer respond to the driver’s
steering intention.
• With ABS car remains steerable even during emergency braking, and thus the
obstacle can be safely avoided.
 Components of ABS
Electronic control unit (ECU)
• It receives signals from the sensors in the circuit and controls the brake
pressure at the road wheels according to the data analysed by the Unit.
• ECU assists the vehicle operator to prevent wheel lockup by
regulating the wheel slip.
Hydraulic control unit or modulator
• It receives operating signals from the ECU to apply or release the
brakes under ABS conditions.
• It executes the commands using three solenoid valves connected in
series with the master cylinder and the brake circuits- one valve for
each front wheel hydraulic circuit, and one for both of the rear wheels.
Thus brakes can be actuated by controlling hydraulic pressure.
 Components of ABS
Power booster and master cylinder assembly
• It is activated when the driver pushes down on the brake pedal. The
master cylinder transforms the applied pedal force into hydraulic pressure
which is transmitted simultaneously to all four wheels.
• It provides the power assistance required during braking.
Wheel sensor unit
• Speed sensors are comprised of a magnet wrapped in a coil and a toothed
sensor ring. An electrical field given off by the contact between the
magnet and the toothed ring creates a AC voltage.
• The voltage frequency is directly proportional to the wheel's rotational
speed.
• It monitors the rotational speed of the wheel and transmits this data to the
ABS control module.
 Working of ABS
• All systems are comprised of three major components:
• Wheel speed sensors (sensors that monitor the speed of wheel
rotation),
• Hydraulic units (the mechanical devices that actually release the
brakes, as needed)
• Electronic control unit (ECU) (the electronic controller that interprets
information from the wheel speed sensors and gives commands to the
hydraulic units).
• In modern ABS setups, the hydraulic units and ECU are attached
together so that while they have different functions, they are
physically one unit.
 Working of ABS
• The ECU is designed to check for rapid wheel speed decelerations:
indicators that a wheel is about to lock up.
• If your ABS computer detects, for example, that the front left wheel is
experiencing a rapid deceleration, it will instruct the hydraulic unit
associated with the front left wheel to reduce the brake.
• The hydraulic unit will continue to reduce the braking force until the
computer senses that the problem wheel’s rate of rotation is accelerating.
• Once the wheel has begun rotating normally, the brakes are once again
applied, and once again released if the wheel shows a risk of locking up.
• The process of releasing and applying the brake can happen several times
a second
 Working of ABS
• ABS setups tend to differ among types of vehicles.
• On cars and sport utility vehicles (SUVs) the most common setup is a four-wheeled
system, where a speed sensor is placed on each wheel.
• When the risk of a lock-up is detected, four-wheeled systems will either release the
brake force on the rear wheels, the affected front wheel, or the affected individual
wheel, depending on the type of system.
• Pick-up trucks and cargo vans, in contrast, tend to have rear wheel only ABS
setups.
• Rear-wheel systems have speed sensors installed only on the back wheels, and will
only initiate anti-lock measures if one of those wheels is experiencing a rapid
deceleration.
• Therefore, pick-up trucks and cargo vans are typically still vulnerable to front
wheel lockage.