Disc BrakesAnalysisMark JettenBrian RichardsSteve Weaver

OverviewParts of disc brake and system

Where disc brakes are used

Materials and performance

Analysis

Uniform PressureUniform Wear

Parts of Braking SystemBrake Pedalforce input to system from driver

Design gives a Mechanical Advantage

Master Cylinderconverts force to pressure

Pressure is used to move brake pads into place

Brake Padsprovide friction force when in contact with rotor

Works to slow or stop vehicle

Caliperholds pads and squeezes them against rotor

Rotorspins with wheel

When used in conjunction with brake pads, slows vehicle

Ventshelp provide cooling to brake

MockupA) Master cylinder

B) Pedal

C) Vents

Mockup continuedD) Brake pads

E) Caliper

F) RotorDEF

Uses of Disc BrakesDisc brakes are often on the front (and sometimes on the rear) wheels of cars

Do the real work of braking

Unlike drum brakes, do not self-energize

MaterialsDifferent materials have different coefficients of friction

Pad material can be chosen for performance or to create a balance between performance and durability

Materials Continued

Asbestos brakes were used for years because of their extremely high friction coefficient, but advances in science has shown that it is a cancer causing substance.

Performance pads often include small pieces of metal in the pads to aid in the pads stopping power. The metal needs to be softer than the rotor, to ensure that the rotor is not damaged over long periods of use.

The friction coefficients in the table are representative of dry conditions. If the rotor and pad are moist, the systems stopping power is greatly reduced until the moisture can be burned off.

Caliper OperationStep 1: Force is applied to by driver to the master cylinderStep 2: Pressure from the master cylinder causes one brake pad to contact rotorStep 3: The caliper then self-centers, causing second pad to contact rotor

123CaliperBrake FluidPadsRotor

Master CylinderForce is applied to brake pedal by driverPrimary piston moves, which in turn pressurizes fluid in front of the first piston. The secondary piston and primary piston are connected through a spring. As the primary piston moves, it causes the secondary piston to move and pressurize fluid in front of it.The pressurized fluid in the brake lines then causes the brake pads to move into contact with the rotor.The secondary and primary pistons are used to give the car essentially two braking systems. Each controls two wheels. If one of the systems were the fail, the other can still stop the car.

Animation

AnalysisBrakes analyzed similarly to clutches

Uniform pressure

Valid for new brakesActuating force is product of pressure and area

Uniform wear

Valid after initial wear occurs

Geometry of Contact AreaF = Force on pads1, 2, r1, r0 = Dimensions of brake pad

Force and Torque Equationsp = pressureri,o = inner, outer radius1,2 = initial, final contact anglef = friction coefficientF=Force on padsT=Torque generated by pads

Uniform PressureWhen brakes are new, p = pa = allowable pressure.

The equations thus look like this:

Uniform WearWith older brake pads, axial wear can be assumed constant. The pressure is expressed as the maximum allowable pressure pa, which occurs at ri.

Thus, p = pari/r

Questions?

ReferencesMechanical Engineering Design by Joseph Shigleyhttp://www.howstuffworks.com