Disc Brakes Analysis

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Disc BrakesAnalysis

• Mark Jetten
• Brian Richards
• Steve Weaver

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Overview

• Parts of disc brake and system
• Where disc brakes are used
• Materials and performance
• Analysis
• Uniform Pressure
• Uniform Wear

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Parts of Braking System

• Brake 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

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Mockup

• A) Master cylinder
• B) Pedal
• C) Vents

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Mockup continued

• D) Brake pads
• E) Caliper
• F) Rotor

E
F
D
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Uses of Disc Brakes

• Disc 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

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Materials

• Different materials have different coefficients
  of friction
• Pad material can be chosen for performance or to
  create a balance between performance and
  durability

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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.

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Caliper Operation
Caliper
Brake Fluid
1
2
3
Pads
Rotor

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

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Master Cylinder

• Force is applied to brake pedal by driver
• Primary 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.

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Animation
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Analysis

• Brakes analyzed similarly to clutches
• Uniform pressure
• Valid for new brakes
• Actuating force is product of pressure and area
• Uniform wear
• Valid after initial wear occurs

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Geometry of Contact Area
F Force on pads ?1, ?2, r1, r0 Dimensions of
brake pad
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Force and Torque Equations

• p pressure
• ri,o inner, outer radius
• ?1,2 initial, final contact angle
• f friction coefficient

FForce on pads TTorque generated by pads
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Uniform Pressure

• When brakes are new, p pa allowable pressure.
  
• The equations thus look like this

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Uniform Wear

• With 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

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Questions?
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References

• Mechanical Engineering Design by Joseph Shigley
• http//www.howstuffworks.com