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Clutches & Brakes

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  • MAE 4353 Mechanical Design IIDr James A. Kidd

    Module 10 Part 1: 10/31/14

  • Clutches, Brakes, Couplings & Flywheels

    Static Analysis of Clutches & Brakes

    Internal Expanding Rim Clutches & Brakes

    External Contracting Rim Clutches & Brakes

    Frictional Contact Axial Clutches

    Disc Brakes

    Energy Considerations

    Temperature Rise

    Friction Materials

    Miscellaneous Clutches & Couplings

    Flywheels

    MAE 4353 Fall 2014 2

  • Introduction

    Elements for managing rotation (transfer & storage)

    MAE 4353 Fall 2014 3

  • Model of Clutch

    MAE 4353 Fall 2014

    Fig. 161

    4

    Friction clutch (or brake) model Slippage between plates causes temperature rise Behaviors include:

    Actuating forces Transmitted torque Energy loss Temperature rise

  • Static Analysis of Clutch/Brake

    General process:

    Estimate, model or measure pressure distribution on friction surfaces

    Identify relationship between largest pressure and distribution at any point

    Use static equilibrium to find braking force or torque and support reactions

    MAE 4353 Fall 2014 5

  • 6MAE 4353 Fall 2014

    Brake/Clutch (Doorstop)

    Fig. 162

  • Doorstop cont.

    MAE 4353 Fall 2014

    Fig. 162

    7

    Leftward movement of

    floor

    Rightward movement of

    floor

  • Internal Expanding Centrifugal-acting Rim Clutches & Brakes

    Fig. 163

    8

    Examples: Expanding Ring

    Textile machinery, excavators, machine tools,

    Clutch may be inside driving pulley

    Centrifugal Typically for automatic

    operations such as electric motor drives

    Magnetic Remote or automatic systems

    and for complex load cycles

    Hydraulic & Pneumatic For complex load cycles and

    remote operations Internal shoe rim clutchSimilar approach in drum brakes

    MAE 4353 Fall 2014

  • Internal Friction Shoe Geometry

    MAE 4353 Fall 2014Fig. 164

    9

    Cant assume uniform normal force distribution due to long shoe length

    Mechanical arrangement will not allow pressure at heel

    Typically omit friction material near heel (no pressure during engagement and reduces interference when disengaged)

  • Internal Friction Shoe Geometry

    Fig. 16510

    Designate max pressure as pa at angle aDetermine pressure distribution via infinitesimal rotation about pivot A on point B

    Evaluate pressure on friction material element at angle from hinge pin

    Deformation & pressure proportional to sinIn terms of pressure at B and at maximum point:

    MAE 4353 Fall 2014

  • Pressure Distribution Characteristics Characteristics:

    Sinusoidal

    For short shoe (a), largest pressure on shoe is pa at shoe end

    For long shoe (b), largest pressure is pa at a = 90

    Note: Material selection is based on maximum allowable friction and maximum imposed pressure pa, so off-shoe pressures are irrelevant

    MAE 4353 Fall 2014

    Fig. 16611

  • MAE 4353 Fall 2014

    Force Analysis

    Fig. 167

    12

    Efficient design concentrates frictional material near maximum pressure point (as shown):

    At any angle from pin there is a differential normal force:

    b is frictional material face width (into the plane)

    Substituting maximum pressure and associated angle gives:

  • Force Analysis, cont.

    MAE 4353 Fall 2014

    Self-locking condition:

    13

    Use normal force with force components to write moment of frictional forces about hinge pin:

    Moment of normal forces about hinge pin:

    Actuating force must balance moments:

    If MN = Mf system is self-locked (no actuating force required)Can determine dimensions for self-energizing action

    Is this correct?

  • Force Analysis, cont.

    MAE 4353 Fall 2014 14

    Torque applied to drum by shoe:

    Hinge-pin reactions (sum of horizontal and vertical forces):

  • Force Analysis, cont.

    MAE 4353 Fall 2014 15

    If rotation is reversed for given geometry (Fig 16-7), self-energizing effect is lost and required actuating force is:

    Pin reactions become:

  • Force Analysis, cont.

    MAE 4353 Fall 2014 16

    simplifying terms:

    Clockwise rotation (Fig 16-7 geometry):

    Counter-clockwise rotation:

  • Assumptions Pressure at any point on shoe proportional to distance

    from pin (zero at heel)

    Centrifugal force effects neglected

    Good assumption for brakes

    Clutch analysis needs to account for centrifugal forces

    Shoes are rigid

    No variation of friction coefficient with pressure

    MAE 4353 Fall 2014 17

  • Discussion Problem

    MAE 4353 Fall 2014 18

    Which shoe limits the maximum actuation force F?

    (F same for both shoes)

    F F

    BA

    F F

    BA

    What if the direction of rotation is reversed?

  • Assignments Mid-term survey Open till 1:00 PM Wednesday

    Quiz for bonus points open till Friday 10:00PM

    By class time Friday (10/31) Read Shigley Chapter 16 (Clutches & Brakes)

    By class time Monday (11/3) Upload Flexible Elements Problem Set (#8) (note revised date)

    Complete quiz and download problem set

    Test Review, Clutches & Brakes cont.

    Tuesday (11/4) TA Study Session 3:30 to 4:40 in EN 208

    Wednesday (11/5) Test #2 Springs, Bearings, Flexible Elements

    19MAE 4353 Fall 2014

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