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

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Introduction

Elements for managing rotation (transfer & storage)

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Model of Clutch

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Fig. 161

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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

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Brake/Clutch (Doorstop)

Fig. 162

Doorstop cont.

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Fig. 162

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Leftward movement of

floor

Rightward movement of

floor

Internal Expanding Centrifugal-acting Rim Clutches & Brakes

Fig. 163

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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

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Internal Friction Shoe Geometry

MAE 4353 Fall 2014Fig. 164

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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:

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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

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Fig. 16611

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Force Analysis

Fig. 167

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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.

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Self-locking condition:

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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.

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Torque applied to drum by shoe:

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

Force Analysis, cont.

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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.

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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

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Discussion Problem

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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

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