Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected]

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[email protected] ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx 1 Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected] Engineering 36 Ch08: Wedge & Belt Friction

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Engineering 36. Ch08: Wedge & Belt Friction. Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected]. Outline - Friction. The Laws of Dry Friction Coefficient of Static Friction Coefficient of Kinetic (Dynamic) Friction Angles of Friction - PowerPoint PPT Presentation

Transcript of Bruce Mayer, PE Licensed Electrical & Mechanical Engineer [email protected]

Page 1: Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx1

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engineering 36

Ch08: Wedge &

Belt Friction

Page 2: Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx2

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Outline - Friction The Laws of Dry Friction

• Coefficient of Static Friction• Coefficient of Kinetic (Dynamic) Friction

Angles of Friction• Angle of static friction• Angle of kinetic friction• Angle of Repose

Wedge & Belt Friction• Self-Locking & Contact-Angle

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Basic Friction - Review The Static Friction Force Is The force that Resists

Lateral Motion. It reaches a Maximum Value Just Prior to movement. It is Directly Proportional to Normal Force:

NF sm After Motion Commences The Friction Force Drops

to Its “Kinetic” Value NF kk

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx4

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction Consider the

System Below

Find the Minimum Push, P, to move-in the Wedge

The Wedge is of negligible Weight

Then the FBD of the Two Blocks using Newton’s 3rd Law

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction For Equilibrium of the Heavy Block

Solve for FA,n

For Equilibrium of the Wt-Less Wedge

sincos0 ,, nAsnAy FFWF

sincos,s

nAWF

cossin0

sincos0

,,,

,,,

nAnAsnCy

nAnAsnCsx

FFFF

FFFPF

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction In the last 2-Eqns Sub Out FA,n

Eliminating FC,n from the 2-Eqns yields an Expression for Pmin:

cossincos

sinsincos

0

sinsincos

cossincos

0

,

,

sssnCy

sssnCsx

WWFF

WWFPF

cos2sin1sincos

2min ss

s

µWP

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx9

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction MATLAB Plots for P when W = 100 lbs

0 2 4 6 8 10 12 14 16 18 2040

45

50

55

60

65

70

75

80

85

(°)

P (l

bs)

W = 100 lbs, µ = 0.2

0 5 10 15 20 25 3010

20

30

40

50

60

70

80

90

µ (%)

P (l

bs)

W = 100 lbs, = 10°

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

MATLAB Code% Bruce Mayer, PE% ENGR36 * 22Jul12% ENGR36_Wedge_Friction_1207.m%u = 0.2W = 100a = linspace(0,20);P = W*((1-u*u)*sind(a) +2*u*cosd(a))./(cosd(a)-u*sind(a))plot(a,P, 'LineWidth',3), grid, xlabel('\alpha (°)'), ylabel('P (lbs)'), title('W = 100 lbs, µ = 0.2')disp('showing 1st plot - Hit Any Key to Continue')pause%a = 10;u = linspace(0,0.3);P = W*((1-u.*u)*sind(a) +2*u*cosd(a))./(cosd(a)-u*sind(a));plot(100*u,P, 'LineWidth',3), grid, xlabel('µ (%)'), ylabel('P (lbs)'), title('W = 100 lbs, \alpha = 10°')disp('showing 2nd plot - Hit Any Key to Continue')pause%u = linspace(0, .50);aSL =atand (2*u./(1-u.^2));plot(100*u,aSL, 'LineWidth',3), grid, xlabel('µ (%)'), ylabel('\alpha (°)'), title('Self-Locking Wedge Angle')disp('showing LAST plot')

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction Now What Happens

upon Removing P

The Wedge can• Be PUSHED OUT• STAY in Place

– SelfLocking condition

Then the FBD When P is Removed• Note that the

Direction of the Friction forces are REVERSED

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction For Equilibrium of the Heavy Block

Solve for FA,n

For Equilibrium of the Wt-Less Wedge

sincos0 ,, nAsnAy FFWF

cossin0

sincos0

,,,

,,,

nAnAsnCy

nAnAsnCsx

FFFF

FFFF

KWFs

nA

sincos,

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx15

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction To Save Writing sub K for FA,n

Eliminate FC,n

Now Divide Last Eqn by Kcosα

0sincos

0sincos

,

,

KKF

KKF

snC

snCs

01sincos20

0sincos0sincos

2,

,

ss

ssnC

snCs

µKKKKF

KKF

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx16

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction Dividing

by Kcosα

Recognize sinu/cosu = tanu

01cossin2

cos01sincos2

2

2

ss

ss

µ

KµKK

222

2

12

12

12tan

21tan

s

s

s

s

s

s

ss

µµµ

µ

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction After all That Algebra

Find The Maximum α to Maintain the Block in the Static Location

Since Large angles Produce a Large Push-Out Forces, and a ZERO Angle Produces NO Push-Out Force, the Criteria for Self-Locking

2max 12arctan

s

s

212arctan

s

sSL

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Push-Out

SMALL PushOut Force• Likely SelfLocking

LARGE PushOut Force• Likely NOT SelfLocking

212arctan

s

sSL

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Friction

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

µ (%)

)

Self-Locking Wedge Angle

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Belt Friction Consider The Belt Wrapped

Around a Drum with Contact angle .

The Drum is NOT Free-Wheeling, and So Friction Forces Result in DIFFERENT Values for T1 and T2

To Derive the Relationship Between T1 and T2 Examine a Differential Element of the Belt that Subtends an Angle • The Diagram At Right Shows

the Free Body Diagram

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx21

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Belt Friction cont Write the Equilibrium Eqns for

Belt Element PP’ if T2>T1

2sin

2sin0

2cos

2cos0

TTTNF

NTTTF

y

sx

Eliminate N from the Equations

2sin

2sin

2sin

2cos

2cos

2cos0

2sin

2sin

2cos

2cos0

2sin

2sin

TTTTTT

TTTTTTF

TTTNF

s

sx

y

Page 22: Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx22

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Belt Friction cont.1 Combining Terms

2

sin22

cos0

TTT s

Divide Both Sides by

2

2sin22

cos0

TTTs

Now Recall From Trig And Calculus

ddLimLim

00

1sin10cos

So in the Above Eqn Let: /2 →0; Which Yields

TdTTTddTdTT

ddT

ss 2 as20

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Belt Friction cont.2 The Belt Friction Differential Eqn

Integrate the Variables-Separated Eqn within Limits• T( = 0) = T1

• T( = ) = T2

From Calculus

Now Take EXP{of the above Eqn}

ddTT

TddT

ss 1Vars Sep

12120lnlnln12

1

TTTTddTT ss

T

T

ss eTTee TT 12ln 12

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx24

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Belt Friction Illustrated This is a VERY

POWERFUL Relationship

Condsider the Case at Right. Assume• A ship Pulls on the Taut

Side With A force of 4 kip (2 TONS!)

• The Wrap-Angle = Three Revolutions, or 6

• µs = 0.3

seTT

1

2

The Tension, T1, Applied by the Worker

lbekip

eTTs

14463.0

21

Page 25: Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx25

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

WhiteBoard Work

Let’s WorkThese NiceProblems

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Bruce Mayer, PERegistered Electrical & Mechanical Engineer

[email protected]

Engineering 36

Appendix 00

sinhTµs

Tµx

dxdy

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

WhiteBoard Work

Let’s WorkThis NiceProblem

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

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[email protected] • ENGR-36_Lec-22_Wedge-n-Belt_Friction.pptx31

Bruce Mayer, PE Engineering-36: Engineering Mechanics - Statics

Wedge Push-Out

SMALL PushOut Force• Likely SelfLocking

LARGE PushOut Force• Likely NOT SelfLocking