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5. Friction
2142111 S i 2011/22142111 Statics, 2011/2
© Department of Mechanical Engineering Chulalongkorn UniversityEngineering, Chulalongkorn University
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Objectives Students must be able toObjectives Students must be able to
Course ObjectiveInclude friction into equilibrium analyses Include friction into equilibrium analyses
Chapter Objectivesp jFor general dry friction Describe the mechanism and characteristics of dry friction,
coefficient of friction and angle of frictioncoefficient of friction and angle of friction Specify and determine the limiting conditions for sliding, toppling,
free rolling and driven wheels under dry friction by appropriate FBDs
Specify the limiting conditions for rear/front wheel drives by appropriate FBDs
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pp p Analyze bodies/structures with friction for unknown
loads/reactions by appropriate FBDs
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Objectives Students must be able toObjectives Students must be able to
Course ObjectiveInclude friction into equilibrium analyses Include friction into equilibrium analyses
Chapter Objectivesp jFor friction in machines Prove, state limitation and apply formula for impending motion in
wedges single continuous threads/screws flat beltswedges, single continuous threads/screws, flat belts, disks/clutches, pivot/collar/thrust bearings, journal bearings
Analyze machines with friction
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ContentsContents Dry friction
Characteristics theory coefficient and angle of friction Characteristics, theory, coefficient and angle of friction Applications
Wedgesg Threads, screws Belts
Di k d Cl t h Disks and Clutches Collar, pivot, journal and thrust bearings
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FrictionDry Friction
Friction Force of resistance acting on a body which prevents or
retards slipping of the body relative to a surface withretards slipping of the body relative to a surface with which it is in contact. The frictional force acts tangent to the contacting
f i di ti d t th l ti tisurface in a direction opposed to the relative motion or tendency for motion of one surface against another
Two types of friction Fluid friction exists when the contacting surfaces are
separated by a film of fluidseparated by a film of fluid. Dry friction occurs between contacting surfaces
without lubricating fluid.
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Dry Friction Theory #1
Dry Friction
Dry Friction Theory #1
Friction forces are tangential forces generated between contacting surfacescontacting surfaces.
Friction forces arise in part from the interactions of the roughnesses or asperities of the contacting surfaces.
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EquilibriumDry Friction
Equilibrium Slipping and/or tipping effects
Frictional force F increases with force P Frictional force F increases with force P.
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Static Friction Equilibrium
Impending MotionDry Friction
Static Friction Equilibrium
Maximum value of frictional force Fs on the object in equilibrium is called the limiting static frictional forceequilibrium is called the limiting static frictional force.
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Static Friction Coefficient
Dry Friction Impending Motion
Static Friction Coefficient
F Nμ=s sF Nμ=
maximum static frictional forcecoefficient of static friction
s
s
Fμ
==
normal forceN =
is the friction forces that can be exerted by dry contacting surfaces
maximumsF
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that can be exerted by dry, contacting surfaces that are relative tstation o each ot rary he .
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Static Friction Angle of Static Friction
Dry Friction Impending Motion
g
N R φcossin
sin
s s
s s s s
N RF R N
F R
φφ μ
φ
== =
sin tancos
s ss s
s
F RN R
φμ φφ
= = =
φ μ−= 1tans sφ μs s
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Static Friction Typical Values
Dry Friction Impending Motion
Static Friction Typical Values
Contact Materials μs
Metal / ice 0.03 – 0.05Wood / wood 0.30 – 0.70Leather / wood 0.20 – 0.50Leather / metal 0.30 – 0.60Al i / Al i 1 10 1 70Aluminum / Aluminum 1.10 – 1.70
sHow to obtain values of ?μ
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sμ
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Frictional ForcesDry Friction
Frictional Forces More force is required to start sliding than to keep it
sliding can be explained by the necessity to break thesesliding can be explained by the necessity to break these bonds.
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Kinetic Friction Coefficient & Angle
MotionDry Friction
Kinetic Friction Coefficient & Angle
μ=F Nμ=k kF N
kinetic frictional forcekF =coefficient of kinetic frictionnormal force
k
k
Nμ =
=
φ μ−= 1tanφ μ= tank k
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Kinetic Friction Angle of Friction
Dry Friction Motion
Kinetic Friction Angle of Friction
At , is impending.At f
sliplidi
sφ φφ φ
=φ φ≥s k
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At , surfa slidinces are relative to eac
gh other.
kφ φ=φ φs k
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Dry Friction Characteristics
Dry Friction
Dry Friction Characteristics
Frictional force acts tangentially to the contacting surfaces opposing the relative or tendency for motionsurfaces, opposing the relative or tendency for motion.
Fs is independent of the area of contact, provided that the normal pressure is not very low nor great enough for deformation of the surfaces.
In equilibrium:Impending slipping: φ = φs
Slipping: φ = φk
Very low velocity: φk ≈ φs
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MotionDry Friction
SlidingRelative sliding (translation motion) between two Relative sliding (translation motion) between two surfaces
Toppling Fall over (rotation) about the edge
Topple tipping rolling tumble trip Topple, tipping, rolling, tumble, trip
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Example Friction 1 #1
Dry Friction
Example Friction 1 #1
Place a sheet of one materials on a flat board. Bond a sheet of other material to block A Place the block onto thesheet of other material to block A. Place the block onto the board, and slowly increase the tilt a of the board until the block slips. Show that the coefficient of static friction between the two materials is related to the angle a by μ =between the two materials is related to the angle a by μs = tanα
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Example Friction 1 #2
Dry Friction
Example Friction 1 #2
α = − = The block is about to slip
0 cos 0yF N WW
αα
=
= − + =
cos (1)
0 sin 0
y
x s
N W
F F Wα
A
μ αμ α
=
=
sin (2)
(2) /(1) tan Ans
x s
s
s
N WN Fs
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Example Friction 2 #1
Dry Friction
Example Friction 2 #1
Will this crate slide or topple over?topple over?
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Example Friction 2 #2
Dry Friction
Example Friction 2 #2
The crate is about to slide.
s sF F Nμ= =
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Example Friction 2 #3
Dry Friction
Example Friction 2 #3
The crate is about to topple.
( )s sF F Nμ<
The crate is about to slide and toppletopple.
s sF F Nμ= =
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Example Hibbeler Ex 8-1 #1
Dry Friction
Example Hibbeler Ex 8 1 #1
The uniform crate has a mass of 20 kg. If a force P = 80 N is applied to the crate determine if it remains in equilibrium Theapplied to the crate, determine if it remains in equilibrium. The coefficient of static friction = 0.3.
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Example Hibbeler Ex 8-1 #2
Dry Friction
Example Hibbeler Ex 8 1 #2
= → + ° − = → =
0
(80 N)cos30 0 69.3 NxF
F F = ↑ + − ° − + = → = 0
(80 N)sin30 196.2 N 0 236.2 Ny
C C
F
N N
= + ° − ° + = →
0
(80 N)sin30 (0.4 m) (80 N)cos30 (0.2 m) ( ) 0 O
C
M
N x
−= − × = −39.0753 10 m 9.08 mm
0 4 Th t ill
x
t t l
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<< →0.4 m The crate willxμ< →
not topple. (70.8 N) The crate, is close to but doesn't not slip.s CF N
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Example Hibbeler Ex 8-3 #1
Dry Friction
Example Hibbeler Ex 8 3 #1
The rod with weight W is about to slip on rough surfaces at A and Bslip on rough surfaces at A and B. Find coefficient of static friction.
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Example Hibbeler Ex 8-3 #2
Dry Friction
Example Hibbeler Ex 8 3 #2
Impending slipF N
3 equilibrium equations
A s A
B s B
F NF N
μμ
==
3 equilibrium equations,3 unknowns ( , , )A B sN N μ
0 cos30 sin30 0 (1)
0 cos30 sin30 0 (2)x s A s B B
y A B s B
F N N N
F N W N N
μ μ
μ
= → + + ° − ° = = ↑ + − + ° + ° =
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0 cos30 ( / 2) 0 (3)A BM N l W l = + − ° =
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Example Hibbeler Ex 8-3 #3
Dry Friction
p
=From (3) 0 4330N W
μ μμ
=
− +=
2
From (3) 0.4330
From (1) & (2) 0.2165 0.21650.228 Ans
B
s s
s
N W
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μ 0.228 Anss
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Example Hibbeler Ex 8-4 #1
Dry Friction
Example Hibbeler Ex 8 4 #1
Find minimum coefficients of static friction that keep the system infriction that keep the system in equilibrium.
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Example Hibbeler Ex 8-4 #2
Dry Friction
Example Hibbeler Ex 8 4 #2
Symmetry of lower pipes about Symmetry of lower pipes about vertical axis passing through O.
Two types of contacts: Pipe – pipe Pipe – ground
-
, ,A B A B
A s p p A
N N F FF Nμ
= =≤
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, -
, -
A s p p A
C s p g CF N
μμ≤
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Example Bedford Ex 9.5 #1
Dry Friction
Example Bedford Ex 9.5 #1
Suppose that α = 10° and the coefficient of friction betweencoefficient of friction between the surface of the wedge and the log are μs = 0.22 and μk = 0 20 Neglect the weight of the0.20. Neglect the weight of the wedge. If the wedge is driven into
th l t t t t bthe log at a constant rate by vertical force F, what are the magnitude of the normal f t d th l bforces exerted on the log by the wedge?
Will the wedge remain in
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place in the log when the force is removed?
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Example Bedford Ex 9.5 #2Driven at constant rateDry Friction
Example Bedford Ex 9.5 #2
= ↑ + Symmetry about the center of the wedge
0 yF
α μ α
α μ α
+ − =
=+
2 sin( / 2) 2 cos( / 2) 0
2sin( / 2) 2 ( / 2)
kN N FFN
α μ α+
=° + °
2sin( / 2) 2 ( / 2)
2sin(10 / 2) 2(0.20)(10 / 2)
k
FN
=
( ) ( )( )
1.75 AnsN F
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Example Bedford Ex 9.5 #3On the verge of slippingDry Friction
Example Bedford Ex 9.5 #3
↑ + Symmetry about the center of the wedge
0F
α μ αμ α
′
= ↑ + + =
= =
0
2 sin( / 2) 2 cos( / 2) 0tan( / 2) 0 087
y
s
F
N Nμ α
μ μ
′
′
= =
<
tan( / 2) 0.087
As , wedge will remain in place. Ans
s
s sμ μ g ps s
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Example Bedford 9.20 #1
Dry Friction
pThe coefficient of static friction between the two boxes andbetween the two boxes and between the lower box and the inclined surface is μs. What is th l t l f hi h ththe largest angle α for which the lower box will not slip.
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Example Bedford 9.20 #2
Dry Friction
Example Bedford 9.20 #2
α = − = 2
FBD of upper block
0 cos 0yF N W
α=
2 cosFBD of lower block
0 0
N W
F N W Nα
α
= + − = =
2 1
1
0 cos 0
2 cos
0 i 0
yF N W N
N W
F N N Wμ μ α
μ α α−
= + − = =
2 1
1
0 sin 0
3 cos sin
tan / 3 or tan (3 ) Ans
x s s
s
F N N W
W W
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μ α α μ= = 1tan / 3 or tan (3 ) Anss s
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Example Bedford 9.30 #1
Dry Friction
Example Bedford 9.30 #1
The cylinder has weight W. The coefficient of static frictioncoefficient of static friction between the cylinder and the floor and between the cylinder
d th ll i Wh t i thand the wall is μs. What is the largest couple M that can be applied to the stationary
li d ith t i it tcylinder without causing it to rotate.
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Example Bedford 9.30 #2
Dry Friction
Example Bedford 9.30 #2
μ ↑ + + Impending rotation
0 0F N N Wμ
μ
= ↑ + + − = = → + − =
0 0
0 0y s w f
x w s f
F N N W
F N N
μ = + − + = 0 ( ) 0O s w sM M N N r
μsW WN Nμ
μ μμμ
= =+ +
+=
2 2, 1 1
(1 ) Ans
sw f
s s
s
WN N
M Wr
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μμ
=+ 2 Ans
1ss
M Wr
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Example Bedford 9.33 #1
Dry Friction
Example Bedford 9.33 #1
The disk of weight W and radius R is held in equilibrium on theR is held in equilibrium on the circular surface by a couple M. The coefficient of static friction b t th di k d thbetween the disk and the surface is μs. Show that the largest value M can have
ith t i th di k t liwithout causing the disk to slip is
sRWM
μ21
s
s
Mμ
μ=
+
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Example Bedford 9.33 #2
Dry Friction
p
Impending slip s sF Nμ=p g p
0 sin 0
0 cos 0
s s
x sF F W
F N W
μ
α
α
= + − = = + − =
0 cos 0
0 0y
O s
F N W
M M F R
α + = + − + =
2
tan , cos
cos 1 tan 1, tan =s N Wμ α α
α α α μ
= =
+ = scos 1 tan 1, tan
cos s sM NR WR
α α α μ
μ μ α
+
= =
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21s sWM Rμ μ= +
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Example Bedford 9.166 #1
Dry Friction
pEach of the uniform 1-m bars has a mass of 4 kg. The coefficient of static friction between the bar and the surface at Bcoefficient of static friction between the bar and the surface at Bis 0.2. If the system is in equilibrium, what is the magnitude of the friction force exerted on the bar at B.
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Example Bedford 9.166 #2
Dry Friction
p
= 0xF + =
=
0
0 x x
y
O A
F + − =
= 4 0
0
y
y y
O
O A g
M
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° − ° − ° =
(1 m)cos 45 ) (1 m)sin45 ) (4 N)(0.5 m)cos 45 ) 0y xA A g
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Example Bedford 9.166 #3
Dry Friction
p
= − + ° − ° =
0
cos30 sin30 0x
x
F
A F N
= − − + ° + ° =
0
(4 N) sin30 cos30 0y
y
F
A g F N
= ° + ° + ° =
0
(1 m)cos 45 ) (1 m)sin45 ) (4 N)(0.5 m)cos 45 ) 0B
y x
M
A A g
40= = − = = = =0, 2 N, 4 N, 2 N, 438 N, 2.63 N Ansy x y xA A g O g O g N F
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Example Bedford 9.166 #4
Dry Friction
p
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Example Bedford 9.163 #1
Dry Friction
Example Bedford 9.163 #1
The coefficient of static friction between the tires of the 1000-kg tractor and the ground and between the 450-kg crate and thetractor and the ground and between the 450-kg crate and the ground are 0.8 and 0.3, respectively. Starting from the rest, what torque must the tractor’s engine exert on the rear wheels to cause the crate to move? The front wheels can turn freely.the crate to move? The front wheels can turn freely.
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Example Bedford 9.163 #2
Dry Friction
Example Bedford 9.163 #2
μ= ,
The crate is about to move.
s cF N
= − + =
0
(450 N) 0yF
g N =
− = 0
0xF
P F
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= =0.3(450 N) 135 NP g g
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Example Bedford 9.163 #3
Dry Friction
Example Bedford 9.163 #3
2
0
2 0xF
P F
= − + =
2 67.5 NF g=
= 0
(0 8 ) 0OM
F T
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− == = ⋅
2(0.8 m) 0(0.8 m)(135 N) 1.06 kN m Ans
F TT g
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Example Bedford 9.165 #1
Dry Friction
Example Bedford 9.165 #1The mass of the vehicle is 900 kg, it has rear-wheel drive, and the coefficient of static friction between its tires and the surface is 0.65. The coefficient of static friction between the crate and the surface is 0.4. If the vehicle attempts to pull the crate up the incline, what is the largest value of the mass of the crate for which it will slip up the i li b f th hi l ’ ti li ?incline before the vehicle’s tires slip?
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Example Bedford 9.165 #2
Dry Friction
Example Bedford 9.165 #2
μ=2 , 2
Impending motion:
s tF N
= − ° + =
2
0
cos20 2 0xF
P F
= + − − ° =
2 1
0
2 2 (900 N) sin20 0yF
N N g P
= + ° + ° − =
2
0
(900 N)(1 m) cos20 (0.8 m) sin20 (3.7 m) 2 (2.5 m) 0OM
g P P N
46= = = =2 2 1563.57 N, 2 814.75 N, 2 529.59 N, 2 2P g N g F g N 78.00 Ng
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Example Bedford 9.165 #3
Dry Friction
Example Bedford 9.165 #3
μ=
= ,The crate is about to move.
0 s c
y
F N
F
− ° + =
= cos20 0
0x
W N
F
− − ° =sin20 0
1 3930 N 7701 1 N
P F W
W P= == =
1.3930 N 7701.1 N/ 785 kg Ans
W Pm W g
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Example Bedford 9.165 #4
Dry Friction
Example Bedford 9.165 #4
What is the maximum tension in the cord that the tractor may tow a crate over a ramp with 45° angle of incline before the tractora crate over a ramp with 45° angle of incline before the tractor topples.
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Example Bedford 9.165 #5
Dry Friction
Example Bedford 9.165 #5
=1Impending topple: 0N
=
− =
1p g pp
0
(2.0 m) / 2 (900 N)(1.5 m) 0
QM
P g =
− − =
(2.0 m) / 2 (900 N)(1.5 m) 0
0
2 (900 N) / 2 0
y
P g
F
N g P =
=
+ =
22 (900 N) / 2 0
0
/ 2 2 0
x
N g P
F
P F
μ≤
=2 , 2, tires do not slip.
955 N Anss tF N
P g
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− + == = =
2
2 2
/ 2 2 0954.59 N, 2 1575 N, 2 675 NP F
P g N g F g
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Friction in MachinesMachine Friction
Friction in Machines Friction helps some machines function.
Machines with frictions Wedgesg Threads, screws Belts
Di k d l t h Disks and clutches Bearings
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WedgesMachine Friction
Wedges Wedges exert a large lateral forces from the faces as a
result of small angleresult of small angle.
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Wedges Analysis #1
Machine Friction
Wedges Analysis #1
The block is about to raise. The slip of the load and wedge are impending.
2 2 3 3,
0
s sF N F N
F
μ μ= =
→ + 3 2 2
0
sin cos 0
0
x
s
F
N N N
F
θ μ θ
= → + − + + =
↑ +
2 2 3
0
cos sin 0y
s s
F
N N N Wθ μ θ μ
= ↑ + − − − =
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Wedges Analysis #2
Machine Friction
g y
FBD of the wedge
0
i 0xF
N N N P
= → + 2 2 1sin cos 0
0
i 0
s s
y
N N N P
F
N N N
θ μ θ μ− − − + =
= ↑ + 1 2 2cos sin 0sN N Nθ μ θ− + =
( )( )
2
2
1 tan 2s sP Wμ θ μ − +
=
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( )21 2 tans sμ μ θ
− −
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Threads/ScrewsMachine Friction
Threads/Screws Assumption: the shaft has a single continuous thread.
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Threads Geometry
Machine Friction
Threads Geometry
1tan2
lr
θπ
−=lead anglepitch of the thread (lead)
di f th th dlθ =
=
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mean radius of the threadr =
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Threads Motion
Machine Friction
Threads Motion
W W
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Threads Moving Upwards
Machine Friction
Threads Moving Upwards
θ φ
= → + 0
i ( ) 0xF
S R θ φ
θ φ
− + =
= ↑ + +
sin( ) 0
0
cos( ) 0y
S R
F
R Wθ φ
θ φ
+ − =
= + =
cos( ) 0
tan( ) and
R W
S W M Srθ φ+tan( ) and S W M Sr
θ φ= +tan( )M Wr
1On the verge of rotating: tans sφ φ μ−= =
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1Uniform upwards motion: tank kφ φ μ−= =
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Threads Moving Downwards #1
Machine Friction
Threads Moving Downwards #1
θ φ
= → + ′ 0
i ( ) 0xF
S R θ φ
θ φ
′ − − =
= ↑ + sin( ) 0
0
cos( ) 0y
S R
F
R Wθ φ
θ φ
− − =
′ ′ ′= − =
cos( ) 0
tan( ) and
R W
S W M S rθ φtan( ) and S W M S r
θ φ′ = −tan( )M Wr
1On the verge of rotating: tans sφ φ μ−= =
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1Uniform downwards motion: tank kφ φ μ−= =
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Threads Moving Downwards #2
Machine Friction
Threads Moving Downwards #2
is removed:Mis supported by friction alone,
providing that W
φ θ≥
Self-locking condition
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Threads Moving Downwards #3
Machine Friction
Threads Moving Downwards #3
φ θ
= → + ′′ 0
i ( ) 0xF
S R φ θ
φ θ
′′− + − =
= ↑ + sin( ) 0
0
cos( ) 0y
S R
F
R Wφ θ
φ θ
− − =
′′ ′′ ′′= − =
cos( ) 0
tan( ) and
R W
S W M S r
Very rough surface :Reversed motion.
θ φ<
φ θtan( ) and S W M S r
φ θ′′ = −tan( )M Wr
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Example Wedges/Threads 1 #1
Machine Friction
Example Wedges/Threads 1 #1
The mass of block A is 60 kg. Neglect the weight of the 5° wedge. The coefficient of kinetic friction between the contacting surfacesThe coefficient of kinetic friction between the contacting surfaces of the block A, the wedge, the table, and the wall is 0.4. The pitch of the threaded shaft is 5 mm, the mean radius is 15 mm, the coefficient of kinetic friction between the thread and the matingcoefficient of kinetic friction between the thread and the mating groove is 0.2. What couple must be exerted on the threaded shaft to raise the block A at a constant rate?
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Example Wedges/Threads 1 #2
Machine Friction
Example Wedges/Threads 1 #2
D FBD f th d th dADraw FBD of the mass and the wedgeThen, follow the same procedure for wedge analysis:
A
( )( )
21 1
21 1
wedge analysis:
1 tan 2
1 2 tank k
k k
F Wμ α μ
μ μ α
− + =
− − ( )1 11 2 tan
Substitute for all known values:667.50 N
k k
F
μ μ α
=
60 N0 N
L
w
W gW
==
1
50.4 for all
non thread contactsk
αμ
= °=
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non-thread contacts
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Example Wedges/Threads 1 #3
Machine Friction
Example Wedges/Threads 1 #3
5 mml
2
5 mm15 mm0.2 for thread/groovek
lrμ
===2
2
garctan( ) 11.310arctan(/ 2 ) 3.0368
k
k k
r
μφ μθ π
= = °= = °
Draw FBD of the thread
θ φ= +° + °
2
Find that will push the thread upwards:tan( )
(667 50 N)(0 015 m) tan(3 0368 11 310 )k
MM FrM
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= ° + °= ⋅ = ⋅
(667.50 N)(0.015 m) tan(3.0368 11.310 )2.5609 N m 2.56 N m Ans
MM
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Flat BeltsMachine Friction
Flat Belts Involve slippage of flexible cables,
belts and ropes over sheaves andbelts and ropes over sheaves and drums.
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Flat Belts Analysis #1
Machine Friction
Flat Belts Analysis #1
Consider line elementConsider line element
0 tF
d dθ θ
= cos ( )cos 0
2 2
cos
d ddN T T dT
ddN dT
θ θμ
θμ
+ − + =
= cos2
0 n
dN dT
F
d d
μ
θ θ
= sin ( )sin 0
2 2
2 sin sin
d ddN T T dT
d ddN T dT
θ θ
θ θ
− − + =
+
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2 sin sin2 2
dN T dT= +
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Flat Belts Analysis #2
Machine Friction
Flat Belts Analysis #2(motion impending)(ongoing movement)
sμ μμ μ
==
0, cos 1:2
dd θθ → →
(ongoing movement)kμ μ=
cos2
ddN dT dT
d d
θμ
θ θ
= ≈
0, sin :2 2
2 sin
d dd
ddN T Td
θ θθ
θ θ
→ →
= ≈2 sin2
T
dN T Td
TdT dT β
θ= ≈2 1T T eμβ=
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2
1
20
1
lnT
T
TdT dTd dT T T
βμ θ μ θ μβ= → = → =
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Example Flat Belts 1 #1
Machine Friction
Example Flat Belts 1 #1
F i i i h
=
Friction resists the lowering of :mT mg
β π
== ==
2
1 / 61.25(2 )
T mgT P mg
A force P = mg/6 is required to lower the cylinder at a constant slow speed with the cord
μβ
β
= 2 1
( )
T T eslow speed with the cord making 1.25 turns around the fixed shaft. Calculate the coefficient of friction bet een
πμ
μ
= 2.5
60 228 Ans
smgmg e
67
coefficient of friction μs between the cord and the shaft.
μ = 0.228 Anss
68
Example Flat Belts 2 #1
Machine Friction
Example Flat Belts 2 #1
Calculate the force P on the h dl f th diff ti l b dhandle of the differential band brake that will prevent the flywheel from turning on its shaft to which the torque M = 150 N·m is applied. The coefficient of friction betweencoefficient of friction between the band and the flywheel is μ= 0.40.
68
69
Example Flat Belts 2 #2
Machine Friction
Example Flat Belts 2 #2
Friction resists the attempt of to rotate M
μβ μ π = = (7 / 6)
2 1 2 1
pthe flywheel in the clockwise direction:
(1)T T e T T e = − ⋅ + − =
− = 2 1
2 1
0 (150 N m) (0.15 m)( ) 0
1000 N (2)centerM T T
T T
69
= =2 1
1 2Solve (1) and (2) 300 N, 1T T 300 N
70
Example Flat Belts 2 #3
Machine Friction
Example Flat Belts 2 #3
= + 0 OM
− + ° + ==
2 1(0.15 m) (0.075 m)( sin30 ) (0.45 m) 0408 N Ans
T T PP
70
71
Disks & ClutchesMachine Friction
Disks & Clutches Use to connect and disconnect two
coaxial rotating shaftcoaxial rotating shaft The shaft can support a couple
due to friction forces between the di kdisks.
Determine the couple by using coefficient of static friction in the flat-ended thrust equation.
71Draw forces on the plane only
72
Disks & Clutches Analysis #1
Machine Friction
Disks & Clutches Analysis #1
2dA r drπ=
μ= 2M PR
2
0 0
2
x
R
F P w dA
P p r dr p Rπ π
= − = μ
3M PR
2
0 0o
x
P p r dr p R
M M wr dA
π π
μ
= =
= − =
72
322
3R
oM pr r dr p Rμ π μ π= =
73
Disks & Clutches Worn Analysis #1
Machine Friction
Disks & Clutches Worn Analysis #1
Distribution of forces on worn plates are not uniformed. For example, linear distribution
2dA r drπ
At
2At , 0
0,
dA r drr R wr w p
π== == =
73
,( ) /
pw p R r R= −
74
Disks & Clutches Worn Analysis #2
Machine Friction
Disks & Clutches Worn Analysis #2
Consider the left plate
2
0 0
( )2 /
x
R
F P w dA
pP p R r r dr R Rππ
= − =
= − =
( )2 /3
0 0
o
x
P p R r r dr R R
M M wr dA
π
μ
= − =
= − =
( )2 /R
oM pr R r r dr Rμ π= −
μ= 12
M PR
74
2
75
Bearings Pivot Bearings
Machine Friction
Bearings Pivot Bearings
μ= 23
M PR3
(motion impending)μ μ= (motion impending)(ongoing movement)
s
k
μ μμ μ
==
75
76
Bearings Collar Bearings
Machine Friction
Bearings Collar Bearings
μ −=
3 3
2 2
23
i oR RM P
R R−3 i oR R
(motion impending)μ μ= (motion impending)(ongoing movement)
s
k
μ μμ μ
==
76
77
Bearings Thrust Bearings #1
Machine Friction
Bearings Thrust Bearings #1
RRi Ro
2 2 dRdA R d R dR
( )2 2
2 2cos
2oR o i
dRdA R ds R
R RRA dA dR
π πθππ
= =
−
PRoRi R
θ
( )cos cosiR
A dA dRθ θ
= = =
77
78
Bearings Thrust Bearings #2
Machine Friction
Bearings Thrust Bearings #2
= 0F
PRoRi
dR
Rθ xθ
= − = 0
cos 0xF
P pAP P
( )θ π= =
−2 2cos o i
P PpA R R
μ
=
− =
0
( ) 0
xM
M R pdA
( )
μ
πμθπ
=−
2 2
( )
2cos
o
i
R
Ro i
p
P RM R dRR R
μθ −
= −
3 3
2 2
23cos
o i
o i
R RPMR R
78
( )
79
Bearings Journal Bearings #1
Machine Friction
Bearings Journal Bearings #1
P P
φ
79
80
Bearings Journal Bearings #2
Machine Friction
Bearings Journal Bearings #2
0
( sin ) 0zM
M P R φ
= + − =
R RRf
sintan
PMM R
PRφφ
=≈
R R
φ μ= ≈sinM PR PR
80
81
Example Hibbeler 8-11 #1
Machine Friction
Example Hibbeler 8 11 #1
The 100-mm-diameter pulley fit l l 10fits loosely on a 10-mm-diameter shaft for which the coefficient of static friction is μss= 0.4. Determine the minimum tension T in the belt needed to (a) raise the 100-kg block and(a) raise the 100-kg block and (b) lower the block. Assume that no slipping occurs between the b lt d ll d l t thbelt and pulley and neglect the weight of the pulley.
81
82
Example Hibbeler 8-11 #2CW rotation
Machine Friction
p
Raise the blockφ −= = =
= +
1
Raise the blocksin (5 mm)sin(tan 0.4) 1.8570 mm
0f sr r
M = + + − − =
=
2
0
(981 N)(50 mm 1.8570 mm (50 mm 1.86 mm) 01056 8 N
PM
TT
82
==
1056.8 N1.06 kN Ans
TT
83
Hibbeler Example 8-11 #3CCW rotation
Machine Friction
Hibbeler Example 8 11 #3
φ −= = =
1
Lower the blocksin (5 mm)sin(tan 0.4) 1.8570 mmf sr r
= + − − + =
2
0
(981 N)(50 mm 1.86 mm) (50 mm 1.86 mm) 0P
M
T
83
==
910.63 N911 kN Ans
TT
84
Example Boresi 10.9 #1
Machine Friction
Example Boresi 10.9 #1
84
85
Example Boresi 10.9 #2
Machine Friction
Example Boresi 10.9 #2
A weight W is lifted at a constant rate by applying a couple Rd to theby applying a couple Rd to the compound jackscrew. Derive a formula for R in terms of
the angle θ, the weight W, the length d of the lever rod, the pitch pof the screw threads the meanof the screw threads, the mean radius r of the screw threads, and the coefficient of kinetic friction μkof the screw. The friction of the vertical wall guides is negligible.
For θ = 15° W = 1000 lb d = 20 in
85
For θ 15 , W 1000 lb, d 20 in, r = 0.75 in, p = 0.333 in, and μk = 0.15, calculate R.
86
Example Boresi 10.9 #3
Machine Friction
Example Boresi 10.9 #3
0 2 cos 0yF T Wθ = − =
86
2 cos (1)W T θ=
87
Example Boresi 10.9 #4
Machine Friction
Example Boresi 10.9 #4
0 cos os 0yF Q Tc
Q T
θ θ = − = =
0 2 sin 0
2 sin (2)xF T P
P T
θ
θ
= − = =
(1) (2) tanP W θ+ =
By symmetry, FBD of the left nut is mirror image of the right nut FBD.
87
88
Example Boresi 10.9 #5
Machine Friction
p
[ ]Pr
φ αφ α
= +
= +
tan( )2 tan( )
kM WrRd Pr φ α
θ μπ
− −
= +
= +1 1
2 tan( )
2( tan ) tan(tan tan ) /2
k
k
RdpR W r d
r
Substitute numbers in the equation4 48 lb AR
88
= 4.48 lb AnsR
89
Example Hibbeler 8-120 (modify) #1
Machine Friction
Example Hibbeler 8 120 (modify) #1
The axle of the pulley fits l l i 50 di tloosely in a 50-mm-diameter pinhole. If μk = 0.30 between the pinhole and the pulley axle and 0.20 between the pulley and the cord, determine the minimum tension T required tominimum tension T required to turn the pulley counterclockwise at a constant velocity if the bl k i h 60 N N l t thblock weighs 60 N. Neglect the weight of the pulley.
89
90
Example Hibbeler 8-120 (modify) #2Case I Journal Bearing
Machine Friction
Example Hibbeler 8 120 (modify) #2
The shaft is about to rotate1 1
,
2
The shaft is about to rotate.tan tan 0.3 16.699
sin 7.1837 mmk b
fr rφ μ
φ
− −= = = °
= =2-1
1 1
11
tan ( / ) 45
sin ( / 2 ) 2.9117
f
f
r r
r r
φθ
β −
= = °
= = °1( )
0 60 sin( ) 0
f
yF R
β
θ β = − + + = 0 cos( ) 0
80.850 N, 54.192 NxF T R
R T
θ β = − + =
= =
90
91
Example Hibbeler 8-120 (modify) #3Case I Journal Bearing
Machine Friction
Example Hibbeler 8 120 (modify) #3
If the cord does not slip over the pulley,
μ β
μ
= ,
, 1
2 1
the minimum is:k f
k f
T T eμ π
μ
==
, 1( / 2)
, 1
60 N (54.192 N)0.064815
k f
k f
e
μ <, 1 0.2, the assumption the thek b
=
cord does not slip is valid.
54 2 N AnsT
91
= 54.2 N AnsT
92
Example Hibbeler 8-120 (modify) #4Case II Flexible Belt
Machine Friction
Example Hibbeler 8 120 (modify) #4
Th b l i b lidμ β =
,2 1
The belt is about to slide.k fT T e
π==
0.2( / 2)60 N43.824 N
TeT
92
93
Example Hibbeler 8-120 (modify) #5Case II Flexible Belt
Machine Friction
Example Hibbeler 8 120 (modify) #5
If the shaft does not rotate,
θ β
θ β
= − + + = = − + =
0 (60 N) sin( ) 0
0 cos( ) 0y
x
F R
F T R β
θ β
= = ° = ° ( )
43.824 N, 45 , 8.8556x
T
βφ φ
−= → == → = °
11
2
sin ( / 2 ) 21.771 mmsin 60.557
f f
f
r r rr r
φ μ μμ
−= → =
>
1, 1 , 1
, 1
tan 1.770.3, the assumpt
k b k b
k b ion that the
93
shaft does not rotate is invalid.
94
Example Boresi 10-86 #1Example Boresi 10 86 #1
A torque T is applied to pulley A, which drives pulley B. The t ll h l dii Th t i i th l k id ftwo pulleys have equal radii. The tension in the slack side of the belt is 2 kN. The coefficient of friction between the belt and the pulleys is 0.30. What is the maximum possible torque that can be applied
to pulley A?? What torque is transmitted to pulley B?
94
95
Example Boresi 10-86 #2
Machine Friction
p=2Given 2 kN
The belt is about to slip:T
μβ
π
= = =
1 2
0.3
The belt is about to slip:
(2 kN) 5 1327 kN
T T e
T e= ==
1
1
(2 kN) 5.1327 kN5.13 kN Ans
T eT
= − + − =
1 2
0
( ) 0OM
T T T r= − = ⋅1 2( )(0.1 m) 0.31327 kN m
As tensions in the belts remain T T T
95
unchanged between the two pu= = ⋅
lley, 313 N m AnsBT T
96
Example Boresi 10-98 #1
Machine Friction
pIn some applications of collar bearings,
lti l ll d T llmultiple collars are used. Two collars are used as shown to support the 10,000 lb load (μs = 0.40 and μk = 0.20)s k
Determine the torque T required to initiate rotation of shaft S, Assume that the load is divided equallythat the load is divided equally between the collars and that the pressure on each collar is uniform.Wh lti l ll d? Why are multiple collars used?
96
97
Example Boresi 10-98 #2
Machine Friction
Example Boresi 10 98 #2
The load is divided equally between the collarsThe load is divided equally between the collars,thus, the load on each collar is 5 kip.
μ −
= 3 32 12 2
For each collar that is about to rotate:
2s
R RT Pμ − −=
2 22 1
3 3
3
3
2 (1 ft) (0.5 ft)0.4(5000 lb)3 (1 ft) (0 5
s R R
T = ⋅3 1555.6 lb ftft)−3 (1 ft) (0.5
= = ⋅
ft)
Total required torque 2 3.11 kip ft AnsT M
97
q q pLess load on each collar, less wear and tear. Ans
98
Concepts #1
Review
Concepts #1
Friction is the tangent resistance force which prevents or retards slipping of the body relative to a surface withor retards slipping of the body relative to a surface with which it is in contact. Maximum friction occurs that the verge of impending motion which can be slipping or toppling or both togethertoppling or both together.
Some machine functions by friction. To use the formula, the physical meanings underlying the friction in machines must be understood.
98