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Friction
Relative velocity
Cause of dry frictionContact between two surfaces.
Hence first task in a friction problem is correct identification of contact surfaces
Identify the surface, the normal and the tangential vectors.
Also important is to get an idea of probable direction of relative motion
The contact force acts along the normal.Gravity is the most common cause of normal force.
Friction acts along the tangent plane opposite to the direction of relative motion
Normal
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Friction problems are essentially equilibrium problems with one f the
forces being functions of another
Friction
Relative velocity
Normal
N
Fr=f(N,V)N
Fr=f(N,V)
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The correct way of writing the dry friction force
r
V ˆF N N VV
N=Normal force vector
V=Relative velocity vector of the bodym= coefficient of dry friction or Coulomb friction
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Problem 1
Knowing that the coefficient of friction between the 13.5 kg block and the incline is ms = 0.25, determine
a) the smallest value of P required to maintain the block in equilibrium,
b) the corresponding value of b.
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Problem 1
x
y
N cos60 mg f sin60 P sin 0N sin60 f cos60 P cos 0
f Nmg sin60 fP
sin sin60 cos cos60N cos60 mg N sin60 P sin 0
mg P sinNcos60 sin60
N sin60 N cos60 P cos 0mg P sin sin60 cos60 P cos 0
cos60 sin60mg P
sin sin60 cos60 P cos cos60 sin60 0
mg sin60 cos60 P sin sin60 cos60 P cos cos60 sin60 0
P sin60 sin cos60 sin cos60 cos sin60cos mg sin60 cos60
sin60 cos60P mg
cos 60 sin 60
cos 60 sin 61 mgP
o
0sin60 cos60
d 1 d0 cos 60 sin 60 0d P d
d sin60 sin cos60 sin cos60 cos sin60 cos 0dsin60cos cos60 cos cos60 sin sin60 sin 0
sin60 cos60tan 2.614 69cos60 sin60
sinP mg
60 cos60 0.866 0.125mg 0.72mg
cos 60 sin 60 cos 9 sin 9
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Problem 2
Knowing that P = 110 N, determine the range of values value of qfor which equilibrium of the 8 kg block is maintained.
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x
y
k
P cos N 0P sin mg f 0f N
N P cosP sin mg N 0
P sin mg P cos 0P sin cos mg
mgPsin cos
Hence downward movement will not start before
mgPsin cos
x
y
Problem 2
mg mg
s
P cos N 0P sin mg f 0f N
N P cosP sin mg N 0
P sin mg P cos 0P sin cos mg
mgPsin cos
Hence upward movement will not start before
mgPsin cos
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Problem 3
The coefficients of friction are ms = 0.40 and mk = 0.30 between all the surfaces of contact. Determine the force P for which motion of the 27 kg block is impending if cable
a) is attached as shown,
b) is removed
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Problem 3
1
1 1
1 1 1
1
1 2
2 1 2
2 1 2 2 1 2
1 2
1 1 1 2
1 2
T N 0N m g 0
T N ,N m gT m g
T N N P 0N N m g 0N N m g 0 N m m gT N N P 0
m g m g m m g P 0P 3 m g m g
TT
mm11ggNN11
ff11
vv
TT
NN11ff11
mm22ggNN22
ff22
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Problem 3
1 1
1 1
1 1
1 1
1 2
2 1 2
2 1 2 2 1 2
1 2
1 1 2
1 2
T N m aNow T 0 N m aN m g 0
N m gN N P 0
N N m g 0N N m g 0 N m m g
N N P 0m g m m g P 0
P 2 m g m g
ff11
00
mm11ggNN11
ff11
vv
00
NN11ff11
mm22ggNN22
ff22
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Additional Problems
The 8 kg block A and the 16 kg block B are at rest on an incline as shown. Knowing that the coefficient of static friction is 0.25 between all surfaces of contact, determine the value of q for which motion is impending.
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Toppling
The magnitude of the force P is slowly increased. Does the homogeneous box of mass m slip or tip first? State the value of P which would cause each occurrence.
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Slip or topple?
mgmg
NN11
ff11
NN22
ff22
1 2
1 2
2
1 2 1 2
1 2
2
1 2
P cos 30 N N 0P sin 30 N N mg 0
P cos 30 d mg d N 2d 0P cos 30 N N N N
P sin 30 N N mgP cos 30 mg 2 N
P sin30 N N mg P cos 30 mg
P sin30 P cos 30 mg P sin 30 cos 30 mgmgP
sin30 cos 300.5mgP
0.448mg0.25 0.866
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Slip or topple?
mgmg
NN11
ff11
NN22
ff22
2
2
2
2
P cos 30 f 0P sin 30 N mg 0
P cos 30 d P sin 30 2d mg d 0
P cos 30 fP sin 30 N mgP cos 30 2P sin30 mg 0
P cos 30 2 sin 30 mgmgP
cos 30 2 sin 30mgP 0.536mg
0.866 1
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Additional Problems
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Additional Problems
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Additional Problems
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Wedge
mg
f=mN
N
P
q
P f cos N sin 0f sin N cos mg 0f N
mgN sin N cos mg 0 Nsin cos
P N cos N sin 0cos sinP cos sin N mgsin cos
P tanmg tan 1P tan tan tan
mg tan tan 1
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A screw thread is a wedge
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A screw thread is a wedge
M=Pa
W
Q
Nf=mN
Q=Pa/r
= equivalent force
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A screw thread is a wedge
W
Q
Nf=mN
Q f cos N sin 0f sin N cos W 0
f NWN sin N cos W 0 N
sin cosQ N cos N sin 0
cos sinQ cos sin N Wsin cos
Q tanW 1 tan
q
Pa tanWr 1 tan
1 Pif tanW
Screw locks
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A screw thread is a wedge
W
P
Nf=mNq
1 PatanWr
1 Wr 0tan Pa
Screw locks
The
There is a critical value of friction coefficient beyond which the thread
does not move irrespective of the force applied.
This happens when a screw is not maintained properly. Because of dirt
and rust m becomes more than critical.
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A screw thread is a wedge
W
Nf=mN
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A screw thread is a wedge
W
Nf
For no movementf cos N sin 0f sin N cos W 0
f sinN cos
sin tancos
q
Self locking
Therefore after raising the load if we let go of the screw the load will not cause the screw to unscrew by itself.
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Terminologies
Lead L npwheren=no. of parallely running threads = starts
Ltan =2 r
Pitch (p)
2pr
Lea
d (L
)
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Turnbuckle
T1 T2
2 1
M tanT T r 1 tan
Used to apply tension. Used to apply tension.
The sleeve is rotated to pull the The sleeve is rotated to pull the threads together. threads together.
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An improved screw jack
q
q
W
W
T T
T
2T cos q
T
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An improved screw jack
f
f
W
W=2T cos f M=Pa
Pa cos sinWr sin cos
M cos sin2Tr cos sin cos
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Worm gear
R
MG
GG
G
G
MM WR W
RPa tanWr 1 tan
M tanM 1 tanrR
MR tanM r 1 tan
M0/R
NNM f
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Belt drives
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x
y
s
s
F 0 T cos F T T cos 02 2
F 0 T sin N T T sin 02 2
F N
T cos F T T cos 02 2
T cos F T cos T cos 02 2 2
F T cos 0 T cos F N2 2
T sin N T T sin 02 2
T sin N T sin2 2
T sin 02
2T sin N T sin 02 2
2T sin N 0 2T sin N2 2
Belt drives
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s
s
s
s0 , T 0 0 , T 0
s
T cos N2
2T sin N2
T cos 2T sin2 2
sin1 T 2cos2T 2
sin1 T 2Lim cos Lim2T 2
1 dT 12T d 2
dT dsT
Belt drives
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2
1
2
1
T
s 0T
2 1 s
2s
1
T
sT 0
dT dT
lnTlnT lnT
TlnT
dT dsT
T2 seT1
Belt drives
Belt is just about to Belt is just about to slide to the rightslide to the right
22 1
1
2 1 1
T s se T e TTTorque required to drive the pulley
sT T e 1 T
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T2 seT1
Belt drives : Important points
Angle bmust be expressed in radians
Larger tension occurs at the end of the belt where motion is about to begin
or is already moving T2 is used to denote the larger
tension
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Belt drives
q
q
q
q
A B
A B
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Belt drives
q
q
2
1
Texp 2 ???
T
T2
T1
T2
T1 1
2
Texp 2 ???
T
A
B
Which expression is correct???? Is T2>T1
Or T1>T2.One pulley must slip.
Friction force is larger for the larger pulley since
angle of wrap is larger. Hence smaller pulley
slips and determines the tension
Always check for mb value for each pulley in a system. The one
with the smallest mb value will
determine the tensions.
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Band brakes
T2
T1
Important!!!In pulleys the belt
is mobile and tries to drive the pulley. Here the drum moves and tries to move the belt. Hence the sense of slack is opposite to that
for a pulley.
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Band brakes
T2
T1
The drum rotates towards the right. If this were a pulley the slack side
would be on the left. But here as the drum is the moving part, the slack
side is on the left.
x x 2
x 2
y y 2 1
y 2 1
B 2 1
1 2
F 0 B T sin( ) 0B T sinF 0 B T cos( ) T P 0
B T cos T P 0
M 0 cT bT a b P 0bT cT
Pa b
a -p
y
x
Ry
Rx Mext
1 2 k
ext 1 2
ext 1 2
x x 2
y y 1 2
T T exp( )M 0 M r T T 0
M T T rF 0 R T sin( )F 0 R T T cos( )
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Band brakes
1 2 k ext 1 2
1 2x 2 y 2 1
T T exp( ), M T T rbT cTB T sin , B T cos T P 0, P
a b
ext 1 2 2 k 2 k 2
ext2
k
2 k 2 k1 22
k ext
k
k
k
M T T r T exp( ) T r exp( ) 1 T rM
Texp( ) 1 r
bT exp( ) cT bexp( ) cbT cTP T
a b a b a bbexp( ) c M
Pa b exp( ) 1 r
c1 exp( )bP
1 exp( )
ext
s kk s
Ma1 rb
cNow 1 exp( ) 0 P 0 if 1 exp( ) 0 k kbP 0 iff b > ce > ce
Otherwise P is negativeOtherwise P is negative. Which means no force is required for braking!
Hence self lockingself locking occurs.Since Mext is equal in magnitude to the torque required
for braking it is also called the braking torque.
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Clutches
2
2
R2
0
Assume pressure is uniform over the contact zonePp=R
Consider a small element of width dr at a radial distance r and spanning an angle d dA r dr ddF pdA prdrddM rdF r prdrd pr drd
M pr dr
2R323 3
0 0 0
32
r p 2d p 2 R pR3 3 3
2 P 2M R PR3 R 3
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Clutches
1
0
2 21 0
2
r22
0 r
Assume pressure is uniform over the contact zone
Pp=r r
Consider a small element of spanning angle d at a radial distance rdA r dr ddF pdA prdrddM rdF r prdrd pr drd
M pr drd p
1
0
2r33 3 3 3
1 0 1 0r 0
3 31 02 2
1 0
3 31 0
2 21 0
r p 2r r 2 p r r3 3 3
2 PM r r3 r r
r r2M P3 r r
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Disc Brake
2
2 2
2 20 i
Assume pressure is uniform over the contact zoneArea of a whole circle (spanning 2 radians) = r
r rArea of a sector = = 2 2
2Pp=R R
Consider a small element of spanning angle d at a radi
oo
i i
2
RR 32 3 3 3 3
o i o i0 R R 0
3 3 3 3o i o i 2 2
0 i
3 3o i
2 20 i
al distance rdA r dr ddF pdA prdrddM rdF r prdrd pr drd
r p pM pr drd p R R R R3 3 3
p 2PM R R R R3 3 R R
R R2M3 R R
dq
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Thrust bearing
PP
qq qq
dd11
qqdd22
dd11
dd11
dd22
qqqq
PP
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Thrust bearing
dN
y
xO
dl
Xdf
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Thrust bearing
2
1
2
1
2 22 1 2 2
2 1
R 2
R 0
R 2
R 0
Assume pressure is uniform over the contact zonePP= R R p p=
R R
M rdf
dN pdA prd dl , dl drcosecdN prd dl prd dr cosec pcosec d rdrdf dN pcosec d rdr
N pcosec d rdr
N p c
22 2
1 1 1
2 2
1 1
22 2
1 1 1
RR R 22
0R R R
2 22 1
R R2 2
R 0 R 0
RR R 32 2 20
R R R
32 1
rosec rdr 2 pcosec rdr 2 pcosec2
N pcosec R R
M rdf r rpcosec d rdr
rM pcosec r dr 2 pcosec r dr 2 pcosec3
2 pM R R3sin
3 32 13
2 22 1
R R2 P3sin R R
dN
y
xO
dl
X
r dr
df
What about q
= 0 ?
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Torque wrench
Smallest m for self locking ?Smallest m for self locking ?
T1
T1
T2
N
N
mN
mN
T2
1 2T T exp 2
1 2
2
T N cos T N sin 0
P N sin T N cos 0
1 2r T T aP
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Torque wrench
Condition for self locking ?Condition for self locking ?
1 2
1 2
1 2
2
2 1
1 1
1 1
1
T T exp 2
r T T aP
T N cos T N sin 0
P N sin T N cos 0
T T sin cos P cos sin
T T sin cos P cos sin exp 2
T T sin cos exp 2 P cos sin exp 2
T sin co
1
1 2 1
s exp 2 1 P cos sin exp 2
P cos sin exp 2 P cos sinT
sin cos exp 2 1 sin cos exp 2
aPr T T aP Tr 1 exp 2
P cos sinaPr 1 exp 2 sin cos e
xp 2
r cos sin a sin cosexp 2
r cos sin a
r cos sin aexp 2
r cos sin a sin cos
acos sin1 rln a2 cos sin sin cosr
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Band brakes
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Band brakes
T1 T3
T2
T4
2 1 1
2 1
3 4 4
4 3
0 3 4 2 1
x 1 3 1 3
y 1 3 B 1
75 75T T exp T exp 0.25180 180
T 1.39T135 135T T exp T exp 0.25180 180
T 0.55TM T T T T RConsider the pin B
F 0 T cos45 T cos 45 T T
F 0 T cos 45 T cos45 T 2T cos45
B 1 B
2 1 3 1 4 1
2 1 3 4 B
T 2T T
T 1.39T ,T T ,T 0.55TThus the largest tension is T 5.6 T T 4.03,T 2.22,T 5.7
M 5.6 2.22 R 3.38 0.16 0.54KNm 540 Nm
B
T1 T3
TB
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Problems
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Tip or Slide
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Problems
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Problems
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