TORSION of Circular shafts. T. Axis of shaft. T. A bar subjected to moment in a plane perpendicular to the longitudinal axis (i.e., in the plane of cross – section of the member) is said to be in ‘TORSION’. This moment is called ‘Twisting Moment’ or ‘Torque’.
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TORSION of
Circular shafts
T
Axis of
shaft
T
A bar subjected to moment in a plane perpendicular to the longitudinal axis (i.e., in the plane of cross – section of the member) is said to be in ‘TORSION’.
This moment is called ‘Twisting Moment’ or ‘Torque’.
Torque = T Unit : N-m, kN-m, etc.
r
r
If ‘τ’ is the shear stress developed in the element, then, the elementary resisting force is,
dF = τ × dA
∴ Elementary resisting torsional moment is,
dT = dF × r = τ × dA × r
∴ Total resisting torsional moment is,
The internal forces develop to counteract the torque.
T
dF1
Axis of
shaft
dF3
dF2
PURE TORSION :
A member is said to be in ‘Pure torsion’, when its cross sections are subjected to only torsional moments (or torque) and not accompanied by axial forces and bending moment.
Consider the section of shaft under pure torsion. The internal forces develop to counteract the torque.
At any element, the force dF developed is in the direction normal to radial direction.
This force is obviously shearing force and thus the elements are in pure shear.
If dA is the area of the element at a distance ‘r’ from the axis of the shaft, then
dF = τ × dA
and dT = dF × r
where τis shearing stress
ASSUMPTIONS IN THE THEORY OF PURE TORSION:
B’
B’
Φ
θ
θ
B
A
B
O
O
T
L
TORSION FORMULA :
When a circular shaft is subjected to torsion, shear stresses are set up in the material of the shaft. To determine the magnitude of shear stress at any point on the shaft, Consider a circular shaft of length ‘L’, fixed at one end and subjected to a torque ‘T’ at the other end as shown.
R
The line AB rotates by an angle Φ and the point B shifts to point Bl, when the free end rotates by an angle ‘θ’ due to the applied torque ‘T’.
R ×θ
L
∴Φ =
If ‘Φ’ is the shear strain (angle BABl) and ‘θ’ is the angle of twist in length ‘L’, then
tan(Φ) = BBl / AB = BBl / L
Since Φ is small, tan(Φ) =Φ= BBl/L
But BBl = Rθ
=> L × Φ = R × θ -----(1)
If ‘τ’ is the shear stress at the surface of the shaft and ‘G’ is the modulus of rigidity, then,
G = τ / Φ
=> Φ= τ / G
Substituting in (1), we have, (R × θ)= (L × τ) / G
τ
R
G × θ
L
=
---------(A)
=>
Now for a given shaft subjected to a given torque T, the values of G, θ and L are constant. Hence shear stress produced is proportional to the radius R
From equation A, it is clear that intensity of shear stress at any point in the cross – section of the shaft subjected to pure torsion is directly proportional to its distance from the centre.
Thus shear stress increases linearly from zero at axis to maximum value of τ at the surface.
R
τ= R × G×θ
L
G × θ
L
B
τr
r
τr ----------(2)
r
τ
R
=> =
=
R
r
If B is a point at a distance ‘r’ from centre instead of on the surface, then
Consider an elemental area ‘dA’ at a distance ‘r’ from the axis of the shaft (or centre).
τ × r
R
τr
=
τ
dA
τr
dF
∴ dT = τ × dA × r2
R
r
r
R
R
If ‘τr’ is the shear stress developed in the element, then, the elementary resisting force is, dF = τr × dA
∴ Elementary resisting torsional moment is,
dT = dF × r = τr × dA × r
But from eq. (2), we have,
τ× r2 × dA τ∫ r2 × dA
R R
∫
T =
=> T =
∴ T ττr
J R r
∴ T = τ × J
R
=
----------(B)
=
∴ Total resisting torsional moment is,
But ∫r2 × dA is nothing but polar moment of inertia of the section, we have J = ∫ r2 × dA .
T τ G × θ
J R L
=
=
From (A) and (B) , we get,
T = Torsional moment
(N-m)
(m4)
J = Polar moment of inertia
τ = Shear stress
(N/m2)
R = Radius of the shaft
(m)
G = Modulus of rigidity
(N/m2)
(radians)
θ = Angle of twist
L = Length of the shaft
(m)
T τ× J τ ×ZP
R
=
=
T τ
J R
=
POLAR MODULUS :
Where ZP = J/R = polar modulus.
Where τ is maximum shear stress (occurring at surface)
and R is extreme fibre distance from centre.
Thus polar modulus is the ratio of polar moment of inertia to extreme radial distance of the fibre from the centre.
Unit : m3
T G × J × θ
L
=
T G × θ
J L
=
TORSIONAL RIGIDITY (OR STIFFNESS) :
Torsional stiffness is the amount of toque required to produce unit twist.
When unit angle of twist is produced in unit length, we have,
T = G × J × (1/1) = GJ.
Thus the term ‘GJ’ may be looked as torque required to produce unit angle of twist in unit length and is called as, ‘torsional rigidity’ or ‘stiffness’ of shaft.
Unit : N-mm2
Polar modulus, ZP = J = πD3
R 16
y
D
IXX = IYY = πD4
64
x
J = IXX + IYY = πD4
32
x
y
POLAR MODULUS :
Solid Circular Section :
R=D/2
y
IXX = IYY = π(D14 –D24)
64
D2
J = IXX + IYY = π(D14 – D24)
32
x
x
Polar modulus, ZP = J = π(D14 –D24)
R 16D1
y
D1
Hollow Circular Section :
R= D1/2
Angle through which shaft moves = N × 2π
60
Power, P = T × N × 2π = 2π NT
60 60
POWER TRANSMITTED BY SHAFTS :
Consider a shaft subjected to torque ‘T’ and rotating at ‘N’ revolutions per minute (rpm). Taking second as the unit of time, we have,
Power, P = Work done per second.
Unit : N-m/s or Watt. 1H.P = 736Watt = 736 N-m/s
T τ G×θ
J R L
=
=
Solution :
T×L
G×θ
J =
J = π.d4
32
NUMERICAL PROBLEMS AND SOLUTIONS
1.What is the maximum diameter of a solid shaft which will not twist more than 3º in a length of 6m when subjected to a torque of 12 kN-m? What is the maximum shear stress induced in the shaft ? Take G = 82 GPa.
L = 6m = 6000mm; T = 12 kN-m = 12 × 106 N-mm
G = 82 × 103 N/mm2; θ = 3º = (3π/180) radians
12× 106 ×6000
82 × 103 ×3π/180
J =
= 16.7695 × 106 mm3
For solid circular shaft,
16.7695×106 = π.d4
32
=>τ T.R
J
T τ
J R
=
=
=>τ 12 × 103 × 0.5716
16.7695 × 106
=
=> d = 114.32mm
=> R = d/2=57.16mm
=>τ = 40.903 N/mm2
=>τ = 40.903 MPa.
Solution :
d
D/2
J = π.(D4 - d4)
32
D/2
2. A hollow shaft 3m long transmitted a torque of25kN-m. The total angle of twist in this length is 2.5º and the corresponding maximum shear stress is 90MPa. Determine the external and internal diameter of the shaft if G = 85 GPa.
L = 3000 mm; T = 25 kN-m = 25 × 106 N-mm
G = 85×103 N/mm2; θ=2.5º=(2.5π/180)rad
τ = 90 N/mm2
and R=D/2
T τ G×θ
J R L
τ
R
=
=
G × θ
L
=
τ×L
G×θ
Taking,
R =
=>
∴ R D (90 × 106 ) × (3)
2 (85 × 109) ×(2.5π/180)
=
=
J T×L
G×θ
=
Also,
T×L
G×θ
=>
=
π(D4 - d4)
32
=> D = 0.1456m
Taking D = 0.1456m, we have,
d = 0.125m
Solution :
Power, P = 2π NT
60
60P
2πN
T
=>
=
(60 × 150×106)
(2 × π × 200)
=> T =
3. A solid circular shaft has to transmit 150kW of power at 200 rpm. If the allowable shear stress is 75 MPa and permissible twist is 1º in a length of 3m, find the diameter of the shaft. Take G = 82 GPa.
L = 3000 mm; P = 150 kN-m/s = 150×106 N-mm/s
τmax = 75 N/mm2; θmax = 1º = π/180 radians;
G = 82 × 109 N/mm2; N = 200 rpm.
= 7161972.439 N-mm
T τ
J R
=
7161972.4 75
D/2
=>
=
T G × θ
J L
=
πD4
32
T×L
G×θ
7161972.4 × 3000
82 × 103 × (π/180)
=> J =
=> =
πD4
32
For maximum shear stress condition,
=> D = 78.64 mm
For maximum twist of the shaft,
=> D = 111.13mm.
Hence, the safe value of diameter which satisfies both the conditions is,
D = 111.13mm.
Note: Relation between ‘D’ and ‘θ’ (or τ) is inversly proportional.
Solution :
τMAX
T
τMAX
T
D1
D
2D1/3
4. Two circular shafts of same material are subjected to same torque producing the same maximum shear stress. If the first shaft is of solid section and the second shaft is of hollow section, whose internal diameter is 2/3 of the external diameter, compare the weights of the two shafts. Both the shafts are of equal length.
TS = TH = T ; τS,MAX=τH,MAX=τMAX ;
LS = LH = L
T τ
J R
=
T J
τR
=>
=
TS TH T
τS,max τH,max τmax
= Constant.
=
=
τMAX
T
T J πD4/32 πD3
τmax R D/2 16
=
=
=
----- (1)
D
Since torque and stress are same,
Considering the solid shaft :
T J π.[D14 – (2D1/3)4] /32 65π.D13
τmax R D1/2 1296
=
----- (2)
=
=
τMAX
T
D1
πD3 65πD13
16 1296
=
2D1/3
Considering the hollow shaft :
Equating (1) and (2),
=> D = 0.93D1.
WHπ.[D12 – (2D1/3)2] × L × ρ
4
WH 5πD12 × L × ρ
36
=
----- (4)
WSπD2 × L × ρ
4
----- (3)
=
Weight of solid shaft,
Weight of hollow shaft,
=
WS
WH
WH
WS
=
=
π.D2 × L × ρ
4
= 0.64 = 64%
=>
5πD12 × L × ρ
36
Dividing (3) by (4), we have,
But D = 0.93D1
= 1.557
Weight of hollow section = 0.64 times weight of solid section
Solution :
τ
τ
TH
TS
D
D1
πD2 . L. ρ
4
π(D12 – D22). L. ρ
4
=
D2
5. Prove that a hollow shaft is stronger and stiffer than a solid shaft of same material, length and weight.
To prove hollow shaft is stronger:
Since material, weight and length are same,
=> D2 = D12 – D22 -----(1)
Weight of the solid shaft = weight of the hollow shaft
TH D14 – D24
TS (D12 – D22)3/2 . D1
∴
=
JH ×τ
R
JS×τ
R
(πD4/32) τ
D/2
τ× πD3
16
TH
TS
TS
=
=>
=
=
=
[π(D14–D24)/32]×τ
D1/2
=
τ× π(D14–D24)
16D1
TH D14 – D24
TS D3 × D1
∴
TH
=>
=
=
For solid shaft, torque resisted is,
For hollow shaft, torque resisted is,
Substituting (1) in the above equation we have,
[ Since D = (D12 – D22)1/2 ]
TH ( D12 – D22 ) ( D12 + D22)
TS (D12 – D22). (D12 – D22)1/2 D1
=
∴
> 1
Hence hollow shafts are stronger than solid shafts of same material, length and weight.
To prove hollow shaft is stiffer:
Stiffness of shaft may be defined as torque required to produce unit rotation in unit length. Let this be denoted by K. Then from torsion formula
K = G J
KH D14 – D24
KS D4
D14 – D24
(D2)2
∴
=
=
KH D12 + D22
KS D12 – D22
=
=>
(D12 – D22)(D12 + D22)
(D12 – D22)2
=
Stiffness of hollow shaft is,
KH = G × JH = G [π(D14-D24)/32]
Stiffness of solid shaft is,
KS = G × JS = G [πD4/32]
=> KH > KS
> 1
Hence hollow shafts are stiffer than solid shafts of same material, length and weight.
Solution :
6. A shaft is required to transmit 245kW power at 240 rpm. The maximum torque is 50% more than the mean torque. The shear stress in the shaft is not to exceed 40N/mm2 and the twist 1º per meter length. Taking G = 80kN/mm2, determine the diameter required if,
a.) the shaft is solid.
b.) the shaft is hollow with external diameter twice the internal diameter.
P = 245 × 103 N-m/s = 245 × 106 N-mm/s
N = 240 rpm; L = 1000mm; G = 80 × 103 N/mm2
τmax = 40N/mm2; θmax = 1º = π/180 radians; Tmax = 1.5T
2πNT
60
P
=
60.P
2πN
Tmaxτmax
J R
=> T =
14622.36×103 40
πD4/32 D/2
=>
=
=
Tmax G×θmax
J L
14622.36×103 (80×103)×(π/180)
πD4/32 1000
=
=>
=
= 9748.24 × 103 N-mm
∴ Tmax = 1.5T = 14622.360 × 103 N-mm
a.) For solid shaft :
Let ‘D’ be the diameter of solid shaft.
=> D =123.02mm
=> D = 101.6mm
Hence, the diameter to be provided is, D =123.02mm.
π(D14 – D24)
32
π(D14 – (0.5D1)4)
32
J =
J =
Tmaxτmax
J R
14622.36×103 40
0.09204D14 D1/2
=>
=
=
Tmax G× θmax
J L
14622.36×103 (80×103)×(π/180)
0.09204D14 1000
=
=>
=
b.) For hollow shaft :
Let ‘d1’ be the external diameter. Then internal diameter, D2 = 0.5D1
=> J = 0.09204d14
=> D1 =125.7mm
=> D1 = 103.21mm
Hence provide D1 = 125.7mm and D2 = 62.85mm
2πNT
60
P
=
Solution :
60P
2πN
=> T =
7. A power of 2.2MW has to be transmitted at 60 r.p.m. If the allowable stress in the material of the shaft is 85MPa, find the required diameter of the shaft, if it is solid. If instead, a hollow shaft is used with 3DE = 4DI , calculate the percentage saving in weight per meter length of the shaft. Density of the shaft material is 7800kg/m3.
P = 2.2 × 106 N-m/s = 2.2 × 109 N-mm/s
τ = 85N/mm2; N = 60 rpm; ρ = (7800×9.81)N/m3
= 350140.874 × 103 N-mm
T τ
J R
T τ
J R
350140.874 ×103 85
πD4/32 D/2
=>
=
=
=
π(DE4 – DI4)
32
J =
350140874.8 85
0.067112DE4 DE/2
=>
=
WSπD2 × L × ρ
4
=
For solid shaft :
=> D =275.802mm
Weight of solid shaft,
= 59743.842 ρ L
For hollow shaft :
=> J = 0.06711166DE4
=> DE = 313.087mm
∴ DI = 234.815mm
WHπ.[DE2 – DI2] × L × ρ
4
=
WS – WH
WS
=
× 100
*************************************************
Weight of hollow shaft,
= 33682.011 ρ L
∴ Percentage saving in weight,
= 43.62%
EXERCISE PROBLEMS :
T49
Contd..
T50
3. Find the diameter of the shaft required to transmit 12kW at 300 rpm if the maximum torque is likely to exceed the mean torque by 25%. The maximum permissible shear stress is 60N/mm2. Taking G = 0.84 × 105 N/mm2, find the angle of twist for a length of 2m.
[Ans : θ = 4.76º.
A solid shaft of circular cross-section transmits 1200kW at 100 rpm. If the allowable stress in the material of the shaft is 80MPa, find the diameter of the shaft.
If the hollow section of the same material, with its inner diameter (5/8)th of its external diameter is adopted, calculate the economy achieved.
[Ans : D = 194mm; % Saving in material = 31.88%]
Contd..
T51
Contd..
T52
7. A solid aluminium shaft 1m long and 60mm diameter is to be replaced by a tubular steel shaft of same length and same outside diameter (60mm) such that each of the two shaft could have same angle of twist per unit torsional moment over the total length. What must be the inner diameter of the tubular shaft if GS = 4GA.
[Ans : Di = 45.18mm]
8.A hollow shaft has diameters DE = 200mm and DI = 150 mm. If angle of twist should not exceed 0.5º in 2m and maximum shear stress is not to exceed 50MPA, find the maximum power that can be transmitted at 200 rpm. Take G = 84GPa.
[Ans : 823kW]
Contd..
*************************************************
T53
9. A hollow marine propeller shaft turning at 110 rpm is required to propel a vessel at 12m/s for the expenditure of 6220kW, the efficiency of the propeller being 68 percent. The diameter ratio of the shaft is to be (2/3) and the direct stress due to thrust is not to exceed 8MPa. Calculate
(a) the shaft diameters
(b) the maximum shearing stress due to torque.
[Ans : DI = 212mm; DE = 318mm; τMAX = 10.66 MPa]