EMA 3702 Mechanics & Materials Science (Mechanics of Materials) Chapter 3 Torsion

Transcription

1 EMA 3702 Mechanics & Materials Science (Mechanics of Materials) Chapter 3 Torsion

2 Introduction Stress and strain in components subjected to torque T Circular Cross-section shape Material Shaft design Non-circular Irregular shape Elastic Elastoplastic Solid Hollow

3 Introduction Component subject to twisting couples, or torques of T & T T is a vector and has two ways of representations Curved arrow Couple vector (right hand rule) Example: Transmission of torque in shafts

4 Stresses in a Shaft under Torsion (1) A shaft subjected to equal and opposite torques (or moments of force) of T and T at A and B In a normal cross-section at C, for an elementary area da, the element shearing force df and local stress τ satisfy: df = da Define as (local) lever arm, i.e., the perpendicular distance from the elemental area to the axis (center), total torque T is: df T

5 Stresses in a Shaft under Torsion (2) From previous df = da df T Therefore, (da) T Complications for torsion: Distributions of and the resulting, i.e., how they change over the cross-section plane is statically indeterminate. Unlike normal stress or simple shearing, distribution of for torsion is NOT uniform!

6 Axial Shearing Stress in Shaft under Torsion Consider a small element as illustrated. Based on previous considerations for shearing stress, if xz 0, axial shearing stress zx = xz 0 zx xz Implication: under torsion, shearing stress exists along longitude planes, and neighboring elements have tendency to slide against each other along axial direction! z y x

7 Axi-symmetric Property of a Circular Shaft For circular shaft under torsion Cross-sections remain planar Cross-section remain undistorted

8 Shearing Strain in Circular Shaft under Torsion Within each cross-section, NO change Along axial direction, there is strain (deformation) due to axial shear zx Define the following terms L Length along shaft axis Radial distance from the shaft axis Angle of twist for a cross-section Shearing strain (change in angle from 90 o ) Far away from location of loading & for small strain and angle L L

9 Shearing Strain in Circular Shaft under Torsion L For a given L, when = c, i.e., radius of the shaft max c L Additionally, for a given L max c c max

10 Shearing Stresses in Circular Shaft under Torsion Hooke s Law for shear G From previous page, torsion shearing strain L Therefore, G shearing stress L For given L and ϕ, when = c = radius of the shaft, shearing stress reaches maximum: The ratio between τ and τ max c max max G max Gc L Use when shaft twist angle, length, AND materials G are known Distribution of shearing stress is linear w.r.t. radius from the axis ρ

11 Shearing Stresses & Torque in Circular Shaft under Torsion c max Recall relationship of T and local shearing stress τ For a circular shaft with fixed radius c under torque T T Recall the definition for moment of inertia: We have Or (da) max T max da da 2 ( ) max da c c 2 max T c Tc and T da Use when shaft geometry AND applied torque known

12 Solid & Hollow Circular Shaft under Torsion For a solid cylinder T max 1 c 2 4 Tc For a hollow cylinder ( c 2 c 2 T min max ) c c 1 2

13 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Example Problem for Torsion 3.1 Shaft ABCD subject to torques as illustrated. Knowing section BC is hollow with ID = 90 mm and OD = 120 mm. Sections AB and CD are solid. Calculate (a) The maximum and minimum shearing stress in section BC; (b) The required minimal diameter for AB and CD if shearing stress should not exceed 65 MPa.

14 Example Problem for Torsion 3.1 Section AB, Net internal torque T AB = 6 knm Section BC, Net internal torque T BC = (6 +14) knm = 20 knm EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion

15 Example Problem for Torsion 3.1 For section BC, T BC = 20 knm section BC is hollow with ID = 90 mm and OD = 120 mm. Max stress in BC occurs at the outer surface BC BCc T N m0.06m outer max 1 BC Min stress in BC occurs at the inner surface min T c 2010 N m0.045m BC 3 BC inner BC EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion m m MPa MPa

16 Example Problem for Torsion 3.1 Section AB, T AB = 6 knm Section BC, T BC = 20 knm Section CD, T CD = 6 knm For both AB and CD, shearing stress should not exceed 65 MPa max T c c 2 Minimum radius for AB or CD: T MPa T c 1 2 max 1/3 1 2 Minimum diameter = 7.78 cm EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion 3 N m 6 N / m 2 1/ cm

17 Class Example A hollow cylinder shaft is 1 m long and has inner and outer diameter of 20 and 40 mm. (a) What is the largest torque that can be applied if shearing stress should not exceed 100 MPa? (b) What is the minimum shearing stress when maximum reaching 100 MPa? From geometry, moment of inertia 1 1 outer inner 2 2 shearing stress should not exceed 100 MPa : Largest torque that can be applied: c c m T 100MPa c out Min shearing stress when max is 100 MPa max outer EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion m m cinner min max c 6 Tc outer 100 MPa N / m N m MPa 50MPa 0.02

18 Normal Stress in Circular Shaft under Torsion For element a at shaft surface max Tc For half of a at 45 o, shear force along BC and BD are: F BC F BD max A 0 To balance, force on CD surface F CD must be normal force F CD Since A CD 2A0 Normal stress due to torsion F F A BC CD CD / cos 45 2 A max Tc max 0 F CD For torsion, significant normal stress still exists and may cause failure!

19 Failure of Material under Torsion For circular shaft under torsion Ductile materials are weaker in shear and fail with 90 o fracture (i.e., fracture surface perpendicular to axis) Brittle materials are weaker in tension and fail with 45 o fracture (i.e., fracture surface at 45 o from axis) Ductile torsion failure Brittle torsion failure Photo by eff Thomas, 11/1997

20 Angle of Twist within Elastic Range For torsion, based on geometry max c L Within elastic limit, Hooke s law max G max Max sharing stress due to torque max Tc max Tc G L T G TL G Angle of twist increases with Increasing T and L Decreasing and G

21 Angle of Twist for Multiple Cross-Section Shafts Overall twist of A w.r.t B i Ti Li G in which T i, i, and L i, G i are obtained and/or analyzed for each section Angle of Twist for Variable Cross-Section Shafts For element dx d Tdx G i i Overall Tdx L 0 G

22 Class Exercise A cylindrical shaft is 0.5 m long and has diameter of 20 mm fixed at one end. If a torque of 76 Nm is applied to its free end, and the measured angle of twist at the loading end is 1.8 o, please estimate the shear modulus knowing moment of inertia is Angle of twist Shear modulus G 1 c 2 TL G G TL 76N m0.5m GPa m EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion 4

23 Example of Statically Indeterminate Shaft (1) A cylindrical shaft AB with two sections is fixed between two rigid end support at A and B. Section AC is solid with A C B diameter of 1.0 in; 10 in 10 in Section CB is hollow with outside diameter of 1.0 in and inner diameter of 0.8 in. Both sections have length of 10 in. If an external torque of 100 lbft is applied at center C, please determine the reaction torque at A and B, assuming the deformation is within proportional limit.

24 Example of Statically Indeterminate Shaft (2) From statics, balance of torque T T T 100lb in A B One equation two variables Statically indeterminate Consider geometry/deformation: For section AC, angle of twist AC For section CB, angle of twist CB AC CB Therefore, TALAC TBLCB AC CB G G AC CB Two equations & two variables, problem could be solved 100 AC CB T A AC T B CB

25 Example of Statically Indeterminate Shaft (3) Therefore, T A T B 100 T A T B TA T B Solving these two T A T B 62. 9lb 37. 1lb ft ft

26 Design of Transmission Shafts (1) Two parameters in transmission shaft design: Power P Speed of rotation For power: P T is angular velocity, = 2, and has unit of radians/s is frequency in unit Hz or s -1 Therefore, power P 2fT has unit of Nm/s = W As a result, for given P and f, the resulting torque will be T P 2f

27 Design of Transmission Shafts (2) Relationship between power output P, angular speed (or frequency f), and torque T P 2f On the other hand, max shearing stress for a given shaft geometry and applied torque max Tc Example of a solid circular shaft, moment of inertia: T c c 3 2 c 2 c T c 2T T max max max_ allowable 1/3

28 Class Exercise What size of solid shaft should be used for motor of 592 hp (1 hp = 6600 lbin/s) operating at 3600 rpm (f = 60 Hz) if the shearing stress is not to exceed 6600 psi in the shaft? Power for the shaft P 2fT P lb in s Torque for the shaft T 2f s Maximum shearing stress Tc max Tc 2T lb / in c 4 c 3 2 Minimum shaft radius: c 2T max_ allowable EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion 1/ lb in lb / in 1 1 1/ lb in 1.00 in

29 Stress Concentrations in Circular Shafts Similar to normal stress, shearing stress may concentrate (i.e., show higher value) at certain locations Concentration factor K max_ actual K Tc / or Tc max_ actual K Local max stress increases with Sharper transition or joining Larger ratio of radius

30 Plastic Deformation in Circular Shafts (1) For a given shaft construction (geometry and material), when torque is low and within linear elastic region, linear stress distribution w.r.t. radius from axis T As torque increases further, shearing stress would reach yield strength Y, it will go into non-linear (e.g., ideal elasoplastic) distribution of w.r.t. radial from axis in certain region T

31 Plastic Deformation in Circular Shafts (2) For angle of twist When torque/shearing stress is low and within linear elastic region, increases linearly with torque T TL G T As torque increases further, shearing stress reaches yield strength Y, it will go into non-linear distribution of w.r.t. T

32 Torsion of Noncircular Members A rectangular or square shaft does not contain axi-symmetry The cross-sections generally do NOT remain flat or planar under torsion Shearing stress at the edge (corner) of the shaft is zero Maximum shearing stress occur in the middle of flat face

33 Thin-Walled Hollow Shafts For arbitrary shaped thin walled hollow tube subjected to torque T, the shearing stress can be approximated as τ = T 2ta a

34 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.0 Read chapter 3 textbook sections 3.1 to 3.5 and give an honor statement confirm reading.

35 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.1 Electric motor applies a torque of 2.8 kn m on shaft EF at E. Knowing each shaft section is solid. Based on the additional torques as depicted, please determine the maximum shearing stress in (a) shaft EF (radius c EF = 25 mm), (b) shaft FG (radius c FG =23 mm), and c) shaft GH (radius c GH =21 mm) E F G H E T F = 1.6 kn m T G = 0.8 kn m T H = 0.4 kn m

36 Homework 3.2 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion A component with both solid rod EF and hollow tube with flange GH welded onto a flat end plate of GFI, and the rod EF and tube GH share the same longitude axis, as illustrated. The hollow tube GH is fixed at rigid flange of AA BB at H using bolts. The solid rod EF has a diameter d EF = 3.81 cm and is made of steel with allowable shearing stress of 82.7 MPa. Tube GH is made of copper alloy with OD d 2 =7.62 cm and ID d 1 =6.35 cm with allowable shearing stress of 48.3 MPa. Determine the largest torque T that can be applied at E. G T H E A A B B F I

37 Homework 3.3 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Two solid shafts GH and I are connected by gears K and L as illustrated. The radius for gear K is R K = 4 in, while radius for gear L is R L = 2.5 in. Both two shafts of GH and I are made of same steel with allowable shearing stress of 8 ksi. The radius for the shafts are c GH = 0.8 in and c I = in. Please determine the largest torque that can be applied at H. K L G R K = 4.0 in R L = 2.5 in I H

38 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.4 Motor applies torque T = 500 N m on the HGFE shaft when it is rotating at a constant speed. Knowing shear modulus G = 27 GPa and the torques exerted on pulleys B and C are shown. Please calculate the angle of twist between (a) F and G, and (b) between F and H. Knowing shaft radius c GF = 2.2 cm; c HG = 2.4 cm; c EF = 2.0 cm; T = 500 N m 0.9 m 1.2 m H G F E Motor T G = 300 N m T F = 200 N m

39 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.5 Solid aluminum rod (G Al =26 GPa) is bonded to solid copper alloy rod (G Al =39 GPa). The radius for both rods are 10 mm. Calculate the angle of twist with respect to base D at E point and at F point, respectively. D copper E aluminum F 0.2 m 0.4 m T F = 50 N m

40 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.6 A 6 foot long composite shaft consists of 0.2 inch thick copper shell (G Cu = psi) bonded to 1.2 inch (G Steel = psi) diameter iron core. If the shaft is subject to 5000 lb in torque. Please calculate the maximum shearing stress in the steel core and the angle of twist of one end versus the other. T Cu shell Fe core 6 ft

41 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.7 Determine maximum shearing stress in a solid shaft of 10 mm diameter as it transmits 2.4 kw at a frequency of (a) 30 Hz and (b) 60 Hz.

42 EMA 3702 Mechanics & Materials Science Zhe Cheng (2018) 3 Torsion Homework 3.8 Two solid shafts EF and I and gears G (R G = 3 inch) and H (R H = 5 inch) are used to transmit 16 hp (1 hp = 6600 lbin) from motor at E operating at 1200 rpm (f = 20 Hz) to machine tool at. Knowing the maximum allowable shearing stress is 7.5 ksi. Please calculate the required radius for shaft EF and shaft I, respectively. H I R H = 5.0 in R G = 3.0 in E F G