8.3^*8.3. DESIGN OF TRANSMISSION SHAFTS^518^526^,,^10865^11128%
Page 518
*8.3
DESIGN OF TRANSMISSION SHAFTS

When we discussed the design of transmission shafts in Sec. 3.7, we considered only the stresses due to the torques exerted on the shafts. However, if the power is transferred to and from the shaft by means of gears or sprocket wheels (Fig. 8.11a), the forces exerted on the gear teeth or sprockets are equivalent to force-couple systems applied at the centers of the corresponding cross sections (Fig. 8.11b). This means that the shaft is subjected to a transverse loading, as well as to a torsional loading.

Fig. 8.11 Loadings on gear-shaft systems.

The shearing stresses produced in the shaft by the transverse loads are usually much smaller than those produced by the torques and will be neglected in this analysis. The normal stresses due to the transverse loads, however, may be quite large and, as you will see presently, their contribution to the maximum shearing stress max should be taken into account.

Page 519

Consider the cross section of the shaft at some point C. We represent the torque T and the bending couples My and Mz acting, respectively, in a horizontal and a vertical plane by the couple vectors shown (Fig. 8.12a). Since any diameter of the section is a principal axis of inertia for the section, we can replace My and Mz by their resultant M (Fig. 8.12b) in order to compute the normal stresses σx exerted on the section. We thus find that σx is maximum at the end of the diameter perpendicular to the vector representing M (Fig. 8.13). Recalling that the values of the normal stresses at that point are, respectively, σm = Mc/I and zero, while the shearing stress is m = Tc/J, we plot the corresponding points X and Y on a Mohr-circle diagram (Fig. 8.14) and determine the value of the maximum shearing stress:

Recalling that, for a circular or annular cross section, 2I = J, we write

(8.5)
Fig. 8.12 Resultant loading on the cross section of a shaft.
Fig. 8.13 Maximum stress element.
Fig. 8.14 Mohr's circle analysis.

It follows that the minimum allowable value of the ratio J/c for the cross section of the shaft is

(8.6)

where the numerator in the right-hand member of the expression obtained represents the maximum value of in the shaft, and all the allowable shearing stress. Expressing the bending moment M in terms of its components in the two coordinate planes, we can also write

(8.7)

Equations (8.6) and (8.7) can be used to design both solid and hollow circular shafts and should be compared with Eq. (3.22) of Sec. 3.7, which was obtained under the assumption of a torsional loading only.

The determination of the maximum value of will be facilitated if the bending-moment diagrams corresponding to My and Mz are drawn, as well as a third diagram representing the values of T along the shaft (see Sample Prob. 8.3).

Page 520

SAMPLE

SAMPLE PROBLEM 8.1

A 160-kN force is applied as shown at the end of a W200 × 52 rolled-steel beam. Neglecting the effect of fillets and of stress concentrations, determine whether the normal stresses in the beam satisfy a design specification that they be equal to or less than 150 MPa at section A-A′.

SAMPLE

SAMPLE

SOLUTION
Shear and Bending Moment.

At section A-A′, we have

Normal Stresses on Transverse Plane.

Referring to the table of Properties of Rolled-Steel Shapes in Appendix C, we obtain the data shown and then determine the stresses σa and σb.

At point a:

At point b:

We note that all normal stresses on the transverse plane are less than 150 MPa.

Shearing Stresses on Transverse Plane

At point a:

At point b:

Principal Stress at Point b.

The state of stress at point b consists of the normal stress σb = 103.0 MPa and the shearing stress b = 96.5 MPa. We draw Mohr's circle and find

Comment.

For this beam and loading, the principal stress at point b is 36% larger than the normal stress at point a. For L ≥ 881 mm, the maximum normal stress would occur at point a.

SAMPLE

Page 521

SAMPLE

SAMPLE PROBLEM 8.2

The overhanging beam AB supports a uniformly distributed load of 3.2 kips/ft and a concentrated load of 20 kips at C. Knowing that for the grade of steel to be used σall = 24 ksi and all = 14.5 ksi, select the wide-flange shape that should be used.

SAMPLE

SAMPLE

SOLUTION
Reactions at A and D.

We draw the free-body diagram of the beam. From the equilibrium equations ΣMD = 0 and ΣMA = 0 we find the values of RA and RD shown in the diagram.

Shear and Bending-Moment Diagrams.

Using the methods of Secs. 5.2 and 5.3, we draw the diagrams and observe that

Section Modulus.

For |M|max = 2873 kip · in. and σall = 24 ksi, the minimum acceptable section modulus of the rolled-steel shape is

Selection of Wide-Flange Shape.

From the table of Properties of Rolled-Steel Shapes in Appendix C, we compile a list of the lightest shapes of a given depth that have a section modulus larger than Smin.

Shape S (in3)
W24 × 68 154
W21 × 62 127
W18 × 76 146
W16 × 77 134
W14 × 82 123
W12 × 96 131

We now select the lightest shape available, namely

Shearing Stress.

Since we are designing the beam, we will conservatively assume that the maximum shear is uniformly distributed over the web area of a W21 × 62. We write

Principal Stress at Point b.

We check that the maximum principal stress at point b in the critical section where M is maximum does not exceed σall = 24 ksi. We write

Conservatively,

We draw Mohr's circle and find

SAMPLE

Page 522

SAMPLE

SAMPLE PROBLEM 8.3

The solid shaft AB rotates at 480 rpm and transmits 30 kW from the motor M to machine tools connected to gears G and H; 20 kW is taken off at gear G and 10 kW at gear H. Knowing that all = 50 MPa, determine the smallest permissible diameter for shaft AB.

SAMPLE

SAMPLE

SOLUTION
Torques Exerted on Gears.

Observing that f = 480 rpm = 8 Hz, we determine the torque exerted on gear E:

The corresponding tangential force acting on the gear is

A similar analysis of gears C and D yields

We now replace the forces on the gears by equivalent force-couple systems.

Bending-Moment and Torque Diagrams
Critical Transverse Section.

By computing at all potentially critical sections, we find that its maximum value occurs just to the right of D:

Diameter of Shaft.

For all = 50 MPa, Eq. (7.32) yields

For a solid circular shaft of radius c, we have

SAMPLE

Page 523
PROBLEMS
8.1 A W10 × 39 rolled-steel beam supports a load P as shown. Knowing that P = 45 kips, a = 10 in., and σall = 18 ksi, determine (a) the maximum value of the normal stress σm in the beam, (b) the maximum value of the principal stress σmax at the junction of the flange and web, (c) whether the specified shape is acceptable as far as these two stresses are concerned.
Fig. P8.1
8.2 Solve Prob. 8.1, assuming that P = 22.5 kips and a = 20 in.
8.3 An overhanging W920 × 449 rolled-steel beam supports a load P as shown. Knowing that P = 700 kN, a = 2.5 m, and σall = 100 MPa, determine (a) the maximum value of the normal stress σm in the beam, (b) the maximum value of the principal stress σmax at the junction of the flange and web, (c) whether the specified shape is acceptable as far as these two stresses are concerned.
Fig. P8.3
8.4 Solve Prob. 8.3, assuming that P = 850 kN and a = 2.0 m.
8.5 and  8.6(a) Knowing that σall = 24 ksi and all = 14.5 ksi, select the most economical wide-flange shape that should be used to support the loading shown. (b) Determine the values to be expected for σm, m, and the principal stress σmax at the junction of a flange and the web of the selected beam.
Fig. P8.5
Fig. P8.6
8.7 and  8.8(a) Knowing that σall = 160 MPa and all = 100 MPa, select the most economical metric wide-flange shape that should be used to support the loading shown. (b) Determine the values to be expected for σm, m, and the principal stress σmax at the junction of a flange and the web of the selected beam.
Fig. P8.7
Fig. P8.8
 
8.9
through  8.14Each of the following problems refers to a rolled-steel shape selected in a problem of Chap. 5 to support a given loading at a minimal cost while satisfying the requirement σmσall. For the selected design, determine (a) the actual value of σm in the beam, (b) the maximum value of the principal stress σmax at the junction of a flange and the web.
  • 8.9

    Loading of Prob. 5.73 and selected W530 × 66 shape.

  • 8.10

    Loading of Prob. 5.74 and selected W530 × 92 shape.

  • 8.11

    Loading of Prob. 5.77 and selected S15 × 42.9 shape.

  • 8.12

    Loading of Prob. 5.78 and selected S12 × 31.8 shape.

  • 8.13

    Loading of Prob. 5.75 and selected S460 × 81.4 shape.

  • 8.14

    Loading of Prob. 5.76 and selected S510 × 98.2 shape.

8.15 The vertical force P1 and the horizontal force P2 are applied as shown to disks welded to the solid shaft AD. Knowing that the diameter of the shaft is 1.75 in. and that all = 8 ksi, determine the largest permissible magnitude of the force P2.
Fig. P8.15
8.16 The two 500-lb forces are vertical and the force P is parallel to the z axis. Knowing that all = 8 ksi, determine the smallest permissible diameter of the solid shaft AE.
Fig. P8.16
8.17 For the gear-and-shaft system and loading of Prob. 8.16, determine the smallest permissible diameter of shaft AE, knowing that the shaft is hollow and has an inner diameter that is the outer diameter.
8.18 The 4-kN force is parallel to the x axis, and the force Q is parallel to the z axis. The shaft AD is hollow. Knowing that the inner diameter is half the outer diameter and that all = 60 MPa, determine the smallest permissible outer diameter of the shaft.
Fig. P8.18
 
8.19
Neglecting the effect of fillets and of stress concentrations, determine the smallest permissible diameters of the solid rods BC and CD. Use all = 60 MPa.
Fig. P8.19 and P8.20
8.20 Knowing that rods BC and CD are of diameter 24 mm and 36 mm, respectively, determine the maximum shearing stress in each rod. Neglect the effect of fillets and of stress concentrations.
8.21 It was stated in Sec. 8.3 that the shearing stresses produced in a shaft by the transverse loads are usually much smaller than those produced by the torques. In the preceding problems their effect was ignored and it was assumed that the maximum shearing stress in a given section occurred at point H (Fig. P8.21a) and was equal to the expression obtained in Eq. (8.5), namely,
Show that the maximum shearing stress at point K (Fig. P8.21b), where the effect of the shear V is greatest, can be expressed as
where β is the angle between the vectors V and M. It is clear that the effect of the shear V cannot be ignored when KH. (Hint: Only the component of M along V contributes to the shearing stress at K.)
Fig. P8.21
8.22 Assuming that the magnitudes of the forces applied to disks A and C of Prob. 8.15 are, respectively, P1 = 1080 lb and P2 = 810 lb, and using the expressions given in Prob. 8.21, determine the values of H and K in a section (a) just to the left of B, (b) just to the left of C.
8.23 The solid shafts ABC and DEF and the gears shown are used to transmit 20 hp from the motor M to a machine tool connected to shaft DEF. Knowing that the motor rotates at 240 rpm and that all = 7.5 ksi, determine the smallest permissible diameter of (a) shaft ABC, (b) shaft DEF.
Fig. P8.23
8.24 Solve Prob. 8.23, assuming that the motor rotates at 360 rpm.
 
8.25
The solid shaft AB rotates at 360 rpm and transmits 20 kW from the motor M to machine tools connected to gears E and F. Knowing that all = 45 MPa and assuming that 10 kW is taken off at each gear, determine the smallest permissible diameter of shaft AB.
Fig. P8.25
8.26 Solve Prob. 8.25, assuming that the entire 20 kW is taken off at gear E.
8.27 The solid shaft ABC and the gears shown are used to transmit 10 kW from the motor M to a machine tool connected to gear D. Knowing that the motor rotates at 240 rpm and that all = 60 MPa, determine the smallest permissible diameter of shaft ABC.
Fig. P8.27
8.28 Assuming that shaft ABC of Prob. 8.27 is hollow and has an outer diameter of 50 mm, determine the largest permissible inner diameter of the shaft.
 
8.29
The solid shaft AE rotates at 600 rpm and transmits 60 hp from the motor M to machine tools connected to gears G and H. Knowing that all = 8 ksi and that 40 hp is taken off at gear G and 20 hp is taken off at gear H, determine the smallest permissible diameter of shaft AE.
Fig. P8.29
8.30 Solve Prob. 8.29, assuming that 30 hp is taken off at gear G and 30 hp is taken off at gear H.
†For an application where the shearing stresses produced by the transverse loads must be considered, see Probs. 8.21 and 8.22.