1. Section VI
Shaft Design

2. Talking Points Shaft? Shaft Design ASME Shaft Equations
Design of Shaft for Torsional Rigidity
Standard Sizes of Shafts
Bending and Torsional Moments

3. Shaft? Rotating machine element that transmits power.
Shafts are usually circular in cross-section, and may be either hollow or solid.

4. Shaft Design
Design of shafts for ductile materials, based on strength, is controlled by the maximum-shear stress theory (Tresca) or distortion-energy theory (von -Mises); while shafts of brittle materials would be designed on the basis of the maximum-normal stress theory.
Consists of the determination of the correct shaft diameter to ensure satisfactory strength and rigidity when the shaft is transmitting power under various operating and loading conditions.
Shafts are usually subjected to torsion, bending, and axial loads.
1) For axial loads: The tensile or compressive stress is:
2) For bending loads: The bending
stress (tension or compression) is:
3) For torsional loads: The torsional stress is:

5. Shaft Design – Cont.

6. Shaft Design – Cont.

7. ASME Shaft Equations
The ASME code equation for hollow shaft combines torsion, bending, and axial loads by applying the maximum-shear equation modified by introducing shock, fatigue, and column factor as follows:
For solid shaft having little or no axial loading, the equation is:
Where:
Where:

8. ASME Shaft Equations – Cont.

9. Designing for Fully Reversed Bending and Nearly Steady Torsion
Designing for Fully Reversed Bending and Steady Torsion ASME Method (ANSI/ASME Standard for Design of Transmission Shafting B106.1M-1985, which is derived from distortion energy theory, can be applied only for:
• constant torque
• fully reversed moment.
• No axial load
d = shaft diameter, m
nd = design factor or safety factor
Kb = stress concentration factor
Mb = maximum bending moment, N.m
Mt = maximum torsion, N.m
Se = actual endurance strength, N/m2
Sy = yield strength, N/m2

10. Design factor or Safety Factor (nd )
A measure of the relative safety of a load-carrying component.
For ductile materials:
nd = 1.25 to 2.0: Design of structures under static loads for which
there is a high level of confidence in all design data.
nd = 2.0 to 2.5: Design of machine elements under dynamic loading
with average confidence in all design data.
nd = 2.5 to 4.0: Design of static structures or machine elements under
dynamic loading with uncertainty about loads,
material properties, stress analysis or the environment.
nd = 4.0 or higher: Desire to provide extra safety to critical components.

11. Estimated Design Values for Kb (bending case)
(a) Profile keyseat
(c) Well-rounded fillet
(b) Sled runner keyseat
Kb = 2,0 for profile keyseat
Kb = 1,6 for Sled runner keyseat
Kb = 1,5 for well-rounded fillet

12. Se = kb ke Se ’ Estimated Actual Endurance Strength, Se
Se = endurance strength
kb = size factor
ke = reliability factor
Se = kb ke Se ’
Size factors, kb
Approximated reliability factors, ke
Size Range
For d in mm
d ≤ 7,62
kb= 1,0
7,62 < d ≤ 50
kb = (d/7,62)-0,11
50 < d ≤ 250
kb = 0,859 – 0,000837d
Desired reliability ke
0.50
1,0
0,90
0,99
0,81
0,999
0,75
NOTE: For simplification in determining the shaft diameter,
the value of size factor can be estimated as kb= 0,75

13. Endurance strength (Se ) versus tensile strength (Su) for various surface conditions of wrought steel ’

14. Yield strength in shear: Sys = 0,50 Sy
Ultimate shear strength (Sus ) and yield shear strength (Sys) for wrought steel
Both the yield strength and the ultimate strength in shear (Sys and Sus) are important properties of materials. Unfortunately, these values are seldom reported. We will use the following estimates:
Yield strength in shear:
Sys = 0,50 Sy
Ultimate strength in shear:
Sus = 0,75 Su

15. Design of Shaft for Torsional Rigidity
It is based on the permissible angle of twist. The amount of twist permissible depends on the particular application, and varies about 0.3 degree/m for machine tool shafts to about 3.0 degree/m for line shafting.
Where:

16. Standard Sizes of Shafts
These sizes vary according to material specifications and supplier. Typical sizes for solid shafts are:
Up to 25 mm in 0.5 mm increments
25 to 50 mm in 1.0 mm increments
50 to 100 mm in 2.0 mm increments
100 to 200 mm in 5 mm increments

17. Bending and Torsional Moment
These are the main factors influencing shaft design. One of the first steps in shaft design is to draw the bending moment diagram for the loaded shaft or the combined bending moment diagram if the loads acting on the shaft are in more than one axial plane. From the bending moment diagram, the points of critical bending stress can be determined. The torsional moment acting on the shaft can be determined from:
1) For belt drive: The torque is found by:
2) For gear drive: The torque is found by:
Where:
Where: