Post on 07-Apr-2020
Chapter 10: Design of
Mechanical Element 4: Shaft
DR. AMIR PUTRA BIN MD SAAD
C24-322
amirputra@utm.my | amirputra@mail.fkm.utm.my
mech.utm.my/amirputra
10.1 INTRODUCTION
The term shaft usually refers to a relativelylong member of round cross section thatrotates and transmits power. One or moremembers such as
(i) Gears,(ii) Sprockets,(iii) Pulleys,(iv) and cams.
are usually attached to the shaft by means of
(i) pins,(ii) keys,(iii) splines,(iv) snap rings,(v) and other devices.
How do the shaft and gear look in the theory ?
How do the shaft and gear look in the practice?
10.1 INTRODUCTION
A shaft can have a non-round cross section, and it need not rotate. It can bestationary and serve to support a rotating member, such as the short shafts (alsocalled spindles) that support the non-driving wheels of an automobile.
10.1 INTRODUCTION
Shafts supporting and driving vehiclewheels are also called axles.
It is apparent that shafts can besubjected to various combinations ofaxial, bending, and torsional loads andthat these loads may be static orfluctuating.
Typically, a rotating shaft transmittingpower is subjected to a constant torque(producing a mean torsional stress)together with a completely reversedbending load (producing an alternatingbending stress).
axle axle
10.1 INTRODUCTION
10.1 INTRODUCTION
10.2 PINS
10.2 PINS
10.2 KEYS
(i) Perhaps the most common of thesetorque-transmitting shaft-to-hubconnections are keys.
(ii) Standard proportions call for the keywidth to be approximately one-fourththe shaft diameter (see varioushandbooks and ANSI Standard B17.1 fordetails).
(iii) Keys are usually made of cold-finishedlow-carbon steel (as SAE or AISI 1020),but heat treated alloy steels are usedwhen greater strength is required.
(iv) Assume that the shaft and key are madeof ductile materials having the samestrength and that (in accordance with thedistortion energy theory—Section 6.8),𝑆𝑠𝑦 = 0.58𝑆𝑦.
10.2 KEYS
10.2 KEYS
10.3 PINS
10.3 PINS
10.4 SNAP RINGS
10.5 SPLINES
10.6 COUPLING
The problems of a small amount of shaftmisalignment can be eliminated by using a flexiblecoupling.
10.6 COUPLING
Universal joints permit substantialangular misalignment of shafts havingintersecting axes. Figure 17.14 showsthe common cross type (known asCardan joint or Hooke’s joint) that isusually used at the ends of the driveshaft on rear wheel-drive automobiles.Plain bushings or needle bearings areused at the yoke to-cross connections.
One ingenious design of fairlyancient origin is the Oldhamcoupling, shown in Figure 17.13.The sliding of the center blockpermits a substantial amount ofshaft lateral offset, and built-inaxial clearance permits someangular misalignment.
10.7 SHAFT DEFLECTION
*Refer to page 206-209
10.7 SHAFT DEFLECTION
*Refer to page 206-209
10.7 SHAFT DEFLECTION
*Refer to page 206-209
10.8 OVERALL SHAFT DESIGN
The following general principles should be kept in mind.
i. Keep shafts as short as possible, with bearings close to the applied loads. Thisreduces deflections and bending moments and increases critical speeds.
ii. Place necessary stress raisers away from highly stressed shaft regions ifpossible. If not possible, use generous radii and good surface finishes. Considerlocal surface-strengthening processes (as shot peening or cold rolling).
iii. Use inexpensive steels for deflection-critical shafts, as all steels have essentiallythe same elastic modulus.
iv. When weight is critical, consider hollow shafts. For example, propeller shaftson rear-wheel-drive cars are made of tubing in order to obtain the low weight–stiffness ratio needed to keep critical speeds above the operating range.
10.8 OVERALL SHAFT DESIGN
The maximum allowable deflection of a shaft is usually determined by critical speed, gear,or bearing requirements. Critical speed requirements vary greatly with the specificapplication. Allowable shaft deflections for satisfactory gear and bearing performance varywith the gear or bearing design and with the application, but the following can be used as ageneral guide.
i. Deflections should not cause mating gear teeth to separate more than about0.13 mm (0.005 in.), nor should they cause the relative slope of the gear axesto change more than about 0.03°. Allowable deflections recommended forbevel gears from 6 to 15 inches in diameter are listed in [10].
ii. The shaft (journal) deflection across a plain bearing must be small comparedto the thickness of the oil film. If the angular deflection of the shaft at thebearing is excessive, the shaft will bind unless the bearings are self-aligning.
iii. The shaft angular deflection at a ball or roller bearing should generally notexceed 0.04° unless the bearing is self-aligning.
Early in the design of any given shaft, an estimate is usually made of whether strength ordeflection will be the critical factor. A preliminary design is based on this criterion; then theremaining factor (deflection or strength) is checked.
10.8 OVERALL SHAFT DESIGN
Shaft and housing shoulder dimensions.
10.8 OVERALL SHAFT DESIGN
Extra Light Series
Light Series
Medium Series
10.8 OVERALL SHAFT DESIGN
Please consult to my chapter-2-slide for Static Failure Prevention Design and also my chapter-3-slide for Fatigue
Failure Prevention Design.
10.9 SAFETY STANDARD
10.10 SAMPLE PROBLEM
10.10 SAMPLE PROBLEM
10.10 SAMPLE PROBLEM
10.10 SAMPLE PROBLEM
10.10 SAMPLE PROBLEM
10.11 HOMEWORK