What is a Shaft?
A (usually) rotating component for the
transmission of power.
The shaft may be hollow or solid, usually (but not only) of circular cross section.
Shafts can carry gears, pulleys and sprockets to transmit rotary motion and power via mating gears, belts and chains.
Press fit, keys, dowel, pins and splines are used to attach these machine elements on the shaft.
A shaft can be stationary and support a rotating member, such as the short shafts
that support the nondriven wheels of automobiles (spindles).
The shaft is generally acted upon by bending moment, torsion and axial force.
Classification of shafts:
Transmission shafts
Machine shafts
Transmission shafts:
Transmit power between the source and the
machines absorbing power.
- counter shafts
- line shafts
- overhead shafts, etc.
Machine shafts:
Form an integral part of the machine itself.
The crank shaft is an example of machine shaft.
Crankshafts:
- Convert linear to rotary motion or vice versa.
- Usually involve eccentric loads.
Camshafts:
Camshafts convert rotational motion into linear (usually reciprocating) motion.
The camshaft controls the opening and
closing of valves through the combustion
process, and the crankshaft takes the
energy from that process and connects it to
the rest of the vehicle.
Four stroke engine:
Intake stroke
Compression stroke
Power stroke
Exhaust stroke
Gearbox shafts:
- Input shaft
- Counter shaft
- Output (or main) shaft
- 1st gear
- 2nd gear
- 3rd gear
- Top gear (direct drive)
Design considerations:
Series of stepped diameters accommodating elements such as gears, sprockets and pulleys as well as keys to prevent rotation, relative to
the shaft, of mounted elements.
Design considerations examples:
- Size and spacing of components
- Material
- Deflection and rigidity (stiffness)
-Bending deflection (e.g. excessive lateral deflection causes gear teeth to disengage)
-Torsional deflection
-Slope at bearings(e.g. excessive angular deflection may cause bearings to fail)
-Shear deflection strength
- Stress and strength
-Static strength
-Fatigue
- Frequency response
-Critical speed
- Manufacturing constraints
Shaft Loads:
Axial Load
Torsional Load
Transverse Load
Stresses:
For bending: SEE EQUATION SHEET
For torsion: SEE EQUATION SHEET
Fatigue in crankshafts:
Fatigue failure occurs at stress levels below the yield stress of the material.
What is the ASME approximate equation for?
Determining shaft diameter.
SEE EQUATION SHEET
Factors of safety:
1.25 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
3.0 – 4.0
4.0 - ???
FOS 1.25 - 1.5 meaning:
Reliable materials under controlled conditions, subject to loads and stresses known with certainty.
FOS 1.5 - 2.0 meaning:
Well known materials under reasonably constant conditions, subject to known loads
and stresses.
FOS 2.0 - 2.5 meaning:
Average materials subjected to known loads and stresses.
FOS 2.5 - 3.0 meaning:
Less well-known materials under average conditions of load, stress, and environment.
FOS 3.0 - 4.0 meaning:
Untried materials under average conditions, or well-known materials under uncertain loads, stresses, and conditions.
FOS 4.0 - ??? meaning:
Untried materials under uncertain loads, (try never to be in this situation!)
Keys and Keyway parts:
Keyseat
Key
Keyway
Types of Circlips:
Basic internal circlip
Basic external circlip
Standard E clip
Light duty push-on fix
Heavy duty push-on fix
Splined shafts:
Straight radial splines
Involute splines
