Shaft
A shaft is a rotating or stationary component which is normally circular in section.
A shaft is normally designed to transfer torque from a driving device to a driven device.
If the shaft is rotating, it is transferring power and if the shaft operating without rotary
motion it is simply transmitting torque and is probably resisting the transfer of
power.
Mechanical components directly mounted on shafts include gears, couplings, pulleys,
cams, sprockets,links and flywheels. A shaft is normally supported on
bearings. The torque is normally transmitted to the mounted components
using pins, keys, clamping bushes, press fits, bonded joints and sometimes
welded connections are used.
Shafts are subject to combined loading including torque (shear loading),
bending (tensile & compressive loading), direct shear loading, tensile loading and
compressive loading. Design of shafts must include assessment of fatigue
loading and unstable loading when the shaft is rotating at critical speeds (whirling).
Note: For information the critical speed of shafts refer to webpage Shaft Critical Speeds
When designing a shaft the following staged procedure is normally adopted
- Produce a free-body sketch of the shaft. Replacing the various associated components with their equivalent load/torque
components
- Produce a bending moment diagram for the xy plane and the xz plane (x = shaft axis direction).
Note: The resulting internal
moment at any point along the shaft = Mx = Sqrt (Mxy2 + Mxz2 )
- Produce a torque diagram.
- Locate the section(s) on the shaft which the internal loading is the highest..This important stage requires significant effort and judgement
- Locate the point on the shaft which the internal loading is the highest. This important stage requires significant effort and judgement
- Assess the strength of the shaft and determine if the safety margin is sufficient.
The failure criteria (ref Failure theories for the shaft depends on the material selection (ductile/brittle) and consideration of the loading
regime, (constant/ variable loading, rotating speed, degree of shock loading )
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Indicative Transmittable Torque Values
This table is provided to allow comparison between shafts and is based on very simplistic assumptions with no allowance for fatigue, additional stresses to Bending Moments etc etc etc
| Shaft Dia |
Pure Torque |
Power (100RPM) |
| mm |
Nm |
kW |
| 30 |
132 |
1.4 |
| 40 |
313 |
3.3 |
| 50 |
612 |
6.4 |
| 60 |
1058 |
10.6 |
| 75 |
2068 |
21.6 |
| 80 |
2510 |
26 |
| 100 |
4900 |
51.3 |
Notes on table values
q = The skin torsion stress of a solid round shaft : T= The torque transmitted by the shaft : T = q.D3/5.1
The table is based on a torsion stress level of 25 N/mm2
Power transmitted by a shaft P = 2 * pi * T * N (N = Revs /sec)
Table power based on pure torque values
Methods of Locking/Driving items on Shafts
| Type |
Description |
Notes |
| KeyWay |
Reliable method based on a machined keyway in shaft and associated bore. Drive is via a fitted key. Results in increased local intensified stresses. A machined keyway results in a weaker shaft.(shaft dia. effectively reduced by 25%). Key can be deformed over time and result is a loose drive |
Keyways
Splines |
| Pin |
Very low cost option. Used for low torque drives. |
Circlips
Pins |
| Spline |
Expensive to manufacture with matching spline in shaft and bore. No additional components required. Allow relative axial movement. High torque capacity. |
Keyways
Splines |
| TaperLock Bush |
A tapered bush that fits into a tapered bore in the Gear/Coupling to be driven by the shaft. The locking action results from forcing the bush into the bore using axial screws. Tendency to stick in position. Limited shaft range (up to 80mm dia) |
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| Shaft Locking Lock Bush |
Similar principles to taperlock bush but has parallel bore and OD. More convenient to install. Much larger size range (up to 500mm dia). These are relatively expensive but required no special shaft machining. |
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| Interference Fit |
Requires accurate shaft and bore. Machined items are assembled using hydraulic press or using thermal methods e.g. Heating the female item or freezing the male item. Difficult to separate |
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| Adhesive |
Low Cost. High torques can be transmitted. No special machining of shaft or bore. Cannot easily be taken apart. Should not be used without extensive development/testing |
Adhesives |
| Hydraulic Locking Bush |
Similar application to locking bush. Large size range. Convenient to apply. No risk of sticking. Relatively expensive. |
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