Why most of the propeller shafts are hollow? Does hollow shaft have more strength than solid shaft?

A hollow shaft has greater strength and stiffness than a solid shaft of equal weight. So most of the propeller shafts are hollow.

The strength of a shaft implies the maximum torque it can transmit. It depends on Torsional rigidity of the shaft since it renders the stiffness to the shaft. Torsional rigidity is defined as the torque required to produce a twist of one radian per unit length of the shaft. and by torsional formula:

T/(θ/L)=GJ=Torsional rigidity.

So, Stiffness and hence strength is proportional to J which is:
by maths, less for solid shaft.


Stress distribution of solid vs hollow shaft.

Hollow shafts are much better to take torsional loads compared to solid shafts. As shown in the figure, shear stress in a “shaft subjected to torsion” varies linearly from zero at the center to the maximum at the boundary.

But Inside a solid shaft, most of the material experiences / carries a shear stress whose value is much below the maximum shear stress [Interior portion of the shaft]. But at the same they are adding to the weight, without contributing much to the capability of the shaft to carry torsional load.

For the resisting the torsion load, the polar moment of area is considered as the resisting area, so if the polar moment of inertia of any shape is more it can withstand more torque  compared to others.

For the same area, Hollow shafts are having more polar moment of inertia, thus they can transmit more torque compared to solid shafts.

Advantages of hollow shaft.

• The stiffness of the hollow shaft is more than the solid shaft with same weight.
• The strength of the hollow shaft is more than the solid shaft with same weight.
• The natural frequency of hollow shaft is higher than the solid shaft with same weight.
• Hollow shafts are having more polar moment of inertia, thus they can transmit more torque compared to solid shafts.
• Hollow shaft is more stronger for same weight of solid shaft.

Disadvantages.

• Hollow shaft is costlier than solid shaft.
• Difficult to manufacture.
• The diameter of hollow shaft is more than solid shaft and require more space.
• Hollow shafts do not transfer more power but power to weight ratio of hollow shafts is more as compare to solid shaft.
• Solid shafts when subjected to bending are stronger than that of hallow shaft.
Depends on the application of the shaft :
For a load bearing type of shaft, then its always better to choose hollow shaft because it has higher stiffness and rigidity and can resist slightly higher bending moments. Used in propeller shafts.
A shaft for torque transmission like crank shafts, drive shafts it is always better to choose a solid shaft because it has higher torsional stiffness