Short Answer:

A solid shaft is a cylindrical rod with no internal cavity, while a hollow shaft has a circular hole along its axis. Both are used to transmit torque, but they differ in strength, weight, and material usage.

In simple words, a solid shaft is heavier and consumes more material, while a hollow shaft is lighter but can transmit almost the same torque. Due to better strength-to-weight ratio, hollow shafts are preferred in modern mechanical systems such as automobiles and aircrafts for efficient performance and material saving.

Difference Between Solid and Hollow Shafts

Detailed Explanation :

Shafts are essential machine elements used to transmit power or rotational motion from one point to another. They are subjected to torsion, bending, and sometimes axial loads. Based on their cross-sectional structure, shafts are mainly of two types — solid shafts and hollow shafts. The difference between the two lies in their geometry, weight, and torsional strength characteristics.

1. Solid Shaft

A solid shaft is a full cylindrical rod without any hollow portion. It is simple to manufacture and widely used in applications where torque transmission and rigidity are more important than weight reduction.

Characteristics of a solid shaft:

• It is made of a single solid circular cross-section.
• The entire cross-sectional area resists the applied torque.
• Polar moment of inertia, , for a solid shaft of diameter  is given by:

• The maximum shear stress in the shaft is:

• It is heavy and consumes more material but offers high rigidity and strength.

Applications:
Solid shafts are generally used in low-speed machines, shafts of small diameters, and where weight saving is not critical, such as machine tools, industrial rollers, and simple transmission systems.

2. Hollow Shaft

A hollow shaft has an empty cylindrical cavity (hole) along its axis. It is designed to provide the same torque capacity as a solid shaft but with less material and weight.

Characteristics of a hollow shaft:

• It has an outer diameter  and an inner diameter .
• Polar moment of inertia, , for a hollow circular section is:

• The maximum shear stress at the outer surface is:

• Material is removed from the central region, where shear stress is minimum, resulting in lightweight but strong shafts.

Applications:
Hollow shafts are used in automobiles, aircrafts, marine drive systems, turbines, and power transmission lines, where weight reduction and high torque-to-weight ratio are crucial.

3. Mathematical Comparison

Let the outer diameter of the hollow shaft be the same as that of the solid shaft . The strength (torque capacity) of both shafts is proportional to their polar moment of inertia .

For solid shaft:

For hollow shaft:

Hence,

If the ratio , then

This means a hollow shaft with 60% internal diameter has 87% strength of a solid shaft but uses only about 64% of the material, giving a much better strength-to-weight ratio.

4. Weight Comparison

The weight of a shaft depends on its cross-sectional area  and material density :

For a hollow shaft, area (and hence weight) is less than that of a solid shaft of the same outer diameter. This makes the hollow shaft lighter and more efficient in design.

Example:
If ,

Hence, the hollow shaft weighs 36% less than the solid shaft while maintaining almost the same torsional strength.

5. Efficiency in Power Transmission

The power transmitted (P) by a shaft is given by:

where  = speed in rpm,  = torque (N·m).

Since a hollow shaft can transmit nearly the same torque as a solid shaft but with less mass, it results in higher specific strength and efficiency. This makes it preferable in rotating machinery and vehicles where dynamic balancing and vibration reduction are vital.

6. Practical Advantages and Disadvantages

Solid Shaft:

• Advantages: Simple to manufacture, rigid, suitable for smaller diameters.
• Disadvantages: Heavier, consumes more material, less efficient for weight-sensitive systems.

Hollow Shaft:

• Advantages: Lightweight, high torsional stiffness per unit weight, efficient in material use.
• Disadvantages: Difficult and costly to manufacture, less rigid for same outer diameter.

Thus, engineers select between solid and hollow shafts depending on design requirements, manufacturing feasibility, and cost constraints.

7. Applications in Industries

• Solid shafts: Used in lathes, milling machines, and light mechanical equipment.
• Hollow shafts: Used in aircraft drive systems, marine propeller shafts, turbines, and automotive drive shafts to reduce weight without compromising torque capacity.

Conclusion

In conclusion, both solid and hollow shafts serve the same function of transmitting torque, but they differ in weight, efficiency, and material usage. A solid shaft is heavier and easier to produce, while a hollow shaft is lighter and more efficient for torque transmission due to its better strength-to-weight ratio. Hence, in modern engineering designs where weight and performance are critical, hollow shafts are preferred for their superior efficiency and torsional characteristics.