Types of shafts in mechanical engineering - Linear shafts, rotating shafts, rods, and how to make the right choice

Shafts, axles, and rods are important components in mechanical engineering. Although they may look identical, they differ significantly in their function. In their basic function, machine elements referred to as shafts transmit rotational motion and torque. In contrast, shafts do not transmit torque. The linear shaft is a special case here. Linear shafts serve as guide shafts and do not transmit torque; however, the name “linear shaft” has become established for this component in the market. The main function of a rod is to act as a connecting element, transmitting forces between components and joining these components together.

Shafts as mechanical components

What are rotating shafts?

Shafts are typically used in applications where rotational motion and torque are transmitted. Typical applications include motors, machine tools, and fans.

A rotating shaft, also known as a motor shaft or torsion shaft, rotates and is held in place by shaft bearings. It transmits torque generated by, for example, gears or a coupling to another component such as a pulley. The rotating shaft (motor shaft) is primarily subjected to torsional stress. In most cases, in addition to the torque, additional transverse forces and thus bending stress on the shaft also occur. Dimensioning is therefore based on the torsional stress, although any bending stress that may occur must also be taken into account.

Rotary shafts are usually made of metal, but can also be made of plastic for lower loads. When designing a rotary shaft, strength, stiffness, vibration behavior, and expected wear play a significant role.

Rotary shafts are available in various designs and dimensions. When designing the system, attention should be paid not only to the shaft and its required minimum cross-section but also to the appropriately sized and correctly aligned shaft bearings. Click here to learn more about optimal bearings for rotary and torsional shafts.

What is a torque shaft?

Torque shafts, also known as ball splines, combine the function of torque transmission with that of a guide shaft. The rolling elements of the linear bearing, which run in longitudinal grooves in the torque shaft, prevent twisting, thereby allowing torque to be transmitted to the shaft. At the same time, the torque shaft provides the linear guidance of a linear shaft. Due to its design, the amount of torque that can be transmitted is usually lower than with pure torsion shafts.

A torque shaft should primarily be designed to withstand bending stress. While it is possible to transmit torque through the shaft, the transfer of torque from the linear bearing to the torque shaft—and vice versa—is limited by the design of the system.

When sizing the system, both the permissible torque and the resulting bending stress must be taken into account. However, vibration behavior and potential wear also play a significant role in the design.

What is a linear shaft?

Unlike a rotary shaft or torque shaft, a linear shaft does not rotate on its own but serves as a precise guide for a linear bearing. Linear shafts are used purely as guide shafts and do not transmit torque.

Linear shafts are typically made of metals with a hardened surface. Their sizing and design are determined in conjunction with the linear bearing, based on bending stress and the required precision. Depending on the application and environmental conditions, the linear shaft may be used with a linear ball bearing or a plain bearing. However, vibration behavior and wear should also be taken into account when designing a linear shaft.

What is an axis?

There are stationary and rotating shafts. What they all have in common is that they do not transmit torque. Shafts are used as elements for supporting and housing components such as pulleys or wheels. Consequently, shafts are primarily subjected to bending stresses caused by transverse forces.

What are rods?

Both rods and bars are manufactured as so-called bar stock. They have an elongated shape with a uniform cross-section throughout. Colloquially, the term "bar" refers to the semi-finished product. The distinction from a rod is not so clear-cut, but rods are generally shorter and have a smaller cross-section relative to their length than bars.

Applications of Axles and Shafts Based on Their Function

Key Terms - A Quick Guide to Types of Waves

• Linear shafts are suitable for simple, linear movements. They serve, so to speak, to guide or act as a rail for attached parts, such as plain bearings. They primarily absorb bending forces.

1= Linear shaft / 2= Linear bearing

• Rotary shafts are suitable for rotational movements. For example, the rotating shaft between the motor and the gearbox. They primarily absorb torsional forces.

1= Rotary shafts

• Rods are suitable for rigid fastenings and connections of individual components.

1= Rods / Spacers