There can be no doubt that shafts - in all of their specialised types and constructions - are among the most fundamental components in mechanical engineering. It is their ability to conduct torque and support rotating parts that makes them so integral to just about any type of machine. In terms of its visual appearance, a shaft resembles an elongated rod or bar and is produced in a wide range of sizes. The material from which it is made is first and foremost selected to suit the intended application and the demands that it has to fulfil in use. Basic requirements such as material strength, hardenability and ductility are satisfied by various kinds of structural steel. Moreover, hardened & tempered or case hardening steels are mostly used in applications in which the shaft has to be able to withstand high load levels and fulfil additional requirements.
Both in terms of its structure and its function, the so-called hollow shaft is an alternative construction that features the same characteristics as the conventional model. However, in contrast to a typical, solid shaft, the fact that it is hollow gives it a number of positive attributes that make this type of shaft a popular and widespread element of many types of machine.
The primary advantage of the hollow shaft’s characteristic construction is the enormous weight saving that it brings about, which is advantageous not just from an engineering but also from a functional point of view. The actual hollow itself has another advantage - it saves space, as operating resources, media, or even mechanical elements such as axles and shafts can either be accommodated in it or they make use of the workspace as a channel. In addition, hollow shafts generally have a relatively high resonant frequency. The CanisDrive® series of servo drives from Harmonic Drive AG also have a large central hollow shaft, which is one of the main aspects of the product series’ varied application spectrum.
The process of producing a hollow shaft is far more complex than that of a conventional solid shaft. In addition to the wall thickness, material, occurring load and acting torque, dimensions such as diameter and length have a major influence on the hollow shaft’s stability. If, for instance, wall thickness and diameter are not mutually configured in a relation that complies with physical laws, the resulting construction may be unstable. In comparison with a solid shaft of the same diameter, the transmissible torque of the hollow shaft is only slightly smaller.
The hollow shaft constitutes an essential component of the hollow shaft motor, which is used in electrically powered vehicles, such as trains. Hollow shafts are also suitable for the construction of jigs and fixtures as well as automatic machines.
Although the terms shaft and axle are often used synonymously in everyday language, in the context of machinery elements, they have different characteristics – including in terms of their functionality. Unlike shafts, axles do not transmit torque, but merely provide support for rotating or oscillating parts. The main task of any axle is to function as a bearing for rotating mechanical components. They are generally mounted in the machine frame and are often subjected to great stress from cross forces and bending moments.
In one aspect of their functionality, axles and shafts do have something in common, however. They are both able to support other components. In contrast, only solid shafts and associated constructions such as hollow shafts are able to transmit torque. Another reason why the two elements are frequently confused is that they are similar in appearance - both axles and shafts have the basic shape of an elongated bar or rod.
The hollow shaft is not the only alternative constructional form. There are others, which are generally designed for specific fields of application and fulfil the requirements of these areas by virtue of their specific construction. One example of a specialised constructional form for shafts are so-called flexible shafts. As far as the basic function is concerned, they are no different to solid or hollow shafts, in that they are responsible for performing the electromechanical transmission of power. However, there is a significant difference in the construction of this type of shaft. A flexible shaft consists of a core wire, around which another wire, such as spring steel wire, is wound. These wires rotate as a cord, also referred to as a shaft core, surrounded by a protective hose. The particular advantage of the flexible shaft’s construction comes into play wherever linear power transmission is not possible or the drive and power take-off shafts are not adequately aligned. Portable operating equipment such as drills or grinders are frequently founded on flexible shafts, otherwise their scope of application would be far more limited.
The group of specialised constructions also includes the crankshaft. As a part of the crank drive, the crank shaft is responsible for the up-and-down motions of the pistons and for transforming these motions into torque. While present-day crankshafts are primarily found as components in combustion engines, their history actually goes back many centuries. The Hierapolis sawmill from the 3rd century AD is regarded as the first machine to transfer a rotational motion into a linear movement with the aid of a crankshaft or piston rod.
After the hollow shaft, flexible shaft and crankshaft, the drive shaft is another construction that has established itself in certain applications by virtue of its particular construction. It consists of a longitudinal rod, the length of which can vary according to the model, with articulated joints or connecting flanges attached to each end to create a dynamic joint. The distinguishing feature of a drive shaft is its precise torque transmission with spatially offset drives and outputs; it is also used for longitudinal adjustment and has proven to be an extremely durable alternative to couplings. The most common types of drive shaft construction are the Cardan shaft, the synchronising shaft and the double-joint drive shaft.