A shaft is an elongated cylinder- shaped rotating machine element that is used to transmit rotary movements and torques . A shaft is usually supported by at least two pivot bearings in the machine frame. When transmitting torque, the shaft is subjected to torsion .
In contrast to a shaft, an axis is usually firmly mounted and does not transmit any rotary motion. It is therefore mainly subjected to bending.
The word wave comes from the Middle High and Middle Low German welle , wille for "roller" . This was derived from the developed Indo-European word * wel- with the meaning "walken", "walzen".
Hollow shafts are mainly used when weight is to be saved or the shaft is to have a high natural frequency . The space in the interior of hollow shafts can also be used, for example, for other components, further shafts or axles , or for the passage of coolant or flushing agent or the like. Hollow shafts are more complex to manufacture. A widespread application is the cardan shaft on automobiles and the hollow shaft drive on electric locomotives .
Flexible shafts are used when the element to be driven is movable (e.g. hand-held devices) and the drive source cannot or should not be moved with it, or when (with switching shafts or screwdrivers) the object to be moved cannot be reached directly. They consist of several layers of wires wound around each other in a helical manner (corrugated core), which rotate as a package in a metal and / or plastic protective hose. They are lubricated with grease .
Flexible shafts have a preferred direction of rotation, which depends on the direction of the top wire layer. The helix in the sense of a left-hand thread is ideal for transferring a right-hand rotation ( clockwise when viewed from behind the drive ), for example for drilling with a twist drill with the usual right-hand twist. In the opposite direction of rotation, only about 40-70% of the power can be transmitted, since a counter-rotating, coiled wire layer must be pulled, which is further inside and can therefore typically transmit less torque. Wire layers in tension develop a force that constricts the wire package. Wire layers that are subjected to compression want to move radially outwards, these make this tolerable limited when opposing coils absorb the torque on tension.
The connection dimensions are standardized in DIN 42995.
Applications: e.g. B. Precision mechanics devices and feeders for analog fittings ( speedometer , rev counter ). The drills at the dentist are now usually pneumatically driven - without a flexible shaft - as this allows higher speeds to be achieved.
Above all for reasons of cost or due to limited installation space, flexible shafts are also used to compensate for a fixed angular offset as an alternative to universal joints or deflection gears. Examples of this are brushcutters or simple delta grinders .
In a piston engine, a crankshaft converts a straight back and forth movement into a rotating movement ( crank drive ). With the reciprocating compressor there is the reverse conversion of motion.
The shaft offset corresponding to half the stroke is typical.
Cardan shafts are used:
- for the transmission of torques with non-aligned axes of rotation
- for the transmission of torques with parts moving against each other
Cardan shafts are used when flexible couplings due to Hardy disks , elastomer couplings or drive articulated disks are no longer sufficient to allow a large angular misalignment or to transmit very high power. A cardan shaft usually consists of two connection flanges, one or two joints and a rigid or adjustable longitudinal rod. Depending on the load from the drive, in addition to the excitation of vibrations , control loop frequencies and imbalance also add up . Special attention must be paid to the resonance frequency when designing cardan shafts . By choosing damping elements with different elastomer hardnesses, the vibration load can be influenced and limited. Depending on the application, intermediate pipes are selected from aluminum, steel, stainless steel or from carbon fiber reinforced plastic (CFRP).
The constant velocity shaft is a special form of the cardan shaft that does not produce any irregularities in the transmission of the rotary movement, even if bending angles occur at the joint. The best-known area of application for synchronous shafts are drive shafts on vehicle wheels, which also serve for steering. Due to the greater angling of the joint when the steering is turned, a noticeable uneven running would occur when using universal joints . Elastomeric joint disks such as Hardy disks cannot be used with a larger angular misalignment. However, these can be used elsewhere as constant velocity joints .
Cardan shafts are widely used in vehicle construction and drive technology. If the shaft only contains one universal joint , however, when the joint is angled, a certain uneven running occurs because there are speed differences between the input and output.
Double cardan shaft
Cardan shafts with two universal joints are used in many motor vehicles. If the joints are installed rotated by 90 °, the uneven angular speeds compensate for each other. The input and output always have the same speed as long as both shafts are parallel. Since the parallel position is not always given when driving steered front wheels, the drive would jerk when driving through narrow bends. If the axis is offset, the intermediate shaft also runs irregularly, which can lead to vibrations. An offset occurs when the input and output shafts are parallel but not in one axis, for example when a vehicle with a rigid rear axle is compressed.
In modern synchronous shafts, homokinetic ball joints are used, in which the angular velocity of the system does not change with the angle. For the drive of the front wheels, a joint according to Alfred Hans Rzeppa is used on the outside of the wheel side , which allows large angular deviations of the axle journal (typically bending angles of up to 47 ° or 50 ° are allowed); on the inside of the differential gear , a sliding joint is usually used that only allows moderate angular deviations of up to about 22 ° (or 31 ° as special versions), but allows the wavelength to be changed by about 45 mm, which is necessary to compensate for the differences when the vehicle is deflected. Alternatively, the front-wheel drive can be equipped with two fixed joints, which are connected to a separate sliding unit. With this, flexion angles of 35 ° and more can also be achieved on the differential side. This variant is particularly suitable for front-wheel drive vehicles with transversely installed engines, since the drive shafts are of unequal length with an off-center differential. In principle, the same designs can be used to drive the rear wheels. For rear-wheel drive sports cars and upper-class vehicles, it is preferred to use synchronous shafts that each have a spherical sliding joint of the so-called type VL (sliding joint Löbro) on both the wheel and the differential side. With these joints, the torsional backlash can be significantly restricted by a special design and construction.
A remote shaft end on which a rotating component is attached is called a shaft journal or (in vehicles) axle journal.
- ^ Kluge, Etymological Dictionary, 23rd edition, 1999, p. 884 "Welle", p. 873 "wallen", p. 874 "Walze"
- Hans-Christoph von Seherr-Thoss, Erich Aucktor, Friedrich Schmelz: joints and cardan shafts. Calculation, design, applications. 2nd, expanded edition. Springer, Berlin et al. 2002, ISBN 3-540-41759-1 .