Press joining
Under press-fitting is defined as a mechanical connection technique .
The parts to be connected are manufactured in such a way that a press fit is created at the connection points after joining . This means that longitudinal and transverse forces can be transmitted in a non-positive manner. Excessive notch effects and long-term frictional loads have a negative impact on the load-bearing capacity of such connections . In order to counteract the notch effect, the shaft is made a few millimeters thicker than the rest of the shaft at the joint and the resulting shoulder is rounded off.
This principle is z. B. used to realize a shaft-hub connection as a so-called cylindrical interference fit. A distinction is made between longitudinal interference fit, cross compression fit and oil compression fit.
Longitudinal compression fit
The hub is pressed onto the shaft seat with high axial force (usually with a hydraulic press ). The mean roughness of the surface of the parts to be joined should be in the range of R z = 3… 10 μm. In terms of design, an insertion bevel or bevel is to be provided, which, however, should not be designed too generously, since it usually reduces the mating surface. A position limitation, for example in the form of a shaft shoulder, is to be provided for axial positioning. It should also be noted that reassembly leads to a reduction in the forces and torques that can be transmitted , as the surface roughness is smoothed during joining and thus the static friction is reduced. As a result, the force that can be transmitted is also reduced compared to an equally large cross compression fit.
Transverse interference fit
Before assembly, the hub is heated and / or the shaft is cooled. As a result, the hub expands or the shaft shrinks, which enables the joining of the two parts with no or greatly reduced force. During the subsequent temperature equalization, the pressure is set, whereby the surface roughness is largely retained, which results in a much tighter fit than with the longitudinal interference fit. The process is then referred to as shrink fitting or cold stretching .
Oil interference fit
The process is a further development of the longitudinal press association. In contrast to this, the shaft and hub must be tapered within a few degrees . In addition, one of the two components has an access hole in the mating surface. During assembly , oil is injected into this hole under high pressure . The oil pressure usually acts on an engineered ring groove. The fluid pressure causes elastic deformation, i. H. the inside diameter decreases and the outside diameter increases. The resulting gap fills with the pressurized oil and thus acts as a separating sliding film between the components, which greatly reduces the axial assembly force required. When the desired sliding path onto the cone is reached, the oil supply is switched off. Due to the elasticity of the materials, the original diameter ratios are restored, the separating oil film is squeezed out and the parts sit firmly. The axial fixation can then also be ended. In addition to the oil bore, annular grooves or spiral-shaped depressions are often also made in the mating surface with the access hole in order to enable better distribution of the oil in the mating surface and also to accelerate the escape of the oil after the push-on operation. One advantage of the oil press fitting is that it is relatively easy to dismantle. If oil is pressed between the mating surfaces again, thus reducing the preload and thus the static friction , the conical surfaces can slide off one another again.
General
Advantages of press joining are function-related good self-centering, and this type of connection is also suitable for shock and alternating loads . The longitudinal interference fit is also characterized by the low overall costs of the shaft-hub connection .
It often proves to be disadvantageous that press-joined parts are difficult to loosen or adjust.