A wheel of a motor vehicle typically has a wheel hub that is rotatably supported on a wheel carrier (e.g. an axle arm or an axle funnel). A tire pressure regulation apparatus is provided for some applications, for example for heavy commercial vehicles, in order to set the air pressure in the tire mounted on the wheel hub to a desired value. In order to be able to supply the tire with the compressed air required for this purpose, the compressed air has to be transferred, for example starting from a central compressed air source, from the respective wheel carrier to the wheel hub supported thereat. The same applies accordingly in the opposite direction for deflating the tire. This is achieved by the rotary feedthrough which comprises a substantially ring-shaped intermediate space between the wheel carrier and the wheel hub in which a compressed air annular space is formed. It communicates both with a compressed air passage provided in the wheel carrier and with a compressed air passage provided in the wheel hub. In this manner, the named compressed air passages are also connected to one another when the wheel hub is rotating so that compressed air can be supplied to the tire via the compressed air passages, independently of the rotational position of the wheel hub.
Very high pressures are consequently generated for the desired tire pressure regulation, for example, up to approximately 7 bar. A compressed air annular space formed in the intermediate space between the wheel carrier and the wheel hub must therefore be reliably sealed with respect to the environment and in particular with respect to the remaining intermediate space. This can take place, for instance, by means of sealing rings, in particular by means of radial shaft sealing rings, which bound the compressed air annular space at both sides and in particular with respect to annular lubricant spaces which are formed in the remaining intermediate space and which can serve, for example, for lubricating the sealing rings and/or bearings for supporting the wheel hub at the wheel carrier.
So that the pressure in the tire of a respective motor vehicle wheel can be varied, controllable valves are employed at the wheel hub, that is, in particular valves that can be selectively controlled either to let compressed air pass through into a respective filling direction or deflating direction or to block the passage of compressed air. In this respect, systems for regulating tire pressure can, for example, be configured as single-passage systems, i.e. having only a single compressed air passage in the wheel hub which leads to the valve, or as multi-passage systems in particular two-passage systems.
With two-passage systems, a compressed air passage in the wheel hub additionally leading to a valve exclusively serves for controlling the switch position of the valve. Two-passage systems have the advantage that a standard valve can be used at the tire whose air pressure should be able to be regulated. As a rule, such a standard valve comprises a compressed air port, a working port (2/2-way valve) and optionally a venting connection (3/2-way valve). For controlling the valve, that is in particular for blocking or (directed) opening of the valve, the valve can also have a control port which is actuated via the additional compressed air passage. In this respect, the valve can be selectively controlled via the pressure applied to the control port or via the power hereby caused at the control port to connect the compressed air port or the venting connection to the working port for the passage of compressed air or to block these connections with respect to one another. In so doing, the valve is preferably preloaded in a blocking state in order to maintain the tire pressure on a failure of the pressure or of the power at the control port.
The disadvantage of such a two-passage system is that the rotary feedthrough must comprise at least two compressed air passages in the wheel hub so that at least two compressed air annular spaces must also be formed in the intermediate space between the wheel carrier and the wheel hub in order to connect the compressed air passages of the wheel hub to associated compressed air passages of the wheel carrier. These compressed air annular spaces, which can have completely different pressures, must be reliably sealed, in particular also with respect to one another; for instance, in that respective sealing rings are provided between them and at the two sides.
To minimize the wear of the sealing rings sliding along the wheel carrier or the wheel hub, lubricants, for example lubricating grease or lubricating oil, can be guided through lubricant passages from the wheel carrier into annular lubricant spaces which are provided in the named intermediate space between the wheel carrier and the wheel hub in addition to the compressed air annular spaces. The lubricant can hereby move to the respective sealing rings bounding the respective annular lubricant space. The wear can admittedly thereby be reduced, but not the comparatively high number of wear parts. Such an arrangement also requires a relatively large construction depth (extent of the named intermediate space along the direction of arrangement of the sealing rings).
A rotary feedthrough of such a two-passage system is known from the laying-open document DE 10 2009 057 158 A1.
In contrast, the valve position in single-passage systems is controlled by pressure impulses within the compressed air passage of the wheel hub. Single-passage systems can thus admittedly have a smaller construction depth of the named intermediate space and fewer wear parts since only a single compressed air passage is provided in the wheel hub for supplying the tire with compressed air and only a single compressed air annular space therefore also has to be formed in the intermediate space between the wheel carrier and the wheel hub. However, a single-passage system requires the use of a special valve which can only be controlled by means of the compressed air port which thus simultaneously functions as a compressed air inlet and a control port. However, such a special valve causes higher costs and necessitates a more complex control of the air pressure in the compressed air passage for a defined generation of the control pressure bursts. Furthermore, there is the risk of a certain proneness to disturbance.