The invention relates to a wheel hub unit comprising a wheel hub, a hydraulic motor and at least one bearing for supporting the wheel hub and the hydraulic motor.
Hydraulic motors for driving wheels of a vehicle are generally known in the art. According to a known wheel hub unit, a steering knuckle is pivotably connected to a kingpin and the hydraulic motor is attached to the steering knuckle at an opposite end of the steering knuckle in relation to the kingpin. The steering knuckle has a cylindrical shape and comprises interior channels for transmitting hydraulic oil to the hydraulic motor. Two bearings are arranged around the steering knuckle at an axial distance from each other.
A typical range of operation of a hydraulic motor is when the vehicle is running at a speed below a speed limit, for instance below 30 km/h. At higher speeds the hydraulic motor is in a freewheeling mode and a counter pressure is applied for disengaging the pistons in the hydraulic motor. When the vehicle decelerates and the vehicle speed passes said speed limit, the hydraulic motor can be engaged again. At the transition between the freewheeling mode and the operation mode pressure spikes in the millisecond range can occur in which the peak pressure is by far exceeding the normal pressure during operation. Such pressure spikes can reach 100 bar, for instance.
According to the known wheel hub unit, a restriction is provided between the bearings and the bearing which is arranged closest to the hydraulic motor is adapted to let the hydraulic oil pass through to said restriction during said pressure spikes. One problem with this design is that the bearing which is arranged closest to the hydraulic motor will have a relatively short life.
It is desirable to provide a wheel hub unit comprising a hydraulic motor, which creates conditions for a long life of the wheel hub unit.
According to an aspect of the present invention, a wheel hub unit comprises a wheel hub, a hydraulic motor and at least one bearing for supporting the wheel hub and the hydraulic motor characterized in that the wheel hub unit comprises a first seal for protecting the at least one bearing from hydraulic oil from the hydraulic motor.
Advantageously, by protecting the at least one bearing of the wheel hub unit with the first seal the bearing is not exposed to high pressure lubricant such as hydraulic oil and is particularly not exposed to pressure spikes with high peak pressure. Further, it creates conditions for using a conventional wheel bearing which employs grease as lubricant instead of hydraulic oil from the hydraulic motor. Grease greased bearings can be sealed to the ambient with well-proven seals. Grease greased bearings have favourably a low friction and less abrasive particles are present in the bearing, thus reducing the fatigue of the bearings. Leakage of hydraulic oil through the seal of the bearing or complex seals for sealing the bearing against the ambient can be avoided.
According to a favourable embodiment of the invention, the wheel hub unit comprises means for protecting the first seal from pressure spikes transmittable by the hydraulic oil of the hydraulic motor. This arrangement creates conditions for using soft seals for sealing the bearing section despite high pressure spikes during operation of the hydraulic motor.
According to a favourable embodiment according to the invention, the protection means may comprise a passage between the hydraulic motor and the first seal, wherein hydraulic oil may enter said passage. The passage may be configured for protecting the first seal from pressure spikes transmittable by the hydraulic oil of the hydraulic motor when passing from a freewheeling mode at high speeds to an operation mode at a speed range where the hydraulic motor is operative. The hydraulic oil passage may comprise a region which causes a change of direction of the hydraulic oil for protecting the first seal from said pressure spikes particularly when entering the oil passage in a pressure spike. By changing the direction of the hydraulic oil entering the passage during a pressure spike counteracts the effects of the pressure spike and reduces stress to the components in the oil passage. The change of direction is preferably a relatively sharp change, such as a 90° bend of the passage. The change of direction is preferably at least a 45° bend, and specifically at least a 60° bend of the passage. The passage may be configured with various means, such as for instance, a long extension of the passage, sealing means, restrictions in the passage. Favourably, by protecting the first seal from pressure spikes the first seal can be a standard soft seal. By providing a region causing a change of direction in the hydraulic oil passage a pressure load of the first seal and the effect of pressure spikes can be effectively reduced in the hydraulic oil passage. The hydraulic oil passage may be arranged so that an entrance of the hydraulic oil passage at the hydraulic motor is positioned at a different radial position than an exit of the hydraulic oil passage at the first sealing. Advantageously, the hydraulic oil may enter the hydraulic motor in a central region of the steering knuckle and the exit may be provided at an outer surface of the steering knuckle, resulting in a compact arrangement.
According to a further favourable embodiment according to the invention, the hydraulic oil passage may comprise a first section extending in an axial direction of the wheel hub unit and a second section extending in a radial direction of the wheel hub unit. At the transition between the first and second section the hydraulic oil has to change its direction thus reducing detrimental effects of a pressure spike in the oil passage. The hydraulic oil passage can be designed in an expedient way to reduce pressure spikes.
According to another favourable embodiment according to the invention, the protection means may comprise a second seal which is arranged between the hydraulic motor and the first seal for protecting the first seal from pressure spikes transmittable by the hydraulic oil of the hydraulic motor. Expediently, the second seal may be arranged in said hydraulic oil passage. The second seal can separate the hydraulic oil passage into a high pressure region at one side of the second seal away from the first seal and a low pressure region between the second seal and the first seal. Favourably, the protection of the first seal is improved by the second seal which may be conditioned to seal against a large pressure difference, and the axial length of the hydraulic oil passage can be reduced. This results in a compact arrangement of the wheel hub unit.
According to a favourable embodiment according to the invention, a first drainage channel may be arranged between the first seal and the second seal. Additionally or optionally a second drainage channel may be arranged in front of the second seal. The first drainage channel between the first and the second seal additionally reduces the action of pressure spikes which may pass the second seal. Such oil is discharged to the first drainage channel. The second drainage channel in front of the second seal further reduces an influence of a pressure spike on the first seal. Hydraulic oil entering the hydraulic oil passage is discharged to the second drainage channel thus reducing the pressure load of the second seal. Favourably, the both drainage channels can join in the same drainage channel in the knuckle on which the wheel hub unit is arranged. The first and second drainage channels can be embodied with of without a flow restriction for the hydraulic oil. Expediently, the first drainage channel may be equipped with a restriction device in order to reduce or avoid a pressure load from the joint drainage channel in the knuckle when oil in a pressure spike is discharged to the second drainage channel.
According to a favourable embodiment of the invention, the second seal can comprise a soft seal portion. Expediently, the soft seal portion can provide a defined leakage thus easing requirements for the design of the second seal for a reduction of a pressure spike. By providing a defined leakage, the pressure of the hydraulic oil between the first and the second seal can be controlled. A complex and expensive seal arrangement for providing a fluid tight second seal can be avoided.
According to a favourable embodiment according to the invention, the soft seal portion can be arranged at a radial inner side of a housing which is connected to the wheel hub and configured to rotate with the wheel hub. It is of advantage that the housing can hold the second seal securely in place even during very high pressure spikes. The housing can also take over load caused by the pressure spike thus protecting the second seal from direct actions of the pressure spike. The soft seal portion is configured to rotate with the housing.
According to a favourable embodiment according to the invention, the soft seal portion can be arranged around a spacer ring which can be connected in a rotatably fixed manner to a steering knuckle. The same spacer ring can be used to distribute hydraulic oil into a high pressure section of the hydraulic oil passage. A compact arrangement is achieved.
According to a favourable embodiment according to the invention, the second seal can comprise a leakage channel with a predefined leakage. In an advantageous development, the leakage channel can be arranged between the soft seal portion and the spacer ring. Expediently, a defined pressure level can be established between the second seal and the first seal by a proper design of the leakage channel. As the soft seal portion can rotate with the housing, hydraulic oil in the leakage channel is at the same time a lubricant for the soft seal portion during rotation.
According to a further favourable embodiment according to the invention, a radial inner surface of the first seal is positioned on a radial different, preferably radial larger distance from a wheel hub rotational axis than the leakage channel. The radial different position of the leakage channel with respect to the first seal is of advantage because if pressure spikes transit the channel in unfavourable circumstances, an oil jet may be generated which hits a wall portion of the hydraulic oil channel in the low pressure region of the oil channel and not the first seal. The radial larger distance from the wheel hub rotational axis is of advantage because the first discharge channel is at a radial smaller distance from the wheel hub rotational axis than the leakage channel. As a result, oil can be discharged to the first drainage channel without passing the first seal.
According to a favourable embodiment according to the invention, the wheel hub unit may comprise two bearings and the hydraulic motor may be arranged beside the two bearings in an axial direction of the wheel hub unit. It is of advantage that the two bearings can be constituents of a standard wheel bearing for wheels without a hydraulic motor.
According to a further favourable embodiment according to the invention, the at least one bearing can be formed by a taper rolling bearing. Advantageously, the at least one bearing can be constituents of a standard wheel bearing for wheels without a hydraulic motor.
According to another favourable embodiment according to the invention, the at least one bearing may be configured to be greased by grease. By lubricating the at least one bearing with grease a long lasting bearing arrangement with low friction and low fatigue can be achieved. As such a standard wheel bearing is sealed with soft seals, the invention provides a favourable combination of a hydraulic motor and a standard grease greased wheel bearing.
According to still another favourable embodiment according to the invention, the hydraulic motor may be formed by a radial piston hydraulic motor. A radial piston hydraulic motor has a compact size which is favourable for wheel motors.
According to a further favourable embodiment according to the invention, the first seal may be a soft seal. Favourably, the first seal can be a standard seal as the pressure differences on both sides of the first seal can be maintained comparably small.
The invention is particularly useful for commercial vehicles, e.g. tractor vehicles, or construction equipment where wheel motors are helpful for providing a sufficient maneuverability of the vehicle.