The disclosure relates to a hydrostatic traction drive in a closed hydraulic circuit and a method for controlling the hydrostatic traction drive.
A generic hydrostatic traction drive has a hydraulic pump and a hydraulic motor, which are connected to each other in a closed hydraulic circuit by means of two hydrostatic working lines. The hydraulic pump can be coupled to a drive machine, for example a diesel engine, and the hydraulic motor can be coupled for example to at least one wheel to be driven or at least one axle to be driven.
There are various methods for using the closed circuit of the traction drive not only for driving, but also for hydraulic braking. In particular, with slow moving vehicles it is known to support the hydraulic pump in the overrun mode of the drive machine in order to utilize the braking effect of the drive machine. The braking power that can be achieved in this way is however limited by the maximum possible braking power of the drive machine. It is therefore important to protect the drive machine against overload resulting from the support.
It is known from the publication DE 198 92 039 A1 to couple a second variable hydraulic pump to a drive shaft of the hydraulic pump. The same is disposed in an open circuit and is configured to feed in just one direction. In cases of a braking process, the hydraulic motor changes to pumping mode and thus drives the two hydraulic pumps. The pressure medium delivered by the second hydraulic pump is then released back into a tank by means of a pressure relief valve. The volumetric flow to the pressure relief valve is limited here by a valve, which feeds a greater volumetric flow to the pressure relief valve with increasing operation of a brake pedal. In this way part of the braking energy is no longer taken up by the drive machine, but is converted into heat at the pressure relief valve.
The publication EP 1960 699 A1 provides a solution with a pressure relief valve, by means of which braking energy can be converted into heat. However, the pressure relief valve is not disposed for this purpose in an extra open circuit provided with a hydraulic pump for this, but during braking pressure medium from the working line downstream of the hydraulic motor is released by means of the pressure relief valve into the other working line, which has the lower pressure, or is released back into the tank. Moreover, during braking the feed volume of the hydraulic pimp is reduced to a so-called braking feed volume and the suckback volume of the hydraulic motor is increased to a braking suckback volume that is dependent on the strength of the braking force demanded by the driver.
The solutions mentioned have in common that the activation of the hydraulic braking has to be actively carried out by the driver or operator.
The disadvantage is that said dependency on the operator/driver is a potential source of error and human failure cannot be ruled out. The discussed overload of the drive machine or accidents can thus occur as a result of late or insufficient deceleration.