Raising and lowering the load-fork of a hand fork-lift truck is normally performed with a hydraulically operated singled-sided lifting cylinder. The lifting cylinder is supplied with a piston pump which is actuated mechanically through a pumping action of a bar. The ratio, pump volume to displacement volume is normally designed so that the load can be raised with an acceptable number of pumping actions on the bar without the operator having to use too much power for lifting a full load. Consequently, this design presents a compromise between the acceptable number of pumping actions on one hand and the power to be used on the other. In many cases, the relatively large number of pumps is not acceptable however, especially with smaller loads. In most cases, the load fork also has to be raised by a certain amount before it takes on the weight of the load from underneath. The operator desires quicker lifting action for this distance.
A so-called quick-lift version is also planned for this kind of pump assembly therefore. The transmission ratio between the piston-pump and the lifting cylinder is reduced in this case. Switching from the quick-lift variation to the heavier ratio is necessary when taking on a certain load. Two different solutions are already known for this. For each of them, an additional valve is necessary. One embodiment has the lifting cylinder in two stages. In one operating version, a limited effective area takes on the pump pressure. In the second operating version, the first effective area is supplemented by a second effective area which is connected with the piston pump through a pilot controlled check-valve. The disadvantage in these cases, is the working length of the lifting cylinder which is almost double the length of standard lift cylinders.
A second solution reversing the pump volume. A reversing valve is situated between the piston chamber and the annulus collector of the piston pump and parallel to this is a check-valve. The reversing valve is controlled by the pressure in the output line of the piston chamber. If this achieves a certain value, the reversing valve opens and connects the piston chamber and annulus collector of the pump. The effective piston surface of the pump piston is therefore reduced to the difference of the effective surfaces in the piston chamber and annulus collector. In one popular solution, the described functionality is made effective in that a valve element is pressed against the sealing surface by a spring which is held in a hole in the pump piston. The hole is connected with the piston chamber and with the annulus collector with a radial opening. If pump pressure exceeds a predefined value, the valve element is opened and the medium flows into the annulus collector. The radial gap that is created here is very small so that significant throttle losses occur. The resulting, system-related loss of force that is characterized by the product from reversing pressure in the annulus collector and the difference from the base and rod surface of the pump piston. Since the reversing valve is opened and closed in time with the pump movement of the bar in this solution, the force that is required is relatively high in normal lifting operation compared with the load forces based on the described loss.