The invention relates to a hydraulic system for controlling or regulating a hydraulic cylinder. A hydraulic system of this type is known, for example, from EP 1 067 296 A1. The invention further relates to a hydraulic unit, a vehicle, a method and a use of the hydraulic unit.

The known hydraulic system is used in self-propelled forklift trucks which are to be operated in an energy-saving manner. A load-dependent pressure potential of the hydraulic fluid is used for this purpose. The known hydraulic system is concerned with the problem that a second supply pump is provided for actuating a consumer for an auxiliary function. In order to control the auxiliary functions, the known hydraulic cylinder has a complex control circuit, which results in throttle losses.

It is the object of the invention to provide a hydraulic system for controlling or regulating a hydraulic cylinder which has a simple, compact and secure structure. In addition, the hydraulic system should be suitable for electric-drive vehicles. It is further the object of the invention to provide a hydraulic unit, a vehicle, a method and the use of the hydraulic unit.

Specifically the object is solved by a hydraulic system for controlling or regulating a hydraulic cylinder comprising—at least one hydraulic cylinder and at least one hydraulic unit. The hydraulic cylinder can optionally be connectable to a pressure source and a tank by the hydraulic unit. The hydraulic system comprises at least one control or regulating device for controlling or regulating the supply of hydraulic fluid to the hydraulic cylinder. The control or regulating device forms a first assembly. The hydraulic unit forms a second assembly. The first and second assembly are structurally separate from one another and fluidically connected. The supply of hydraulic fluid to the hydraulic cylinder can be predominantly controlled or regulated by the control or regulating device from outside the hydraulic unit. The hydraulic unit is rigidly fastened on the hydraulic cylinder.

The invention has the advantage that the hydraulic device has a simple and compact structure and is secure against hose rupture. As a result of the simple structure and the division into hydraulic unit and control or regulating device, throttle losses during the raising process which in the prior art are caused by the control units usually integrated in the hydraulic unit are avoided. The control units are not required or only to a lesser extent in the invention. To this end, it is provided according to the invention that the two assemblies formed by the control or regulating device and the hydraulic unit are structurally separate from one another. The structural separation can for example be achieved by a spatial separation and mechanical delimiting of the assemblies. To this end the two assemblies can, for example, be accommodated in different, spatially separate, in particular spaced apart, housings. The assemblies form mechanically separate structural units which can be handled independently of one another, in particular which can be mounted independently of one another.

The hydraulic system according to the invention is space-saving and enables a flexible use of the available installation space because a simple control and regulating device can be accommodated centrally which is connected to one or more small simple hydraulic units which are mounted in a decentralized manner on the hydraulic cylinders. This avoids the fact that a larger installation space needs to be prepared at a single position of the vehicle. The hydraulic units can each be fitted with pressure storage devices for the suspension.

For the control or regulating function the two assemblies are fluidically connected to one another. This is accomplished, for example, by lines which interconnect the two structurally separate assembles. In other words, the two assemblies are on the one hand mechanically separate from one another and on the other hand fluidically connected to one another. In this case, the hydraulic unit is primarily responsible for the control/regulation of the outflow of hydraulic fluid from the hydraulic cylinder and the control and regulating devices are primarily responsible for the control/regulation of the inflow of hydraulic fluid to the hydraulic cylinder. According to the invention, it is therefore provided that the supply of hydraulic fluid to the hydraulic cylinder can predominantly be controlled or regulated from outside the hydraulic unit by the control or regulating device.

It is sufficient if the supply of hydraulic fluid to the hydraulic cylinder is predominantly controlled or regulated from outside the hydraulic unit. A maximum reduction in the throttle losses is achieved if the supply of hydraulic fluid, i.e. the lifting process is completely controlled or regulated from outside the hydraulic unit, wherein safety units, for example, a check valve can be integrated in the hydraulic unit. In extreme cases, the entire control or regulation of the supply is external. This means that in extreme cases at least one lifting branch of the hydraulic unit, i.e. the region of the hydraulic unit provided for the supply of the hydraulic fluid, is free from switching members, control members and regulating members.

Other regions of the hydraulic unit, for example a lowering branch of the hydraulic unit, which is used for the outflow of the hydraulic fluid has switching members, control members and regulating members for the hydraulic fluid.

Predominantly means that insignificant control activities in the lifting branch of the hydraulic unit, i.e. in the supply of hydraulic fluid to the hydraulic cylinder, are allowed. For example, up to 25%, in particular up to 20%, in particular up to 10%, in particular up to 5% of the supply power for the cylinder stroke can be controlled inside the hydraulic unit. At least 75% of the supply power is controlled or regulated from outside the hydraulic unit by the control or regulating device.

The rigid fastening of the hydraulic unit on the hydraulic cylinder has the advantage that a structural unit of hydraulic unit and hydraulic cylinder which can be handled uniformly is created, which has a robust, simple and particularly safe structure because flexible hose connections can be dispensed with between the hydraulic cylinder and the hydraulic unit. As a result safety against hose breakage is achieved. In a preferred embodiment the rigid fastening of the hydraulic unit on the hydraulic cylinder is achieved by arranging the hydraulic unit directly on the hydraulic cylinder and fluidically connecting to this. This reliably eliminates a hose breakage at this point. It is also possible that the hydraulic unit is spaced apart from the hydraulic cylinder by a gap and the fluid connection is made by fixed pipe pieces which bridge the gap.

The structural unit of hydraulic unit and hydraulic cylinder is mounted together and dismounted together or exchanged for maintenance purposes. The structurally separate control or regulating device is provided at a different spatial position of the hydraulic system.

The invention has the further advantage that the structurally separate control or regulating device can be coupled and operated with an electrical drive of the vehicle.

