Previous valve designs already satisfied the above demands for a crane slew drive and were systematically orientated towards motor-driven load requirements. A key advantage of the MSC16 slew drive module, Figure 3, over previous designs is pressure compensation downstream of the recirculation duct and the separation and option of a separate control system for meter-in and meter-out functionality. Downstream recirculation pressure compensation is similar to flow sharing. It is based on the principle of load-independent flow distribution (LIFD), Figure 4.

Here, the oil flow is parallel-partitioned between the circuit and the slew drive motor via two metering orifices (A1 and A2) with corresponding cross-sectional areas and curves. A load-recording circuit transmits the working pressure P2 to the pressure scales downstream of the recirculation flow so that the pressure after metering orifice A2 (recirculation) is the same as the working pressure. If production tolerances and friction in the pipes are neglected, the pump oil flow is partitioned as follows according to the cross sections of the metering orifices:
Q1 ÷ Q2 = A1 ÷ A2

Linear oil flow (corresponding to the spool piston cross-sectional configuration) is beneficial with this switching control system regardless of the pump's meter-in flow, and for the fine control range to remain constant. As described above, no reduction of resolution across the piston spool stroke occurs. Therefore, we can assume that:

Q 1 = flow rate with reduced pump drive speed,
Q 2 = flow rate with maximum pump drive speed, and
Q 3 = flow rate with additional pump summation.

An important feature of the slew drive module is that it accounts for negative loads, such as those encountered in the braking phase on slew drives. Meter-in and meter-out notches are consistently separated, and they can not be actuated separately. Because pressure differentials at the metering orifices depend on the pressures encountered during acceleration and deceleration, resolution of the metering notches by means of variable hydraulic loading can be employed to induce either optimized acceleration or cavitation-free deceleration. Pressure sensors on the slew drive motor can be used to realize further optimization, including driver-dependent and driver-selectable braking characteristics and torque regulation.

A float position can still be achieved by charging the two meter-out valves, without having to accept additional elements or compromise fine control.

Strightforward results

The MSC16 slew drive module contains all functions needed for a crane slew drive, Figure 5, and a typical system incorpoarting the valve is illustrated in Figure 6.  The MSC16 module provides a compact and cost-effective control system for slew drives. Because it was developed on the basis of fundamental hydraulic circuits, the module compensates for speed-related pump input and  load fluctuations on the slew drive caused by the moments of inertia. The result is fine control without additional extension of sensors and electric control circuits.

Electrohydraulic piloting means the slew drive module can be incorporated into existing vehicle designs and wiring and system architectures. Incorporating pressure sensors into the slew drive control subsystem produces the highest precision and range optimization of a crane. This future-orientated solution ensures that crane operator's requirements with respect to function, safety, and costs will be satisfied even as they become more demanding.

Contact Helmut Fischer, Bosch Rexroth AG., Lohr, Germany via e-mail at helmut.fischer@boschrexroth.de. Click here to view and download a PDF brochure from Bosch Rexroth: "Drive and Control Systems for Cranes."