静液压制动和泊系功能

5.4 Closed Circuit

The basic features of the closed circuit are:

x Each function has its own circuit and its own pump

x Both the direction and the flow (i.e. the speed of the movement) are controlled by the pump.

x There is no valve block. There may only exist a flow switch which allows to switch one pump between two circuits that never run at the same time.

Hoist

GFW + A6VM

Swivel drive

GFB + A2FE

Travel drive GFT + A6VE

Luffing GFW + A2FM

Hoist GFW + A6VM

Travel drive GFT + A6VE

Diesel engine

+ Variable pumps

A4VG Direction

+ Speed

Figure9:C osed circuit means that both the direction and the f ow (i.e. speed) is controlled by the pump.

Application Areas

Criteria that favor closed circuit applications are:

x Rotating movements (winches, swivel)

x High load capacities of the crane

x Heavy duty cargo handling (harbor mobile cranes, cargo-, marine-, offshore cranes) A closed circuit crane only becomes competitive with electronic control, i.e. microcontroller application. A hydraulic system will be too expensive.

Today, closed circuits are mainly used in bigger cranes:

x AT crane winches above 150 tons capacity,

slew drives above 80 tons capacity

x Crawler crane winches, slew and travel drives above 80 tons

capacity

x Deck cranes all movements in case of winch luffing

x Offshore cranes all movements in case of winch luffing

x ailway cranes with a hydrostatic drive unit running above 40

km/h

Advantages

x Smaller pumps because higher rpm rates are possible than in open circuit.

x No valve blocks. The pump controls flow and direction.

x No counterbalance valves. The load is supported by the Diesel engine via the pump. x Smaller oil tank. Only the boost volume has to be fed into the circuit (approximately 25% of maximum flow).

x Smaller cooler. The lowering energy is only partly converted into heat.

x Better fine control characteristic due to electronic control.

Disadvantages

x Number of independent movements depends on number of pumps installed.

x Increased piping work.

x High no-load hook speed only possible with bigger pump or variable motor.

x Diesel engine must be protected against overload condition

Consider

The Diesel engine must be protected against any overload condition. The returned energy while lowering load may be handled in different ways:

E ectric engines, e.g. on deck cranes of cargo ships, easily consume the lowering energy by generating electric energy and feeding it back into the ship’s electric circuit. Obviously, the electric system on board needs to be able to accept this additional electric energy.

All cranes with diese engine require a power balance calculation since the diesel engine can at maximum provide a counterbalance torque of 20% of the nominal drive torque. Since the load often exceeds this counterbalance torque, an additional brake pump becomes necessary.

Do not use counterbalance valves in a closed circuit. It is impossible to lead off the gen-erated heat. The closed circuit would quickly overheat.

5.4.1Central Boost and Cooling System

Although the A4VG pumps offer internal boost pumps, it is more economical and safer to install a central boost and cooling system:

One central boost pump may be of smaller size than the sum of three or four individual boost pumps in each circuit. This saves money both in purchasing and in reduced energy consumption.