Off-Line Filter Circuit
By using a pump and an electric motor specialized for filtration, this circuit filters contaminants even when the main hydraulic pump is not turned on. This circuit has the best filtration of all the filtration circuits.
Pressure Line Bleed-Off Filter Circuit
This circuit filters a small amount of the by-passed outlet flow from the pump (about 1 to 2 L/min (0.26~0.53 U.S.GPM)).
Return Line Filter Circuit
This circuit filters working fluid going back to the reservoir. The filtration rate is approximately 10 //m to 20 ^m.
Pressure Line Filter Circuit
This circuit protects the directional and other control valves in the line. Its filtration rate ranges from about 10^m (solenoid/proportional valve) to 3 ^m (servo valve, etc.).
This circuit has an objective similar to that of Fig. 15.16, but it filters out contaminants from the cylinder. It is better to filter out contaminants from only one direction with the anti-reverse-flow valve.
Pump Filter Circuit
This circuit’s objective is to protect the hydraulic pump. This circuit has two types: in one type, a filter is set inside a reservoir on a pump suction port (sometimes it is called a strainer), and in the other, a filter is set outside a reservoir to aid in maintenance. The filtration rates of these circuit types are limited to around 100 ^m because of the influence on pump suction resistance.
Pre-Fill Valve Circuit
In this circuit, the subsidiary cylinders and the pre-fill valve help the main cylinder achieve pumping function. This circuit drastically reduces the pump-output volume required for the high-speed up-and-down cylinder motion of the press machine.
As the subsidiary cylinders move downward, the main cylinder is pulled down with them, sucking fluid from the reservoir through the pre-fill valve. At the end of the downward motion, the sequence valve is opened, and working fluid is directed to the main cylinder, which then generates a great pressure force on the press.
After the pressure is released, the solenoid valve is set for the high-speed upward motion. The main cylinder is pulled up again with the subsidiary cylinders, pushing fluid back to the reservoir through the pre-fill valve.
Differential Circuit
When the cylinder is pushed forward, working fluid discharged from the cylinder head is added back to the cylinder cap because of the difference in surface area between the cap and the head of the cylinder. This achieves a faster-cylinder-forward motion, compared to a circuit with only one channel of incoming flow.
The relationship between forward speed V and the rod area, and between output force F and the rod area, are obtained as follows. V and F are functions of the rod area. The size of load pressure and pressure loss require due attention.
Circuit with Proportional Electro-Hydraulic Directional and Flow Control Valve
Optimal flow rate (speed) is achieved by controlling the amount of the spool shifted in the proportional electrohydraulic valve; the spool is shifted proportional to the amount of electric signal received. The actuator is controlled smoothly with this valve, and the hydraulic circuit is simplified, as shown in Fig. 15.12.
Speed Change Circuit
This circuit changes the speed of the cylinder motion by employing two flow control valves. Shifting to high speed, change over the solenoid operated directional valve for low speed first then change over the solenoid valve for high speed so that shock is kept small as shown in Table 15.1.
Weight Balancing Circuit
This circuit balances (holds up) the weight by using the balancing valve. The balancing valve cannot hold up the weight if the pump does not work and does not generate enough energy or pressure to hold up the weight. Therefore, a pilot operated check valve is commonly used in the circuit.
