Unloading System for Energy Saving
An “unloading” system is used to divert pump flow to a tank during part of the operational cycle to reduce power demand. This is done to avoid wasting power idle periods. For example, it is often desirable to combine the delivery of two pumps to achieve higher flow rates for higher speed while a cylinder is advancing at low pressure. However, there may be considerable portions of the cycle, such as when the cylinder is moving a heavy load, when the high speed is no longer required, or cannot be sustained by the prime mover. Therefore, one of the two pumps is to be unloaded resulting in a reduction of speed and consequently, power. The components of this system are: A, B: Hydraulic pumps, C, E: Pilot operated Spring loaded Relief valves, D: Check valve
Mode 1: Both Pumps Loaded
In Figure 28.1 below, when both pumps are delivering, oil from the pump A passes through the unloading valve C and the check valve D to combine with the pump B output. This continues so long as system pressure is lower than the setting of the unloading valve C.
Mode 2: One pump unloaded
In Fig. 28.1, when system pressure exceeds the setting of the unloading valve C, it makes pump A to discharge to the tank at little pressure. Although the system pressure, supplied by pump B, is high, the check valve prevents flow from B through the unloading valve. Thus only pump B now drives the load at its own delivery rate. Thus the load motion becomes slower but the power demand on the motor M also reduces. If the system pressure goes higher, say because load motion stops, pump B discharges when its relief valve settings would be exceeded.
Closed Circuit for Vehicle
Closed circuits are widely employed in vehicles performing running, circling or HST (Hydro Static Transmission: no shift change for speed change) functions. One of the characteristics of this circuit is to use a pump as a hydraulic motor to absorb the power: this is a reverse use of the pumping function of a motor found in the previous section on brake circuits. In addition, pressure inside the line is low because the hydraulic pump controls the speed of the vehicle. This system is more efficient, and achieves less heat generation when compared to valve control systems. The feed pump fills and replaces working fluid internally, and it supplies clean fluid through a filter. The circuit must be made in such a way that safety valve ® has a higher pressure than that of safety valve ®, and that working fluid from the feed pump is discharged to the reservoir via the flushing valve.
Brake Circuit with Hydraulic Motor
This figure is an example of a motor that turns both directions. With the solenoid valve in position 1, the hydraulic motor turns right. After that, the solenoid valve is in the middle position, but the hydraulic motor keeps working as a pump because of inertia. Discharged working fluid runs through the check valve 4, and returns to the reservoir with back pressure given by the relief valve. The primary side of the motor becomes low pressure, thus working fluid through the check valve ® is supplied into the line. In the case of a left turn, the check valve 2 and 5 are used.
Circuit with a Cylinder as Intensifier
This circuit intensifies pressure by using the difference between cap and head area in cylinders. In the following figure, the solenoid valve for adding pressure is turned ON. Working fluid channeled through the sequence valve ® pushes the working cylinder head forward until it hits an object. The contact between the cylinder head and the object eventually increases the pressure inside the line. Then, the circuit delivers the pressurized working fluid to the intensifying cylinder in which the fluid is pressurized yet further. The highly pressurized working fluid in the intensifying cylinder is then supplied back to the working cylinder. The decompression valve on the primary side of the intensifying cylinder adjusts the output power. Also, in the process of returning the cylinders, it is important to note that the intensifying cylinder is returned by the sequence valve © (using the counter balance valve as the sequence valve).
Circuit with Servo Valve
This circuit detects the position of two cylinders and uses two servo valves to control the amount of working fluid required to adjust synchronization errors. The following figure is an example of such feedback-synchronization control. Rather than detecting the position of one cylinder, and giving the feedback to the other cylinder for synchronization, it is better and more accurate with less time lag if each cylinder works separately and their positions are controlled by different servo valves.
Circuit with Synchronized Cylinders
This circuit realizes a very accurate synchronized motion via combined synchronizing cylinders. But, sometimes, spacing becomes an issue because it requires the volume of all the cylinders to be the same.
Circuit with Synchronized Hydraulic Motors
With the shafts combined, these motors can displace the same amount of working fluid to and from each cylinder. The accuracy of the amount of displacement controls the accuracy of synchronization. Therefore, if the volumetric efficiency is the same, setting the circuit with high speed motors reduces synchronization errors.
Synchronizing Circuit with Flow Control Valves
In this circuit, the flow control valve controls fluid flowing in and out of the cylinders. Generally, a high accuracy valve is employed.
Synchronizing Circuit with Mechanical Combination
This circuit realizes a synchronized motion by mechanically combined cylinder rods. In the following figure, the relationship between the two main cylinders and two other auxiliary cylinders is also a mechanical combination. This circuit does not necessitate a control valve for synchronization. Synchronization errors would be happened by production accuracy and rigid of mechanism.
