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Pistons Pumps Used in The Hydraulic Drive for AGC Systems

The automatic gauge control (AGC) systems of hot and cold rolling mills determine the thickness of steel strips processed by the mill. While the position of hydraulic cylinders or actuators do the work to ensure gauge thickness is maintained within tolerance, the fluid power to position these cylinders is provided by large hydraulic drive systems (up to 6,000kW) that use piston pumps to develop the fluid power.

Piston pumps are contained within large hydraulic power units (HPU). A diesel engine or electric motor is the prime mover of the HPU, while the pump in conjunction with an accumulator, reservoir tank, relief valves and hydraulic fluid develop the power to drive the entire system. The most common types of piston pumps used in these applications are radial and axial piston pumps. All piston pumps consist of inlet and outlet valves, a piston assembly, fluid chamber, cylinder block and drive shaft. Basically, the inlet valve lets fluid into the pump. The stroke of the piston creates a suction force that pulls fluid through the pump to be discharged via the outlet valve.

Axial Piston Pumps

An axial piston pump is a variable, positive displacement pump that functions as a hydraulic motor. It is constructed with a number of pistons arranged in a circular pattern inside a rotating cylinder block. The pistons’ stroke is parallel to the plane of the rotating cylinder block. The cylinder block is driven by a prime mover via a drive shaft. As the cylinder block turns, the pistons follow the movement of the angled swashplate, which guides the piston stroke to regulate pressure and flow. As the inlet and outlet valves open and close in synchronization with the reciprocating pistons, hydraulic fluid is forced through the pump. The number of pistons and the angle of the swashplate determine the displacement capacity as well as the stroke of the pump. It is worth noting that the hydraulic fluid pumped through the axial piston pump is also being used for pump lubrication. Axial pumps are used for medium to high pressure applications, typically, in the range of 280 bar (4000 psi) to 320 bar (4600 psi), depending on whether the pump is rated for continuous or intermittent duty. The benefits of axial piston pumps include, quiet operation, low pressure ripple, low cost and minimal maintenance.

Radial Piston Pumps

A radial piston pump is a fixed (variable in special circumstances), positive, displacement pump that also functions as a hydraulic motor; they are noted for their smooth fluid flow at high pressures.  The pump is constructed with a number of pistons mounted in a radial orientation around an eccentric, spindle-like, drive shaft with a fluid chamber, suction (inlet) valve and pressure (outlet) valve. The piston and suction valve is spring-pressed to the drive shaft. As the drive shaft rotates, the pistons will reciprocate. When the piston moves down, pressure causes the suction valve to open so the chamber fills with hydraulic fluid. Conversely, when the piston moves up, the suction valve closes and the pressure valve opens to force fluid to be discharged. The benefits of radial piston pumps include quiet operation, self-priming, high reliability, high pressure operation, high efficiency, low pressure ripple. They can be operated in a low speed/high load condition safely.