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Hydraulic Bushings, Caps, Plates, Brackets and Other Accessories Used In AGC Systems

Any discussion of hydraulic automatic gauge control (AGC) systems for hot or cold rolling mills usually focuses on these systems’ critical components such as, roll position hydraulic actuators (cylinders), electro-hydraulic servo valves, roll position/pressure transducers (sensors), screw-down mechanisms, mill drive spindles, closed loop controller and other major components. But these major components use a variety of mechanical hardware such as, bushings, caps, plates, brackets, fittings, couplers, among others. This hardware should be part of a mill’s in-house inventory stock or replacement parts kits since they are not modular, replacement items as are cylinders and valves. They may be required during mill refurbishments, overhauls, upgrades or emergency repairs.

Mill Housing Hydraulic Accessories

Top cover plates, spindle guard & support brackets, cylinder caps, roll & suspension assemblies, roll coupling sub-assemblies, work roll bearings, screw-down assemblies are all used within the mill housing compartment. For older model rolling mills that primarily use screw-down AGC control with short stroke hydraulic actuators, much more hardware is used than with the current generation of hydraulic gauge control systems since the later models eliminate mechanical screw-down assemblies and rely solely on long stroke, hydraulic actuators. It is worth noting that another trend in AGC systems is the use of modularized control blocks that integrate the actuator, transducer and servo valve into one component; these control blocks also use less hardware and accessories.

Hydraulic Cylinder Hardware and Accessories

While the roll position cylinder is typically replaced as a complete unit, hydraulic cylinders may be disassembled for testing and preventive maintenance. One will quickly notice upon disassembly that the hydraulic cylinder contains numerous types of hardware including, retainer nuts, cap screws, washers, retaining rings, piston seals, extension spring assemblies, ram couplers, o-rings, tensioner plates, straight fittings, bushings, bearing sleeves and stop rings. These sub-components should be inspected for integrity and replaced as necessary. Since hydraulic cylinders used in hot and cold rolling mill applications can experience hydraulic pressures as high as 10,000 psi (700 bar), it is critical that they are reassembled according to the manufacturer’s instructions. Prior to reinstallation, the cylinder should be tested. Improper installation of hardware can lead to a variety of cylinder problems. For example, if the cylinder isn’t advancing properly, the coupler may not have been tightened to the proper torque. Leaking or damaged seals can cause the cylinder to operate but not to hold.

Servo Valve Filters, Seals and Accessories

The current generation of servo valves used in AGC systems have modular designs and consist of an electric torque motor & associated circuitry and the hydraulic flow control valve assembly (flexure tube, spool, nozzle orifices and control ports) to provide cylinder position control. The mechanical accessories associated with servo valves include hardware (bolt kits) for manifold or sub-plate mounting, seals, cable clamps, hydraulic inlet filters, o-rings and end caps. The importance of the inlet filter cannot be overstated. A plugged (or partially plugged) inlet filter can cause erratic operation of the servo valve including, the valve’s inability to follow an input command signal properly, jerky valve operation or poor valve response.