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Servo Valves Used in Hydraulic Looper Controls of Hot Rolling Mills

 

Electro-hydraulic servo valves are an essential control component in the tension control system or looper control of hot rolling mills. They are used to improve the time response characteristics of the hydraulic looper system so constant tension can be maintained during the rolling process. Constant tension of rolling mills is directly related to the mill’s productivity as well as the quality of the rolled strip. Too much tension can cause the strip to break, while too little tension can cause undesirable folds in the roll.

Hydraulic Looper Controls

Hot rolling mills have used two kinds of looper controls: electrical and hydraulic. Electrical looper controls were standard on older mills and performed an adequate job of maintaining rolling strip tension but had a higher amount of maintenance associated with the D.C. motors and drives. Currently, rolling mills are designed with hydraulic looper controls because they provide a faster response to changing mill conditions. They also are easier to maintain than electrical looper controls. Hydraulic looper control systems consist of a system controller, servo valve, hydraulic cylinder, sensors (transducers/load cells) and piping. They are mounted between two mill stands of a tandem rolling mill, which is a type of rolling mill where rolling is done in one cycle, rather than in several cycles. The hydraulic looper controls the strip tension by changing the angular position of the looper arm to increase or decrease the height of the hydraulic cylinder’s ram to maintain tension. Pressure transducers, attached to the piston rod of the hydraulic actuator senses and calculates the changing forces on the actuator and sends a signal proportional to these forces to the looper controller.

Hydraulic Servo Valves

In hot rolling mils, hydraulic servo valves are used to accurately position the hydraulic actuator for tension control. Servo valves receive an input from a closed loop controller, a transducer (feedback signal) to position the servo to drive the cylinder to a specific height to maintain constant tension on the strip. While the primary advantage of servo valves is that a small electrical signal from the controller can accurately position the hydraulic actuator, servo valves are complex and expensive control components

Servo valves consist of the following parts: spool (single, double or three-stage), a torque motor and a flapper nozzle or jet pipe. The electromagnetic torque motor receives an electrical signal from the controller and applies a torque on the flapper to change the size of the orifice/nozzle. In turn, this changes the hydraulic force applied to the hydraulic actuator of the looper. Single-stage valves are directly connected to an electric torque motor. Two-stage servo valves are constructed with pilot and main valves. The pilot valve is driven by an electric torque motor, which receives an electrical signal from an actuator position transducer. The motor drives the flapper plate of pilot valve to control the pressure differential of the valve orifices that send fluid flow to the main valve stage. Three-stage servo valves have the best dynamic/time response and are similar to two-stage valves with one modification: the first or pilot stage is really a two-stage valve.

There are two types of orifice/nozzle systems of servo valves: jet pipe or flapper.  The jet pipe type doesn’t get contaminated as easily as the flapper type. This feature is important due to the nature of a servo valve’s failure mode. A clogged or contaminated flapper type will operate in a maximum flow direction while a jet pipe type generally fails in a neutral direction. When specifying a servo valve for a hot rolling mill application, a variety of factors must be considered to the complex nature of the system. These factors include, pressure, temperature, rate flow, rated current, controller characteristics, materials, failure modes, interface requirements, mounting type and hydraulic fittings.