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The Use of Short and Long Stroke Hydraulic Cylinders in AGC Systems

Automatic gauge control systems (AGC) for the control of steel strip thickness in rolling mill applications have used two types of hydraulic cylinders for roll positioning operations: short stroke and long stroke. Short stroke, hydraulic cylinders have been used in older mill designs prior to the 1970s that use pressure-type, screw-down mechanisms to regulate rolling strip thickness, while long stroke cylinders are used with later version mill designs. Long stroke cylinders along with advanced computer control eliminate the need for screw-down mechanisms and provide improved accuracy and precision. They can achieve gauge tolerances ranging from 0.5 to 2 percent.

Short stroke cylinders are large diameter cylinders and referred to as “pancake” cylinders. They have a very small range of motion but develop immense force due its piston, which has a much larger diameter relative to its rod. They are designed to be fitted at top of a mill and underneath the load screw mechanism. The screw-down mechanism gives a rough or coarse gauge regulation with the short stroke cylinder providing the roll gap fine-tuning control. But short stroke cylinders are noted for long response times giving them less accuracy than their long stroke counterparts.

It should be noted that even prior to the use of short stroke hydraulic cylinders, gauge control was accomplished in two ways:

(1) screw-down mechanisms driven by an electric D.C. motor with manual control or automatically with using X-ray gauges with closed feedback control; or

(2) reel drives operated in a tension control mode. But these control methods suffered from a limited maximum rolling speed, diminished quality and no means to control the flatness of the roll.

Summed together, they kept mill productivity low. But in the late 1970s, short stroke cylinders gained acceptance in mill design and thickness-control response times improved. But even though the combination of the short stroke cylinder and screw-down mechanisms provided gap control of a wide variety of steel slab widths, their response time was still too long. Long-stroke cylinders resolve the response time problem.

Long stroke cylinders (sometimes referred to as “top hat” cylinders) have replaced screw-down mechanisms with short stroke cylinders common in older mills. A primary advantage of long stroke cylinders is that since they can completely replace roll load screw mechanisms, making the mill easier to maintain, and allowing the mill to be operated at faster rolling speeds. Long stroke cylinders not only can control the steel strip’s thickness but also its flatness. They can be designed to deal with load variations that affect rolling quality, such as roll flexure and mill deformations. Presently, long stroke cylinders are designed with modularity, a variety of mounting styles and interchangeability. But both short and long stroke hydraulic cylinders are usually custom-designed and manufactured to specific requirements. In general, mill duty hydraulic cylinders are rated up to 5000 psi with bores between 2 to 40 inches and temperature ranges between -60° F to 600° F (-50° C to 315°C). The stroke can be any size but lengths as short as 2 inches or as long as 60 feet are possible.