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Hydraulic Roll Balance Cylinders in Rolling Mills

Rolling balance cylinders are primarily used in rolling mill stands to ensure that the upper work and back up rolls are maintained in position relative to the lower rolls. They are also used to perform a secondary functions as the actuators of the roll eccentricity compensation control system, which counters the deformation of a circular roll into an ovalized form due to bearing wear and improper manufacturing and thereby creating undesirable variations in rolled strip gauge.

Work and Back Up Rolls

The rolling process can reduce the thickness of metal ingots (600 mm) to rolled strips of the desired gauge or thickness (< 5mm). To do so, rolling mills use an automatic gauge control system (AGC) that controls the vertical pressure applied through the work and back up rolls’ to the metal ingot or slab through pass line speed and roll gap or gauge adjustment. The thickness reduction is a function of roll speed as well as the type, temperature and amount of rolling lubricants.

Work and back up rolls are finely machined and polished to ensure a circular shape, flat surface finish, high polish, and are slightly crowned to compensate for the bending of the roll as rolling forces act upon it during the rolling process, as well as from the thermal expansion and wear-and-tear from the work roll’s normal use. Rolls with adjustable crowns are part of the mill’s flatness control system. While the work rolls are where the metal strip passes through and where gauge reduction occurs, back up rolls are used in conjunction with the work rolls to provide support, additional pressure and stability (dampening & roll deflection) to maintain the tight gauge tolerances and product quality.

Hydraulic Cylinders

Hydraulic cylinders are utilized with the rolling mill’s work roll gap positioning, roll bending, and backup roll balancing, making them part of a complex and interrelated control system. As a discrete sub system, roll balance cylinders act on the backup roll chocks to balance the rolls and to allow them to be raised or lowered to maintain their position. In essence, they are the actuators of a closed loop pressure regulation. They act on the upper and lower working roll chocks to press them up and down so the same roll balance and roll bending force is applied. Depending on the type of mill stand, intermediate rolls maybe used with to provide the roll balancing forces to maintain the desired gap between the work rolls. Roll balance cylinders are usually custom designed but can have bores of 1.8 meters and lengths of 24 meters. They have a variety of designs including, bolted, welded, tie-rod.

While hydraulic roll balance cylinders have been the standard since replacing counterweight-balancing systems, it merits noting that spring cylinders have been introduced as an option to hydraulic cylinders. Spring cylinders are designed to provide a constant force during the rolling process. They eliminate the need for hydraulic systems and the energy use and increased maintenance (fluid leakage) associated with hydraulic cylinders. They are designed for a specified range of loads, roll diameters and roll gaps.