Roll chocks in Rolling Mill
Roll chocks in Rolling Mill
Rolling mills for rolling of steel differ in many aspects with each other. The rolling mills are of different sizes and capacities. The mills roll steel materials of different cross-sections, sizes and qualities and in material conditions which are either hot or cold. The mills have different configurations and speeds of rolling. The configurations of the mills can vary from cross country, reversing, semi continuous to continuous. The equipments of rolling mills can have manual operations, mechanical operations, electro-mechanical operations, pneumatic operations, hydraulic operations, or a combination of all of these. The controls provided in the mills can be manual controls, remote controls, instrumented controls, or fully automated controls. Further in many types of mills even heat treatment processes are integrated.
In spite of the so many differences, all the rolling mills have in common some basic technologies and equipments. All the rolling mills have rolls for the rolling of materials which are fitted in roll stands. Rolls are either driven by electric power or friction driven and are to resist many forces for normal rolling. The roll stands can have two rolls, three rolls, four rolls, six rolls, or a set of multiple rolls mounted on them depending on the types of mills.
Rolls for their smooth rotation as well as for resistance to different forces need ‘bearings’. Roll bearings are to meet the basic need of the rolling mill which is the smooth rolling of the steel products. They are friction reducing devices which provide support to the rolls for effective rolling with minimum of energy loss. The bearings are designed to withstand high rolling loads, heavy shocks, varying speeds, and high temperatures. Apart from this, bearings are designed to endure ingress of scale, dirt, and water. They play important role in reducing the power consumption and improving the rolling condition.
The housing of a work roll or backup roll bearing is known as roll chock or simply chock. Chock is the basic part of a rolling mill. It is mounted in the window of the roll stand housing between the posts with a small clearness. The roll stand housing encloses and supports the chock assembly. The chock also prevents scale getting into the roll neck bearing. The process stability of rolling mill is significantly influenced by the clearance between the chocks and housing in mill stands.
The prime function of roll chock in rolling mill is to house and accommodate the roll neck bearings. Roll neck bearings serve for accurate mounting of roll necks, in both horizontal and vertical plane. Roll chocks are designed to fit into the window of the housing in such a way that they are important component in a rolling mill for maintaining accurate positioning of the rolls. During rolling, the load on the rolls gets transferred to the roll neck bearings and their assembly (chocks).
The important features of roll chocks are that they have special holes (bores) for slider bearings, antifriction bearings, or oil film bearings. They have side support planes. For increasing the wear resistance and to prolong chocks work life, these planes are equipped with lining plates having different hardness of surfaces. The surface of the roll chock in contact with the bearing needs precision, and accuracy, as well as close and smooth finish. A cross section of a four high rolling mill stand showing roll chocks and antifriction bearing is shown in Fig 1.
Fig 1 A cross section of a four high rolling mill stand showing roll chocks and antifriction bearing
Roll chocks are very high stressed components of the rolling mill. Any failure of the chocks leads high production loss in a rolling mill. Failure of chocks in a rolling mill is basically because of high stresses generated during start up and shut down condition of rolling mill. Hence, for the design of chocks, it is essential to know the maximum load acting on a chock to prevent its failure. Also fatigue failure can occur in the chocks because of its cyclic loading.
Another important function of the roll chock is that it has to work as an isolator. Hence, the design requirement of the chock is that it needs to have a required stiffness to transmit a vibration from a source to receiver. So while designing a chock the basic two requirements are (i) the chock is to have strength to sustain a maximum load, and (ii) the chock is to have minimum deflection.
Roll chocks of different design are needed for top roll and bottom roll in a two high mill because of the roll screw down mechanism and adjusting and balancing equipment. Upper roll chock has a support with an additional lining for holding up the top roll when the rolling mill is idling. In a four high mill, there are four types of roll chocks namely (i) top back up roll chock, (ii) top work roll chock, (iii) bottom work roll chock, and (iv) bottom back up roll chock. Further the design of the roll chock is also influenced by the type of housing which is used in the rolling mill. Housing-less roll stands need rigid chocks connected by solid and pre-stressed joints.
The forces, which act on the rolls during rolling, are completely transferred on to the housing through the roll chock. Forces acting on the top roll chock are the force of the hydraulic cylinder transferred through the bottom rolls through the material to be rolled and through the top work roll. Top roll chock is constrained at the top of the rolling mill by the screw down mechanism.
Roll chocks are made either by castings or forgings. In case of cast iron roll chock, only minor welding repair is possible. The material used for the manufacture of roll chocks is either gray cast iron or steel. The design of the roll chock and the material of construction of the roll chocks depend on (i) type of mill housing, (ii) type of roll such as work roll or back up roll, (iii) loads generated during the rolling process, and (iv) type of roll neck bearing. The design of the roll chocks is to provide easy assembling and disassembling of rolls and roll neck bearings. Also, the design of roll chocks is to ensure that the ingress of rolling fluids and other contaminants to influence the bearings are prevented as well as there is proper lubrication available for bearing.
The axes of the upper and lower work rolls and the large upper and lower backup rolls (in case of a four high mill) are contained in a common vertical plane so that the extremely heavy workloads exerted by the screw down mechanism, through the backup roll chocks and rolls, to the work rolls is expected to theoretically produce only a vertical load on the work bearing chocks in the static load condition. However, minor misalignments inherent in such equipment as a result of manufacturing tolerances, wear, strain, and the like, and as a result of loads produced by the roll drive and by the work piece moving through the mill stand, produce very heavy loads on the work roll chocks tending to upset the coplanar relation of the roll axes and, as a result, the mill stand housings is required to place heavy restraining loads on the work roll chocks. These heavy loads have, in the past, caused wear on the bearing surfaces of both work roll chocks and the mill housing. To minimize this wear, and to facilitate maintenance of the roll stands, it is the normal practice to provide wear plates, or liner plates, in the form of high-strength hardened steel plates on the face of the mill housing and on the adjacent face of the roll chocks. While these liner plates have generally been effective in reducing wear and keeping the chocks centered in the mill housing, there have been instances where the chocks have been permitted to move sufficiently to produce a hammering effect causing excessive wear on the liner plates and in extreme cases to cause wear or damage to the face of the mill housing beneath the liner. This has been particularly true in the case of the lower work roll chock which generally has been provided with substantially smaller liner plate area than the top work roll chock.
The process stability of finishing mill stands is significantly influenced by the assembly accuracy between the mill stand components. In a four-high mill assembly, work roll chocks and backup roll chocks are mounted in the windows of mill housing. During the finishing process, the high-speed of the steel material being rolled generates a disturbing load and causes the chattering of the roll chocks in the housing.
For absorbing the undesirable chattering and axial thrust and to avoid the wear of contacting surfaces between mill components, consumable steel liner plates which are installed on chocks and housing faying surfaces need to have a sufficient narrow range of clearances between them. The liner plates also provide space for lubrication and thermal expansion. Adequately designed narrow clearances ensure good axial alignment of rolls and guarantee a precise shape and gauge of the rolled material after finishing. During the finishing process, the initial clearance is gradually broadened over time due to liner plate wear. The asymmetric wear of chock liner plates results in the misalignment of rolls, which is commonly referred to as a roll cross. This misalignment leads to a deviation of roll axes from a common plane, and results in additional horizontal loading of the chocks and the housing surfaces, which further degrades the steel liners. The wear condition and the residual thickness of the liners are crucial to the maintenance strategy of clearances between components. An inadequate clearance range leads to the dynamic instability of the rolled material during finish rolling.
Roll chocks used in a mill are quite large in number especially in a continuous rolling mill with large number of rolling stands. Hence the inventory of chocks and their maintenance needs a close management. The functioning of the chocks in a rolling mill needs close tracking since malfunctioning of chocks can results into increase in bearing consumption, delays in the rolling mill, and product quality defects.
Damaged chocks can also be repaired but the repair of chocks needs skill, experience, technology and a disciplined process and normally it is carried out under the supervision of experienced persons. If a damaged chock is repaired properly then the performance of the repaired chock is similar to the performance of a new chock.
The repairing process for the chocks consists of thorough cleaning and inspecting of the damaged chocks and documentation of the inspection findings. Chocks are then carefully disassembled to ensure components are not damaged and parts are thoroughly cleaned. After this, the critical dimensions are measured and then a plan to repair the chock in the best, most cost-effective way possible is developed. The repair process normally includes pre-machining of the surfaces to ensure quality welds, expert welding, arc wire spray welding, stress relieving, and precise finish machining to ensure proper contact surfaces. To ensure top performance, bore diameters are verified and the width of shim liners is set. Final steps in the refurbishment process are inspection, deburring, and sanding. After this, further steps are cleaning and painting, and careful reassembly, including bearing load zone rotation.