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between the rolls, frequent burning losses of the end-thrust roller bearings, deviations
in the rolling load acting on both ends of the backup roll (BR), and short service life
of the four-row radial roller bearings in the BR system, to mention a few. As a
consequence, rolling mills designed for heavy loads and high speeds cannot be
operated at their design load and speed in a safe and stable way, and their actual
production performance (rolling load and speed) is far lower than the design targets.In
the above rolling mills, the work roll (WR) and BR axes in a four-high mill (including
the intermediate roll axes in a six-high mill) are not in the same vertical plane. The
horizontal distance between the roll axes is defined as the offset distance. The offset dis- tance has been regularly referred to in text books and reference books [2,3]
since 1965, and it is a common theoretical design vari- able in the design of four- or
six-high plate rolling mills. Due to the offsetting between the rolls, a horizontal force
in the rolling direction is generated to maintain the rolls parallel during the rolling
process. Different offset distance values are used in different rolling mills.
In addition to offsetting, in four- or six-high plate rolling mills, there is a
small clearance, generally 1–1.2 mm, between the chocks and the side
surfaces of the housing window to ensure free space that allows convenient roll
removal and provides for the thermal expansion of the chocks. The effects of this
clearance, which is very small as compared to the dimensions of the chocks, have
received little attention in the technical literature. However, in the analysis of the
horizontal vibrations of a four-high rolling mill [4], some research works have found
that this small clearance is an important reason for the instability of the rolling
systems. Observations revealed that the rolling mill encounters serious
vibrations when the work piece is entering or exiting. The greater the clearance, the
more serious rolling mill vibrations are gener- ated. Therefore, it is suggested that
a damping device be installed to minimize the negative effects of the clearance [5].
However,such practice limits the free thermal expansion of the roller bear- ings and
results in the burning of the bearings.The analysis of the degrees of freedom (DOF)
of the spatial mechanism representing the roll system reveals that the traditional four-
or six-high plate rolling mills are statically indeterminate structures. The problems
caused by the statically indeterminate characteristics cannot be solved solely based
on the proper application of the theories of strength, rigidity, and vibrations as
applied in traditional rolling mill design [6]. Thus, the theory of statically
determinate characteristics in mechanisms [7] is proposedto be used to solve these
problems. In this regard, statically determinate four- or six-high plate rolling mills
that maintain the rolls parallel and even enhance the load performance of the radial
roller bearings will be explored and developed.
The 2200mm aluminum foil four-high mill is taken as an example to demonstrate
the analysis and proposed developments.Some of the parameters of the roll system
and the rolling conditions are shown in Table 1.According to the analysis of the DOF
of a spatial mechanism, the existence of the clearance in the roll system results in
statically indeterminate characteristics of the system. For illustrative purposes, the
lower BR system is chosen for DOF analysis. The lower BR system and its skeletal
structure are shown in Fig. 1.The left side is the drive side (DS) while the right side is
the operation side (OS). For the lower BR system, the chocks are set in the housing