Page 177 - Root Cause Failure Analysis

Process Rolls 165

Roll
Figure 13-1 Typical process roll.

Conveying-roll bodies normally are cylindrical, but work-roll bodies may have a vari-
ety of shapes or profiles. In many of these applications, the roll body will have a spe-
cific profile, commonly referred to as a crown, that enhances the work performed by
the roll. The profiles range from convex to concave, which determines the force trans-
mission the roll provides.

Face
The roll face is the surface of the roll body. This is the area that performs work. Typi-
cally, the roll face is ground and polished to provide a smooth surface that does not
affect the product when it is in contact with the roll.

A variety of finishing techniques are used to prepare the roll face. In work-roll appli-
cations, the face may be chrome plated, rubber coated, etched, or corrugated. The fin-
ishing method is determined by the specific application and the work to be performed.
For example, coatings such as rubber commonly are used to increase friction between
the roll face and the transported product. A corrugated surface is used to impart a pat-
tern to the product (e.g., paper towels).

Neck
The neck is the transition area on both ends of the roll body that reduces the roll’s
diameter to that of the bearing-support shafts. The design methodology used for roll-
neck construction varies with the intended function of the roll. For example, rolls used
in a cold-reduction mill have a cast-steel body and neck. Because the roll must bend
in normal operation, the necks are not hardened, to facilitate bending.

Neck design is critical to roll reliability, and many failures can be directly attributed to
poor design. On large-diameter rolls, the reduction in diameter from the body to the
final shaft size should be in steps rather than as a single reduction. Each step down
should have stress-relief cuts at the transition points to prevent stress failure. Smaller-
diameter rolls can be reduced in a single step, but they also must have stress relief by
undercutting to prevent failure.