CATEGORY:
JOURNAL BREAKAGE
TYPE:
BEARING FAILURE
AFFECTS:
WORK ROLL AND BACK UP ROLL
CHARACTERISTICS
The journal surface and or the barrel end face displays mechanical and thermal
damage that can consist of scoring, deformation, cracking and surface spalling. The
bearing contact area may also show bearing material pick up if significant heat has
been present. In extreme cases a full cross sectional fracture of the journal may also
result.
EXAMPLE
Figure 1
Journal surface of a work roll following a bearing failure. Extensive scoring,
deformation and bearing material pick up can be seen.
1
Figure 2
Close up of the bearing surface showing scoring and bearing metal pick-up.
Figure 3
Bearing surface spalling resulting from excessive heat and bearing material peeling
off.
2
Figure 4
Full cross sectional journal fracture resulting from extreme heating during a bearing
failure. The mechanism of failure is similar to that of a barrel thermal breakage (see
section V.A)
3
Figure 5
Journal fracture that occurred following a bearing failure.
Figure 6
Thermally induced cracking (firecracking) in the barrel end face of a back up roll.
Failure of the thrust bearing assembly resulted in the bearing contacting the barrel end
face.
4
MECHANISM
The purpose of the bearing is to reduce rotational friction and support the radial and
axial loads applied to the journal during rolling. If the efficiency of the bearing is
reduced then a resultant increase in frictional force will occur with a resultant increase
in the heat generated within the bearing. If this temperature reaches a critical level the
disintegration of the bearing components may result. This disintegrating can lead to
direct contact between the bearing components and the journal resulting in mechanical
damage such as scoring and gouging of the journal surface and or the barrel end face.
If sufficient heat is generated then thermal cracking at the journal surface or barrel
end face may also result (firecracking) especially when in contact with cooling water.
High temperatures can also result in melting of the bearing components which then
becomes welded to the journal surface. In extreme cases where sufficient heat has
been applied to the journal and the resultant temperature gradient and internal tensile
stress generated within the journal cross section are sufficient, then a thermally
induced full cross sectional fracture can occur.
PREVENTION
It is not within the scope of this manual to give advice on bearing maintenance
procedures. It is however recommended that all necessary maintenance procedures are
put in place to ensure where possible reliable operation of the roll bearings. When
bearing failures are encountered with resultant damage to the roll journal the
following practices are recommended.
 Where bearing material pick up is present on the journal or barrel end face this
should be removed to allow for assessment of any damage present.
 Examination of the journal surface for the presence of cracks should be carried
out. Dye penetrant can be useful in aiding crack identification.
 All cracks must be removed from the journal before it can be returned to
service
 Where significant damage such as scoring or deformation have occurred a
repair by welding may be carried out to return the journal to its drawing size.
It is recommended that the guidelines in section IV.E on such repairs are
followed.
 Where scoring or cracking at the barrel end face is present it will be safe to use
the roll as long as the following criteria are met:
o The cracking is not within the working diameter of the roll
o The cracking does not extend into the barrel face to journal radius
o The integrity of the bearing seal will not be compromised.
5