Failure Analysis
Surface Damage Failures
Everything possible should be done to prevent mechanical surface
damage to sucker rods, pony rods and couplings. Surface damage
increases stress during applied loads, potentially causing
rod string failures. The type of damage, and its orientation,
contributes to this increased stress effect. The orientation
of the damage contributes to higher stresses with transverse
damage having increased stresses over those associated with
longitudinal damage. A sharp nick will create a higher stress
concentration and would be more detrimental to load than a
 shallow, broad-based depression. Sucker rods with indications
of surface damage must not be used and must be replaced. Care
should be used to avoid all metal-to-metal contact that might
result in dents, nicks, or scratches. To prevent potential
sucker rod damage, place strips of wood between metal storage
racks and between each layer of sucker rods so metal-to-metal
contact can be avoided. Use sucker rods for what they were
designed for-to lift a load. Never use sucker rods as a walkway
or workbench. Keep metal tools not intended for use on sucker
rods and all other metal objects away from the rods. Make sure
the tool you use is intended for the purpose and ensure that
it is in proper working order.
Figure 8 is an example
of various surface
damage failures.
The example on the
left in Figure 8
shows a slight depression
from a wrench, tool,
or other metal object.
The second example
from the left in
Figure 8 is damage
from a pipe wrench
used in applying
field-installed rod
guides. The second
example from the
right in Figure 8
has a small longitudinal
scratch, through
metal-to-metal contact,
by allowing sucker
rods to run down
other rods in a rod
bundle during installation.
The example on the
right in Figure 8
exhibits transverse
surface damage.
 Figure 9 is an example
of surface damage
caused by sucker
rod elevators. The
bottom example in
figure 9 is damage
from worn or misaligned
elevator seats. After
an extended period
of service, the elevator
seats become so worn
that they develop
an oval shape rather
than a round shape.
As the oval shape
grows, the tangency
ring of the rod upset
to the elevator seat
face is lowered in
the front half of
the seat. As the
seat continues to
wear the seating
position of the rod
upset is moved forward
of the elevator trunnion
centerline. This
causes an offset
in the hook load
and tilts the elevator
body forward. When
the elevator lifts
the rod string load,
the hook load will
bend the sucker rod
centerline to coincide
with the elevator
trunnion centerline.
As the rod string
weight increases,
the hook load will
bend every sucker
rod engaged by this
elevator. Bent sucker
rod failures that
occur below the surface
upset bead may be
from bad elevator
seats. The top example
in Figure 9 is damage
caused by the elevator
latches. This type
of damage normally
occurs as a result
of picking up or
laying down in doubles.
Never pick up or
lay down anything
more than one single
sucker rod. Anything
else causes the elevator
latches to act as
a fulcrum and allows
bending stresses
to concentrate in
the transition zone
of the rod body and
the forged upset.
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