Bolt Failure Problems 1

[Matador]

I work at a pulp mill in Saskatchewan Canada and we have been struggling with downtime due to bolt failures.

Case Study

The bolts being used are Gr 8, 5/8" X 2" Lg (CSH*C58-2P, UNC) and we use lock nuts. (LNS9C58P, UNC)

(1) - The bolts secure the pans to a chain links and the conveyor is used to remove hot smelt from a recovery boiler. The bolts can be exposed to hot molten metal at the recovery boiler spout and during winter months -25 C outside temperatures at the conveyor discharge. Cooling water is also sprayed on the bottom of the conveyor pans.

(2) - In the past our mtce people tried Gr 12 bolts thinking it was a strength problem. The bolts failed right after they were put in service. Now they leave the Gr 8 bolts loose, by using a locknut and damage the threads to help prevent them from backing off. The downtime problem still exists because the nuts eventually come loose and the conveyor pans fall off.

Questions:
(a) - Can I change from a UNC thread to try and keep the bolts from backing off?
(b) - Does anyone have experience using diiferent types of nuts to ensure they will not come loose and back off?
(c) - Is the idea of leaving the connection loose a good one. Maybe bolt with different metallurgy would stand the expansion and contraction without failing.

Any help is appreciated.

 

[diamondjim]

I am questioning the grade 12 fasteners that
were used. Are you speaking of ISO or Din Grade
Fasteners? How did these fail? Break at the head
of the bolt or thru some area of the clamped part?
What torque level were used for these?
If you can incorporate a longer bolt design
by increasing the washer thickness to at
least one inch thicker, you will decrease the
stress in the bolts. I like the double nut
approach also. Can you spot weld the nuts?

The only place you indicated that it might have
been to some cylic load conditions were that some
failed within a 5 year period. Were these extremly
rusted?

It may be as simple as following the chain
manufacturers suggestion. You cannot beat experience.
If the bolts can be loose, why use bolts?