Some quick facts about the right ways to ensure nuts and bolts don’t come loose.
Be aware when installing nuts or bolts that a split washer can distort final torque values. The “open” ends of a split washer can dig into the metal surface and cause a torque wrench to misrepresent torque values. That’s why there are no lock washers on torque-sensitive bolts like engine cylinder head bolts. Technically, the threads on a bolt or nut torqued to full specified value are preloaded enough to prevent loosening.
Understand what type of lock nut you’re using. “Lock nuts with nylon inserts can be re-used up to a dozen times,” says Andrew Brown, engineer with Whittet-Higgins Company, an international manufacturer of industrial fasteners. “A deformed-thread lock nut is designed to mechanically distort the threads on a bolt. Using a deformed-thread lock nut on the same bolt again can damage the threads enough to create a weak connection.”
Use the right type of lock nut for the right location. Deformed-thread lock nuts are often used where engine heat, exhaust heat or frictional heat could weaken the grip of nylon-insert lock nuts.
If a bolt or nut with damaged threads must be re-used, it’s better to use a thread cleaning tap or die rather than a thread cutting tap or die. Thread cleaners re-shape damaged threads. Thread cutting tools slice through damaged threads, resulting in weaker threads.
As a farmer repairing equipment, be suspicious of previous repairs. “Ag machinery is notorious for being repaired under field conditions,” says Brown. “There’s a tendency to use whatever nuts and bolts that are available, so lock nuts get swapped for common nuts and vice versa.” With catch-as-catch-can field repairs in mind: never use a lock nut on a fastener that needs to be torqued with a torque wrench because lock nuts distort torque values.
Ultimately, never assume the nuts and bolts used in previous repairs are correct. The current breakdown may be due to inadequate performance of incorrect nuts and bolts.
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