Here are a few good facts and reminders about nuts and bolts.
1. Torque To The Proper Value
Engineers with fastener manufacturers assure me if nuts and bolts are torqued to their proper value, they will hold within specifications without a lock washer. In fact, they warn lock washers can cause problems when using a torque wrench. If the ends of a lock washer dig into the metal surface under them, it can leave the fastener under-torqued.
2. Screws Versus Bolts
Screws have threads to the bottom of their heads. Bolts have unthreaded shanks between the underside of their heads and the beginning of their threads.
3. Know Your Washer Needs
Flat washers are necessary only in association with holes slotted to allow adjustment of components. If there’s no need for adjustment, a combination of flanged-head nut and flanged-head bolt eliminates the need for flat washers.
4. Watch For Signs Of Stress
One edge of the holes in quality flat washers has a subtle bevel around the inside circumference. The underside of bolts where the shank meets the bottom of the head is not a perfect right angle. The beveled edge of the hole in the “top” of a flat washer should nest against the beveled junction of the bolt’s head and its shank. Acute angles between the edges of bolts and washers can cause “stress risers” that increase failure rates.
5. Two Threads Are Ideal
That’s the optimum protrusion of a bolt’s threads through a nut. If less than two are showing, the taper at the end of a bolt’s threads might reduce the strength of the connection. If more than two threads are showing, the threads are wasted.
6. Rules Of Thumb
When tightening nuts and bolts without a torque wrench:
- For bolts or screws smaller than ¼" (6 mm), use only your little finger on the wrench. Stop tightening when your finger is uncomfortable.
- For nuts and bolts 5/16" to 3/8" (8 to 10 mm), center the end of the wrench in your palm and quit when uncomfortable.
- Fasteners from ½" to 5/8" (10 to 12 mm) are adequately torqued when an average-sized man pulling an average-length wrench can no longer turn the nut or bolt with arm-strength alone.
- Fasteners larger than 5/8" (12 mm) generally can’t be over-torqued by an average man using average tools. Jumping on the end of a wrench doesn’t count. When in doubt, use a torque wrench.