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Now that Owner Maintenance may have more individuals involved in maintenance, the onus is on owners to learn some of the procedures which mechanics have had to learn to obtain their licences. One of these important topics is the torquing of nuts and bolts.

Bolts are used for joints which are required to be removable. Bolts transfer loads through tension or shear. In either application, it is important not to change bolt specifications as you do your maintenance replace type with type. Don't substitute.

From where does the strength of a bolted joint come? If the joint is in shear i.e. the parts are trying to slide past one another - then the friction between the bolted parts produced by the bolt tension allows the joint to hold more than the shear strength of the bolt itself. 'Friction joint' might be a better name than 'bolted joint'. Builders and maintainers may be fooled into thinking they are automatically getting the full nominal shear value of the bolt regardless of torque applied. This is far from the case. Not only would full strength not be developed, but the fatigue life of the loosely bolted joint is much reduced from what might be expected under ideal conditions.

Job number one is to obtain a torque wrench, and to use it, following recommended values for the fastener. "Knuckle White" tight is not an acceptable standard. If you don't believe this, tighten a bolt using your method. Then check it with a torque wrench. 95% of bolts installed this way are over-tightened. The problem is particularly acute when tightening a nut or bolt against aluminum. Aluminum is softer, more plastic, and 'gives', thinning out the material under the fastener.

As you pull on a wrench, and the bolt turns, two simple machines are at work; the 'Lever' of the wrench, and the 'Inclined Plane' of the pitch on the bolt. The mechanical advantage produced by each is not additive, but multiplicative. In English this means that if the Mechanical Advantage of the wrench on the bolt is 30, and that of the pitch is 40, then the Ideal Mechanical Advantage of the system is 1200! In other words, if there were no friction, the tension produced by a 10 lb. pull on the wrench would produce a force of 12000 lb. in the bolt. Snap. Luckily there is friction!

You should know that:

It is absolutely essential that appropriate torques are applied to nuts and bolts so that the design strength of joints is maintained.

Frank Hofmann, AME
Retired Prof. of Aircraft Maintenance
EAA Technical Counsellor

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Frank Hofmann
Copyright © 1991