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No, you don’t have to use a torque wrench but it is important to stress the joint to the designed specifications. An alternative to a torque wrench, and much more accurate, is an extensometer. Using this device you measure the length of the bolt and tighten it until it reaches a specified length, which depends on the bolt. However, a torque wrench is much easier to use and is cheaper. If you are in a bind there is also the "turn of the nut method" which many people are referring to here when they say that "their father/uncle/grandfather has rebuilt engines for so many years without any problems.” The thing that people let out is that this individual had 30+ years of experience and learned from an individual with 30+ years of experience who learned from an individual with 30+ years of experience. With 90+ years of experience stored in the grey matter, they didn’t really need any special tools to get the job done. Anyhow with this method you use the pitch of the bolt and calculate how many degrees you need to turn the nut to reach the desired load. You can look up or measure the pitch and have to consult a table or figure out the extension. Here is a list of the Proof Strength of some of the common bolts:
Grade Proof Strength (psi)
1 33000
2 55000
4 65000
5 85000
5.2 85000
7 105000
8 120000
8.2 120000 The above numbers are good to about a ¾ inch bolt. You can then calculate the extension by dividing the Proof Strength by the Modulus and multiplying this by the shank length of the bolt (the distance between the head and the nut). The Modulus for steel is 30,000,000 psi. Extension = Length x Proof Strength / Modulus With a pitch of say 20 threads per inch and a desired extension of say, .075" you would calculate the turns required as follows: 20 threads per inch = .05 inch per thread (1/20)
.075"/.05" = 1.5 turns of the nut (540 degrees) to reach the desired bolt extension. The turns of the nut start from a snug-tight condition which by engineering definition is "the full effect of a person using an ordinary wrench." For a 120 pound engineer it’s all they can do to snug a bolt, so for you big fellers, don’t get too crazy, leave off a bit. If the front wheels start sliding you done went to far, as my uncle used to say. Now if your plane spins a bearing and crash lands on a deserted island, and you just happen to have another set of bearings, you could polish the crank with coconut peel and torque the bolts this way. However, using a torque wrench is well worth the extra assurance that the job is done right. (and if something would go wrong you could always blame it on a bad torque wrench) If not for anything else use a torque wrench for the sake of the poor engineer who spent about a day and a half calculating that torque spec for you with a slide rule, pencil and paper. Poor guy.
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