[rotorwrench]

Fasteners are classified in two categories. They can be either tension or shear and applied torque depends on which category it is. In aviation, there are three acceptable methods of torque up depending on the application. The most common is still the torque wrench.

The next is stretch torque where the fastener length is measured loose then tightened until it reaches a specific stretch length. These type fasteners are generally high tensile strength types that have their full strength potential at the stretch specification. These fasteners are also reusable if still in serviceable overall condition. Sometimes the nut threads get stretched causing too tight a fit so that would require replacement of the nut. Some of these types have an initial torque requirement to check for excessive yield of the bolt prior to final stretch. It's the only way to see if it meets specs for the design. The fastener is replaced if the mechanic stretches it too far.

The final type of torque up is relatively new and was developed for the military to simplify maintenance requirements. This is the wrench arc torque method. The engineers developed this method through testing of each fastener to find out how much the nut had to be turned after the fastener is drawn up snug but not yet torqued. Each different size and type of thread has a different amount of degrees it has to be turned to reach the desired torque. If it is 45 degrees for example, you snug the nut till there is no shake or movement by hand then you turn it another 45 degrees to reach the torque spec. It's simple and it works. The military currently uses it more than the civilian world but it's just a matter of time before it will end up to some degree in both worlds.

With the helicopters, the clamping force isn't always a all that much since most fasteners are small to reduce weight and the torque will be in the lower inch pound settings. It's where the shear loads increase where the fastener strength comes into play. Main rotor blade bolts have to take a lot of shear load so they are mostly stretch torqued for full fastener load capability. Some tail rotor blade & hub fasteners are also in this category but the rotor is a lot smaller so the centrifugal loading is a lot less.

The big load on the early Fords is the rear axle on that old tapered shaft with key application.