Torquing hardware is an everyday occurrence. In our day-to-day routine, numerous tasks are performed that require a specific torque to be applied. Why are these torques so important? How can using a "calibrated hand" when torquing hardware pose a problem? This article will cover some basics on torque and discuss proper torquing techniques.
So what is torque? Well, Webster's Dictionary has two definitions. The first is a torture device that was worn around the neck, either a metal collar or neck chain. The second is a measure of force that consists of the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation. Although the first definition may hit home in that torquing hardware can be a torturous, time-consuming task, the second definition is the one that applies to our job tasks. Put simply, torque is force times distance.
When engineers design an aircraft, thorough analysis is done on the type of stresses that will affect each part of the aircraft. When the engineer designs a particular assembly, he takes into account all of these stresses that will be encountered as well as other factors like temperature changes, fatigue, and corrosion possibility. Fasteners are then chosen that will best fit the application and provide the necessary preload.
In order for a fastener to perform properly, it must be tightened accurately. Under-torqued hardware will provide inadequate preload. It can result in unnecessary wear of nuts and bolts as well as the assemblies that they secure. Over-torqued hardware can exceed the design limitations of the structure or hardware and lead to a failure. Failure of the bolt or nut can result from overstressing the threaded areas.
There are several methods to measure the preload on a given fastener. The method most commonly used by mechanics is a torque wrench.
When choosing which torque wrench to use, try to select one that has the required torque in the second or third quarter of the wrench's torque scale. The torque settings at the first and last quarter of a torque wrenches scale are not as accurate as those in the middle quarters.
AC43.13-1B gives the following important procedures to keep in mind to ensure that correct torque is applied:
1. Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.
2. Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.
3. Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.
4. Add the friction drag torque to the desired torque. This is referred to as final torque, which should register on the indicator or setting for a snap-over type torque wrench.
5. Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs during the final torque, back off the nut and re-torque.
Note: many applications of bolts in aircraft/engines require stretch checks prior to reuse. This requirement is due primarily to bolt stretching caused by over-torquing.
6. When installing a castle nut, start alignment with the cotter pin hole at the minimum recommended torque plus friction drag torque. Do not exceed the maximum torque plus the friction drag. If the hole and nut castellation do not align, change washer or nut and try again. Exceeding the maximum recommended torque is not recommended.
7. When torque is applied to bolt heads or capscrews, apply the recommended torque plus friction drag torque.
8. If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly. Determine the torque wrench indication or setting with an adapter installed as shown in Figure 1.
One of the basic things to remember when torquing hardware is to work in the proper measurement. It is sometimes necessary to convert the values given in the maintenance manual so that you can use them on your torque wrench. Typical conversion charts can be used to convert from one unit to another, say from ft./lbs. to in./lbs. In addition, if you have internet access, numerous conversion calculators are available to give you conversions instantly (www.ex.ac.uk/cimt/dictunit/cctorq.htm is an example). Once you type in the torque value you want to convert, it provides all of the conversions for you. It doesn't get much easier than that.
Due to the effect that tightening a bolt has on other bolts in a group (known as elastic interaction of bolt crosstalk), with multiple fastener applications you should not tighten bolts in a series. A criss-cross tightening sequence should be used. This is especially important in gasket application like fuel cell covers. In gasket applications like that, it is a good practice to go back and re-check the torque a second time around to ensure settling hasn't occurred. In many cases, a specific torque sequence is called out for critical components like propellers.
Sometimes it is necessary to use an extension on the drive end of a torque wrench. This may be to fit a unique fitting or just for accessibility. It is important to remember to use the formula shown in Figure 1 to calculate the torque setting for the torque wrench. If you don't, you'll end up with an inaccurate torque of the item.
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