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.
Care of your torque wrench
As mentioned earlier, torque wrenches should be calibrated at least once a year. However, torque wrenches may have to be calibrated more often depending on usage and user needs.
When storing torque wrenches, try to store them in their original boxes. Also avoid storing them in areas of extreme hot or cold temperatures, or in areas of high humidity.
On clicker type torque wrenches, always turn them down to the lowest setting when storing them. Storing it at any settings other than the lowest one will cause main spring wear and failure. But be careful not to go past the lowest setting or it may come apart in your hand, and you'll have to ship a box of springs and parts to the torque wrench repair shop.
Keep torque wrenches clean. They should be kept free of grease, moisture, and debris. All of these can be destructive to a torque wrench and throw off calibration.
Don't abuse torque wrenches. They are precision measurement equipment, and dropping them or using extensions on them can cause serious damage.
Watch those calibration records
When your torque wrench comes back from calibration, it is a good practice to look at the calibration records. The shop will include the readings indicated on the torque wrench versus the actual torque applied. Even though the torque wrench comes back to you within limits, it may have been significantly off before the lab adjusted it. Normally, the errors are not far off, but if a gross error exists, such as the torque wrench applying 70 in./lbs. of torque when it was set at 40 in./lbs., it could warrant going back and re-checking work that was done using that torque wrench before it was sent in.
Keep in mind that external factors can affect the accuracy of torque. As mentioned earlier, torque specifications take into account the friction on the given bolt or nut. If lubricant is called out to be used, it is important to use it. Not using lubricant will result in more friction during the torquing sequence, and even though the torque is achieved on the torque wrench, the actual amount of preload on the part will be less than called for. Along the same line, if lubricant is not called for but is used on the application, then when the specified torque is reached the actual preload will be greater than spec, and failure could result. Needless to say, cross threaded bolts or nuts will result in erroneous torque values as well.
So in the end it's not so hard after all. With a little knowledge on torquing procedures, you don't have to feel like you have that torture device tied around your neck.
Always Preload Your Torque Wrench
Preloading a torque wrench is an important process in overall accuracy of these tools. It must be performed each time the torque wrench is used after periods of non-use or whenever torque direction is changed. There are several reasons for preloading your torque wrench. First, it will set internal components so that when force is applied, torque begins immediately with no internal settling. Second, it distributes lubrication to moving internal parts. The final reason is the hysteresis characteristic of the steel, initial stress should be applied to the steel in order to moderate the hysteresis presence.
How to do it:
2. Mount torque drive in a stationary fixture (i.e. socket welded to bench, vise).
3. Exercise the torque wrench three to five times in the direction you will be verifying.
4. Perform torque measurement.
Remember to store torque wrenches (click-type) in the low setting. Otherwise calibration will be needed at shorter intervals.
Duncan Intelligence, Fall '99 (For a complete listing of Duncan Intelligence Newsletters, you can visit them online at www.duncanaviation.com).