H O F T E C
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:
- When you wrench a bolt it experiences both tension and torsion. Only the tension remains after wrenching, so if it hasn't snapped during the torquing it won't fail in service if the correct size bolt is installed.
- If at all possible, turn the nut, not the bolt. If nothing else, turning the bolt will remove the protective Cadmium finish on the bolt, enabling corrosion.
- As you torque a bolt, about 90% of the effort is used to overcome friction. This has the effect of protecting the bolt from being over-stretched during the tightening operation. Observe whether the torque value you are using is for 'wet' or 'dry' installation, or for plated hardware. If 'dry', it implies no lubricant – oil or other additive. Installation in 'wet' conditions may lead to over-stretching of the bolt. A residual film of oil under a nut may also make it easier for it to come loose again. Normally threads should be clean and free of all contaminants.
- When using a table of torque values notice that bolt values are higher than nut values. The reason for that is that bolts experience greater friction. Again, turn the nut, not the bolt.
- As a bolt is tightened, it stretches and becomes a little thinner. Thus its clearance in the hole is further increased. It is important that bolts are pre-stressed during the torquing to values near the load they are expected to carry. This assures maximum clamping pressure so that any 'necking down' of the bolt diameter won't allow relative movement between the clamped parts.
- As you apply the final torque, hold that torque for 4 seconds to make sure that there is no further stretching or 'following'.
- Nuts have to be matched to bolts so that as tension builds, the stretch between the threads of the bolt are matched by 'give' in the threads of the nut. Otherwise undue stresses are built up in some of the threads of the bolt. Ideally each engaged thread should carry its share of the total load.
- As a bolt stretches and a nut takes the load, there is a tendency for the nuts to dilate, exposing the forces to a smaller cross section of the triangle making up the threads. This leads to stripping of threads at the weaker tips of the triangle. It is therefore a good practice not to re-use nuts and bolts in highly loaded joints. The pitch diameter limits on a 1/4" nut are measured in ten thousandths, 0.0001", the permissible range being 0.0003".
- If you torque a castellated nut, and the holes don't line up, back off, change nut or washer thickness and try again. Don't over torque to get a fit.
- Understand that if you add more washers than required because you don't have a bolt of the right grip length, the longer bolt will stretch more for a given load. In other words, a longer bolt will 'give' more, making it more likely to 'work' and to come loose. Good practices dictate no more than 2 washers. Beyond that you should change bolt length. If the bolt is too long, it is possible for the nut to bottom out before producing the required tension, giving a false reading of the proper torque having been applied.
- The best guarantee against a fastener coming loose is proper tension. The friction produced by tension prevents rotation. Once a nut turns on a spring washer, it is only a matter of time before the nut loosens itself totally. That is why we safety-wire in the manner we do. It absolutely prevents rotation.
- In the light structures used in aircraft, concern is not only with static joint strength, but the effect vibration, differential expansion rates and materials' differing elasticities.
- If you re-use a bolt in an aluminum structure, it is likely that the protective Cadmium plating will have been scratched, exposing the corrosive steel to the aluminum. With moisture, corrosion will result. How do you know that the bolt you have removed was not already over-stretched? Don't re-use bolts. Don't substitute, even if it is a 'stronger' bolt.
- Elastic stop nuts are ideal fasteners in applications where temperatures stay under 150ºF. These nuts tend not to scrape the Cadmium plating off the bolt as do the 'pinched metal' type of self-locking nut.
- After you finish torquing a fastener it is a good idea to put a dab of witness paint on it to remind yourself of what has and has not been done.
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