Quick measurement method of aviation fastener size and use of torque wrench
Quick measurement method of aviation fastener size and use of torque wrench
1. What is fastener torque?
Moment, which is the product (M) of the force (F) and the force arm (L). For aerospace threaded fasteners, it is necessary to have proper contact between the fasteners to overcome the tension or shear force between the two separate components, while not damaging the internal structure of the fastener. Therefore, it is particularly important to set the appropriate torque for threaded fasteners. A torque wrench is required when setting or applying torque.
2. What are the aviation fasteners?
Bolts can hold two or more parts together, and most bolt heads are hexagonal or dodecagonal. The size of the bolt is determined by the length and diameter of the rod, and can be divided into coarse thread bolts and fine thread bolts according to the distance between the accumulated grains. In practical applications, the magnitude of the torque is only related to the diameter of the bolt and the type of thread teeth. Under normal circumstances, the diameter interval of different types of bolts is 1/16; the thread teeth are expressed in the number per inch. A thread size of 0.375-24 in the circle means that this bolt is 3/8 in diameter and has 24 threads per inch. And when we get a bolt, how can we quickly determine the size of the bolt?
We know that the size interval of the commonly used open-end wrench is generally 1/16, so the diameter of the bolt can be quickly determined by clamping the screw with a suitable open-end wrench.
For the determination of the size of the teeth, it is necessary to use a special tool thread gauge (screwpitchgage) or pitch gauge (see Figure 4). The thread gauge is composed of many slices, and the Custom Steel Screw Anchor teeth on each slice are complementary to the thread teeth of a certain size. When measuring, clamp the toothed steel sheet of the tooth gauge on the bolt. If it is not tight, replace it until it is tight.
The screw is another kind of threaded fastener that is widely used in aircraft. Its structure is similar to that of the bolt. The main difference between it and the bolt is that the strength of the material is low, so it is hardly used in the bearing structure.
There are three types of screws commonly used in aircraft structures, namely, widely used machine screws, structural screws with strength properties and close bolts, and self-tapping screws commonly used in lightweight materials.
Nuts are another commonly used threaded fastener, often used in conjunction with bolts and screws.
In the aircraft, especially in some high vibration areas, the landing gear engine area must take some measures to prevent it from accidentally loosening or falling off. Common methods include installing cotter pins, fuses and using self-locking nuts. For cotter pin nuts, the torque cannot be changed arbitrarily in order to force the proper installation position of the nut. A different shim should be tried to obtain the required torque in the proper mounting position.
For the self-locking nut, the torque must be applied after the nut is screwed in place. In addition, due to the self-locking ability of the self-locking nut, it has a gross moment that cannot be ignored, that is, a large torque is required to screw the nut in place. Therefore, when applying torque, the gross torque must be added to the target torque value in order to obtain the correct torque value. For the measurement of gross moment, there are many methods. The most common is to screw the self-locking nut until the teeth are exposed, then slowly tighten the self-locking nut using a deflection beam or dial torque wrench to obtain a gross torque reading on the dial. For the reusable self-locking nut, its self-locking ability needs to be measured to determine whether the self-locking nut can be used again.
The washer provides a bearing surface for the nut and also provides the proper clamping section length for the nut and bolt. At the same time, it is also often used to adjust the installation position of the cotter pin nut. In addition, it can increase the tension between the nut and the member, thereby preventing the nut from loosening due to vibration. Three types of gaskets are commonly used in aircraft construction - large area gaskets, self-locking gaskets and special gaskets.
3. Precautions for the use of torque wrench
Torque wrenches are valuable precision instruments. In order to standardize aviation maintenance and prolong the service life of torque wrenches, the following points need to be paid attention to.
(1) The torque wrench cannot be used for disassembly
A torque wrench is a measuring instrument, especially a mechanical preset torque wrench commonly used in the industry. It can only be used to determine the torque when tightening a screw, and can never be used as a disassembly tool to loosen a tightened screw. Although some torque wrenches can be switched to counterclockwise operation via the paddles of the ratchet, they can only be used to unscrew loose screws. The static friction force of the thread in the tightened state is much greater than the dynamic friction force in the loose state, especially after the screw is installed for a period of time, the self-spoon torque required to loosen the screw is much greater than the torque of the tightening (up to 1.5 times), and the reverse The use of the wrench is easy to damage the internal structure of the wrench, resulting in failure to use it normally. Some torque wrenches, such as dial-type torque wrenches, can measure torque in both directions, but the counterclockwise operation is not used to loosen the screw, but to tighten the reverse thread. Therefore, when removing the screw, you should use a common wrench to loosen it first, and then use a torque wrench or a ratchet wrench to increase the removal speed.
(2) Select the correct range
Torque wrenches have torque ranges, called ranges. In order to prevent damage to the wrench and parts, before using the torque wrench, be sure to correctly understand the use range of the wrench. Generally speaking, when choosing a torque wrench, it is best to have a working value between 20% and 80% of the wrench range, so as to ensure that the internal structure of the wrench will not frequently touch the critical point, increase the service life of the wrench, and ensure long-term use. Accuracy.
(3) Maintain the correct force application process
Before applying force to the torque wrench, make sure that the head of the wrench and the fastener are in close contact, and the part held by the wrench should be at the rear of the wrench handle, and the adjustment knob should be locked at the same time. In the process of applying the torque, the closer the preset torque is, the more stable and even the slow force should be applied, and the medium force should not be used, so as not to exceed the standard torque. After reaching the preset torque, stop applying force immediately. If the torque applied to the fastener exceeds the required torque value, the fastener should be loosened and then retorqued. In addition, in order to ensure that the torque meets the requirements, at least two more torque tests should be performed.
(4) Return to zero in time after use
For the drag-jump torque wrench, the torque value needs to be preset before use. Timely reset to zero after use is beneficial to avoid plastic deformation of the internal spring, thereby prolonging the service life of the torque wrench and maintaining the accuracy of the torque wrench. For torque wrenches that are not designed to be zeroed, they should be reset to the minimum zero scale after use.
The torque wrench is a precision instrument, and its accuracy is easily affected by vibration. In order to maintain the accuracy of the torque wrench and prolong its service life, it must be handled with care. For the torque wrench with a box, the torque wrench should be placed in the box during transportation.
