The common mistakes in the selection of fasteners, see how many of you?

Update:21-07-2018
Summary:

The common mistakes in the selection of fasteners, see […]

The common mistakes in the selection of fasteners, see how many of you?

Core Tip: When using fasteners, first use them from the national standard specifications, and then select them in the industry. If necessary, design according to the special needs of the host. Among the requirements, anti-loose and fracture prevention are two main considerations.

First, in the coarse

The important couplings on the machine, such as the drive shaft, the flying shaft, etc., the bolts are mostly made of fine thread. When there is a missing piece in the repair, someone replaces it with a coarse bolt. Due to the large inner diameter of the fine bolt, the small pitch and the outer angle are small, the strength is high, the self-locking performance is good, and the ability to withstand impact, vibration and exchange load is strong; after being replaced by the coarse-toothed bolt, it is easy to loosen or take off and break. , causing mechanical accidents.

Second, the pores do not match

Bolts (such as drive shaft bolts and flywheel bolts) that are subjected to lateral loads and shear forces on the machine, and their mating with the bolt holes are transition fit, and the assembly should be strong and reliable, and can withstand lateral forces. Some people do not pay attention to the inspection during assembly. When there is a large gap between the bolt and the bolt hole, it is still installed. As a result, the bolt is loose or cut off.

Third, thick nuts

Some people often mistakenly believe that thickening the nut can increase the number of working turns of the thread, thereby improving the reliability of the coupling. In fact, the thicker the nut, the more uneven the load distribution between the threads of each ring, and the more easily the coupling is loose.

Four, one mother and more mat

In the installation, sometimes the bolts after assembly are too long, so some people put a lot of spring washers on one bolt, so that the spring washers are unevenly stressed during the sturdy process, and some will break, so that the bolts are pre-tightened. The force is reduced; or an eccentric load is generated, which reduces the reliability of the bolt connection.

Five, the gasket is too large

Sometimes, due to the lack of a suitable gasket, someone replaces it with a gasket with a larger inner diameter. In this way, the contact area under the bolt head with the washer is small, the bearing pressure or the locking force of the washer is lowered, and the bolt is easy to loose under the action of vibration and impact load.

Sixth, the torque does not match

Many people think that the bolts should be “never tight”, so they consciously increase the tightening torque of the bolts, resulting in bolt slips. On the other hand, for the important bolts that need to be tightened by torque, there are some people who use the adjustable wrench. To unscrew, the result is often loose due to insufficient torque, resulting in mechanical failure.

Seven, improper lockup

Important bolts should be locked after installation. When using the split pin to lock, the common mistake is to use a thin open lock or a half-piece open lock; the common mistake is to lock the washer opening too small, lose the elasticity; lock with the lock piece The common mistake is to lock the locking piece at the corner of the nut; the common mistake of locking with a double nut is to put the thinner nut on the outside and not tighten it tightly.

Eight, false and solid

Bolts, nuts or screw holes have rust, scale, iron filings, sand and other impurities, or the joint surface of the coupling has burrs, sand and other impurities, which are not removed before assembly. When tightening the bolts, due to impurities Blocking effect, the surface is tightened very tightly, the torque value has reached the requirement, but in fact the coupling is not really compressed; in addition, there is a case, due to the action of the sludge, scale and other impurities in the screw hole, Although the bolt torque is in compliance, the work piece is not compressed. Under such false sturdiness, vibration, load shock and temperature changes often cause the bolt to loosen quickly.

When selecting fasteners, first use them from the national standard specifications, and then select them in the industry. If necessary, design according to the special needs of the host. Among the requirements, anti-loose and fracture prevention are two main considerations.