Loose Connection of Threaded Fasteners: Causes&Precautions

The connection of threaded fasteners is easy to assemble and disassemble and can be used repeatedly.

Its disadvantage is that it may become loose in the working condition, especially in the case of vibration, impact, variable load and excessive temperature difference, it is more likely to become loose, or even the nuts fall off, making the equipment invalid, even causing malignant faults.

Therefore, it is one of the important tasks of designing and selecting threaded fasteners to find out the causes of loose connection of threaded fasteners and take effective preventive measures.

Related reading: Everything About Fasteners You Should Know

Loose Connection of Threaded Fasteners: Causes&Precautions 1

1. Reasons for loose connection of threaded fasteners

1.1 Looseness caused by initial tightening

After tightening the threaded fasteners, the unevenness and micro roughness of each contact surface (including the thread profile, bearing surface and the contact surface of the connected parts) of the threaded connection will be further reduced and run in during the working process, such as under the vibration state, or even the contact surface will be partially depressed, which will change the fastening state of the threaded connection, thus losing the pre tightening force and causing the threaded connection to be loose.

This loosening is called initial loosening.

The solution is to retighten the threaded fasteners after a short period of work to make up the pre tightening force lost due to initial loosening.

1.2 Looseness caused by the collapse of the bearing surface

In the connection of threaded fasteners, if the contact pressure of the bolt or nut bearing surface is too large, the surface of the connected part will produce plastic annular depression at the contact with the bolt or nut bearing surface.

Severe compression or compression aggravated due to the continuous reading of plastic deformation in the working process will reduce or lose the pre tightening force of the tightened fasteners, resulting in loose connections.

This looseness is called indentation looseness. Collapse deformation is unavoidable.

The measure to prevent collapse and loosening is to make a flange with increased contact area on the bolt or nut or a hardened washer with higher strength and larger contact area under the bearing surface.

1.3 Self loosening of threaded connection

Experience shows that the most common failure cause of threaded connections subjected to dynamic load is self loosening, and the failure caused by self loosening is the most frequent.

Self loosening mechanism: in thread connection, friction is generated on the contact surface of internal and external threads and the contact surface between the bearing surface of threaded fasteners and the connected parts.

When the threaded connection starts to loose, the torque M1 required to overcome the friction on the thread contact surface is:

Loose Connection of Threaded Fasteners: Causes&Precautions 2

Where: Q is the pre-tightening force acting on the bolt or screw, also known as axial force or clamping force; d2 is the pitch diameter of the thread; ρ is the friction angle.

For triangular threads,

Loose Connection of Threaded Fasteners: Causes&Precautions 3

μ 1 is the friction coefficient between thread contact surfaces; β is the half angle of tooth form; α is the rising angle of the thread helix.

(The rise angle of helices with different diameters is different, and it is based on the rise angle of pitch diameter helices.).

After the threaded fastener is tightened, the additional torque M2 generated by friction on the bearing surface of the nut or screw head is:

Loose Connection of Threaded Fasteners: Causes&Precautions 4

Where: μ2 is the friction coefficient between the bearing surface of the nut or screw head and the contact surface of the connected piece; D2 is the average diameter of the bearing surface (contact surface) of the nut or screw head.

When the threaded connection starts to loose, the total torque M required to overcome friction is:

Loose Connection of Threaded Fasteners: Causes&Precautions 5
Formula 3

According to the analysis formula 3, only when the total torque M is equal to or less than zero, the threaded fastener will automatically loosen.

For threaded connection, when subjected to static load, its friction angle p is always greater than the lifting angle a (satisfying the self-locking condition), the total value in the brackets in Formula 3 will not be equal to or less than zero, and the threaded fastener will not automatically loosen.

However, under dynamic load, such as under the action of vibration and impact, the normal pressure on the thread-bearing surface will instantly reduce or even be equal to zero.

When the friction constraint is lost, the nut will slide downward along the slope with the vibration, gradually making the nut loose.

Like a heavy object on an inclined plane, it will not slide down when it does not vibrate.

When it vibrates, it will slide down at the moment when it is bounced up due to the reduction or disappearance of friction.

This loosening is called the self-loosening of threaded connections.

Tens of thousands of vibration cycles have exhausted the anti-loosening friction resistance of the threaded connection, from slight loosening to complete loosening.

2. Common locking methods

2.1 Destroy the relationship between thread movement pairs

It is a very reliable anti-loose method to change the detachable threaded connection into the non-detachable threaded connection by welding, bonding or punch riveting (Fig. 1) to make the thread pair lose its movement characteristics.

Its disadvantage is that threaded fasteners cannot be reused, and the operation is troublesome.

It is often used in some occasions where it is required to prevent looseness without disassembly.

2.2 Locking with mechanical fasteners

Use mechanical fasteners to fix and lock between threaded parts and connected parts or between threaded parts and threaded parts to prevent looseness.

The reliability of this method depends on the strength of the mechanical fixings.

The disadvantage of the utility model is that the weight of the fastening connection is increased, the manufacturing and installation are troublesome, and the mechanical installation cannot be carried out.

See Fig. 2 for common locking methods of mechanical fasteners.

2.3 Increase friction

The purpose of preventing looseness is achieved by increasing the friction between threads or between the bearing surfaces of bolts (screws) and nuts or both.

The biggest advantage of this method is that it is not limited by the use space, and it can be assembled and disassembled repeatedly for many times and can be assembled mechanically.

Therefore, this kind of method is the most widely used.

2.3.1 Double nut

Loose Connection of Threaded Fasteners: Causes&Precautions 6

Tighten two nuts and bolts of the same height to increase thread friction to prevent loosening (see Fig. 3).

The assembly method is to first tighten the inner nut with 80% of the assembly torque, and then tighten the outer nut with 100% of the torque.

This can make the nut closely fit with the bolt thread, significantly increasing the anti-loosening friction resistance.

The utility model is characterized by simple structure, convenient assembly and good anti-loosening effect.

The disadvantage is that the weight is increased with two nuts, and larger installation space is required.

2.3.2 Toothed end face lock nut and lock screw

The lower-end faces of nuts and screw heads are knurled or made into serrations.

Under the pre-tightening force, “serrations” are embedded on the surface of the connected parts to increase the friction resistance between contacts (see Fig. 4).

This method has a good anti-loose effect.

The premise is to apply enough pre-tightening force to fasteners, because without a large pre-tightening force, there will be no anti-loosening friction resistance between end faces.

This method cannot be used with washers, and attention should be paid to the reasonable matching of hardness.

Generally speaking, the hardness of clamped parts should be lower than that of fasteners.

The disadvantage is that the sawtooth under the end face is easy to damage the surface of the connected piece.

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Fig.3
Loose Connection of Threaded Fasteners: Causes&Precautions 8
Fig.4

2.3.3 Elastic washers

Such washers include spring washers, saddle-shaped or wave-shaped elastic washers, tooth-shaped lock washers, etc. (see Fig. 5).

The utility model is characterized in that the spring tension of the washer or the friction resistance generated by the tooth warping is used to provide the locking function for the threaded connection.

The utility model has the advantages of simple structure, low cost and convenient use.

However, this kind of method has a poor anti-loosening effect and is not suitable for parts subject to severe impact and vibration.

2.3.4 Effective torque lock nuts and screws

The so-called effective torque refers to that before tightening, the nut needs to apply a considerable torque when it rotates overhead on the screw (while the general threaded fasteners can rotate freely during idling before tightening).

Effective torque-type lock nuts are divided into all metal lock nuts and non-metallic insert lock nuts.

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Fig.5

(1) Effective torque type all metal lock nut

This kind of nut is made by non-circular necking at the upper end of the nut or by grooving the upper end of the nut to make the fitting diameter locally smaller and deformed, thus increasing the friction resistance between matching threads, so that the bolt and nut are firmly locked together (see Fig. 6). This kind of nut has good locking effect.

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Fig.6

(2) Effective torque type non-metallic insert lock nut

This kind of nut, commonly known as nylon ring lock nut, is a nylon ring embedded in the upper end of the nut body (see Fig. 7).

When the bolt is screwed, the nylon ring is squeezed out of the internal thread.

The nylon material with good elasticity and toughness forms a large and stable friction resistance with the bolt, and has a good absorption and damping effect on external impact and vibration.

Therefore, the locking performance of the nut is much better than that of the all-metal locking nut, and its anti-loosening performance is excellent, and it has high anti-loosening reliability.

Moreover, the nylon material has good resilience, so it can be repeatedly assembled and disassembled for use, and is suitable for use under severe impact and vibration.

It can be used with any bolt from low precision to high precision;

It can also be used with any bolt from low strength to high strength.

The disadvantage is that the use temperature of the nut is limited by the use temperature of the nylon material.

The service temperature of the nut is generally – 50 -+100 ° C.

Too high temperature will soften the nylon material, and too low temperature will make the nylon material hard and brittle and accelerate aging, which will significantly reduce the mechanical and working performance of the lock nut.

(3) Nonmetal insert flange face lock nut

The nut is a flange with an increased contact area based on the non-metallic insert lock nut (see Fig. 8).

The nut has the advantages of non-metallic insert lock nut and flange nut, and has better anti-loosening performance.

However, the applicable temperature of the nut is also limited by the temperature resistance of nylon.

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Fig.7
Loose Connection of Threaded Fasteners: Causes&Precautions 12
Fig.8

(4) Bolt with pre-coated locking layer

The bolt (screw) with pre-coated locking layer refers to the threaded surface of the bolt (screw) rod coated with a layer of nylon and other macromolecular materials.

The locking principle of the bolt (screw) is similar to that of the nylon ring lock nut.

When the bolt is screwed into the nut, the nylon layer on the thread profile is squeezed, and the extruded material is filled into the gap between the internal and external threads, thus increasing the friction and achieving the purpose of locking.

However, there is no national standard for this product.

3. Application and review of anti-loosening methods

There are many methods to prevent the connection of threaded fasteners from loosening, which should be selected according to the specific situation in practical application.

What needs to be reminded here is that the spring washer and flat washer (see Fig. 5) commonly used in the mechanical industry are actually not ideal.

Statistics and anti-vibration tests of threaded connection show that the effective torque type non-metallic insert lock nut and the effective torque type non-metallic insert flange face lock nut have the best anti-vibration and anti loosening performance and the longest anti-loosening life.

After the nut has been subjected to a long period of intense impact and vibration, there is still no sign of looseness.

Some foreign experts call it the lock nut that never loosens.

Its disadvantage is that the use temperature is limited by the temperature resistance of nylon material.

At present, the nut has been widely used in non-high-temperature parts of cars.

It is believed that with the progress of technology, the improvement of understanding and the improvement of temperature resistance of nylon, this kind of nut will be widely used in a wider range.

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