For tightening bolts, friction is a factor that cannot be underestimated. There is always friction in the bolted joints, which can be part of the cause of the problem and can also be used to solve the problem.

Friction is a force that occurs when two objects are in contact with each other. It is opposite to all capacity. No matter which direction the object moves, the friction drags the object in the other direction.

However, we need friction. Without friction, we can't walk, we can't sit in a chair or climb stairs. Everything just keeps sliding around.

FRANK NAUMANN is a former Managing Director of the German Fasteners Association and has more than 50 years of experience in the field of friction. He explains that friction is always present in bolted joints. On the one hand, it limits the efficiency of converting the torque to the required preload; on the other hand, it is indispensable for maintaining the preload in the joint to prevent loose parts.

Naumann said: "There are two coefficients that describe the friction between the rotating parts during the tightening process. The first is the friction coefficient μb between the bearing faces under the screw head or nut (μb = washer friction); The two are the friction coefficient μth between the threads (μth = thread friction). Both frictions consume a lot of energy and convert the energy into useless heat. For example, if μb=μth= 0.10, then only 16% of the torque is converted into a pre-tightening force. People can control the lubrication to influence the coefficient. In the automotive industry, the range used is mostly between 0.08 and 0.16."

In terms of bolting, friction can sometimes be used to solve the problem or it can be part of the cause. Of course, the clamping force of the bolt is mainly determined by the torque required to tighten the bolt, but it is also affected by many factors, such as the diameter and length of the bolt, the shape of the thread, and of course, it must be forgotten in the thread and the bolt. The coefficient of friction under the head and nut.

The coefficient of friction is necessary when calculating the tightening torque and the resulting bolt tension and stress, and calculating the friction generated between the joint faces.

However, the approximation of the coefficient of friction shown in the chart is only a representative value and should be verified against other sources of information and suitable tests.

The torque value is dependent on the friction force and is present between the thread and the bolt head and nut being tightened, with the fastening material or gasket (if used). In fact, almost all of the input energy is lost in the tightening process against the friction of the bolt head, nut and thread. Only a small fraction of the torque is converted to a clamping force load or tension.

The torque wrench cannot directly measure the preload in the bolt. When applying torque, it is necessary to overcome the bolt head and nut (depending on which end is twisted) and the static friction in the thread.

Most of the applied torque (about 50%) will be lost in friction against the bolt head and nut under the applied torque, and about 40% of the torque loss is over the friction of the thread, only 10% of the torque is effective Stretch the bolts and provide a preload.

Increasing friction is a common method of avoiding loose bolts. The most common examples include the use of deformed threads, or the use of nylon locknuts, or the use of serrated/toothed/star washers. For these solutions, the principle of friction locking is to increase the friction in the threads or under the bolt heads and nuts.

However, factors such as torsion, seizure and metal wear can have a negative impact on these friction-based solutions.

In friction-based locking methods, increasing the torsional stress in the joint can also cause problems. High torque may cause the fastener to be damaged below the estimated preload. However, due to uneven friction conditions, the required preload may not be achieved.

Fasteners made of stainless steel, aluminum, titanium and other alloys can also experience unpredictable thread wear (cold welding). When the bolt is tightened, the thread may wear during the fastening process of the fastener due to the increasing pressure of the contact surface and the sliding surface of the thread.

In extreme cases, metal wear can cause seizures, that is, the threads and bolts are locked and fixed together. Continued tightening may result in broken fasteners or tearing of the threads.
Lubricating oils are often used to reduce friction and achieve a uniform clamping force load. Frequent lubrication of internal and external threads eliminates thread wear.

According to Michael Stähler, product manager at Dörken, the current trend is to apply a suitable coating on the bolt or nut that is designed to perform the intended work. In fact, this coating contains lubricating oil; it is no longer necessary to lubricate it separately after applying the coating, as this is all that is necessary. In addition, this coating is unstable to temperature and can cause the joint to loosen itself.

However, the lubricating oil will significantly reduce or even eliminate the locking effectiveness of any friction-based locking method. Therefore, the lubricating oil must not be used in combination with the friction-based locking method because the lubricating oil will weaken the locking effect.

Controlling the clamping load is very important and can be achieved by reducing the instability of the friction coefficient. When using a bolted joint solution, lubricating oil can be used if it is not locked based on friction.

By reducing the friction of the lubricant, we can increase the control of the preload and reduce the instability of the friction coefficient. This will allow the bolt to reach full load and extend the life of the bolt joint. Bolt failure due to fatigue wear can cause annoying production downtime.

NORD-LOCK's innovative solution for friction

The NORD-LOCK washer uses tension rather than friction to tighten the bolt joint. The design of this gasket creates a wedging effect, as evidenced by the increase in tension observed when loosening the bolt.
This wedging effect avoids loosening caused by thread vibration or shock. Due to the normal settlement between the contact surfaces, the bolt will only lose a small portion of the preload during the initial period.

Reducing thread friction while safely tightening bolts is often considered impossible. However, with the help of Nord-Lock's tension-based wedge-type locking method and lubricants, the possibility is made possible.

Nord-Lock application engineer Lena Kalmykova said: "The wedge-in locking method is based on tension rather than friction. The most common wedge-type locking system is a pair of cam-shaped washers whose height is higher than that of bolts. The pitch is also large. The washer pair is mounted by mounting the cam surface opposite the cam surface. When the bolt/nut is tightened, the teeth bite and engage the locking engagement surface, allowing only movement on the cam surface. Any rotation of the bolt/nut It is blocked by the wedge effect of the cam. The wedge-type locking ability is not affected by the lubricating oil. In addition, by using this gasket, you can fully utilize the full performance of each bolt."

Facts about friction

Friction depends on the characteristics of the surface - the degree of flatness, roundness or roughness. Friction also depends on the medium on which the surface is located—wet or dry, or particles in the medium.

This is a multidisciplinary field as it relates to fields such as mechanics, chemistry and fluid dynamics. Only by reaching the atomic level can you have a full and detailed understanding of friction.

The classic rules of sliding friction were first discovered by Leonardo da Vinci and later rediscovered by Guillaume Amontons. Amon East proposed the nature of friction from the unevenness of the surface and the force required to lift the weight of the pressed surface.

In the last century, at the micro level, the actual contact area was only a small fraction of the apparent contact area. The development of atomic force microscopy has enabled scientists to study friction at the atomic scale.

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