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Friction
Since torquing methods are the most common means of loading a bolt, friction is one of the critical factors in a majority of bolted joints. There are three general ways friction can cause problems in a bolted joints; it prevents a good portion of the applied force from being used to stretch the bolt, it decreases the accuracy with which all the bolts in an application can be loaded, and it causes galling of the threads.
A commonly accepted general statement says that only 10% of applied torque is transferred to force that actually stretches the bolt. Various friction losses account for the other 90% of the applied force. The friction between the threads of the bolt and nut accounts for roughly 40% of the losses, and the other 50% occurs between the nut and the mating surface on the flange. The word roughly is used here because in actuality, some of the friction forces are transferred to heat and other forms of energy. That figure of 10% can be drastically reduced by any number of factors such as poor surface finishes, lack of a proper lubricant, etc. This narrow margin of useful load has to be protected so the bolt can do its job, and to do this, care has to be taken to keep the effects of friction to a minimum.
Not only does friction reduce the amount of load that can be applied to the bolt, it also reduces the consistency with which a load can be applied. A mechanic using a torque wrench can not tell which bolts are subjected to more friction; the wrench indicates the torque applied to the bolt, not the actual tensile load the bolt is subjected to. Even if all the bolts on a single flange are initially torqued to the same value, friction can cause a wide variation in loads from bolt to bolt. The result is uneven compression on a gasket, damaged flanges, or the compression on the flange and gasket is nowhere near the minimum required to achieve a seal.
If enough pressure is applied, it is possible to actually fuse the threads of a nut and bolt assembly, and also the contact area between the bolt head (or nut) and flange. This is called galling; the load between the surfaces increases to a point where they actually weld themselves together. If galling occurs during assembly, a dramatic drop occurs in the amount of applied torque that is actually transferred to tensile stress. Most, if not all, of the applied energy is absorbed by friction as the metal parts fuse together. If severe galling occurs, the threads can no longer move relative to each other and the bolt can no longer be stretched via torquing. Increasing the torque applied to the bolt only increases torsional stresses but doesn't create any increase in tensile stress in the bolt. Galling can be reduced by decreasing the thread pitch, changing the bolt or nut material and by using a lubricant.
These friction losses are caused by a number of things. Surface finish is one; rough finishes can create higher friction in both the threads and the contact area between the bolt head (or nut) and flange. This is one reason flat washers are sometimes recommended under the head of the bolt and the nut; they provide a flatter, smoother surface resulting in lower surface friction.
The base material of the nut and bolt is another consideration. Given the same size and thread design, there can be considerable differences in the coefficient of friction of a bolt depending on its base material. Coating or plating a bolt can dramatically change its coefficient of friction, even if the base material is the same. Also consider that the nut is not necessarily made from the same material as the bolt. To decrease the coefficient of friction and to reduce the chance of galling, the engineer may specify a different nut material (with suitable strength characteristics) instead of one made from the same material as the bolt. For example, an ASTM A193, grade B7 bolt is a high strength, low alloy steel is sometimes used with a 300 series stainless steel nut to reduce galling.
A proper lubricant is one of the primary tools to reduce the problems caused by friction. In most instances, the use of a lubricant on a bolt lowers the coefficient friction when compared to the same bolt assembled dry. As coefficient of friction decreases, more of the torque applied is converted to tensile stress. If a mechanic uses a torque wrench to tighten two bolts, one with lube and one without, the unlubricated bolt will have a lower load than the lubricated bolt, even though the torque wrench indicates both are torqued to the same value. On the other side of the coin, the lower coefficient of friction makes it possible to apply the same load to the lubricated bolt with a lower torque than the unlubricated bolt would require.
Lubricants increase the accuracy of the torquing method used. The engineer calculates the torque required to achieve a certain load on a bolt, assuming a certain coefficient of friction which can vary widely on a dry bolt. The use of a lubricant increases the consistency in the coefficient of friction, and reduces the chances of galling. This consistency makes it more likely that the desired preload is achieved at the calculated torque value. On a flange with 12 bolts coated with a proper lubricant, it is more likely that the applied load will be closer to the calculated value, ensuring that the gasket stress is sufficient to maintain a tight seal. Not only is the achieved load closer to what is anticipated, but the variation in load between the bolts in the flange is reduced, ensuring uniform gasket compression. Without the accuracy provided by lubricants, the engineer and mechanic would be shooting in the dark trying to calculate bolt loads.
The choice of lubricant is based on, among other things, the operating temperature, media the bolt could be exposed to and coefficient of friction of the lube. If a lube is applied beyond its recommended temperature, the coefficient of friction and its lubrication properties will change. Note that the lubricant may start to break down at higher temperatures, so the coefficient of friction is usually only accurate at or around room temperature. Some applications require that only certain lubricants can be used because of safety or operational concerns. For example, nickel anti-seizes and other similarly made types of lube cannot be used if they are going to be exposed to chlorine, because of the risk of chemical attack or a severe reaction. The coefficient of friction for a lubricant can vary, the degree of which depends on its design. A product with minimal variation between the minimum and maximum coefficient offers the best consistency. Better efficiency, meaning lower friction losses, is achieved with lubricants having the lowest coefficient of friction.