Bolts with an M20 designation are use for medium-duty applications. M20 bolts are use when a bolt must hold significant weights, but does not need to be the size of an M24 bolt or larger. Due to many different uses of M20 bolts, it is essential to understand the thread pitch that the bolt will use.
The thread pitch of an M20 bolt will impact the way the joint behaves under a load, as well as the amount of clamping force that the bolt will provide when tighten. A thread pitch chart will show the various pitches that can be used for M20 bolts, the impact that the pitch will have on the minor diameters of the bolt, the pitch diameter of the bolt, as well as the stress area of the bolt. The stress area is the part of the bolt that is use to provide tension to the bolt.
Choosing and Using M20 Bolts
Coarser thread pitches will remove more material from the base of the bolt threads than finer thread pitches. Therefore, the stress area will be lower for bolts with coarse thread pitches than bolts with fine thread pitches, as the stress area can better handle the loads that is placed upon it. Coarser thread pitches will allow for the bolts to handle more of the load before the bolt begins to yield.
Many fabricators will opt for the coarse 2.5 mm pitch of M20 bolts because it is faster to assemble the bolt with this thread pitch, as well as because it is more likely to tolerate any debris that may be present in the joint threads. The coarse 2.5 mm thread pitch, however, will have a lower tensile capacity with the joint, as well as be more likely to become loose with vibrational movement of the joint. Fine pitch M20 bolts, in contrast, will advance at a slower rate with each turn of the bolt, which allows for the bolts to more easily reach the desired preload for the joint.
Additionally, the shallower angle of the flanks of the fine pitch M20 bolts allow for better resistance to vibrational movements of the joint. The precision that fine pitch M20 bolts allow, however, require more manufacturing tolerances for the bolts, as well as requires that the bolts are handled with more careful during installation. The type of M20 bolt that is to be used will impact the torque that should be applied to the bolt head.
For example, a Grade 8.8 bolt will have a recommended dry torque of 190 Nm for the bolt threads if they are clean and dry. By lubricating the threads of the bolt, however, that same preload can be achieved with only about three-quarters of the dry torque. Both lubricated and dry torque values can be seen on a thread pitch chart for M20 bolts.
Using the wrong torque will either under-clamp the joint with the bolt, or the bolt will be stretched beyond it’s yield point. The geometry of the threads of the M20 bolt will impact the way that the bolt fits within the joint. The diameter of the M20 bolt’s major diameter will always be 20 mm.
The pitch diameter, however, will impact how the bolt fit within the joint. If the pitch diameter of the M20 bolt is not within the proper tolerance to that of the mating threads within the joint, the bolt will either become stuck within the joint, or the bolt will not feel secure within the joint. Therefore, the pitch diameter is another value represented on a thread pitch chart for M20 bolts.
The manner in which the M20 bolt is to be installed will also impact the formation of a successful joint. The joint should be brought to a snug-tight state, after which the final torque should be applied in a star pattern to the bolt head. Using a star pattern when applying the final torque will ensure that the joint is even in its compression, preventing failures of the gaskets that are placed within the joint.
For structural joints, the turn-of-nut method is often used to apply the final torque to the M20 bolt. The turn-of-nut method measures the number of revolutions of the bolt head until the joint is properly tighten, which is more reliable than simply applying torque with a wrench. While a thread pitch chart will not replace proper installation techniques for M20 bolts, the chart will provide the values for the proper installation of the bolts.
Bolts with an M20 designation are used in a variety of applications. For instance, M20 bolts are often used in the formation of steel structures, pipes, wind turbines, and heavy vehicle. The requirements of the various applications will dictate the type of M20 bolt that should be used in each of those applications.
For instance, because wind turbine components are subjected to the cyclic loads with the rotation of the turbines, bolts with fine pitches may be required to provide the longevity of the structures. In contrast, bolts with coarse pitches can be used in the formation of joints within pipes, especially if the pipes are within an area that does not experience significant vibrations; the coarse pitch will allow for the workers to complete the installation of the bolts at a faster rate. It is also important to be able to recognize the differences between an M20 bolt and an imperial 3/4-10 UNC bolt.
Each of these types of bolts are of similar diameters, but have different thread pitches, stress areas, and wrench size. M20 bolts have a larger stress area than a 3/4-10 UNC bolt, and require a wrench size of 30 mm. The 3/4-10 UNC bolt, in contrast, requires only a 1-1/8 inch wrench size, and has a smaller cross section in relation to the M20 bolt.
If the distinction between these two types of bolts is not recognized, it is possible for workers to select the wrong bolt for the joint to be form. Finally, the proper configuration of the M20 bolt should be selected for the joint that is to be formed. The decision between the various pitches for M20 bolts will depend upon the requirements for the joint.
The thread pitch chart provides the data for the bolts to be configured in an appropriate manner. After determining the correct type and pitch for the M20 bolt, it is also important to ensure that the threads are clean, that the proper amount of lubrication is used, and that the desired amount of torque is applied to each bolt. Ensuring the proper use of all steps will transform an M20 bolt into a reliable joint for the components that are to be connected.
