Flange bolt/nut, as shown in the figure below, is replacing the bolt/nut + gasket form and becoming more and more popular.
We know that in the traditional sense, we add a gasket under the bolt/nut, and its main purpose is to disperse the load and prevent damage caused by excessive pressure on the joint surface. The flange plate added under the flange bolt/nut actually has the same purpose. So why is this design a better choice?
The method we are familiar with and most commonly used to control bolt tension is still the indirect control method through torque, but we also know that this method is not particularly accurate in controlling axial tension, because most of the torque is used to overcome the friction between the contact surface and the thread under the bolt head during tightening, and only about 15% is actually used to generate axial tension.
Compared with the bolt + gasket form, flange bolts can effectively improve the variables in the above tightening process. Let's take a look.
Standard washers are usually softer than the nuts or bolts they match, and often undergo plastic deformation under high compressive stress, as shown in the figure below. Once deformed, the clamping force will be significantly reduced. The nuts and bolts on the flange surface will not be affected by this, because the washer surface and the bolt/nut are integrated and have the same material strength.
Some students may say, then it is okay to use hardened gaskets, right? There is another problem here, that is, the gasket is installed misplaced. The opening in the middle of the gasket is usually much larger than the bolt diameter, because it is necessary to ensure that the transition fillet under the bolt head does not touch the gasket. The existence of this large gap will cause the gasket to be eccentric when tightening, resulting in local stress concentration, increasing the risk of indentation and joint surface damage, as shown in the figure below.
In addition to the above two factors, studies have found that when tightening bolts or nuts, gaskets will also rotate. Although it is not common, it is not easy to detect when it occurs. Once it rotates, the relationship between the applied torque and the bolt tension may change. Under a given tightening torque, the actual bolt tension generated does not achieve the expected effect.(来源：iMechanics机械)
In summary, flange bolts/nuts have obvious advantages over washers. Are you still using washers?
