There exist two main types of fastened joints: rigid and gasketed. Gasketed joints are used when sealing the flow of gas or liquid is a primary design concern, such as in piping, or in aerospace applications. This course only aims to cover rigid joints for the simple reason that gasketed joints are uncommon in the industrial automation industry. Custom machine building can often be viewed as “integrating”, or the integration of existing components like sensors, robots, or pneumatic cylinders to accomplish a design task. for those purposes, rigid joints are sufficient. It’s rare that the mitigation of fluid flow has to be taken into account in light industrial automation. Still, a quick explanation of gasketed joints will be included in the course.
The main difference between gasketed joints and rigid joints is the presence of a gasket, a part made of material more flexible and willing to deform than the parts being joined. This compliant material forms a seal when compressed to stop the flow of fluids, either gas or liquid. Gasketed joints are much more complicated to accurately design than rigid joints due to this deformation. The main difficulty is correctly estimating the appropriate preload, or initial tensile force, of the fastener. We’ll explain preload more in depth later, but for now, understand that any fastened joint will have some amount of compressive force which binds the mated parts together, and this compressive force deforms the gasket.
If too much force is applied, the gasket will crush, warp, or split, which will cause a leak. Or, the joint will become too rigid when it was meant to remain slightly flexible, either to accommodate mechanical stresses or thermal expansion. Then again, if the force is too little, the fastener won’t hold and the joint will leak anyway. Further complicating design, the elastic materials gaskets are made out of show considerable hysteresis on compression and recovery. They also will have a coefficient of thermal expansion which differs from the parts that are being joined.
We won’t go any deeper into gasketed joints here, but correct design procedures can be found in ASME Boiler and Pressure Vessel Code, Section VIII.