Product engineers seldom receive training on fasteners, so some common mistakes are made in the design phase. The next five blogs will focus on some of these common mistakes.
What are Soft Joints?
A joint is considered “soft” when one or more of the materials being joined together is softer or weaker than the fasteners used to secure the joint. This may result in low preload which could lead to loosening or fatigue failure depending on the outside forces acting on the joint. Some examples of soft joints are gasket joints and plastics.
These are soft joints which sandwich a soft gasket material between two surfaces to seal against leaking. They must not be over tightened, or the gasket could fail. Over time, these gaskets can break down and begin to leak or allow the fasteners to loosen. An alternative method to help prevent failure of this joint is to design a step or groove in the gasket surface which allows the gasket material to compress and seal. This provides the hard surfaces to contact one another and achieve a higher clamp load, once again making a hard joint.
Many designs now incorporate plastics, which can be tricky to assemble without cracking. When attaching plastics to metal, the use of a shoulder screw or some type of compression limiter that prevents too much stress on the plastic material can be incorporated. Another alternative is threading specialty screws such as Delta PT® screws directly into the plastic. With the proper hole size and design, this method has been proven to be highly effective and economical.