Coupling Types - Pin Joints

The simplest coupling design consists of surface contact, which generally occurs with direct contact between large, relatively flat interface features. Because surface contact couplings have many possible contacts spread over the interface, deterministic design becomes difficult and repeatability depends mostly on flatness, finish, and other tolerances. Generally, direct interface contact can constrain only three degrees of freedom, which requires the incorporation of additional alignment features into the design for complete constraint of the six degrees of freedom.

The most common alignment geometry are pins, due to low cost and easy implementation for non-precise applications; however, strict tolerances or compliant geometries are required to achieve a reasonable repeatability. Without compliance, pin joints can easily destroy parts when the alignment features jam or deform. Compliant geometries, such as dovetails, grooves, and rails, improve repeatability of an interface by enforcing geometric congruence through predictable elastic deflections and the removal of irregular surface features through wear-in. Elastic averaging represents a subset of surface coupling types where improved accuracy is derived from the averaging of error over a large number of contacting surfaces.

In many industrial applications, preload to secure the joint is applied using bolts normal to the interface surfaces. Because of the frictional contact of the surfaces and the bolting load, surface contact couplings provide maximum stiffness and load capacity, which makes them suitable for most industrial applications where precision is not the crucial functional requirement.