Understanding terminology commonly used in an industry is crucial to comprehending the nuances of the products and services of that industry. Within the expansion joint industry there are a collection of terms that carry significance when making decisions on product selection. In 1971 the Fluid Sealing Association (FSA) surveyed leading engineering firms to better define some of the key terms used in the expansion joint industry. This list was studied by members of the non-metallic expansion joint division and reduced to the following terms.

Operating Pressure
The actual pressure in which the system performs under normal working conditions. This value may be positive or negative (vacuum).

System Design Pressure
The maximum pressure expected during operation. Typically, the calculated operating pressure plus safety allowance.

Expansion Joint Design Pressure
The maximum pressure the expansion joint can handle.

Surge Pressure
Operating pressure with added increase the joint will be subjected to for short increments due to pump start up, valve closings, etc.

Maximum Allowable Pressure
Defines the maximum pressure recommended for a specific expansion joint.

Hydrostatic Test Pressure
This is used to demonstrate system or expansion joint capability. The standard for this test is 1-1/2 times the maximum allowable pressure held for ten minutes without leaks.

Some other terms commonly used in expansion joint selection are spring rates. Spring rates are defined as the force required to move the expansion joint one inch. Movement in expansion joints is defined as:

Axial Compression
Movement along the central axis reducing the face-to-face measurement.

Axial Elongation
Movement along the central axis resulting in an increase of the face-to-face measurement.

Lateral or Transverse
Movement is a shear force perpendicular to the central axis.

Angular
A bending force where the faces of the joint are no longer parallel.

Torsional
A twisting motion not recommended for most expansion joints.