Grooves and Glands

The physical space occupied by the seal is usually referred to as a groove, but it can also be referred to as a gland. In most cases, it’s a rectangular groove cut into the hardware whether it be on a piston, in the housing for a rod, or in the face of two mating surfaces.

Compared to O Ring and elastomer seals, PTFE and polymer seals are critically dependent on hardware conditions for proper functionality. Configuring the gland for proper seal installation and preparing the mating surfaces with the necessary finishes and hardness is necessary for successful sealing.

Diameters

OD – Outside Diameter

ID – Inside Diameter

Almost all the polymer seals we manufacture are round parts machined on CNC lathes. Therefore, the diameters are key dimensions for both the seal and mating hardware.

Diameter dimensions are proceeded by the symbol “Ø” on manufacturing prints. Many seal prints feature cross-sectional views where the diameters are not shown completely. This leads to some confusion about whether the radius or diameter is being called out.

A radius would be proceeded by the symbol “R” and would involve finding the theoretical center of a part to measure/inspect. Therefore, diameters are almost exclusively used to describe the external dimensions of parts.

Nevertheless, the diameter of thin flexible parts is sometimes difficult to measure. Tools such as a Visual Measurement Machine, which provides a non-contact method that also takes into account the inevitable part out-of-roundness, can be invaluable for accurate inspection.

The diameters of PTFE and polymer seals are also typically the most difficult dimensions to tightly control while machining, especially for larger sizes. This is due to the inherent instability of polymer materials. Fortunately, most seals are not critically dependent on diameter precision for functionality.

Radial Cross-Section

The radial cross-section of a seal sometimes referred to as the “wall” is equal to the OD minus the ID, divided by two. The radial cross-section is often the most critical dimension for the functionality of a seal. This dimension will determine the compression or squeeze on a spring or O-Ring energizer which is responsible for providing the contact and sealing force of the seal.

Too little or too much compression or interference can cause a number of problems from leakage to excessive wear. Fortunately, the cross-section dimension is typically easy to hold within tight machine tolerances and easy to inspect.

Seal Width or Height

The axial length of the seal can be referred to as either the seal width or seal height. JST typically uses width rather than height because this dimension is most often checked with calipers while holding the seal in your hands, rather than with a height gauge on a calibrated flat surface.

The seal width will always be dimensioned to have clearance in the hardware gland, but how much clearance can be is important. In reciprocating applications, a seal too undersized for the groove can shuttle back and forth in the gland causing accelerated wear.

This also creates an opportunity for the seal to become cocked or misaligned in the groove which can lead to premature failure.