Fittings and Couplings (Fitting)
A fitting is the connecting element that joins a hose or tube to the valve, cylinder, and pump, and provides the seal. Standards such as metric, BSP, and JIC look alike but have different thread pitches and different cone angles. A mismatched fitting first leaks, then ruins the thread.
Sealing is achieved in two different ways, and knowing this distinction solves most problems:
- Cone-sealing types: JIC 37° and metric 24° cone types. What seals is not the thread but the metal-to-metal seating of the two cones. Overtightening crushes the cone.
- O-ring-sealing types: ORFS and O-ring boss types. Here the elastomer provides the seal; tightening torque only brings the faces into contact.
- Thread-sealing types: tapered threads such as NPT. The interference fit is made by the thread itself and the number of assembly cycles is limited.
When BSP and NPT threads, which look almost identical, get mixed up, the connection feels like it is threading in but starts to leak a few hours later. When in doubt, measure with a thread gauge and calipers. During retrofit kit installation there is no such risk because the hydraulic connections are not touched; but if hoses are also being replaced during the same service, dimensional checks must not be skipped.
Where this term is used: Connect Plus and Minus, Run: Installation That Does Not Depend on Technical Staff