There are two main types of threads widely used in the industry defined by the nature of the thread form.
– Tapered (Pipe) Threads
– Parallel Threads
Tapered (Pipe) Threads
Pipe threads, as the name suggests, were first invented to join pipes together. These threads take a tapered form achieving a metal-to-metal pressure tight seal when assembled. Most common tapered threads used in the industry include NPT (National Pipe Taper), BSPT (British Standard Pipe Taper) and Metric Taper.
One of the limitations with some tapered threads such as NPT is that when assembled, the gap between the threads of the male and female connectors will leave a leak path. Therefore, a thread sealant will be required to achieve a leak-free seal. Thread sealants will fill any gaps and imperfections of the thread to enable full contact and makes it easy to assemble averting any galling effects.
Common thread sealants used in industrial applications include Teflon thread tape and chemical sealant. Teflon tape is a non-sticking tape that is easy to apply on the thread surface without damaging the thread. However, the tape is prone to tear while assembling which could lead to clogging and contamination of hydraulic systems. Therefore, care must be taken to apply the tape in the direction of the thread so that it would not unravel and tear while assembling. Chemical sealant minimises the likelihood of system contamination but can have its own drawbacks due to incompatibility with connector material, temperature limits etc.
There are however modifications to the tapered thread such as NPTF and NPSM with modifications in thread crest and root design to facilitate full contact between the surfaces establishing a seal without thread sealant. Although, these types of threads may also require thread sealant to achieve complete sealing of the joint.
When designing connections with tapered threads, following limitations should be considered:
- Lack of sealing in larger thread sizes
- Not ideal for dynamic pressure systems
- System clogging and contamination due to sealant
Due to its reliability and ease of maintenance, parallel threads are often chosen as the preferred connection method in hydraulics.
The prime function of parallel threads is to facilitate a joint (hold) between two connectors. A sealing method will need to be accompanied to complete a leak-free connection. Therefor the following sealing methods are present in connectors with parallel threads.
- Metal Seal
- Soft Seal
Metals seal type connectors achieve sealing by means of a machined face on one connector and a matching, inverted face on the mating connector. These machined “seats” provide better sealing compared to traditional tapered connectors and more importantly, no thread sealant is required.
Common seat angles featured on connectors are 30° (e.g. BSPP), 37° (e.g. JIC), 45° (e.g. SAE).
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For port applications, soft seal connectors are used due to their ability to retain sealing at high pressures and vibrations.
Soft seal connectors feature an elastomeric material such as O-Ring, Encapsulated Ring or bonded washer. Some of the widely used soft seal connections include SAE Code 61 / 62 Flange, ORFS and UNO.
For an overview of the different hydraulic connector types, check out Hydraulic Connectors 101.
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