The thread titles can pretty much explain themselves, but it’s still important to understand the difference between parallel and tapered threads because mixing up the two can lead to disastrous results!
Parallel threads are threads that have a parallel profile and maintain a consistent diameter all the way down the part. You can either measure the parallelism, or simply look at the profile and see if the sides of the thread are straight down the part. Pretty much all bolts have straight threads, except for self-cutting bolts, but ultimately the threads turn into a straight profile. Common straight thread fluid power fittings are SAE O-Ring Boss (ORB), British Standard Pipe Parallel (BSPP), or Joint Industry Council (JIC).
Notice that BSPP is a pipe thread, but does have parallel threads, so be attentive of which fitting you actually need.
Tapered Threads
Tapered threads are threads that taper along the thread profile and decrease in diameter as you travel down the part. In the same fashion as the straight threads, you can either measure the tapered profile, or simply observe the decreasing diameter of the threads. Tapered pipe fittings are the most common fluid power connection with National Pipe Thread (NPT) and British Standard Pipe Thread (BSPT). Especially for pneumatic applications, NPT fittings are a great option for functionality, availability and price.
Thread Mechanics
For fluid power fittings, parallel threads are not effective in holding pressure by themselves because the threads have a small clearance between the fitting and the mating threads. Because of this clearance, any air or fluid could work itself past the threads. You either need to apply a sealant such 3M adhesive (not suggested for high pressure) or utilize a mechanical seal such as an O-ring. SAE ORB fittings use an O-ring seal against the shelf of the fitting head, while SAE ORF use a seal at the face of the fitting.
JIC fittings do not have any rubber seals, but use a metal-seal against the mating cone of the thread body. To help ensure a good metal-seal, you can install a soft metal washer to fill in any imperfections of the surrounding steel faces.
Tapered threads utilize a metal-to-metal seal against the mating threads (which can also be tapered). When installing a tapered fitting, you can feel the resistance increasing because the threads are increasingly interfering with mating threads. Since the “seal” is strictly dependent on the metal interference, you’ll want to apply a sealant, such a “thread tape” or “pipe dope”. These sealants fill in the gaps between the threads and allow for consistent sealing properties.
Now that you know the difference and function between parallel threads and tapered threads, you can be confident in selecting a fitting or repairing a damaged assemblies. If you happen to run into any issues with identifying an existing fitting or selecting a new one, feel free to contact us at RG Group!