When truck and trailer OEMs select DOT air brake fittings, a number of considerations frequently come into play including performance, price, assembly time and the ability to customize to their specifications. Another important consideration is the type of thread pattern utilized on the port-ends of DOT air brake fittings. There are two main types of thread patterns typically offered on-air brake fittings, American National Standard Taper Pipe Threads (NPTF) and the straight thread O-ring (STO). Within the heavy-duty trucking industry, pipe threads are a known source for warranty claims. Find out why straight thread O-Ring fittings provide a more reliable seal.The challenges of pipe threads
Straight thread O-Ring (STO) fittings originally started in the hydraulic industry as a more reliable replacement for traditional pipe threads. Pipe fittings, or NPTF, feature tapered threads providing the mechanical strength needed to hold the joint together as well as provide the sealing of the connection. Pipe threads require the use of a thread sealant to create a proper seal. This can lead to several issues that will result in leakage. Inconsistent application of thread sealant can lead to leaks and correct application of the thread sealant can be problematic. Pre-applied thread sealant overcomes many of these challenges but is not without its own concerns.
Because pipe threads seal metal-to-metal, there can be some damage to the mating parts. The can lead to damage of either the female or male threaded part, resulting in an inability to remake the connection without leakage and requiring a replacement of one or both components.
Within the heavy-duty trucking industry, pipe threads are a known source for warranty claims. Many air brake system components, such as brake valves and air tanks, utilize NPT threaded ports. Air tanks are manufactured by machining the ports and then projection welding these to the air tank. The welding process can cause distortion to the threads during the heating and cooling of the welding process. When a pipe thread is installed into a port with distorted threads, a proper seal can be hard to achieve. A poor fit will result in air leakage. In addition, backing out the pipe thread to change position can cause issues. This is easily remedied with an STO fitting.A more reliable alternative
STO fittings have a male straight thread used to connect and hold the fitting in a female straight threaded port. The threaded connection does not act as a sealing mechanism. Instead, an elastomeric seal or O-Ring is utilized to create the seal and is able to compensate for most variations in the threads of the port. STO fittings are recommended to be assembled to a torque value to ensure accuracy of the seal, which is beneficial for consistent leak-free assembly in the current manufacturing environment.
Pipe threads have been around for many years and are functional. However, in today’s global market place the drive to automate and eliminate variability in products makes the STO fitting a preferred option.
The various types of STO fittings frequently used in the truck market include:
- Inch STO: Commonly used on components manufactured in the US, inch STO fittings meet SAE J1962 and ISO 11926 specification with the seal being made in a conical chamfer above the threads.
- Metric STO: Commonly used on components manufactured outside of the US, metric STO fittings can meet several different ISO specifications including:
- ISO 6149 – Seals in a conical chamfer above the threads. This meets the same specifications as SAE J2244 and DIN 3852-3. This specification is frequently used in coolant and fuel applications. (Parker FSC designation is -MIxx)
- ISO 9974 – Seals on the face of the port. This meets the same specifications as DIN3852-1 and ISO 4039-1. This specification is frequently used for coolant and fuel applications. (Parker FSC designation is -MxxR)
- ISO 4039-2 – Seals in a conical chamfer above the threads. This is similar to ISO 6149. However, there are slight differences in the male stud, port and O-Ring dimensions. This specification is based off an OEM design used in Europe for pneumatic braking systems on-road vehicles. (Parker FSC designation is -MAxx)
- Universal Design – Seals in both face seal and conical chamfer style ports at restricted pressures. This was one of the first STO designs to be manufactured in the US when metric threads started appearing in the North American truck market. Many customers were not sure which STO fitting their port required. The universal design was made to work in any port, which also allowed for the same fitting to be used in multiple applications. (Parker FSC designation is -Mxx)
As OEMs continue to eliminate pipe threads from their specifications in order to align with the global industry, this brings up another point. Metric port-ends may be becoming a standard, but the North American trucking industry continues to use imperial size tubing. Converting from a metric port-end to an inch push to connect tube end has generally required the use of an adapter, adding an additional cost. Parker has manufactured one of the only metric STO to inch push to connect DOT rated fittings in the market place to alleviate this issue, eliminating the need for one-off connectors or similar solutions with smaller, less reliable seals.
Parker’s new Push-to-Connect Metric Straight Thread O-Ring fitting is certified to ISO 4039-2 and meets or exceeds D.O.T. FMVSS571.106 and SAE J2494-3 standards for truck and trailer applications. Now available in 1/4”, 3/8”, 1/2” and 5/8" inch tube diameters and MA12, MA16 and MA22 thread studs in straight, 45 Degree Elbow, and 90 Degree Elbow configurations.
If you are looking for a more reliable alternative to pipe threads, Parker can help. Request a sample of the new PTC Metric Straight Thread O-Ring Fitting or locate your nearest Parker distributor to find out more about our full range of STO fittings.
See how Parker is going the extra mile for the transportation industry. View the video now.
Article contributed by Samantha Smith, marketing services manager, Fluid System Connectors Division, Parker Hannifin Corporation.
New Testing Results Give Way to Changing Specifications for Engine OEMs