Heavy-duty vehicles, such as trucks and trailers, use air brake systems to assure safe and effective stopping. Air brake tube fittings are a virtually invisible component on these vehicles, but they are instrumental to the reliable operation of the air brake system. A typical truck requires between 60-100 air brake tube fittings, and these components can have a significant effect on brake system performance, cost and assembly line productivity. In this blog, we will explore the major types of DOT air brake fittings used by truck and trailer OEMs and how they compare on price, performance, ease of assembly and customization as well as new technology trends.
The U.S. Department of Transportation (DOT) standards have been in place for over 50 years and they specify how air brake hose and tubing, assemblies and end fittings used on trucks and trailers are expected to perform. The official requirements for DOT-certified air brake fittings are published in Title 49 of the Code of Federal Regulations, Section 571.106.
These regulations were developed to reduce injuries and fatalities that could occur as a result of brake system failure. DOT-certification is required for the air brake hoses and tubing used to control the air brakes, and for the end fittings directly connected to those hoses and tubing. There are certain applications for fittings and tubing outside of the air brake system on heavy trucks, including air horns, air-ride seats, axles and others. Fittings and tubing used for these applications do not need to be DOT–rated. However, most OEMs choose to avoid the mistake of applying a simple industrial fitting into an area of use requiring DOT conformance. They will standardize on DOT conforming parts, adsorbing the higher cost to ensure safety.
Air brake tubing, tube fittings and plastic air brake tubing assemblies must be able to meet the following code requirements:
There are several key factors truck and trailer OEMs should consider when choosing DOT conforming air brake fittings:
An industry staple for decades, brass compression fittings are trusted for their dependable, versatile application and comparatively low price point. While they are priced competitively, installation tends to be more labor-intensive than other types of fittings. Once installed though, they are considered excellent in terms of forming joint connections and offering superior sealing performance.
Brass compression fittings withstand high heat. As such, they are often chosen for environments near air compressors and in engine compartments.
Customization lends itself well to brass compression fittings. Brass can be manufactured into various shapes with relative speed, however, its physical properties can add some manufacturing complexity.
Introduced in the 1990s, brass push-to-connect fittings offer reductions in fitting time assembly and complexity. They maintain the performance profile of brass compression fittings, yet allow for relatively foolproof assembly with no additional tools other than a tool cutter.
The biggest benefit to brass push-to-connect fittings is the assembly time savings. Installation time can be reduced by as much as 90%. Compounding this savings over time and installations can represent an enormous cost improvement for the user. In addition, the brass push-to-connect fittings can be color-coded. This helps to identify where the fittings are to be installed, saving more time and reducing installation errors.
Although the unit cost is higher for a brass push-to-connect fitting than a compression fitting, the labor savings more than make up the difference.
Brass push-to-connect fittings offer similar customization attributes to compression fittings. But the advances in simplicity, fool-proof design and color coding, make them an exceptional choice, adding value to truck OEMs.
Manufactured with a polymer body and brass screw threads, composite push-to-connect fittings offer all the labor saving features of brass push-to-connect fittings. However, due to the polymer, the material is lighter in weight and lower in cost than brass. The 43% weight reduction, when considered in aggregate on board a truck where 100 or more DOT fittings may exist, can be a substantially positive design factor. Customization is also enhanced as the polymer is formed using versatile injection molding processes. This allows for freedom in creating uniquely shaped bodies for specific applications.
In recent years, engine temperatures have increased, due to design enhancements that offer other positive benefits. This has pushed the upper-temperature limits of some DOT push-to-connect fittings and tubing. When maximum operating temperatures are close to, or surpassed, the probability of problems occurring, such as leaks and premature hardening of elastomeric seals, increases.
This challenge can be met in several ways. Some manufacturers have opted for brass compression fittings in high heat areas. Others have chosen to modify the seal used in push-to-connect fittings. They have increased the thickness of the elastomeric seal, thereby offsetting the accelerated setting activated by heat. Still, others have designed completely new DOT-rated push-to-connect fittings, with a higher maximum temperature specification.
Two main types of thread patterns are typically used on air brake tube fittings: American National Standard Taper Pipe Threads (NPTF) and Straight Thread O-ring (STO).
While NPTF threads are far more common, STO fittings are found to be more reliable connections where they are employed. This is largely due to the sealing being completed by an elastomer O-ring as compared to the metal-to-metal sealing in an NPTF fitting. In NPTF fittings, voids in the connection remain, increasing the chance of leaks.
An STO fitting does present an increased cost to truck OEMs. They may require some customization of the fittings or their respective components. The added reliability of the STO fitting will need to be weighed against the increased cost by the truck OEM.
Truck OEMs want to avoid performance issues with air brake assemblies at all costs because these may lead to accidents, complaints, investigations, warranty claims and recalls – with significant financial implications and public repercussions. Careful consideration should be given to selecting the right type of DOT fitting to optimize brake system performance while at the same time keeping costs in check.
This blog was contributed by the Fluid and Gas Handling Technology Team, Parker Hose Products Division.