Posted on March 16, 2021

What is a D.O.T. Air Brake Fitting?

Ally Pulskamp
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Ally Pulskamp

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Have you ever noticed a Runaway Truck Ramp on the side of steep, mountainous highway or road? These off-ramps that end in a complete dead-end are for trucks and other large vehicles that lose control of their brakes. The ramp has an uphill slope that uses the power of gravity to slow the truck. 

There are many factors to help a driver when safely slowing or stopping a large heavy vehicle. Experienced drivers will know to apply the drum brakes and, if necessary, activate the jake brake on a downhill slope to reduce acceleration. Yet for normal stopping power, before beginning the incline and in everyday driving, drivers of large vehicles rely on a compressed air brake system. These are friction brakes that use compressed air that presses on a piston, which applies pressure to the brake pad or brake shoe to slow and stop the vehicle. Air brakes are mandated by the U.S. Department of Transportation for large, heavy vehicles, such as those with multiple trailers that require linking the brake system. Examples include trucks, semi-trailers, trailers, buses and railroad trains. d.o.t. air brake fittings 
The fittings that connect lines in an air brake system are vital to the control in stopping the vehicle. Referred to as D.O.T. fittings, the importance of these connections must be compliant to Department of Transportation standards. D.O.T. fittings follow the designation and approval for commercial motor vehicles set by the Department of Transportation’s Section 571.106 title 49 of the Code of Federal Regulations for both push-to-connect and compression fittings. 
Though compression fittings are sometimes used in air brake lines, push-to-connect fittings prove to be much more effective in these applications. 
  • D.O.T. push-to-connect fittings are faster and easier to install and disassemble in the compact spaces where air brake hoses run. They are easily connected by hand, where compression fittings require tools and special installation procedures.
  • By simply pushing the tube or hose into the fitting, they provide a leak free seal. Yet compression fittings require torquing the nut to a specified rate-- if over torqued, the fitting can cut into the hose or tube causing leaks.
  • Push-to-connect fittings make a sturdy connection able to withstand frequent vibration experienced by vehicles on the road, while compression fittings can easily become loose under these conditions.
  • As for assembly efficiency, it can take up to ninety percent less time to install a push-to-connect versus compression fitting. That means 900 push-to-connect fittings could be assembled in one hour, as opposed to only 90 compression fittings. In other words, one person can assemble the same number of push-on connections versus ten employees who are working with compression fittings. 

Air brake push-to-connect fittings are available in both nickel-plated brass and composite materials. 
Brass push-to-connect fittings eliminate complex assembly and do not require the time it takes to install brass compression fittings. However, they still retain the other benefits of a compression fitting, such as resisting rust and corrosion. They also don’t become brittle from the cold temperatures often experienced on the road. Their design of a collet and O-ring seal provides a leak-free assembly without requiring special tools. 
Composite push-to-connect fittings are made of a polymer body with brass threads. Composite fittings are about 43% lighter, nearly as durable and cost less than brass push-to-connect fittings. In addition, their lower weight is substantial enough to positively impact fuel efficiency and payload capacity. For instance, the average truck will use between 60 to 100 D.O.T. fittings. When using composite style instead of compression fittings, the weight reduction is around 7 to 9 pounds per vehicle. This doesn’t seem like much on a fully loaded 80,000 pound truck, but if that truck, or bus or train is one of a fleet of vehicles, the savings will quickly add up. 
A number of years ago a 40-foot moving van carrying 30,000 pounds of tightly-packed furniture was exiting the highway near Syracuse, New York. The ramp was a steep incline to a tee in the road at the bottom. The driver was frantically shifting down and jumping on the brakes trying to pump the air lines. Straight ahead at the bottom was a four-foot dirt embankment. The driver knew he had to turn right or left, otherwise the cab would be crushed into the embankment. He cranked the wheel to the right when reaching the tee in the road. The truck leaned over and laid down on its side with a crash and slid with the roof smashing into the embankment. The passenger was lying on top of the driver, though fortunately neither were hurt. After scrambling to get out of the passenger door, now above them, they saw a 20-foot drop over the embankment to railroad tracks and another 30 feet down to a river. Obviously, the air brakes failed. If he hadn’t thought quickly enough, the cab would have been crushed and the 30,000 pounds behind them would have flipped the truck over to slam down onto the tracks, and likely roll to the river below. 
It was later discovered a compression fitting came loose from the air brake line. Learn more about D.O.T. fittings for truck and trailer applications in this article
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