Interpretation ID: nht94-6.12

DATE: April 25, 1994

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Christopher S. Spencer -- Engineering

TITLE: None

ATTACHMT: Attached to letter dated 9/8/93 from Christopher S. Spencer to R. C. Carter (OCC-9128)

TEXT:

This responds to your letter about the brake reservoir requirements of Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems (49 CFR S571.121). I apologize for the delay in our response. You stated that you are developing a new reservoir design to improve reservoir volume without increasing the need for space. You asked how to test your reservoirs since you believe that "(t)he safety standard does not clarify the test criteria specifically how the reservoir is to be sealed."

By way of background information, the National Highway Traffic Safety Administration does not provide approvals of motor vehicles or motor vehicle equipment. Under the National Traffic and Motor Vehicle Safety Act, it is the responsibility of the manufacturer to ensure that its vehicles and equipment meet applicable requirements. The following represents our opinion based on the facts provided in your letter.

Standard No. 121 establishes performance and equipment requirements for braking systems on vehicles equipped with air brakes. The Standard's reservoir requirements for trucks and buses are set forth in section S5.1.2. That section requires these vehicles to be equipped with one or more service reservoir systems that meet specified performance requirements. Section S5.1.2.2 specifies the following:

Each reservoir shall be capable of withstanding an internal hydrostatic pressure of five times the compressor cutout pressure or 500 psi, whichever is greater, for 10 minutes.

The purpose of this requirement is to ensure that an air brake system reservoir has a minimum level of structural integrity. NHTSA has long interpreted the term "withstand" to require that there be no rupture or permanent circumferential deformation of the reservoir exceeding one percent. At one point, the agency issued an interpretation concluding that the term "withstand" meant that a reservoir can deform only slightly and must contain the applied pressure with only a limited pressure drop at any time during the test. However, NHTSA later withdrew that interpretation because it inadvertently increased the severity of the requirement. See 42 FR 64630, December 27, 1977, and 43 FR 9149, March 6, 1978.

You asked about this requirement in connection with a reservoir design that includes a bushing on the inside of an endcap. A weld is placed around the bushing. You describe two different procedures you have used to seal the reservoir.

In what you describe as "Test Criteria 1," a socket head plug is put into

the bushing with 3 full wraps of tape. With this first method, you state that as the pressure is applied to the reservoir, the endcap starts to expand out. The bushing stretches with the endcap, and as the bushing stretches the threads are pulled away from the plug. The plug must therefore be retightened several times before the required pressure is reached. In your "Test Criteria 2," you state that a rubber grommet or washer is placed on the inside of the bushing and forced to expand to seal the bushings from the inside. You stated that this method checks the weld but removes the threads from the test. With the second method, you state that there was no failure at over five times the working pressure.

While Standard No. 121 does not specify a particular test procedure for this requirement, the language of S5.1.2.2 makes it clear that a reservoir must "withstand" for 10 minutes a condition where the reservoir is pressurized at the specified level. Therefore, in conducting a compliance test, NHTSA would pressurize a reservoir to the specified level. This would necessitate sealing the reservoir.

In considering how a particular reservoir would be sealed, it is important to bear in mind that the purpose of the test is to evaluate the reservoir's structural integrity and ability to withstand pressurization. I can offer you the following comments on the two alternative test methods you described. The first method (Test Criteria 1) would appear to evaluate a reservoir's ability to withstand pressurization. The threaded plug would appear to reasonably approximate how the reservoir would be sealed in an actual use situation. I note that the mere fact that the plug needs to be tightened during the test to achieve the specified level of pressure would not indicate a failure but would simply reflect minor air leakage around the plug.

The second method (Test Criteria 2) would not fully evaluate a reservoir's ability to withstand pressurization, since it would, as you recognized, remove the threads from the test, thereby creating an artificial seal. It is our opinion that a reservoir would not be capable of "withstanding" the specified hydrostatic internal pressure if the threads failed under such pressurization. This would represent a structural failure equivalent to a rupture.

I hope this information is helpful. If you have any questions about NHTSA's safety standards, please feel free to contact Marvin Shaw at this address or by telephone at (202) 366-2992.