Interpretation ID: nht74-2.6

DATE: 09/26/74

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Eagle International Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your July 16, 1974, request for approval of the Bendix "dual circuit air brake system" for use on your buses in satisfaction of Standard No. 121, Air brake systems. In a subsequent phone call with Mr. Herlihy of this office, you stated that your only concern was whether the standard requires a parking brake system that meets the axle-by-axle retardation force requirements of S5.6.1 and the grade holding requirements of S5.6.2.

The National Highway Traffic Safety Administration is unable to "approve" plans or prototype systems for compliance with a standard in advance, because there is no way to establish that a vehicle so equipped actually meets the requirements until it has been manufactured.

With regard to your specific question, S5.6 states that each vehicle shall have a parking brake system that meets the requirements of S5.6.1 or S5.6.2 at the manufacturer's option. This means that you are free to choose a system which meets either of these requirements but does not meet both.

Yours truly,

ATTACH.

July 16, 1974

Sid Williams; 400 7th Street, S.W.; Washington, D.C. 20024

Dear Sid:

Enclosed is a copy of our Dual Circuit Air Brake System as developed by the Bendix heavy vehicle system group (their drawing number SA-8117-99). With this is an explanatory writeup describing a circuit operation for normal running, parking position and for service brake failure on the rear or front axle.

We would appreciate receiving your comments and possibly your approval for use in compliance with safety standards #121.

As previously mentioned we would like to avoid the use of spring brakes especially on the tag axle because these wheels are independently sprung and have a tendency to lock up because they do not have a load dividing connection with the drive wheels.

I would appreciate your help on these matters and hope to hear from you soon.

With warmest personal regards,

Sincerely yours,

Harry L. Cuthbert -- Chief Engineer, Eagle International Inc.

Enc: 2

SA-9117-99

EXPLANATORY WRITE-UP

NORMAL RUNNING

1. All reservoirs charged and accessory pressure protection valve open.

2. Parking Control Valve handle in release position. DD-3 Valve (Item 20) does not deliver air. DD-3 locks are disengaged. Service Interlock Valve controlled from its delivery line.

3. "Stand-by" Valve (Item 15) does not deliver any brake valve air, and low pressure indicator switches are open.

4. Service brakes can be applied by treadle valve; primary section of the brake valve applies rear axle and secondary section of the brake valve applies front axle.

SECONDARY BRAKES

1. A service brake failure, which would result in a rear axle circuit reservoir pressure loss will cause the "Stand-by" Valve (Item 15) to open. This will supplement the front axle brakes with rear axle brakes by applying modulated from axle circuit service pressure to the parking diaphragm.

2. A front axle circuit service brake failure will only cause a loss of front axle braking. Full service brakes can be applied on the rear axle through the treadle valve (Item 14).

3. In event of a broken treadle, the parking brakes can be applied as a "Back-up" emergency.

PARKING POSITION

1. When Parking Control Valve Handle (Item 18) is moved to the "park" position, the DD3 locks engage. The parking DD3 Valve will first deliver adequate pressure to the DD3 parking diaphragm, and then exhaust it automatically. In this condition, the parking application will remain applied strictly by "a mechanical means."

2. Service Interlock Valve being decontrolled, is closed and parking brakes cannot be released until PP-1 is pushed and a full service application is made and released, in this sequence.

RCF: dw Current for Drawing Revision #7; 6/18/74 (Graphics omitted) ITEM QTY DESCRIPTION 1 1 AIR COMPRESSOR 2 1 GOVERNOR 3 1 RESERVOIR-SUPPLY 4 1 (Illeg.) 5 1 (Illeg.) 6 1 (Illeg.) 7 1 (Illeg.) 8 3 DRAIN COCK 9 1 SAFETY VALVE 10 1 PRESSURE PROTECTION VALVE 11 3 SINGLE CHECK VALVE 12 3 LOW PRESSURE SWITCH 13 2 AIR PRESSURE GAUGE 14 1 (Illeg.) 15 1 (Illeg.)

TWO-STEP RELEASE OF DD-3 ACTUATORS.

2. (Illegible Words)

3. (Illegible Words)

(Graphics omitted)

(Illeg. table)