nht91-5.28 | NHTSA

DATE: August 19, 1991

FROM: Thomas D. Turner -- Manager, Engineering Services, Blue Bird Body Company

TO: Paul Jackson Rice -- Chief Counsel, NHTSA

COPYEE: Robert Hellmuth -- OVSC

TITLE: Re Request for Interpretation: 1. 49 CFR Part 571.217 Sections S5.3 and S5.4 2. Report No. 217-MSE-90-10-TR0009-10 Bus Window Retention and Release Compliance Test-FMVSS 217 1990 Blue Bird (33 Passenger) 3. NEF-31KNu/NCI 3189 4. NEF-31KNu/NCI-3189.2

ATTACHMT: Attached to letter dated 11-13-91 from Paul Jackson Rice to Thomas D. Turner (A38; Std. 217)

TEXT:

A 1990 Blue Bird bus was compliance tested to the requirements of FMVSS 217 as reported in Reference 2. As a result of the test, NHTSA's Office of Vehicle Safety Compliance (OVSC) issued NCI letters, including references 3 and 4. Blue Bird Body company responded as requested to reference 3 and requested and attended a technical meeting on July 2, 1991 in Washington, D.C. in response to reference 4. As a result of this activity, there is one issue that needs to be formally resolved before NCI 3189 can be closed. The following paragraphs discuss the issue, provide the OVSC position, provide the Blue Bird Body Company position and request an interpretation of which position is correct according to Standard No. 217.

49 CFR Part 571.217-Sections S5.3.2 and S5.4.1

Discussion:

Section S5.3.2 provides two force application options for options for emergency exit release mechanisms. Option (a) LOW FORCE APPLICATION specifies type of motion as "Rotary or straight", and magnitude as "Not more than 20 pounds." Option (b) HIGH FORCE APPLICATION specifies type of notion as "Straight, perpendicular to the undisturbed exit surface.", and magnitude as "Not more than 60 pounds." Section S5.4.1 establishes emergency exit extension reach distances and force levels based on Section S5.3.2.

OVSC Position:

As stated in Reference 4, the OVSC position is: "In this case the release handle is pin jointed at the top much like a piano hinge. The only possible movement the handle can experience is to rotate around the pin joint. The release mechanism neither slides nor translates, IT VERY CLEARLY ROTATES. The video that Blue Bird supplied this office shows that the handle rotates during the release of the window. An occupant of the bus would then have to apply a force in a rotary notion to operate the release mechanism. The maximum release should therefore be 20 lbs."

Blue Bird Body Company Position:

Blue Bird Body Company agrees with the OVSC that the release mechanism of the pushout window design used by Blue Bird is rotary; however, it is our understanding that Standard 217 regulates the "type of motion" of the FORCE APPLICATION used to manually operate the release mechanism and does not regulate the "type of motion" of the RELEASE MECHANISM itself. Blue Bird Body Company disagrees with the OVSC that an occupant would have to use a rotary motion to operate the mechanism. The introduction of the book The Way Things Work by David Macaulay supports Blue Bird's position by stating:

MOVEMENT AND FORCE Many mechanical machines exist to convert one form of movement into another. This movement may be in a straight line (in which case it is often backward-and-forward, as in the shuttling of a piston-rod) or it may be in a circle. Many machines convert linear movement into circular or rotary movement and vice versa, often because the power source driving the machine moves in one way and the machine in another.

But whether direction is altered or not, the mechanical parts move to change the force applied into one-either larger or smaller-that is appropriate for the task to be tackled. (1)

The essence of our argument and position is the principal that a straight linear force can be used to operate a rotary mechanism. A weight put on a lever arm and used to lift an object against the force of gravity is an example of this principle. The straight linear force of gravity acting on the weight is the only application force acting on the lever arm. The lever arm's reaction to this force will be rotary motion and will involve other motions and forces; however, the application force of gravity, since it acts only in a direction toward the center of the earth, remains straight and linear. As another example, a latch string, extending through the latch string hole of a door, can be pulled straight out and perpendicular to the surface of door to activate the latch bar and allow opening of the door. Note that in this example, the latch bar pivots or rotates in a different plane and direction from the application force.

Since the pushout window release mechanism of the Blue Bird pushout windows is in the high force application region and is releasable by a single occupant, operating one mechanism, using one force application that is straight, perpendicular to the undisturbed exit surface, we must be allowed the use of option (b) HIGH FORCE APPLICATION and, therefore, are allowed a magnitude of 60 pounds.

The report, reference 2, determined the type of motion of release mechanism of the Blue Bird pushout window to be rotary and, therefore, determined the required force application to 20 pounds maximum. It determined the type of motion for extension of the pushout window to be

straight and perpendicular and, therefore, allowed a 60 pound application force. The pushout windows are hinged at the top and must swing out (rotate) to open. In a nearly identical way, the release mechanism is hinged at the top and must swing in (rotate) to unlatch. The only difference, from a principle of mechanics viewpoint, is the length of the lever arm from the hinge to the point of force application. For the latch mechanism, the lever arm is short and the motion appears rotary. For the pushout window, the lever arm is long, and the motion appears linear in reality, both motions are rotary but both can be actuated by straight linear force applications. The discrepancy and inconsistency in applying different requirements to mechanisms that operate in the same manner must be resolved and the resolution must be based on established principles of mechanics.

Request for Interpretation:

Blue Bird Body Company requests confirmation that the requirements of reference 1, with regard to motion, apply to the application forces and not to the release mechanisms being activated by the forces. Further, we request confirmation of our understanding of the principles of mechanics, as applicable to FMVSS 217 requirements, that straight linear forces can cause rotary notion to occur and can be used to manually operate a rotary mechanism. Blue Bird's desire that our pushout rectangular transit window design be allowed the use of Section S5.3.2 (b) HIGH FORCE APPLICATION of 60 pounds is consistent with both the letter of FMVSS 217 and the intent of the standard. We believe the requirements as written, acknowledge the limitations in typical human physical capabilities. When rotary motion (involving gripping and forearm rotation) is required to operate a release mechanism (such as rotating a common door knob) the allowable force levels are restricted to 20 pounds. Similarly, if reach distances for straight motion are such that body and arm extension are required to reach the mechanism, the allowable force levels are restricted to 20 pounds. Hence in these cases, the "Access Region for Low Forces" and LOW FORCE APPLICATION are required. When a push or pull force is required and the reach distances are small, as is the case with the Blue Bird pushout transit window latch, a human being can easily exert higher forces and the "Access Region for High Forces" and HIGH FORCE APPLICATION are allowed. If NHTSA believes that it is more appropriate for pushout windows to operate within the Low force application requirements, such a change should be implemented through Rulemaking Action and not by interpretations made in Compliance Test Procedures.

Blue Bird Body Company requests that thoughtful and timely responses to the above requests for interpretation be provided so that NCI 3189 can be satisfactorily resolved and closed in the best interests of motor vehicle safety.

If additional information or further discussion will assist in timely resolution of the issues involved, we will be glad to visit NHTSA to provide whatever information or assistance possible.

Interpretation ID: nht91-5.28