Design flaw in Window Mechanism of Chennai Metro Bus

Chennai is India’s one of the booming Metro City. The Bus service run by the Tamil Nadu Government (State Government) known as Metro Transport Corporation (MTC for short) handles around 40,000 services and 5.5Lakh (0.55 Million) passengers each day. Each bus has a seating capacity between 45-60 and standing space fot 25-30 more. The MTC recently acquired a new set of Tata Marco Polo  Buses to replace  their old buses. But it is found that there is a lot of design flaw in the window mechanism.

The Window mechanism

The movable window moves on vertical rails. The Vertical rails are attached to a stationary window glass pane.  2 Stops attached to the stationary glass prevents the movable glass from coming down when in open position. To Close the window, the stop is rotated such that it moves our of the path of moving window, causing it to coming down. The following picture should help in understanding.

Window mechanism of MTC Bus to help understand the design flaws

Detailed look on the Window stop

window stop

The stop attached to the stationary glass consist a base with horizontal flange to increase bending strength of the stop. The cylindrical stop is hinged so that it can be moved out of the way of the movable glass whenever required.  A depression (shown by yellow highlight) in the surface of the cylinder prevents accidental movement of the stop when the window is in open state.

The Design flaws

  1. The Stop is not strong enough to hold the window over time. The Stop hinge and the base bends over time.
  2. There is a safety problem when a passenger accidentally  moves the stop (even with the depression meant for prevention of accidental movement)
  3. If one stop fails, stress on the second stop increases causing to fail much earlier.
  4. If both stop fails, there is no way to hold the window in open state.

The following image shows outside view of the bus. The Window on the right side has one failed stop and so, the glass is dropped more on the left side compared to the right side of the window.

Outside view of bus

The bending of the stop and the flange is the root cause and is given in detail below.

Bending of Stop Hinge

The stop hinge yields causing deflection of stop much as shown in the photo below. Note that the bending causes the depression for safety to be useless.

Deflection of Stop

The reason could (no experiment was conducted) be one or more of

  1. Improper dimension and Material
  2. Failure of Creep Consideration
  3. FMEA not considered. (ASQ on FMEA, Wikipedia on FMEA)
  4. Improper identification of requirements of Safety and usage

 Bending of Flange

Deflection of Flange

The design flaw in the flange is Improper dimension of bottom flange.  The small deflection of the flange even though it does not cause much problem, has to be considered a design flaw due to

  1. It adds to the bending of the stop.
  2. When flange is deflected, the stress on hinge increases.

Suggestion on Design changes

  • The Stop acts as a cantilever beam with a point load. The spacing between the Stationary and moving glass could be reduced, thereby making the load on stop closer to the fixed side. This means the Bending Moment is lower and so is the deflection.
  • The Bottom, flange could have been thicker
  • Use a material with higher bending strength.
  • Switch to a different mechanism.

Conclusion

With the design flaws primarily affecting the safety of passengers, this design should not be used. Another design where  the movable glass is “hanged” on a hook is seen on some of the newer buses. This is a better design and the safety is much higher.  Also noted that FMEA is not used. If it had been, the flawed design would have been modified. It is hoped that the authorities do the necessary to change the design.

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