EXAMINE WHY A MANHOLE COVER JUMPED OUT OF ITS CASTING

THE TIDDLY WINK EFFECT

REVIEW OF THE EVENT

A vehicle was driving along a main road when it struck an open sewer manhole, lost control and crashed into a tree. The road had been re-surfaced a few months prior. The sewer manhole cover had been raised to match the new roadway surface with a casting riser. After the re-surfacing the manhole cover began banging whenever a vehicle drove over it. A few weeks prior to the crash a neighbor had seen the manhole cover pop out of the casting and roll away. The neighbor found the cover and replaced it . As cars drove over the manhole cover it banged until one night the cover either broke and fell into the manhole, rolled away or someone took it.

      
 

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Photograph of manhole cover.
 

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View video of tire rolling over properly seated manhole cover at 30 mph
tire_over_good_mh.wmv
 

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Vieω video of tire rolling over improperly seated manhole cover at 30 mph
tire_over_mh.wmv
 

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Graph of Kinetic Energy v Potential Energy
When the Kinetic Energy exceeds the Potential Energy of the manhole cover, the cover will jump/slide out of its casting.
Manhole_Energy_Motion2.avi
 

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After the MH cover has been given the initial rotation because of the tire rolling over it, it is free to continue to rotate.

What is the limit of rotation and what is the height that one end of the MH cover can attain as it raises up from the casting?
Procedure: Convert linear velocity to angular velocity:

V_f²=V_o²+2xgxh

Where h = Height that the MH cover must reach to clear the casting.

ω_f²xr²=ω_o²xr²+2xαxrxh

Where ω_f = Final angular velocity of the MH Cover = 0 (Vertical Direction)

ω_o = Initial angular velocity of rotation of the MH cover in rad/sec.
r = Radius of the MH cover in inches, and;

ω_f=ω_o+αxt; α=ω_f/t

If we estimate that the tire rolls over a section of the manhole equal to the radius then the equation reduces to:

ω_f=2xh/r/t

Where t is the time for the tire to roll over a portion (chord), of the MH cover equivalent to a function of the radius.

Resulting graph:

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After the MH cover has been given the initial rotation because of the tire rolling over it, it is free to continue to rotate. What is the limit of rotation and what is the height that one end of the MH cover can attain as it raises up from the casting?

Procedure: Convert linear velocity to angular velocity:

V_f²=V_o²+2xgxh;

Where h = Height that the MH cover must reach to clear the casting.

ω_f²xr²=ω_o²xr²+2xαxrxh

Where;
ω_f = Final angular velocity of the MH Cover = 0 (Vertical Direction)
ω_o= Initial angular velocity of rotation of the MH cover in rad/sec.
r = Radius of the MH cover in inches and;
ω_f=ω_o+αxt; α=ω_f/t

If we estimate that the tire rolls over a section of the manhole equal to the radius then, the equation reduces to:

ω_f=2xhx/r/t

Where t is the time for the tire to roll over the MH cover.

 Last update: 09/112/2009 Top of Page