Posts Tagged ‘vibration’

Rapidly Imploding Bubbles Create Problems

Monday, March 12th, 2018

    Last time we learned how both low and high pressures exist within a single centrifugal pump, and if water pressure at the inlet is low enough, the cavitation process begins.   Today we’ll see how these rapidly imploding water vapor bubbles create serious problems in the pump’s high pressure area.

Rapidly Imploding Bubbles Create Problems

Rapidly Imploding Bubbles Create Problems


    Water flows from low pressure at a centrifugal pump’s inlet to high pressure upstream when it meets up with the pump’s impeller. This high pressure causes cavitation bubbles formed at the inlet to rapidly implode, that is, collapse in on themselves.   Implosion occurs because pressure outside the bubbles is much greater than the pressure inside them.   This pressure difference exists because the bubbles were formed in the low pressure area of the pump.

    When cavitation bubbles meet up with high pressure areas deep inside the pump, they get squeezed hard and burst rapidly, creating multitudes of shock waves, grinding noise, and vibration so intense it sounds as though gravel, not steam bubbles, are passing through the pump.   The noise and vibration are bad enough, but cavitation has far worse consequences.

    Rapidly imploding bubbles form tiny but powerful micro jets of water which hold an enormous amount of kinetic energy.   When these jets hit the pump’s metal interior, their kinetic energy causes minute fragments of metal to break away.  Over time these tiny water jets wear away enough metal to cause damage to the pump’s interior and interfere with function.

    Next time we’ll see how cavitation bubbles flowing through the low pressure area of a pump degrade its performance.

opyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog




Spur Gears In Motion

Wednesday, February 12th, 2014

      Last time we learned about forces generated when spur gear teeth mesh and move along a specific line of action.   Today we’ll see them in movement.

      Looking at the illustration below it might appear that there are three teeth in contact, but this isn’t the case.   As the gears rotate, only two teeth make contact at any given time, although the third tooth comes very close.   The actual point of contact between the teeth is represented by a black dot on the illustration below.   This is where two opposing forces, F1 and F2, meet.

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      Now let’s animate the illustration to see how the line of action remains constant the entire time the gear teeth are in motion.   By constant I mean that this imaginary line’s position and angle does not change relative to the gears throughout the course of their movement.

mechanical design engineer

      In the animation, the point of contact moves along the line of action as the gears turn.   Each tooth’s involute profile ensures that smooth contact is maintained along the faces and flanks of the gear teeth.   The involute profile’s unique shape facilitates opposing teeth remaining in constant contact along the line of action for the duration of their movement together.

      If the gear tooth profile wasn’t involute in its shape, say for example it was square or triangular, the forces acting upon the meshed teeth during movement would vary in direction and intensity as a result of uneven contact between the teeth.   For example, consider the square shaped tooth profile in the gear train below.

Gear expert

      As the gears rotate, the pointed tip of one tooth strikes the flat face of another.   As they  continue to turn, the two flat faces of the teeth slap together, then the pointed tip of one tooth will strike the flat face of the other tooth, and so forth.   The result is movement that is jerky and destructive.   There would be excessive vibration and wear and tear on the teeth, resulting in rapid gear tooth erosion and decreased efficiency overall.

      Next time we’ll introduce the gear ratio, a formula which allows us to alter the rotational speed of the driven gear in relation to that of the driving gear, something which comes in handy when designing things that require this differential.