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CRITICAL SPEED
Why Shaft Design Is So Important
Every rotating shaft has a point where it becomes unstable at a given speed or rpm, which is called critical speed. Critical speed of the shaft changes depending on may variables such as shaft diameter, length, number of impellers and weight of impellers. Theoretically every shaft assembly has an infinite number of critical speeds, but the lowest critical speed (the first) and the second critical speed (the next higher) are of primary interest to mixer shaft design.
Some mixers are designed to operate above first critical speed. When designed this way the shaft passes through critical speed with nothing more than a slight tremor at start up or shutdown. Generally speaking, it’s good practice to stay below or above first critical speed by 20% or more. The ratio of operating speed to critical speed (N/Ncr) is called the critical speed ratio. A critical speed ratio of 0.8 would indicate that the operating speed is 20% below the critical speed. A ratio of 1.2 indicates the operating speed is 20% above critical speed.
The Brawn Mixer application and engineering group will design your mixer/shaft assembly with critical speed in mind so you don’t need to get caught up on all the technical aspects, but it’s important that you are aware of critical speed when it comes to potential field modifications or operating the mixer with a variable speed controller. Keep in mind by changing the shaft length (longer or shorter), adding an impeller or varying the speed could put your shaft assembly into critical speed.
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