What is Shaft Deflection?
Shaft deflection is the bending of a pump shaft that occurs when the velocity of liquid being pushed by an impeller is not equal at all points around the impeller.
When a pump operates at the best efficiency point (BEP), velocity and pressure are relatively equal all the way around the impeller. When operation moves dramatically away from BEP in either direction, a pressure imbalance is produced and one side the impeller experiences dramatically higher velocity than the other side. The result of this imbalance is a radial load is placed on the shaft and the shaft bends in response to the radial load.
The amount of bending is called shaft deflection, and can be calculated by a competent pump Engineer. Excessive shaft deflection can cause:
- Mechanical seal leakage and premature failure
- Bearing damage and premature failure
- Wearing ring wear
- Increased vibration
- In extreme cases, damage to the volute and impeller
Pressure imbalances within the volute and resulting shaft deflection can be decreased dramatically by utilizing a dual volute type of casing. Most high-pressure centrifugal pumps utilize a dual volute casing for this reason.
Deflection is a concern in horizontal centrifugal pumps, especially overhung designs such as end-suction and vertical inline pumps. Pumps with a diffuser type casing, as opposed to a volute, such as vertical turbines, do not experience the same sort of radial loading and shaft deflection due to pressure imbalances within the casing are not a concern for pump models that incorporate a diffuser style casing.
These articles from the McNally Institute provide an excellent technical introduction to shaft deflection.