‘Lift’ refers to a condition whereby an upwardly directed force acts on the impeller/shaft unit of the pump during operation. This condition can be observed when the pump is pumping more water than what it is rated for, i.e. if the pump is operating far to the right on the curve
However, in normal well applications where a non-return valve is installed, lifting forces generally do not occur. This is because the static liquid column remaining in the pipeline after the pump is turned off by the non-return valve generates a counter-pressure that acts on the pump immediately after restart so as to prevent the pump from operating far to the right on the curve in this phase as well.
However, if no non-return valve is installed, or if the non-return valve is leaky, there will be no static liquid column acting on the pump during start-up. In most pumps, a lifting force will act on the impeller/shaft unit inside the pump in this case. The resulting upward movement continues to the pump-motor coupling, meaning that lifting force is also generated in the motor.
Although most motor manufacturers equip their motors with lifting bearings that are able to accommodate a limited lifting force without damaging the motor, lifting forces should be avoided in order to minimise wear on the pump and motor components. Lifting forces occurring at every start-up may lead to premature wear on both the pump and motor.