Downforce is a downwardly directed force acting on the pump's impeller/shaft unit during operation. Most pumps and motors are designed so that a downforce must always be present during operation. However, downforces that are too high may also lead to problems, if for instance the pump is operating at a duty point with a low flow rate and thus high delivery pressure. In the event of continuous operation in this range, the motor's axial bearing may be damaged. Furthermore, the pump and motor may overheat due to the absence of sufficient cooling flow.
In order to prevent these problems from occurring, the pump must be operated within the specified flow rate range. The curves for Grundfos submersible pumps represent the permitted flow rate range in the form of a solid line, with the impermissible operating range shown as a dashed line.