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In most cases it cannot be bought off the shelf. In common-shaft (close-coupled) pumps, the impeller is mounted on the motor's extended, specially machined shaft; the shaft of a standard IEC motor does not have these dimensions. A separate motor can be considered only if the pump design accepts a standard flanged motor via an intermediate shaft or an adaptor.
First, identify the pump's construction
| Construction | How to recognise it | Standard motor? |
|---|---|---|
| True common shaft | Impeller mounted directly on the motor shaft, no coupling between motor and pump casing | Usually not |
| Close-coupled with stub shaft (rigid coupling) | An extension shaft or sleeve clamped onto a standard motor shaft | Suitability can be checked by measurement |
| Coupled pump | Motor and pump on separate bearings, with a flexible coupling between them | Yes, if the dimensions match |
To tell them apart, it is enough to check the manufacturer's catalogue or the type code on the pump nameplate before separating the motor flange from the pump.
Options for a true common-shaft pump
- Repairing the motor: If there is a winding fault, rewinding the special-shaft motor keeps the pump in service.
- The same unit from the pump manufacturer: Since the shaft dimensions are specific to the pump, this is often the quickest route.
- Switching to a coupled arrangement: Moving to a pump that runs with a standard motor connected by a coupling on a baseplate makes future motor replacement easier. In this case the baseplate and coupling are planned again together with the pump.
Checklist for a stub-shaft arrangement
- The bore of the extension shaft matches the motor shaft diameter (D) and key size
- The motor flange (B5 or B35) is the same size as the pump lantern
- The shaft length (E) does not change the impeller position; the impeller must not rub against the casing
- The mechanical seal is renewed along with the replacement
A motor fitted with the wrong dimensions can damage the seal and impeller even if it shifts the impeller position by only a few millimetres; dimensions must be confirmed before ordering.
Horizontal pumps with a separate motor and coupling are covered on the end-suction pump motor page, and vertically arranged pumps on the in-line pump motor page. For applications whose dimensions are confirmed, you can browse flange and foot-mounted models in the shop.