Electric Motor Replacement on a Horizontal Water Pump: Coupling, Shaft and Mounting Fit

Replacing the motor on a horizontal water pump is often thought of as "find a motor with the same power and speed, then fit it". In reality, the main part of the job is making sure the new motor's shaft end matches the coupling hub, its shaft height matches the pump axis and its foot holes match the baseplate. If any of these three points is off, the pump will show it within the first days through vibration, gland leakage and bearing noise.

This article focuses on renewing horizontal-shaft motors that sit on a baseplate and drive the pump through a flexible coupling. We explain, step by step, which information to take from the old motor, what the IEC frame code tells you and why alignment is an inseparable part of the replacement.

First identify the pump layout: coupled or close-coupled?

Horizontal pumps are found in two main layouts on site. In the first, the pump and motor stand on a common baseplate; the motor is bolted down by its feet and a flexible coupling connects the two shafts. In this layout the motor is a standard IEC-dimensioned, foot-mounted (B3) motor, and once the correct frame is chosen the replacement is mechanically simple.

The second layout is the close-coupled (monobloc) pump: the pump casing sits on the motor's front flange and the impeller is often mounted directly on the motor shaft. In these designs the shaft length, shaft-end machining and flange are specific to the pump manufacturer. Such a motor is not equivalent to a standard motor; a special-shaft motor is required, which may fall outside the scope of the standard range in HEM Motor's 2026 catalogue. In that case, ask us with the nameplate and a photo of the shaft.

In the coupled layout, check one more detail: if the coupling is a spacer type, the pump can be removed without moving the motor. The gap between the two shaft ends is then set by the spacer length, and the shaft shoulder of the new motor must come to the same point.

Information to take from the old motor

• Nameplate values: Power (kW), rated speed (e.g. 2,930 rpm), voltage and connection (Δ/Y), frequency. Do not change the speed, because the pump's impeller diameter and head are designed for it.
• IEC frame code: such as 132S or 160M. If the code is the same, the shaft height and foot hole spacing are the same by standard.
• Shaft end: Shaft diameter, shaft extension length and key width. Measure with a caliper and also note the bore of the coupling hub.
• Direction of rotation: Look at the arrow on the pump casing. On a three-phase motor the direction is reversed by swapping two phases; it must always be checked at first start-up.
• Baseplate condition: Shim plates, foot bolts, cracks or corrosion on the baseplate.

On motors whose nameplate cannot be read, the most reliable way to find the frame code is to measure the height from the shaft centre to the bottom of the feet. The IEC frame number gives this height in millimetres; on a 132 frame the shaft height is 132 mm.

Shaft end and coupling hub fit

On standard IEC motors, shaft diameter and length depend on the frame code and the number of poles. On some of the larger frames, the shaft of a two-pole motor is thinner than that of a four-pole motor in the same frame. So "same frame" does not always mean "same shaft". If you plan to take the coupling hub off the old motor and fit it on the new one, the shaft diameter and key size must match exactly.

If the dimensions differ, there are two options: renew the motor-side coupling hub for the new shaft diameter, or choose another model whose dimensions match. Forcing the hub on with a hammer stresses the shaft and bearings from the very first installation; the hub should be fitted by heating it or with a suitable puller/press tool. You can compare shaft and key dimensions in the 2026 technical and product catalogue. We do not give dimension values here, because they must be confirmed for the frame before ordering.

Shaft height, foot holes and baseplate

Since the pump axis is fixed, the shaft axis of the new motor must be at the same height. The same IEC frame code ensures this. If a motor with a different frame is chosen, an adapter plate must be made for the baseplate or the baseplate must be re-drilled; both options lengthen the labour and alignment time.

The letter in the frame code (S, M, L) indicates the longitudinal distance between the foot holes. 160M and 160L have the same shaft height but their foot holes are in different places. On some frames the feet may have holes for both dimensions; confirm this from the drawing. The 2D technical drawings prepared by frame size can be used for this comparison.

Coupled pump motors usually run foot-mounted (B3). According to the general information on our site, B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15 arrangements are offered according to the frame; the flange size is confirmed with the order.

Alignment: the last and most critical step of the replacement

Once the motor is in place, the misalignment between the two shafts is measured in two directions: the parallel offset of the axes relative to each other (radial) and the angle between them (angular). A flexible coupling tolerates small misalignment, but constantly compensating for it wears the coupling element, the motor and pump bearings and the gland packing.

• Do the alignment with the pipe connections fitted; the load that the suction and discharge pipes put on the pump shifts the axis.
• A foot that does not sit fully on the baseplate (soft foot) is corrected with shims; the frame must not flex when the bolt is tightened.
• Use a dial indicator or a laser alignment tool; checking with a straightedge is only for rough adjustment.
• At first start-up, check the direction of rotation, the current and the vibration. The commissioning checks article describes these steps in detail.

Example models and a hypothetical replacement

Two-pole (3,000 rpm synchronous) motors are common on coupled horizontal centrifugal pumps. The cast iron frame models below represent powers often seen on this type of pump. All have IP55 protection, class F insulation and a 400/690 V (Δ/Y), 50 Hz supply.

Hypothetical example: suppose the old 7.5 kW, 2,930 rpm, 132S-frame motor on a plant's baseplate-mounted pump has failed. A photo of the nameplate and the shaft dimensions are taken; the 132S2-2 with the same frame is chosen as the new motor. If the shaft end matches the dimension in the catalogue, the coupling hub is transferred to the new shaft, the baseplate stays as it is, the shims are renewed and alignment is done with a dial indicator. Had the shaft end not matched, only the coupling hub would have been renewed; moving to a model with a larger frame, however, would have brought the baseplate into the job as well.

Browse suitable HEM Motor models and current prices in the online sales section. General information on pump applications is available on the pump electric motors page.

Conclusion: what to send with your replacement request

On a horizontal pump, the right motor is found with the same power and speed, the same IEC frame and a shaft end that fits the coupling hub. Send the following information together with your request:

• A photo of the old motor's nameplate (power, speed, voltage, frame code)
• Shaft diameter, shaft length, key size and coupling type
• Mounting arrangement (foot or flange) and the condition of the baseplate
• Supply voltage and starting method
• Where the pump is used and its environment (outdoors, humidity, pump room)

Send a photo of your current motor's nameplate, its speed and mounting details through the nameplate submission page and request a quote for the right model. If you have already made your choice, you can compare models in the buy electric motors section, and use our contact page for any other questions.

Frequently Asked Questions

If the old and new motors have the same frame code, does the baseplate change?

With the same IEC frame code, the shaft height and foot hole spacing are standard; the baseplate usually does not change. Even so, renew the shims and redo the alignment from scratch.

Can I replace the pump motor with a 1,500 rpm motor instead of 3,000 rpm?

The same impeller at half the speed gives a markedly lower flow and head. If the speed changes, the pump selection changes too; the impeller and the pump curve must be re-evaluated.

What should I do if the coupling hub does not fit the new motor's shaft?

Do not force the hub. Renew the hub for the new shaft diameter or choose a model whose shaft end fits; confirm the dimension in the catalogue.

Can a standard motor be fitted to a close-coupled pump?

If the impeller is mounted directly on the motor shaft, a special shaft is usually required. In that case a special motor is considered instead of a standard one; ask us with the nameplate and a photo of the shaft.