Checking Shaft and Foot Dimensions When Replacing a 15 kW Electric Motor

When a 15 kW (20 HP) motor fails, most plants ask for a new motor of the same power and assume the problem ends there. Yet on replacement day, what costs the most time on site is not the power but the mechanical fit: if the new motor's shaft does not go into the coupling or pulley hub, if the foot holes do not line up with the holes in the base frame, or if the shaft axis sits higher than the machine's axis, the motor cannot be installed no matter how well it was chosen. This article explains what to check on the shaft and feet, and in what order, before ordering a 15 kW replacement.

Three models and three different frames at 15 kW

At HEM Motor, 15 kW is offered with a cast iron frame only; the aluminium series ends at 11 kW. Each of the three speed options sits in a different IEC frame. This means that even at the same power, changing the speed changes the mechanical dimensions completely.

All three models have IP55 protection, class F insulation and a 400/690 V (Δ/Y), 50 Hz supply. The frame sizes in the table are the starting point for the mechanical check during replacement.

Why compare the frame size first?

The IEC standard defines the shaft height, foot mounting distances and shaft end dimensions for a given frame size. So if the old motor's nameplate says 160L and it is a standard IEC motor, the shaft height and foot holes of a new 160L motor will most likely land in the same place. Comparing the code is the quickest check to do before measuring on site with a tape measure.

There are two points in this table that are easy to miss:

• The M and L difference: 160M and 160L have the same shaft height, but the front-to-rear distance between the foot holes is different. If you want to replace a 3,000 rpm 15 kW motor (160M) with a 1,500 rpm 15 kW motor (160L), the rear foot bolts will not line up with the holes in the base even though the shaft stays at the same height.
• Same power, different height: the 180L frame of the 1,000 rpm model is taller than the 160 frames. Moving from a 1,500 rpm old motor to a 1,000 rpm motor changes the coupling alignment and the base height from scratch.

Where the nameplate is unreadable, the motor was built to an older standard, or the machine builder used a special frame, comparing codes is not enough; you then have to work from on-site measurements.

Shaft checklist

The shaft is the part that transmits power to the machine, and even the smallest mismatch stops the installation. Before or after removing the old motor, check the following:

• Shaft diameter: it must match the bore of the coupling, pulley or gear hub. Measure with a caliper and compare it with the hub bore.
• Shaft length: the length of shaft protruding from the front end shield determines whether the hub will sit fully. A short shaft only half-holds the hub; a long shaft may touch the mating part.
• Keyway and key: the key width and height must fit the keyway in the hub. Simply moving the key from the old motor to the new one is not appropriate; check its size.
• Shaft end hole: some pulleys and couplings are secured with a bolt in the threaded hole at the shaft end. If this arrangement exists, confirm that the hole is present and check its thread.
• Effect of the number of poles: in some frame sizes, the shaft end of the 2-pole model may differ from the 4- or 6-pole model in the same frame. Read the row for your own number of poles in the technical drawing.

If the shaft end is outside standard IEC dimensions (stepped shaft, special length, double-ended shaft, tapered end and so on), it cannot be matched by a standard motor. Such a shaft requires a special motor; ask us with a nameplate photo and the shaft dimensions.

Foot checklist

In a foot-mounted (B3) 15 kW motor, the second major risk in replacement is the foot mounting. Things to check:

• Shaft height: the height from the foot base to the shaft centre. If it differs, the motor and machine shafts will not be on the same axis; with a coupled connection this directly means bearing and coupling failure.
• Distance between foot holes (across): the distance between the holes in the right and left feet.
• Distance between foot holes (lengthwise): the distance between the front and rear holes; this is where the M and L frame difference shows.
• Distance from the front foot hole to the shaft shoulder: if this changes, the coupling gap changes even if the motor sits on the same holes.
• Bolt hole diameter: whether the existing anchor bolts pass through the new motor's holes.

While taking measurements, also inspect the base frame. On a bent, cracked or uneven base, the new motor will also run under stress. Adjustment can be made with shims; however, that does not solve foot holes that do not line up.

Where to compare dimensions

Shaft diameter, shaft length, key, shaft height and foot hole distances by frame are given in the 2D technical drawings and in the 2026 technical and product catalogue. To see at a glance which power corresponds to which frame, you can check the frame dimension table. Comparing the measurements you take on site with these drawings is far safer than ordering on guesswork. For foot-and-flange (B35) or flange-mounted motors, the flange size is confirmed separately with the order; mounting options are offered according to the frame and are summarised in the mounting arrangements question.

Example: replacing a 15 kW fan motor

The following sequence is a hypothetical example. Suppose the 15 kW motor of a belt-driven radial fan at a plant has burned out.

• Read the nameplate: the old nameplate shows 15 kW, a rated speed of about 1,465 rpm and a 160L frame. This information points to the 160L-4 model.
• Measure the shaft: measure the old shaft diameter and length with a caliper and compare them with the pulley hub; then compare them with the shaft dimension in the technical drawing for the 160L frame.
• Measure the feet: note the across and lengthwise distances between the foot holes, the shaft height and the bolt diameters. If there is a belt-tensioning slide rail, also write down the hole spacing on the rail.
• Think about belt alignment: if the distance between the front foot hole and the shaft shoulder differs, the pulley alignment shifts; check whether the pulley hub can be moved along the shaft.
• If there is a mismatch: if the dimensions do not match, whether a base adapter, a spacer plate or a special shaft is needed must be discussed before ordering.

In the same example, if the old motor had been 3,000 rpm, the 160M2-2 would come into question, and at 1,000 rpm the 180L-6. But changing the fan's speed also changes the power the fan draws; so keep the speed the same as the old motor during replacement and confirm the power from the actual load, not from the machine's name.

Information to send with your order

• A legible photo of the old motor's nameplate (power, speed, frame size, voltage)
• Shaft diameter, shaft length and key size; the shaft end hole, if any
• Shaft height and foot hole distances, or the flange size
• Mounting arrangement (B3, B5, B35, etc.) and connection type (coupling, pulley, gearbox)
• Supply voltage, starting method and operating environment

You can review suitable 15 kW models and current prices on the buy electric motors page. If you are unsure about the dimensions, send us the nameplate and the measurements and we will check the fit together; for other questions, reach us through our contact page.