Close Is Not Drop-In: Making Shaft Tooling for 4.5 x 26.5mm

What the OEM needed matched

A European OEM wanted a second source for a shaded-pole motor in a cabinet ventilation assembly. It ended up needing new motor shaft tooling.

Usefully they supplied the complete drawing for the part being replaced, which is the best thing a customer can send. It made a form, fit and function analysis possible rather than a guess.

The constraints that could not move

• First, a 4.5mm shaft diameter and 26.5mm shaft length.
• Next, 1500 RPM no-load on a 220V, 50Hz supply.
• Meanwhile the mounting had to match exactly: four M4 holes on a 48mm square pattern.
• Ball bearings, requested explicitly, for longer life than sleeve.
• Finally, speed control by simple potentiometer rather than a digital signal.

The mounting requirement is what made this rigid. Their chassis already exists. Consequently any deviation means the redesign they were trying to avoid.

Why motor shaft tooling was unavoidable

We cross-referenced the requirement against our existing shaded-pole platform first, because a stock match would have been the fastest answer.

Close, and not close enough

Certainly several models in the series were dimensionally near. One even had the correct 4.5mm shaft diameter, although its power and other dimensions differed.

However, none combined the exact shaft dimensions with the required performance.

On a form-fit-function replacement, that gap matters more than it would elsewhere. The whole point is that the new part bolts into an existing assembly untouched, and a shaft 2mm too long stops it doing that as surely as a wrong mounting pattern would.

What we changed, and what we did not

A full new product cycle was unnecessary. Instead the base platform was modified in two specific places.

First, new motor shaft tooling was made to produce the 4.5mm diameter to a -0.005mm tolerance, at 26.5mm length. Second, the copper winding was recalculated to reach 1500 RPM no-load on 220V at 50Hz.

Everything else came from the existing design, which is why this ran as a minor development rather than a new product.

Why AC, confirmed by how they control it

Here the customer runs speed control from a simple potentiometer. That single detail confirms a shaded-pole AC motor is the right technology.

An EC motor with digital control would be over-engineered here, and the control architecture could not use it. Consequently proposing one would have added cost for capability that stays switched off.

What it cost

Tooling and time. Making shaft tooling is a real investment, and it is the reason a second-source project is not simply a catalog lookup.

Motor shaft tooling pays back on volume rather than on a first order. So be clear about annual quantity before committing. Below a certain number the arithmetic simply does not work.

Technical Specifications

The shaft rows below are the ones that needed new tooling. Everything else came off the existing platform.

Parameter	Value
Model	LWSPM6116-02
Voltage	220 V
Frequency	50 Hz
Max Input Power	35 W
Max Output Power	7 W
No-Load Speed	1500 RPM
Insulation Class	Class F
Ambient Temperature	-15°C to +120°C
Bearings	Rolling Bearing (Ball Bearing)
Shaft Diameter (D)	4.5 mm
Shaft Length (E)	26.5 mm
Stator Stack Height (A)	16.5 mm
Overall Depth (B)	43.5 mm
Frame Width	60.0 mm
Mounting Hole Pattern	48.0 x 48.0 mm (4x M4)
Rotation Direction	CCW (Viewed from shaft end)
Approvals	CE
Winding Material	Copper Wire

Technical Documentation

Meanwhile the following documents were central to the project, outlining the transition from the legacy component to the final Longwell solution. Procurement managers and design engineers can use these for verification and integration.

Legacy Component Drawing (ebm-papst EM3015-94)

The document details the original German-made motor, establishing the baseline for all critical form-fit-function requirements.

Final Specification Drawing (Longwell LWSPM6116-02)

This is the final Longwell engineering drawing for the approved drop-in replacement, confirming all dimensions, electrical specifications, and material choices.

Here the standard product datasheet for the LWSPM61 motor series, from which the custom LWSPM6116-02 was derived.

A view of the customer’s sub-assembly production, showing the integration of the final Longwell motor into their product chassis.

If you are second-sourcing a motor

Second-sourcing a motor succeeds or fails on how precisely the requirement is stated.

• Send the drawing, not the part number. A drawing gives shaft diameter, length, mounting pattern and tolerances in one document. A part number tells a supplier only what to search for, and cross-references between manufacturers are often approximate.
• Say which dimensions are absolute. Here the mounting pattern and shaft were fixed because the chassis exists. Naming that up front stops a supplier proposing a near match and wasting a round.
• Ask whether motor shaft tooling is needed. It is the single biggest cost and schedule item in a motor second-source. If an existing shaft fits, the project is days; if not, it is weeks and a tooling charge.
• State your annual volume honestly. Tooling amortises over quantity. A supplier who knows the real number can tell you whether the route makes sense, rather than quoting it and letting you discover the economics later.

See our fan motor range, the shaded pole motor section, or AC motors. Appliance safety follows IEC 60335-1.

Questions about replacements

Why not use the nearest stock motor?

Because the customer chassis already exists. A motor with the right shaft diameter but the wrong length forces a sheet metal change. So does the right performance on a different mounting pattern. That change costs more than motor shaft tooling, and it defeats the purpose of a drop-in.

How tight is a -0.005mm shaft tolerance?

Tight enough to need dedicated tooling rather than general machining. It matters because the impeller is pressed onto that shaft, and the fit determines whether the assembly runs true. A loose fit shows up as vibration that no amount of balancing corrects.

When does a second-source project not make sense?

When annual volume is too low to amortise tooling, or when the incumbent part is still readily available at an acceptable price. Second-sourcing buys supply security and cost leverage, and both need volume behind them to pay for the setup.

Why did the customer specify ball bearings?

Longer life and less sensitivity to mounting orientation than sleeve. In a cabinet ventilation assembly that may sit in any position and run continuously, that difference is worth the small cost increase. It was their call, and it was the right one.