What the OEM sent us
A European maker of compact air handling units asked us for a 145mm forward-curved wheel, 130mm high, with a precision 8mm bore. Then they asked about centrifugal impeller balance.
The enquiry was short. It contained the core dimensions, a request for a 2D drawing, and one question that mattered more than the rest: what is your dynamic balancing specification?
Why that question came first
Asking about centrifugal impeller balance before asking about airflow tells you something. This customer has had a vibration problem before, or knows someone who has.
An out-of-balance wheel produces noise their end users complain about, and it works the motor bearings until they fail early. Neither shows up in a datasheet comparison, and both show up in warranty.
Additionally they wanted RoHS confirmation for European market entry, which is routine.
What the request implies about their build
They asked for the impeller alone, not a fan. Consequently they source their own motors and fabricate their own scroll housing. What they need from us is the one part they cannot easily make.
Answering the centrifugal impeller balance question
Here the request matched a part already in our library, so no development was needed. Instead the work lay in confirming our standard part met requirements they had not fully stated.
Why forward-curved
A forward-curved wheel moves high volume at low to medium static pressure in a compact envelope. That is exactly what a compact air handling unit needs, and it is why this geometry dominates the application.
It carries 45 blades. Consequently the pressure pulses spread more evenly around the wheel, producing a smoother sound than a wheel with fewer, larger blades.
The material choice
The part is injection-molded in ABS with 15% glass fiber.
Unfilled plastic would flex and lose shape. Meanwhile aluminum weighs more and costs more, and at these speeds it buys nothing structurally. So glass-filled ABS sits between them, suiting the temperature and load a commercial HVAC unit imposes.
The answer to their actual question
Residual unbalance does not exceed 0.30 grams measured at 1000 RPM.
More usefully, that figure comes from two-plane dynamic balancing after molding, on every unit rather than a sample. In short, a centrifugal impeller balance figure without a process behind it is a claim. With 100% checking behind it, it is a measurement.
What it cost
Nothing on this project, since the part existed. The general trade is that a 45-blade forward-curved wheel is efficient at low pressure and falls away quickly as resistance rises.
If their air handling unit later gains a denser filter, this impeller will not carry it. That is a limit of the geometry rather than of the part, and it is worth knowing before someone specifies a filter upgrade.
Technical Specifications
Meanwhile the bore tolerance and centrifugal impeller balance rows below are the demanding ones. By contrast, diameter and height are not.
| Parameter | Value |
|---|---|
| Model | LWF-φ145×130-01 |
| Impeller Type | Forward-Curved Centrifugal |
| Diameter | 145 mm (±1 mm) |
| Height | 130 mm (±1 mm) |
| Bore Diameter | Ø8 mm (+0.022 / 0 mm tolerance) |
| Number of Blades | 45 |
| Material | ABS GF15 (15% Glass-Fiber Reinforced) |
| Color | Black |
| Dynamic Balancing | ≤ 0.30 g residual unbalance @ 1000 RPM |
| Runout Tolerance (Radial / Axial) | ≤ 1.0 mm |
| Compliance | RoHS |
Technical Documentation
A 3D CAD rendering of the LWF-φ145×130-01 impeller, used for initial design visualization and fitment checks.
Open Technical Drawing: LWF-φ145×130-01PDF
The final 2D technical drawing provided to the customer, containing all critical dimensions, material specifications, and balancing tolerances for OEM integration.
If you are buying impellers as components
Buying impellers as loose components differs slightly from buying fans. So these help.
- Ask for the balancing process, not just the number. A figure alone tells you what a good unit measures. Asking whether it is 100% inspected or sampled tells you what the worst unit in your batch might be, which is what actually reaches your customer.
- Give the bore tolerance and the shaft it mounts on. The bore decides how the wheel sits on the shaft, and everything else follows. A loose fit lets the wheel run slightly off-center, and no amount of balancing corrects that.
- State the operating speed with the balance requirement. Residual unbalance makes force proportional to the square of speed. Consequently 0.3g at 1000 RPM and 0.3g at 3000 RPM differ greatly in effect. So state the speed you will actually run.
- Check the pressure capability against your future plans. Forward-curved wheels are excellent at low pressure and poor above it. If filtration might be added later, say so now, because the geometry choice changes.
See our fan impeller range, the forward curved fan section, or centrifugal fans. Balance grades follow ISO 21940.
Questions about impellers
Is 0.3g at 1000 RPM a tight specification?
For an injection-molded wheel of this size, yes. It is a working production standard rather than a marketing figure. However, what matters more is that centrifugal impeller balance is checked on every unit in two planes. A tighter number checked on a sample is worth less than this one checked on all of them.
Why does blade count affect noise?
Because each blade passing the scroll cutoff makes a pressure pulse, and those pulses combine into a tone at blade passing frequency. More blades push that tone higher and reduce the energy in each pulse, which usually reads as a smoother, less intrusive sound at the same overall level.
Can you supply the matching motor and housing?
Yes, though this customer did not want it. Some OEMs buy the impeller only because they already have motor sourcing and sheet metal capability. Others prefer a complete matched assembly. Both are normal, and the impeller specification is identical either way.
Why not use aluminum for a more rigid wheel?
Because at 145mm and 1000 RPM there is no structural case for it. Glass-filled ABS is stiff enough, weighs less, costs less and moulds to a repeatable shape. Aluminum earns its place on larger diameters, higher speeds, or where temperature exceeds what the plastic tolerates.
Technical Documentation & Resources
Need the same thing for your own unit?
Give us the numbers and the envelope, and you get a shortlist rather than a catalog. Meanwhile custom spec sheets and samples run to 90 days. Since 1990 Longwell has supplied EC fans and blowers to OEMs in HVAC, cold chain, data center and industrial markets.
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