DEV Community: Nikolai

Dynamic unbalance of a narrow rotor

Nikolai — Fri, 04 Sep 2026 12:03:34 +0000

Usually dynamic unbalance is characteristic of long rotors, such as shafts, and static unbalance — of narrow ones. However, if a narrow rotor is mounted with a skew relative to the axis of rotation, or is deformed (a "figure-eight" wobble), a hard-to-remove dynamic unbalance appears.

The masses in the upper and lower parts of such a rotor create forces F1 and F2, directed in opposite directions and not lying on one line. These forces do not compensate each other, and in rotation they form a moment applied to the rotor.

Since the rotor is narrow, the distance between the correction planes is small, and the arm available for creating the compensating moment is short. Correction weights of considerable mass may therefore be required. This in turn produces an additional effect — the so-called induced unbalance, caused by deformation of the narrow rotor under the centrifugal forces of the correction masses themselves (see ISO 21940-11, the standard on balancing procedures and tolerances for rotors with rigid behaviour).

This is noticeable on narrow fan impellers, where aerodynamic unbalance acts in addition to the mass unbalance. The aerodynamic force is directly proportional to the angular speed of the rotor, while the centrifugal force of a correction mass is proportional to the square of the angular speed. Therefore the compensation is exact only at the specific speed at which the rotor was balanced; at other speeds an additional error appears. The same can be said about electromagnetic forces in an electric motor, which are also proportional to the angular speed.

It follows that it is impossible to eliminate all causes of machine vibration by balancing alone. Balancing removes mass unbalance; the vibration that remains points to other sources — this is what the FFT spectrum in Balanset-1A is for.

DIY Balancing Machine — Lumitech, Slovenia

Nikolai — Thu, 03 Sep 2026 12:41:39 +0000

Lumitech, a company from Slovenia, repairs pumps, blowers, vacuum pumps, servo motors and spindles — as complete motor assemblies. Their rotors range from 0.3 to 100 kg, with operating speeds up to 12,000 rpm.

With such a spread in rotor mass, finding an off-the-shelf balancing machine is difficult — so they built their own. Entirely in-house: design, CNC, machining — all done within the company.

The foundation was made properly: 1,050 kg of concrete and 400 kg of steel, topped with a 25 mm steel plate and two pedestals for two correction planes.

The pedestals sit on flat springs made of laser-cut 65Mn spring steel. An elegant solution: the springs are replaceable, and the thickness is selected to match the rotor mass. One machine covers the entire range, from a fraction of a kilogram to a hundred. By changing the plate thickness, the machine can behave either as an above-resonance or a below-resonance stand.

For the measuring system, they installed our Balanset-1A.

The result: residual unbalance of 0.73 g·mm in the first plane and 0.46 g·mm in the second, with correction masses of 0.702 and 0.462 grams.

It turned out beautifully. It's a pleasure when the instrument ends up in the hands of people who know how to calculate, machine, and see a job through to the end.

Lumitech: Facebook · Instagram · LinkedIn

Why not just balance it on knife edges?

Nikolai — Wed, 02 Sep 2026 10:25:10 +0000

A question I hear regularly: why an instrument at all, when rotors were balanced on knife-edge prisms long before any electronics?

For some rotors that still works. Put the rotor on the prisms: the heavy spot turns down under gravity. Add a correction weight on the opposite side until the rotor rests indifferently in any position. That is static balancing — and for a narrow, disc-like rotor it can be enough.

Now take a long rotor with two equal unbalanced masses, M1 and M2, sitting in different planes along the length — on opposite sides of the axis. At standstill only gravity acts, and the masses balance each other: on knife edges this rotor looks perfectly balanced.

Spin it up. Centrifugal forces Fc1 and Fc2 appear — equal in magnitude, opposite in direction. But they are applied at different points along the shaft, not on one line, so they do not cancel each other. Together they form a moment that rocks the rotor once per revolution — which is why this is called couple (moment) unbalance. The uncompensated forces land directly on the bearing supports, can exceed the design loads considerably, and shorten bearing life.

This unbalance exists only in rotation. It is physically impossible to detect — let alone remove — on knife edges. To correct it, you need two weights creating a moment equal in magnitude and opposite in direction to the unbalance moment. They do not have to sit exactly opposite M1 and M2, nor match them in size — what matters is the moment they create.

In the general case M1 and M2 are not equal, so a real rotor carries a combination of static and couple unbalance — dynamic unbalance. And the theory here is clean: for a rigid rotor, two correction weights spaced along the length are necessary and sufficient — they compensate both the net centrifugal force (the static part) and the moment (the couple part).

That is exactly why Balanset-1A measures at two bearings and corrects in two planes: correction masses and angles are calculated from how the rotor behaves in rotation, not from how it hangs in gravity.

What is a rotor, and where unbalance comes from

Nikolai — Tue, 01 Sep 2026 12:50:35 +0000

A rotor is a body that rotates about an axis and is held by its bearing surfaces in supports. The bearing surfaces — the journals — transmit the loads to the supports through rolling-element or sliding bearings.

In a perfectly balanced rotor the mass is distributed symmetrically about the axis of rotation: for every element of the rotor there is another element located symmetrically on the opposite side. During rotation each element is acted upon by a centrifugal force directed radially, perpendicular to the axis. In a balanced rotor the centrifugal force on any element is counterbalanced by the centrifugal force on its symmetric counterpart: forces F1 and F2 are equal in magnitude and opposite in direction, and the total centrifugal force on the rotor is zero.

If the symmetry is broken, an uncompensated centrifugal force F3 appears. As the rotor turns, this force changes direction together with it. The resulting dynamic load is transmitted to the bearings and accelerates their wear. The same alternating force cyclically deforms the supports and the foundation — this is what we perceive as vibration.

Balancing is the operation of restoring the symmetry by installing counterbalancing masses. The task of balancing is to find the magnitude and the angular position of one or several correction masses.

This is exactly the calculation the Balanset-1A performs from the measured vibration and phase. More at vibromera.eu.

Balanset-1A: balancing a rotor where it works

Nikolai — Tue, 01 Sep 2026 09:56:30 +0000

Since I've introduced myself, a few words about what we build.

The Balanset-1A is a portable two-channel instrument for dynamic balancing and vibration measurement. It measures the vibration of a rotating machine and calculates the correction mass and angle for one or two planes.

The reason it exists is practical. The traditional way to balance a rotor is to disassemble the machine, remove the rotor and take it to a balancing machine. For a mulcher shaft, a fan impeller or a crusher rotor, the disassembly often costs more than the balancing itself. The alternative is to balance the rotor in its own bearings, at its working speed, without removing it. That is what this instrument is built for.

The method is influence coefficients: an initial measurement, then a measurement with a known trial weight, and the correction is calculated from how the vibration vector responded. The rotor mass is not needed for the calculation. For repeat rotors of the same type, the coefficients are stored, and subsequent jobs take a single run.

Besides balancing, it works as a vibration meter with FFT spectrum and phase readout. In practice this part matters just as much: not every vibration is imbalance, and the spectrum usually shows whether the problem is imbalance, misalignment, looseness or a resonance — before any weights are installed.

Typical applications among our users: mulcher and flail mower shafts, industrial fans, crushers, combine harvesters, centrifuges, electric motor rotors, driveshafts, CNC spindles. The same instrument is also used as the measuring system of stationary balancing stands, including self-built ones — like the Lumitech stand from the previous post.

The kit consists of the measuring unit, two accelerometers, a laser tachometer, a magnetic stand, digital scales, the software and a case. A Windows laptop is required.

Questions about specific machines or setups are welcome — that is usually where the interesting part starts.

More at vibromera.eu.

From web dev to hardware founder: I build portable balancing instruments

Nikolai — Mon, 31 Aug 2026 18:18:56 +0000

Hi, I'm Nikolai Shelkovenko — CEO and co-founder of Vibromera, based in Porto, Portugal.

I got into hardware the way many developers do: sideways. I joined the family engineering project to build its website — and stayed. Then came production, sales, and 13+ years of hands-on field balancing: mulchers, fans, crushers, centrifuges and spindles. Along the way I even ran my own mulchers, clearing right-of-way corridors for power lines, gas and oil pipelines — so I'm not only a developer, but also an operator who drove and maintained this equipment himself.

Today Vibromera designs and builds the Balanset line of portable instruments for dynamic balancing and vibration analysis. The idea is simple: a rotor should be balanced where it works — on its own bearings, without taking the machine apart — and the software should do the math so the engineer can focus on the mechanics.

Here I'll write about vibration diagnostics, balancing practice, measurement hardware and its Windows software, and running a small engineering company — including how we use AI tools in day-to-day work with customers.

More at vibromera.eu.