Vibration Basics in Plain Language

← Balanset-1A Knowledge Base — all chapters

This chapter is the minimum theory you need to work with the Balanset-1A consciously, rather than just pressing buttons.

What vibration is and why the machine shakes

Any rotating rotor (a mulcher drum, a fan impeller, a shaft) ideally has its centre of mass exactly on the axis of rotation. In reality there is always a slight mass offset — дисбаланс. As the rotor spins, the “heavy spot” pulls it sideways on every revolution, creating a centrifugal force that shakes the machine at the rotation frequency.

Important: unbalance is only one of the causes of vibration. A machine also shakes because of:

• bearing play and wear,
• shaft misalignment (couplings, belts),
• cracks and loose fasteners (bolts, frame),
• a bent shaft,
• structural resonance,
• aerodynamic and electromagnetic forces (fans, variable-frequency drives).

Balancing removes only the unbalance. Everything else must be repaired, not balanced. Hence the golden rule: the device cannot “balance away” a broken machine.

What vibration is measured in

The Balanset-1A measures vibration velocity — the RMS value in мм/с. This is the standard quantity for assessing machine condition in the working frequency range.

A rough quick-assessment scale (based on ISO 10816/20816, details in chapter 06):

For agricultural machinery (mulchers, mowers) the limits are softer: up to 4.8 mm/s is fine, up to 7.1 mm/s on soft supports, and considering working conditions “below 8 mm/s is already good”.

Frequency, RPM and harmonics

Say a rotor spins at 3000 rpm. That is 50 revolutions per second = 50 Hz. Vibration is decomposed into frequency components — the spectrum (FFT). The spectrum shows “who is doing the shaking”:

• 1x (rotation frequency) — the classic signature of дисбаланс. This is the component balancing reduces.
• 2x, 3x (second, third harmonic) — more often несъосност на вала, loose fasteners, a bent shaft. If 2x is higher than 1x — it is too early to balance; look for misalignment/looseness first.
• High frequencies, “noise” — bearing and gear-mesh defects.
• Low frequencies not related to RPM — loose foundation/mounts, aerodynamics.
• Multiples of the mains frequency (e.g. 6 × 50 Hz = 300 Hz) — electromagnetic interference from a variable-frequency drive. Test: kill the VFD during coast-down — if the peak disappears, it’s electrical, not mechanical.

A practical rule from tech support: if a spectrum peak shifts proportionally when you change RPM — it is a rotation harmonic; if it stays put — it is a structural resonance or an external source.

What it looks like on screen:

Phase — “where the heavy spot points”

Besides amplitude the device measures фаза — the angle between the tachometer pulse (the reflective mark passing the laser) and the vibration maximum. Phase tells you WHERE the unbalance sits. From the change of amplitude and phase after a trial weight is installed, the device calculates the mass and angle of the correction weight — the метод на коефициента на влияние.

Practical consequences:

• the tacho mark must not be re-stuck between runs — the phase “zero” shifts. The tachometer itself must not be moved or re-aimed either;
• RPM must be the same in all runs of a series;
• phase must be stable run to run (scatter no more than ~10–20°). If phase wanders — the measurement cannot be trusted; look for a mechanical cause or resonance.

Resonance — balancing’s worst enemy

Every structure (rotor + supports + frame) has natural frequencies. If the operating RPM coincides with a natural frequency, резонанс occurs: vibration shoots up, phase jumps around, the device’s calculations stop converging, and “the device keeps asking for more weight”.

Signs of resonance:

• a small RPM change alters vibration several-fold;
• after installing the calculated weight, vibration grows instead of dropping;
• amplitude and phase are unstable under unchanged conditions.

What to do: balance at different RPM — usually под the resonance. The typical mulcher recipe: rough balancing at 600–900 rpm, then move to working RPM. Recent software versions have a RunDown (coast-down) chart — spin the rotor up, cut the drive and record the coast-down: the peaks reveal the resonance frequencies, and you pick balancing RPM between them.

Real example: a customer’s rotor would not balance at 1600 rpm but balanced fine at 1000 rpm. The cause — a resonance near 1600.

Linearity, and why you install half the mass

The device’s calculation assumes the system is Линейна: double the weight — double the response. On rigid machines with a good foundation this is close to true. But a mulcher on a tractor, a machine on soft supports, or a structure with play and cracks behaves nonlinearly: the response to a weight is often not proportional.

Hence the practice-proven rule: install 1/2 (on bad machines 1/3) of the calculated mass and take a check run. The device will then refine the remainder. That is faster than re-welding a huge weight that “overshot” the target.

Static and dynamic unbalance

Статично — the heavy spot is on one side of the rotor. A stopped rotor on free supports will roll heavy-side down by itself. Corrected with a single weight in one plane.

Динамичен — heavy spots at different ends and on different sides of the rotor. The rotor doesn’t just vibrate, it “wobbles” as it spins. Weights in two planes are needed. You cannot detect it statically: with the rotor turned 90° the two weights seem to counterbalance each other, but in motion each heavy spot pulls its own way.

Една коригираща равнина или две?

• One plane — narrow rotors (discs, pulleys, narrow impellers). ISO 21940-11 rule of thumb: if the rotor width is less than half its diameter (ratio ~1:2; in practice support recommends 1:3) — one plane is enough.
• Два самолета — long rotors (mulcher drums, shafts, combine rotors, fans at 3000 rpm and above). Unbalance at the two rotor ends cross-affects each other, so the device measures two channels simultaneously (sensors X1 and X2) and calculates both weights at once.

G balance-quality grades (ISO 1940 / ISO 21940-11)

Residual unbalance is specified in г·мм by G grades:

The higher the RPM and the smaller the rotor — the tighter the grade. The Balanset software has a tolerance calculator: enter rotor mass, RPM and G grade — it returns the permissible residual unbalance in g·mm per plane. Real example: an 80 kg rotor at 3000 rpm, G6.3 → tolerance ≈ 802 g·mm per plane.

Don’t confuse the two assessment systems:

• мм/с (vibration velocity) — assessment of the assembled machine in place, per ISO 10816/20816;
• г·мм (residual unbalance) — assessment of the rotor itself, per ISO 1940/21940. Balancing machines work in g·mm; field balancing usually watches mm/s.

Mini-glossary