Compare rotor balance or measured vibration with documented category limits to find tolerance margins.
Concepts behind this calculation
Check documented rotor residual moments or compare factory and installed fan velocity readings with explicit AMCA 204-20 category, support and measurement conditions.
Residual balance assesses the rotor’s remaining mass eccentricity, whereas vibration assesses motion of the assembled or installed machine. In the balance relation, angular speed comes from service RPM and the agreed grade limits the permissible eccentricity. Multiplying that tolerance by rotor mass gives a residual unbalance moment, which is allocated to documented correction planes. It does not supply the angular position of a correction weight.
Vibration comparisons depend on the measurement method, peak or RMS quantity, support behavior and location or axis. Factory filter-in readings and installed overall readings use different limits. Broadband peak and RMS are not automatically interchangeable, and readings on different axes should remain separate. A rotor may satisfy its balance tolerance while the installed assembly still experiences aerodynamic excitation or support-related vibration.
How to use this calculator
Select Assessment method, Agreed fan balance/vibration category and Rotor behavior; record Category, measurement and source reference.
For vibration, choose Actual measured velocity quantity and Measured velocity unit, confirm Measurement basis documented, and enter Measurement rows for separate locations and axes.
For installed readings, enter the actual low and high measurement cutoffs and Documented historical shutdown limit when required.
For residual balance, enter Mass of the rotor assembly being balanced, Specified service/design operating speed and Plane rows with documented shares, radii and measured residual moments.
Calculation method and applicability
Balance: ω=2πN/60, eper=1000G/ωµm; Uper(lb-in)=mass(lb)G/(25.4ω); multiply by16 foroz-in and allocate documented plane shares. Vibration: compare each measured peak or RMS velocity to the matching SI-primary Table 8.2/8.3 cell; convert in./s×25.4 to mm/s. No broadband peak/RMS conversion.
N = service speed (RPM); ω = angular speed (rad/s); G = selected balance quality (mm/s); eper = allowable specific residual unbalance (µm); Uper = permissible residual unbalance (lb-in, converted to oz-in where shown).
Input definitions and units
Assessment method
Residual rotor balance, assembled factory filter-in vibration and installed overall vibration are different assessments. They share a page and do not predict one another. Choices: Installed overall vibration; Assembled factory vibration; Rotor residual balance.
Category, measurement and source reference
Required: agreed BV category and edition, rigid-rotor applicability, fan/model, test/report, rated operating point, sensor/axis ID map and documented support class. For balance record operating speed, plane allocation and residual-test basis. Example data are constructed; source text does not authenticate a test.
Agreed fan balance/vibration category
Use the manufacturer/purchaser agreement and AMCA 204-20 Table 6.1. HVAC at or below 5 hp typically maps to BV2, above 5 hp to BV3, but this selector does not determine contractual category or infer driver power. Choices: BV-1 — G16; BV-2 — G16; BV-3 — G6.3; BV-4 — G2.5; BV-5 — G1.
Rotor behavior
The pinned standard covers rigid rotors. A flexible support is not a flexible rotor. Severe impacts, extreme temperatures and excluded operating conditions require a separate method. Choices: Documented rigid rotor; Flexible rotor — unsupported.
Actual measured velocity quantity
Use the measured quantity and matching table column. Broad-band peak and RMS have no automatic square-root-of-two conversion; displacement/acceleration are not accepted here. Choices: Measured peak velocity; Measured RMS velocity. Applies to Assessment method: Installed overall vibration or Assembled factory vibration.
Measured velocity unit
Inputs use this selected unit. Limits use the standard’s SI-primary mm/s values; inch-per-second outputs are calculated with exact 25.4, not copied from rounded/conflicting source parentheses. Choices: in./s; mm/s. Applies to Assessment method: Installed overall vibration or Assembled factory vibration.
Measurement basis documented?
Confirm agreed rated operating point and a suitable direct mechanical bearing signal path. Factory requires filter-in velocity at rotation frequency; installed requires overall filter-out velocity. A reading on an unsupported guard or at an unagreed VFD point is outside this comparison. Choices: Not established — withhold comparison; Documented for selected method. Applies to Assessment method: Installed overall vibration or Assembled factory vibration.
Measurement rows: ID, support, velocity (CSV)
One to100 location/axis rows. Unique integer ID; support 0 =rigid or1=flexible for that axis, established from the fan/support fundamental frequency relative to running speed; nonnegative measured velocity in the selected quantity and unit. Map every ID to a real location/axis in the source note. Do not average or vector-sum axes. Applies to Assessment method: Installed overall vibration or Assembled factory vibration.
Actual overall measurement low cutoff (Hz)
Installed filter-out measurements must cover at least 10–1,000 Hz. Enter the positive actual lower cutoff, no higher than10 Hz. This is not a single 1× rotational-frequency reading. Applies to Assessment method: Installed overall vibration.
Actual overall measurement high cutoff (Hz)
Enter the actual upper cutoff, at least1000 Hz. Meeting the frequency span does not authenticate the sensor or mounting. Applies to Assessment method: Installed overall vibration.
Required for BV1/BV2: Table 8.3 supplies no shutdown number. Use project/historical data matching the selected peak/RMS quantity and units; it must be above the alarm threshold for every entered support. Example 0.9 in./s is constructed. Applies to Assessment method: Installed overall vibration; Agreed fan balance/vibration category: BV-1 — G16 or BV-2 — G16.
Mass of the rotor assembly being balanced (lbm)
Use the actual rotating assembly mass, not total fan/shipping weight. Below 227 g, the source cautions that BV1 residual unbalance may be difficult to measure. Applies to Assessment method: Rotor residual balance.
Specified service/design operating speed (RPM)
Operating speed used for the specified balance tolerance, not the balancing-machine test speed. Use the applicable documented design/service requirement. Applies to Assessment method: Rotor residual balance.
One or two documented correction planes. Unique ID; fraction of total tolerance (all shares sum to 1); positive correction radius in inches; nonnegative residual moment in oz-in from a suitable assessment. Equal halves are an example, not automatic allocation. Radius converts a tolerance moment to equivalent mass; no correction angle is solved. Applies to Assessment method: Rotor residual balance.
How to interpret the result
Inspect each plane or measurement margin and the critical normalized reading. A below-alarm comparison does not diagnose the machine, and a residual balance pass does not establish acceptable installed vibration. Preserve the measurement basis and investigate trends separately.
Worked examples
Installed overall vibration
Inputs: Constructed BV3 installed RMS scenario, in./s: ID1 rigid0.20, ID2 flexible0.30, ID3 rigid0.40; documented rated-point/direct-bearing basis assumed and overall range 10–1,000 Hz. These are teaching readings, not a field test.
Calculation: Convert by 25.4: 5.08, 7.62, 10.16 mm/s. Rigid RMS startup/alarm/shutdown=4.5/7.1/9.0 mm/s; flexible=6.3/11.8/12.5 mm/s. All three exceed startup, but only ID3 reaches alarm and shutdown. Highest alarm-normalized ratio=10.16/7.1=1.430985915.
Result: 1 reading reaches the shutdown level. The first two readings remain below alarm despite exceeding startup. Apply the agreed operating/corrective-action procedure; this arithmetic comparison does not establish a cause or replace a field assessment.
Assembled factory vibration
Inputs: Constructed assembled-factory BV3 peak test with filter-in readings at the agreed rotational frequency: rigid ID 1 =0.12 in./s and flexible ID 2 =0.18 in./s. This differs from an installed broadband test.
Calculation: Measured velocities=3.048 and4.572 mm/s. Source SI-primary factory limits are 3.8 rigid and 5.1 flexible peak; margins are0.752 and0.528 mm/s. The higher normalized ratio is4.572/5.1=0.896470588 at ID2.
Result: 0 of 2 readings exceed their factory criteria. Calculated limit conversions are0.149606299 and0.200787402 in./s; the rounded source 0.15/0.20parentheses are not used as exact boundaries.
Rotor residual balance
Inputs: Constructed rigid rotor: BV3/G6.3, 10 lbm, 1, 800 RPM service speed. Two documented equal-share planes, radii 5 and 6 in; measured residuals0.10 and0.12 oz-in.
Calculation: Angular speed=188.495559215 rad/s; specific permissible unbalance=33.422538049 µm. Total moment=0.210535672751 oz-in. Each half gets0.105267836376 oz-in. Plane1 margin=+0.005267836376; plane2=−0.014732163624 oz-in.
Result: 1 of 2 planes exceeds its allocated residual tolerance. Equivalent tolerance masses are0.021053567275 and0.017544639396 oz at the entered radii. These are not correction-weight/angle instructions and do not predict vibration.
Assumptions and limits
The pinned model is AMCA 204-20. It covers rigid rotors and selected fan balance/vibration criteria, not flexible-rotor balancing, structural foundation design, personnel exposure or severe impact/extreme-temperature applications. Current contractual edition and adoption remain user decisions.
SI is primary in Section 5. Table 8.3 contains conflicting I-P parentheses, including 11.8 mm/s versus 0.28 in./s for BV3 flexible RMS alarm, 14 mm/s versus 0.5 in./s for BV2 flexible RMS alarm, and5.7 mm/s versus 0.20 in./s for BV5 rigid peak alarm. This page uses SI values and exact conversion; it does not silently correct the source or mix unit columns.
Peak and RMS are separately measured quantities. Broadband velocity is not automatically convertible by√2; displacement or acceleration are not accepted as broadband velocity. Single-frequency conversion and sleeve-bearing proximity-displacement guidance are outside this page.
Support class is based on the documented fan/support fundamental frequency relative to operating speed and may differ by axis. A spring, concrete base or simple bump test alone does not authenticate that classification.
The manufacturer and purchaser establish category, rated point and applicable limits. No automatic application/driver-power category is assigned. Off-design VFD points and modified contractual criteria need manufacturer review rather than an automatic 1.6 factor.
Source readings belong at bearings or an appropriate shortest solid signal path. Unsupported guards/panels and arbitrary frame locations may not represent bearing motion; the2024 official pickup interpretation is linked.
BV1/BV2 have no standard shutdown number. A historical/project value must match the instrument quantity, units and support; the example is constructed. No zero, infinity or invented standard shutdown is substituted.
Residual balance tolerances require suitable measurement and documented plane allocation. The source notes half-tolerance per plane is often suitable, but this tool does not infer that arrangement or solve a correction phase/angle.
Factory/startup equality is within the stated criterion; reaching alarm/shutdown triggers that threshold. Trends, sudden increases, transient conditions, wear, aerodynamic effects and maintenance history require assessment even if a number is below alarm.
All complete worked operating/balancing scenarios are constructed. Source table values and equations are checked separately; no actual fan test, field diagnosis or AMCA certification is claimed.
FAQs
Does a flexible support mean the rotor is flexible?
No. Support behavior describes the fan/support dynamic relationship and can differ by axis. Rotor behavior is a separate applicability condition; the implemented balance and vibration profile requires a documented rigid rotor.
Why must I supply a shutdown limit for some categories?
The implemented installed-vibration table does not supply a shutdown number for BV-1 and BV-2. A documented historical or project limit must match the selected measured quantity, units and supports; the tool cannot infer it from the alarm number.
Sources
ANSI/AMCA 204-20 standardPublic PDF accessed without login. Sections5–8, Table 7.1, balance formula and SI-primary Tables8.2/8.3; unit-column conflicts are retained explicitly. Full standard text is not redistributed.
AMCA 2024 interpretation — pickup signal pathApproved February 16, 2024 for 204-20: bearing pickup or shortest direct mechanical signal path. The interpretation does not decide a specific frame from incomplete information.
AMCA 204-2005 interpretation1 — factory versus installedApproved August 5, 2020; historical 2005-edition interpretation distinguishes fan manufacturer factory testing and installation in an AHU. Not relabeled as a2020-edition amendment.
