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Impeller Balance Calculator

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About This MicroSim

An impeller is balanced until its residual unbalance is within a limit set by a balance quality grade G. The sim uses the formulas from the chapter section "Impeller Balancing":

  • Permissible residual unbalance: U = 6.015 × G × W / N, with G in mm/s, W the impeller weight in lb, N the maximum service speed in rpm and U in oz·in.
  • Correction mass at radius r: m = U / r, with r in inches and m in ounces.
  • Rotating force from that unbalance: F ≈ 1.77 × U × (N/1000)², in lbf.

The sim starts with three challenges, in this order. All values are illustrative.

# W (lb) N (rpm) G r (in) U (oz·in) m (oz)
1 200 1,200 G6.3 15 6.32 0.42
2 200 1,200 G2.5 15 2.51 0.17
3 600 1,800 G6.3 20 12.63 0.63

For each challenge you type both U and m. The challenge is correct when both values are within ±2% of the unrounded model values. You get two attempts. A wrong answer shows the worked calculation for that challenge (and says which of your two values was within 2%); a second wrong answer shows the model values and the challenge counts as missed. Once a challenge is resolved, your answers, the model values and the worked calculation appear together, and the end view and force bars are revealed.

The drawing shows an end view of the impeller with an orange correction mass at radius r (40 in is drawn at the rim) and an arrow for the outward force. The bars compare the force from the same residual unbalance U at half, the selected and double speed, when those speeds lie within 300–3,600 rpm. Mastery is 3 of 3 challenges with both values correct on the first attempt. After challenge 3, Explore mode unlocks the weight, speed and radius sliders and the grade menu (G16, G6.3, G2.5, G1.0); U, m and F update at once. Explore mode is never scored.

How to Use

  1. Read challenge 1. Type U in the U (oz·in) box and m in the m (oz) box, then press Check (or Enter).
  2. A correct answer reads "✓ Correct: U = 6.32 oz·in, m = 0.42 oz." in a solid green box. A wrong one shows the worked calculation in a dashed red box; use your second attempt.
  3. Press Next challenge (or Enter) and repeat for challenges 2 and 3. "Challenges correct: n of 3" keeps the running count.
  4. After challenge 3, press Start exploring. The panel shows the first-attempt score and the mastery line. Set N to 1,200 rpm, then 2,400 rpm: U halves, because the grade allows less unbalance at a higher speed, while the bars show that the same impeller, with the same U, would push four times as hard at double the speed. Start over returns to challenge 1.

Keyboard: Enter checks the answer, and Enter again goes to the next challenge. Tab reaches both answer boxes, every button, the grade menu and the three sliders; the arrow keys move a focused slider.

Data label: "Illustrative values", at the bottom left of the drawing.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://moroshek.github.io/FanTASTIC-textbook/sims/ch08-impeller-balance-calculator/main.html"
        height="642px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will calculate the permissible residual unbalance and the matching correction mass for an impeller from its weight, speed, balance grade and correction radius, to within 2% of the model value (Bloom: Apply, verb: calculate).

Audience

Adult professional development: new application engineers, sales staff, representatives and facilities staff.

Duration

10-15 minutes

Prerequisites

  • Impeller balancing and residual unbalance (Chapter 8)
  • Balance quality grade G (Chapter 8)
  • Ounce-inch and correction radius (Chapter 8)

Activities

  1. Challenges (5-7 min): Solve the three challenges with a calculator before pressing Check. Write down U first, then divide by r.
  2. Review (2 min): Compare challenges 1 and 2, which differ only in grade, and say which grade is tighter.
  3. Explore (3-5 min): Double the weight and watch U double. Then double the speed and compare the change in U with the change in the force bars.

Common Misconceptions Addressed

  • A heavier impeller is allowed less unbalance: U rises in proportion to W, so a heavier wheel is allowed more.
  • Force from unbalance grows in proportion to speed: the bars show four times the force at double the speed for the same U.
  • A larger grade number means tighter balance: G2.5 gives a smaller U than G6.3, so the smaller number is the tighter grade.

Assessment

  • The running line "Challenges correct: n of 3" counts challenges solved on either attempt. At the end the sim shows "First-attempt score: n of 3" and the mastery line; mastery is 3 of 3 challenges with both values within ±2% on the first attempt.
  • Moving the sliders is not evidence. Follow-up question: a customer wants to run an impeller balanced at 1,200 rpm at 1,800 rpm. What happens to the force from the same unbalance, and what has to be checked first?

References

  1. Balancing of rotating masses (Wikipedia) - background on static and dynamic unbalance and how it is corrected.
  2. ISO 21940-11:2016, Mechanical vibration - Rotor balancing - Part 11 - the standard that defines balance quality grades G and permissible residual unbalance.
  3. AMCA International - publisher of AMCA 204, which assigns balance grades to fan application categories.
  4. FanPedia online - Twin City Fan's reference on impellers and fan construction.