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Spring Isolator Deflection Calculator

Run the Spring Isolator Deflection Calculator MicroSim Fullscreen

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About This MicroSim

A spring isolator compresses under the share of the fan's weight it carries. This MicroSim asks you to calculate that static deflection before it shows the answer, then lets you change every input and watch the whole isolation calculation respond.

The model follows the chapter's rules:

  • Load per isolator = W / n, and static deflection δ = (W / n) / k.
  • Natural frequency fₙ = 3.13 / √δ (Hz), running frequency f = rpm / 60, and frequency ratio r = f / fₙ.
  • Above r = 1.41 the springs isolate: transmissibility T = 1 / (r² − 1) and isolation efficiency = (1 − T) × 100%.
  • At or below 1.41 the status reads "Amplifying: springs too stiff for this speed", T = 1 / |1 − r²| and efficiency is "none". Within ±0.02 of r = 1.00 the status reads "Resonance" and T is "very large".
  • Above 2.00 in of deflection a note says that higher deflection requires engineering review (FanPedia p. 134).

The three challenges come in this order:

# W (lb) n k (lb/in) Model deflection (in)
1 2,400 4 600 1.00
2 3,600 6 300 2.00
3 1,600 4 1,600 0.25

An answer within ±0.02 in of the model is correct. Each challenge allows two attempts: a wrong first answer shows the calculation hint, and a second wrong answer shows the model value and counts as missed. While you answer, every result that depends on the deflection shows "?" and the loaded spring is hidden. After you check, the loaded spring appears to scale beside a free spring (an illustrative 4-inch free height), together with its natural frequency and the isolation results at the default 1,180 rpm. Mastery is 3 of 3 on the first attempt.

After challenge 3, Explore mode unlocks the four sliders with the default values (2,400 lb, 4 isolators, 600 lb/in, 1,180 rpm). Explore mode is never scored.

How to Use

  1. Read the challenge: the total weight W, the number of isolators n and the spring rate k of each isolator.
  2. Type the deflection in inches and press Check (or Enter). If you get a hint, use your second attempt.
  3. Read the loaded spring and the results panel, then press Next challenge (or Enter).
  4. After challenge 3, press Start exploring. Move the sliders and notice that raising n lowers the deflection, that raising the fan speed raises the frequency ratio, and that a 0.25-in spring (W = 1,600 lb, n = 4, k = 1,600 lb/in) on a 300-rpm fan reaches the Amplifying status.
  5. Start over returns to challenge 1 and clears the score.

Keyboard: Enter checks the answer or goes to the next challenge; Tab reaches every control; the arrow keys move a focused slider.

Data label: "Illustrative values · Typical deflections: FanPedia p. 134", shown 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/ch23-spring-isolator-deflection-calculator/main.html"
        height="702px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objective

The learner will calculate the static deflection of equally loaded spring isolators from total weight, isolator count and spring rate, to within 0.02 in (Bloom: Apply, verb: calculate).

Audience

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

Duration

10-15 minutes

Prerequisites

  • Spring isolator, static deflection and spring rate (Chapter 23)
  • Natural frequency, running frequency and frequency ratio (Chapters 22 and 23)
  • Transmissibility and isolation efficiency (Chapter 22)

Activities

  1. Challenges (5 min): Work the three deflection challenges. Write the load per isolator before typing each answer.
  2. Review (3 min): Compare the loaded springs of challenges 2 and 3. Which spring isolates better at 1,180 rpm, and why?
  3. Explore (5 min): Set the challenge 3 spring and slow the fan to 300 rpm. Then find the slowest fan speed at which that spring isolates.

Common Misconceptions Addressed

  • Doubling deflection halves the natural frequency. Change k to double δ and read fₙ: it falls by a factor of 1.41, not 2.
  • Any spring isolates, whatever the fan speed. A 0.25-in spring on a 300-rpm fan shows the Amplifying status and a transmissibility above 1.
  • Adding isolators adds deflection. Raising n spreads the same weight over more springs, so each one deflects less.

Assessment

  • First-attempt score shown as "First-attempt score: n of 3"; mastery is 3 of 3 on the first attempt. The running line reads "Challenges correct: n of 3".
  • Follow-up question: a 600-rpm fan sits on 1-inch springs. Is it isolated, and what would 2-inch springs change?

References

  1. FanPedia: The Fan & Blower Encyclopedia (PDF) - Twin City Fan & Blower; p. 134 lists the typical spring deflections (under 1 in, 1 in or 2 in; higher by engineering review).
  2. FanPedia online - Twin City Fan & Blower's online version of the encyclopedia.
  3. Chapter 23: Spring Isolators, Restraints and Field Measurement - the "Spring Isolators" section, with the equations and the worked example.
  4. Chapter 22: Fan Bases and Vibration Isolation - deflection, natural frequency and transmissibility.
  5. Vibration isolation (Wikipedia) - background on passive isolation and transmissibility.