Knowledge check. Twenty-two auto-scored questions on application, calculation, and judgment across the balancing series, open-book with lookup links, a per-article score breakdown, and a printable certificate at 80 percent.
Knowledge check for the balancing series: twenty-two auto-scored questions on application, calculation, and judgment across the six articles and seven calculators. The check is open-book. Every question carries lookup links to the article and the calculator it draws on, and calculator questions open the tool blank. Three of the questions are shown here with their answers.
Sample question · Two-Plane Balancing: The Fundamentals
A rotor rests in any orientation on rollers, yet at speed its two bearings vibrate at equal amplitude and 180° out of phase with each other. Which condition does this describe, and can one correction plane remove it?
(a) Static unbalance; yes, one mass corrects it.
(b) Couple unbalance; yes, one mass in the plane of the center of mass cancels it, because the resting test shows no net force to remove.
(c) Dynamic unbalance; yes, provided the single plane is placed at the anti-node of the first mode.
(d) Couple unbalance; no, two planes are needed.
Answer: (d) Couple unbalance; no, two planes are needed.
Couple unbalance produces no net force, so the rotor has no resting tendency and passes the resting test, while its equal and opposite bearing reactions appear at speed. A mass added in one plane changes the net force on the rotor, which a pure couple does not have, so removing a couple requires two planes.
Sample question · Single-Plane Balancing: The Fundamentals
A rotor of mass W = 100 kg has a maximum continuous speed N = 3,000 RPM and a correction radius r = 100 mm. Using the Balance Tolerance Calculator on ISO grade G2.5, find the allowable residual unbalance. Answer in g·mm.
Rotor weight W, maximum continuous speed N, and the correction radius r measured from the axis of rotation.
Answer: 796 g·mm
U = 9,549 · G · W / N = 9,549 · 2.5 · 100 / 3,000 = 796 g·mm. Switch the calculator to SI, select G2.5, and enter W, N, and r; the radius affects only the allowable unbalance mass row.
Sample question · At-Speed Balancing: The Fundamentals
A uniform rotor carries a single unbalance at midspan. On the low-speed machine it is balanced to tolerance in two correction planes near its ends, yet as it approaches its first bending critical it vibrates heavily. Which correction-plane choice would have addressed both the rigid resultants and the first mode?
(a) A plane near midspan in addition to the end planes.
(b) Larger corrections in the same two end planes.
(c) A plane at the coupling hub, outboard of the bearings, where access is easiest.
(d) Planes at the bearing centerlines.
Answer: (a) A plane near midspan in addition to the end planes.
The end corrections cancel the rigid resultants of the midspan unbalance but sit near the first mode’s nodes, where they have almost no authority, while the midspan unbalance drives the mode with full effect. A correction’s authority over a mode is proportional to the mode’s deflection at its plane, so the first mode needs a plane near midspan.
The full check scores each answer, breaks the score down by article, explains every question answered incorrectly with links to the relevant sections, autosaves in the browser and to a file so it can be finished in more than one sitting, and prints a certificate carrying the results summary at 80 percent or better.
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