Background
An aggregate quarry near Johannesburg operates two double-deck vibrating screens for gravel classification. Each screen is driven by a pair of counter-rotating unbalanced motors at 980 RPM. After a motor rewind at a local shop, one unit returned with a residual imbalance that pushed frame vibration to 16.2 mm/s RMS at the drive end — well beyond ISO 10816 Zone D.
The symptoms were unmistakable: housing bolts worked loose every shift, the motor ran hot, and aggregate throughput dropped because operators reduced feed rate to limit the shaking. Sending the motor back to the shop meant 5 days of downtime on the primary classification line.
Procedure
The screen was isolated and the motor decoupled from the counterweight shaft. Balanset-1A sensors were mounted on both bearing housings. Two-plane balancing was selected given the motor’s 260 kg rotor length-to-diameter ratio.
Baseline: DE 16.2 mm/s, NDE 13.4 mm/s — both at 1× RPM, confirming mass imbalance.
Trial run: 120 g trial weight on the DE fan-end correction plane. Influence coefficients captured for both planes.
Correction weights:
- DE plane: 85 g at 312°
- NDE plane: 55 g at 78°
Both weights were welded to the motor fan shroud per the motor manufacturer’s approved correction method.
Final measurement: DE 2.0 mm/s, NDE 1.7 mm/s — both within Zone A.
Impact
| Metric | Before | After |
|---|---|---|
| DE vibration | 16.2 mm/s | 2.0 mm/s |
| NDE vibration | 13.4 mm/s | 1.7 mm/s |
| Motor temperature | 92 °C | 71 °C |
| Bolt re-torque interval | every shift | none required |
| Balancing time | — | 45 min |
The screen returned to full feed rate immediately. Maintenance reported zero bolt re-torques over the following two weeks — previously a daily task. Motor winding temperature dropped 21 °C, significantly extending insulation life.
"The screen ran through a full shift without the housing bolts loosening for the first time in months"
Resultados obtenidos
16.2 mm/s 2.0 mm/s
DE vibration
13.4 mm/s 1.7 mm/s
NDE vibration
45 min
Time
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