A customer called about a helical gearbox that kept tripping its thermal switch. 5.5 kW, 20:1 ratio, driving a screw feeder. The gear oil was black. The bearings were worn. The customer wanted a bigger gearbox. We went on-site and found something they hadn’t considered.
The screw feeder started from a plugged condition. Every cycle, the screw was buried in product. The starting torque was 3x the running torque. The gearbox was sized for running torque (5.5 kW). But every start dumped thermal energy equivalent to about 30 seconds of running at full load. At 12 starts per hour, that’s 6 minutes of equivalent heating per hour. The gearbox oil never cooled down between starts. It heated up cumulatively. After 4 hours, the oil was at 95°C. The thermal switch tripped.
We didn’t upsize the gearbox. We installed a VFD and ramped the feeder from 0 to full speed over 6 seconds. The starting current dropped from 6x to 1.5x. The starting heat dropped by 75%. The gearbox ran at 55°C instead of 95°C. The VFD cost $400. A bigger gearbox would have cost $1500 plus installation. The VFD also let the customer adjust the feed rate for different products. It paid for itself in energy savings within a year.
The customer’s maintenance team had been replacing the gearbox every 8 months. They blamed the gearbox quality. But the gearbox was fine. The problem was the duty cycle. A gearbox rated for S1 (continuous) at 5.5 kW can handle 5.5 kW forever. It cannot handle 15 kW peaks 12 times an hour. That’s S4 duty. The rating plate said S1. The application was S4. The mismatch was the issue, not the gearbox.
If your gearbox overheats after a few hours of operation, count the starts per hour. If it’s more than 6, you’re in intermittent duty territory. A VFD with a soft start will fix it cheaper than a bigger gearbox. And check the oil — if it’s black within 6 months, the heat is killing the lubricant, not the other way around.
