Sight glasses are accurate on a gearbox when?

• A. When the Gearbox Is Not Running
• B. During Operation
• C. When The Gearbox Is Cooling
• D. At High RPM

Answer: (A)

The level shown in a sight glass is only reliable when the oil is at rest. In operation the oil is circulating, foaming can occur, and pressure differences throughout the gearbox push oil around, so the level you see through the sight glass isn’t the true static fill. When the gearbox is not running, the oil settles and any air or bubbles dissipate, giving an accurate indication of the actual oil level at the marked point.

That’s why the correct practice is to check the level with the gearbox off, after it has cooled and settled on a level surface, so you’re reading a stable and correct amount. Checking during operation, while cooling, or at high RPM would all distort the reading due to movement, aeration, and thermal effects.