Industrial gear oils

What fails in a gearbox, and why

An enclosed gear drive does not wear uniformly. The failure modes are distinct and ask different things of the oil: scuffing appears at high load and high speed, when the film breaks down thermally — countered with EP additives and enough viscosity. Micropitting appears at the opposite end, at low speed and thin film, and leaves the flank matt, as if lapped; more EP does not fix it, a formulation tested explicitly against it does. Abrasive wear comes from contamination, not from formulation. And on worm drives the problem is sliding friction, which makes heat — hence PAG oils, which cut the friction coefficient in a way a mineral oil cannot approach, at the price of being incompatible with almost everything else.

Specifications, grades and base oil types

The European reference is DIN 51517-3, which defines the CLP class — a gear oil with anti-wear, antioxidant and anti-corrosion additives. The international equivalents are ISO 12925-1 CKC and CKD, and AGMA 9005 on the American side. Usual viscosity grades run from ISO VG 68 on high-speed units to ISO VG 680 on very slow, heavily loaded drives; VG 220 and 320 cover most industrial gearboxes. The base oil matters just as much: mineral is the default, PAO buys a drain interval several times longer and good cold behaviour, and PAG is used on worm drives and at high temperatures. Where the gearbox sits in a food plant, the equivalent is an NSF H1 registered gear oil.

Common mistakes on gearboxes

Most of them are made with good intentions, which is what makes them hard to catch:

Moving up a grade "to be safe". A VG 320 in place of a VG 220 raises friction losses, and the gearbox runs hotter than before.
Topping up a PAG with mineral or PAO. PAGs are incompatible with most other base oils, and with some internal paints and seals.
Choosing on EP where the problem is micropitting — the flank keeps degrading however high the declared load capacity.
Ignoring the breather: a gearbox breathing through an unfiltered vent draws in dust and moisture on every thermal cycle.
Checking the level only cold and stopped, with all the oil in the sump. The difference from the real running level has left many gearboxes half full.