Industrial Oils / 5 min
Why Viscosity Matters in Gear Oils
In gear oils, viscosity is the most critical physical property determining oil film thickness. Using the wrong viscosity causes gear breakage, overheating and energy losses. In enclosed gearboxes, matching viscosity and additive package is essential to extending reducer life.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
Review our editorial guidelinesTable of Contents
- - The Effect of Load and Speed Factors
- - Extreme-Pressure (EP) Additives
- - Micropitting Protection
- - The Effect of Temperature Changes on Viscosity
The Effect of Load and Speed Factors
Gear systems operating at low speeds and high torque (such as crushers and mixers) carry a risk of metal contact, so they need higher-viscosity oils (e.g. ISO VG 320 or 460). High-speed systems, on the other hand, should be lubricated with thinner oils (ISO VG 150 or 220) to overcome frictional resistance.
Extreme-Pressure (EP) Additives
The immense pressure generated where gear teeth contact each other pushes the fluid out instantly. EP (Extreme Pressure) additives form a protective phosphorus- or sulfur-based chemical layer on the gear surface, preventing metal-to-metal welding.
Micropitting Protection
Especially in wind turbines and heavy-industry reducers, microscopic surface fatigue can lead to gear breakage over time (micropitting). Using the correct viscosity is a primary factor supporting the additive package in preventing the formation of these micro-cracks.
The Effect of Temperature Changes on Viscosity
The temperature of the operating environment or gearbox directly changes the oil's fluidity. Under extremely hot operating conditions, the oil's protective film thins and tears. In such cases, a switch should be made to synthetic (PAO or PAG) gear oils with a high viscosity index (VI).
Frequently Asked Questions
What happens if I use a low-viscosity gear oil?
Because the oil film will be too thin, full separation between gear teeth cannot be maintained. Metal-to-metal friction increases, wear accelerates, and the reducer fails much sooner.
Why are synthetic gear oils preferred in reducers?
Synthetic gear oils maintain their viscosity much better in extreme hot and cold environments. Their oxidation life is also longer (3-4 times that of mineral oil), extending maintenance intervals.
Do PAG-based gear oils mix with other oils?
No. Polyalkylene glycol (PAG) based synthetic gear oils must never be mixed with mineral or PAO-based oils. If switching a system to PAG oil, a thorough flushing procedure is mandatory.
Why does the oil in a gearbox foam?
Foaming results from causes such as incorrect viscosity, water ingress into the system, depletion of anti-foam additives, or overfilling the reducer beyond its capacity.