Understand How Temperatures Affect Gearboxes
Jane Alexander | December 22, 2016
Understanding the impact that the environment can have on the long-term well being of gearboxes is key to keeping them healthy. According to experts at Philadelphia Gear (philagear.com, King of Prussia, PA), environmental factors can significantly influence gear-drive service life and associated maintenance costs. A white paper from the company offers tips on how to deal with several of those factors, including temperature and temperature variations.
When choosing lubricating and cooling oil for a particular gear-drive application, consider viscosity under normal and “cold iron” conditions. The viscosity must be capable of providing adequate oil film to support gear-tooth and bearing loads under all operating conditions. When a gear drive is in the “cold iron” state, viscosity must be low enough so that during the unit’s operation, the splash-lubrication or force-feed lubrication system is capable of distributing the proper amount of oil to where it’s needed. If oil is too thick (due to the cold), there may be no splash, or the pump in a force-feed-lubrication system might stall, thus failing to supply oil to critical surfaces.
Several oil characteristics must be taken into account when selecting products for specific applications and temperature differentials. Some formulations have a flatter temperature index than others, meaning that, as oil temperature increases, viscosity will decrease at a lesser rate than comparable fluids. For example, synthetics have a flatter temperature index than mineral-based oils and, at a low temperature, will be less viscous than similar-viscosity mineral-based products.
Many locales, of course, experience severe temperature swings from summer to winter that can dramatically affect oil viscosity. Sites in such regions should change to different-viscosity products as seasons change. Oils that aren’t changed out seasonally could thin to a kerosene-like consistency in severe summer heat, and thicken to a molasses-like substance in frigid winter cold. Either scenario puts gear drives at risk. (NOTE: To help overcome environmental extremes, oil heaters are often employed to maintain a minimum oil temperature of 40 F to 50 F during colder months, and air-to-oil or water/glycol-to-oil coolers are used to control oil operating temperatures during extreme heat.)
When gear operation generates higher-than-normal operating temperatures, mineral-based oil can break down and lose some of its lubricating capability. Over time, this deterioration can cause the fluid to separate into its constituent organic parts, i.e., carbon, hydrogen, and oxygen in various chemical combinations. In such cases, the carbon manifests as fine grit that can be introduced into the oil. If this condition persists and the sludge approaches the thickness of the oil film between gear and bearing components, a loss of film can be expected. This situation, in turn, could result in metal-to-metal contact between gears and bearing components, eventually leading to gearbox failure.
Extreme Pressure (EP) mineral oils, in particular, contain additives such as phosphorous and sulfur, that enhance the products’ ability to support load, but, when broken down, introduce additional, potentially corrosive and abrasive, materials into the mix. Thus, when using EP oils, it is extremely important to monitor the oil through periodic sampling.
While the additives in EP products increase load-carrying capacity, they can be depleted over time. In that event, such oils no longer exhibit their original load-carrying capabilities. How can you tell if this is happening/has happened? Extended gearbox operation with EP oil that has lost its load-carrying capabilities can result in gear-tooth overload symptoms such as pitting.
For more information from Philadelphia Gear’s experts, and/or to request a copy of “The Impact of Environmental Conditions on Gearbox Lifecycle,” (the white paper on which this article is based), visit philagear.com.