What Is Open Gear Lubricant? Types, Uses and How It Works

How does open gear lubricant stay on exposed gear teeth?

Open gear lubricant stays on exposed gear teeth through tackiness, very high viscosity and, in some products, solid lubricants such as graphite. In open gear applications the lubricant must resist being thrown off by centrifugal force or scraped off by the action of the gear teeth, and a highly adhesive lubricant is required for most of them (USACE, 2016). Tackiness agents, such as polymers of long-chain molecules or aluminum soaps of long-chain fatty acids, increase the adhesiveness of an oil or grease (USACE, 2016). Open gears on navigation structures, which move very slowly under large loads, typically operate in the boundary lubrication regime (USACE, 2016), and one lubricant maker explains that a high viscosity helps build film thickness between tooth flanks, which protects them from wear, scuffing and pitting (Klüber Lubrication, 2021). Adhesion has a cost: adhesive lubricants may attract and retain dust and dirt that act as abrasives, so the gears should be cleaned periodically (USACE, 2016).

What is open gear lubricant made of?

Open gear lubricant is made of a very viscous base fluid plus a solvent diluent that evaporates, a thickener with solid lubricants, or neither. Most open gear lubricants are heavy oils, asphalt-based compounds or soft greases, and oxidation inhibitors or extreme pressure (EP) additives may be added depending on service conditions (USACE, 2016). Residual compounds are heavy straight mineral or EP oils mixed with a diluent for ease of application; once applied, the diluent evaporates and leaves a heavy adhesive coating (USACE, 2016). Special greases for low-speed, high-load gears usually contain base oil, a thickener and a solid lubricant such as molybdenum disulfide (MoS2) or graphite (USACE, 2016). High-viscosity transparent fluids are based on mineral or synthetic hydrocarbon oils or on polyglycol oils (Klüber Lubrication, 2021). One maker states that asphaltic, or bituminous, compounds can be carcinogenic and are banned by many countries (Klüber Lubrication, 2021). Check the product's Safety Data Sheet (SDS) and the rules of your jurisdiction before buying, applying or disposing of any open gear lubricant.

Typical open gear lubricant classes and viscosities

One maker groups open gear lubricants into asphaltics, black and white solid-containing greases, and high-viscosity fluids based on hydrocarbon or polyglycol oils (Klüber Lubrication, 2021); the table merges the two fluid types into one class. Solvent-cutback data sheets report viscosity without solvent (Phillips 66 data sheet, 2026). That convention matters because a diluent lowers viscosity only until it evaporates, and one maker notes that diluted products seldom reach full viscosity during operation when the next spray comes before the diluent has gone (Klüber Lubrication, 2021). The table below shows the typical viscosity or makeup of each class and the application methods it suits.

The residual compound row is the one with a published engineering range: 400 to 2000 mm2/s at 100 °C without diluent (USACE, 2016). Maker data sheets often cite ANSI/AGMA 9005-E02 Annex D minimum viscosity recommendations for open gearing with intermittent application (Bel-Ray data sheet, 2026). The active edition is ANSI/AGMA 9005-F16, reaffirmed in May 2026, which still covers open gearing (AGMA, 2026), and ISO 12925-2:2020 sets the ISO minimum requirements for the open and semi-enclosed gear categories CKH, CKJ and CKM (ISO, 2026). One supplier reports that high-viscosity fluids without solid lubricants have shown significant disadvantages on slow-running kiln gears, and that where kiln seals are not designed for oil, the lubricant can creep onto the hot kiln shell and create an ignition risk (Indian Cement Review, 2018).

Where is open gear lubricant used?

Open gear lubricant is used on large gear sets with no oil-tight housing, where the gear mesh and its bearings are lubricated separately: mill and kiln girth gears, mining shovels and draglines, and slow movable structures such as lock gates. The foreword of ANSI/AGMA 9005-F16 refers to open gearing as gearing "where the bearings are lubricated separately" (AGMA, 2026). In the raw materials industry, open gears most often drive tube mills and rotary kilns in cement and gypsum production (Indian Cement Review, 2018). Kiln gears run rather slowly but are exposed to higher radiation temperature, while mill gears run comparatively fast with lower gear tooth temperatures (Indian Cement Review, 2018). In mining, one maker recommends its residual compound for open gears on mining shovels, draglines and excavation equipment (Phillips 66 data sheet, 2026). Open gears on U.S. navigation structures generally move very slowly, carry large loads, are prone to dirt and debris and must perform under adverse conditions, including submersion (USACE, 2016). Where gates stand still for long periods, lubricants formulated for continuous operation can be too thin and run off, leaving the teeth dry when the gate moves again (USACE, 2016).

How is open gear lubricant applied?

Open gear lubricant is applied mostly by intermittent spray systems, with circulation, dip lubrication and manual brushing or dripping used on some equipment. In cement plants the majority of open gears are still lubricated by intermittent spray units, some equipment makers install circulation systems that lubricate continuously, and some minor drives use splash lubrication, where the girth gear dips into the lubricant and carries it to the pinion (Indian Cement Review, 2018). Solvent-containing asphaltic lubricants are restricted to intermittent spray systems aimed at the gear rim, and are not suited to circulation, dip or pinion spray (Indian Cement Review, 2018). One maker adds that the diluent keeps evaporating when equipment stops, which can clog spray nozzles and starve the gear on restart (Klüber Lubrication, 2021). Before spraying or brushing a solvent-cutback product, read its Safety Data Sheet for flash point, vapor and personal protection. In the United States, OSHA 29 CFR 1910.1200 requires employers to keep the SDS for each hazardous chemical readily accessible to employees during each work shift (US OSHA, 29 CFR 1910.1200(g)(8)); other jurisdictions have equivalent hazard communication rules.

How does open gear lubricant differ from enclosed gear oil?

Open gear lubricant differs from enclosed gear oil because it must stay on the teeth by adhesion with no sump, while enclosed gear oil sits in an oil-tight housing and also carries heat away. Enclosed gears, those encased in an oiltight housing, usually require an oil with rust, oxidation and foam inhibitors, plus EP additives when loads are severe (USACE, 2016). Open gear lubricants have extra requirements instead: drip resistance, especially at high temperature, and resistance to embrittlement at very low temperature (USACE, 2016). ISO 12925-2:2020 covers lubricants intended for open and semi-enclosed gears (ISO, 2026), while ANSI/AGMA 9005-F16 covers both kinds of gearing in one document (AGMA, 2026). Related products are covered in industrial gear oil versus automotive gear oil and ANSI/AGMA 9005-F16 requirements for industrial gear oils. When a girth gear shows wear or a change of lubricant class is planned, have the gear maker and a lubrication specialist review it; Lubinpla can organize such a case from your own data sheets, SDS and site history and return one cited document for review.

Related terms

• Girth gear: the large ring gear mounted on a mill or kiln shell and driven by a pinion.

• Residual compound: a heavy mineral or EP oil cut with a diluent for application, which leaves a thick adhesive film once the diluent evaporates.

• Diluent: the volatile solvent in a cutback open gear lubricant that makes it sprayable and then evaporates.

• Tackiness agent: a polymer or soap additive that makes an oil or grease adhere to metal and string between surfaces.

• Boundary lubrication: the regime in which the oil film is too thin to separate the surfaces fully, typical of very slow, heavily loaded open gears such as those on navigation structures.