Gear reducers need lubricant in order to function properly and deliver the torque, efficiency and performance that you expect when you invest in your machinery. With so many different types of lubrication out there, it’s hard to determine which one is the correct one for your application.
How Lubricants Work
To tackle this issue, we first have to understand what lubricants do for gear reducers. In all gear reducer applications, gears and bearings experience friction. Therefore, the primary function of any lubricant is to minimize the friction that is caused by the sliding or rolling action between gears and bearings. The right lubricant will develop an elastohydrodynamic film that completely keeps the faces of these rotating/meshing elements from touching.
Additionally, lubricants cool operating elements. Gear reducers transmit torque from prime movers, with this torque multiplication producing heat within the gear box itself. The amount of heat created is related to the efficiency of the box, with higher efficiency boxes operating at cooler temperatures and lower efficiency boxes generating more heat.
Lubricant also helps prevent corrosion between the mating steel surfaces and carries away any particulates that may accumulate during normal operation.
Choosing the right lubricant is dependent on the type of gearing, operating environment, operating speed and operating temperature. These conditions drive the viscosity – the resistance to flow – when subjected to a force. Less viscous lubricants flow more easily, while more viscous lubricants flow more slowly.
Types of Gearing
Worm gear units that use sliding engagement reduce speed experience more tooth contact and will higher friction. This higher friction creates more heat, and a thicker (higher viscosity number) lubricant would be appropriate for this application. Conversely, helical gearing that use rolling engagement, are more efficient and less viscous lubricants are more appropriate. It is important to note that higher speeds (lower ratio units) create more heat, and viscosity numbers may need to change accordingly.
For either type of gearing, operating environments and ambient temperatures will drive changes that will affect your selection of the proper lubricant viscosity. Hotter operating temperatures require lubricants with higher viscosity numbers, because as oil warms, its ability to resist flow decreases.
Choosing the Right One
All manufacturers supply operating instruction manuals with their products. These manufacturers have conducted the laboratory testing to determine the right oil viscosity for your operating circumstances. Generally, most helical gear units operating at the industrial ambient of 40C (104F) will function well with SAE10W for higher speeds (3000RPM) to SAE 40 for lower speeds. Cooler ambient temperatures would change this range to SAE 10W to SAE 20W.
For worm reducers, SAE140 (600W) would be appropriate for 40C ambient temperatures, while higher weights would be required for higher temperatures.
Using the Improper Lubricants
Using lubricants with incorrect viscosity can damage your gear box. SAE numbers that are too low will prevent proper formation of the elastohydrodynamic film and allow direct metal-to-metal contact between gears and bearings. Too-viscous lubricants will build up a thick film that can build internal (sump) temperatures, leading to damage.
It is equally important to never use motor oil in a gear reducer. Motor oils are formulated with multiple viscosities, for example SAE 10W40, and additives that trap byproducts of combustion. Motor oil will foam in a gear reducer, accelerating gear wear as entrained air insulates and disrupts the oil film, leading to metal-to-metal contact that can quickly cause a reducer to fail. Viscosity numbers for motor oils are different than gear oils. SAE 40 motor oil is the same viscosity as SAE 90 gear oil. These differences are caused by additives contained in motor oil.
Changing Your Oil
Oil is a wearing component in a gear reducer. You should regularly change your gear reducer oil as part of your maintenance program. Schedule your oil changes based on the stresses your machine experiences related to operating hours and environmental conditions.
Most manufacturers recommend that you change your oil after one month for newly installed units and then at 2,500 operating hours or 6 months, whichever occurs first. This cycle may be extended for synthetic oils, with some manufacturers noting that, depending on conditions, gear reducers can run up to 25,000 hours before an oil change.
You may want to replace your lubricant more frequently if your machine operates at higher ambient temperatures or in dirty or wet environments, as this can increase the rate of lubricant degradation.
Optimally, an oil testing program should be considered and employed for all of your gear reducers. These programs are designed to identify units that risk failure before breakdown and downtime occurs.
Contact your Kaman Representative for help in selecting proper lubricants and recommending an oil testing program.
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