Constant Level Oilers: Best Practices For Optimal Lubrication

EP Editorial Staff | March 1, 2007

Constant level oilers have been utilized on process pumps and other rotating equipment for more than half a century. Although some oiler designs haven’t changed much over time, others have become more advanced—as have the equipment and applications they serve.

Regardless of the types of oilers in your plant, you’re counting on them to maintain proper levels of lubrication—however and wherever they are deployed. That’s why proper selection, installation and maintenance of these devices should be an utmost concern. By taking steps to eliminate additional points of contamination ingression and avoid installation and application errors with your constant level oilers, you can ensure both proper quality and quantity of the lubrication they supply.

0307_lubrication_img1The right amount and condition
The right condition and the right amount are two critical elements of lubrication. The right condition refers to the quality of oil—specifically oil that is contaminant free. Having the proper quantity of contaminated oil is no better than having an insufficient quantity of contaminant-free oil. Having the proper quantity of oil would be considered more important than maintaining the quality of the oil, but both have significant impact on the life of the lubricant and component being lubricated. Oil sump lubrication doesn’t require that a specific level be maintained for proper bearing load—only that oil levels do not reach critically low or high points (Fig. 1).

In a low-level operating condition, the bearing will not receive enough lubricant necessary for proper film strength—a precursor to surface contact, skidding and possible catastrophic failure. Without enough oil to prevent friction, thermal runaway can happen quickly to a steel bearing. As the temperature of the bearing increases, the ball and race both expand, which creates an even tighter fit. This further increases the temperature, and the cycle continues to a rapid, catastrophic failure.

In a high-level operating condition, churning of the lubricant will occur, accelerating the oxidation rate as a result of excessive air and elevated temperatures. It is a common mistake to believe that more is better—especially when it comes to oil sump lubrication. Too much oil can affect the operation of oil rings, flingers and direct bearing contact. Leaking seals are another result of high lubricant levels.

Solving the problem
Maintaining the proper quantity of lubricant is perhaps the easiest means of increasing lubrication life and effectiveness. Consult with your equipment manufacturer for recommended oil levels, optimum lubricating equipment and preferred practices. For direct contact lubrication, such as in rolling element bearing applications, the ball at the deepest point of submersion in a static condition should not be submerged more than one-half the diameter.

The constant level oiler (Fig. 2) continues to be one of the most widely used methods of maintaining the proper level lubricant in a bearing housing. These devices replenish oil lost by leakage through seals, vents and various connections and plugs in the bearing housing. Once the proper level has been set, replacing the oil in the reservoir is the only required maintenance.

Constant level oilers have a “control point” that must align with the proper oil level of the equipment. The device is installed on the equipment and oil is filled to the predetermined level. All constant level oilers require air to function properly. Consequently, if the oil level within the sump lowers, the seal at the control point is broken, allowing air to enter the reservoir and displace the oil until the seal is reestablished. As long as a constant level oiler is set correctly and there is oil in the its reservoir, the equipment will always have the optimum oil level within the sump.

Vented & non-vented designs
Constant level oilers can be put into two categories: vented and non-vented. As mentioned previously, all oilers require air to function properly.

  • In vented types, the air comes from the surrounding atmosphere.
  • In non-vented types, the air necessary for proper operation is the same air found within the sump.

Vented oilers, which entered the marketplace over 60 years ago, still are the most used type today. That’s because they provide great value in maintaining proper oil levels and extending equipment life. It is important to note that vented oilers will provide the proper quantity of oil, but not necessarily proper quality, as they can be a source of contamination ingression.


Pressure differentials between the equipment housing and surrounding atmosphere is a leading cause of contamination ingression. In outdoor applications, for example, or where housing temperature fluctuations occur during frequent on/off running conditions or process fluid temperature changes, air flow over the equipment create this atmospheric exchange as pressure is equalized. During this air exchange, contamination from the surrounding environment is “breathed” into the oil sump.

Non-vented oilers do not exchange air with the atmosphere. One common non-vented model uses a pressure balance line that connects the air within the oiler base to the air within the sump (Fig. 3). Non-vented types of oilers operate under the same principle as vented. Their “control point” is set to align with the desired oil level and if it lowers, air enters the reservoir displacing the oil until the seal is re-established.

Proper specification
Determining which type of oiler to use for an application requires consideration of the surrounding environment, type of seals and design of the housing, including port locations and vents. In highly contaminated environments, it is recommended that a non-vented type of oiler be installed to minimize particulate and moisture ingression. Other sources of ingression are through seals and vents located on the top of the housing.

Certain types of seals are better at preventing ingression than others. Spending money to upgrade seals to prevent contamination ingression and oil leakage will not be as effective if vented types of oilers are used. The ideal configuration for oil sump housing lubrication is to eliminate the potential for contamination ingression by “closing” up the sump. Some seals are not capable of handling the pressures due to equalization and would require an expansion chamber. An expansion chamber has a rolling diaphragm that expands as the air within the sump heats up minimizing pressure increases (Fig. 4).

Proper installation & maintenance The leading causes of incorrect oil sump levels when using constant level oilers include:

  • Incorrect oiler settings
  • Pressure differential (vented types only)
  • Oiler location
  • Blocked or plugged fittings
  • Improper filling methods

These problems can be overcome, as follows:

0307_lubrication_img2Incorrect oiler settings. . .

Review the instruction sheets provided with the oiler for better understanding of how to adjust and set the device for proper use. Understanding where the control point is can greatly reduce problems associated with high and low oil levels. On adjustable-type oilers, knowing the adjustment parameters relative to the port and oil level are equally important.

A vertical stem pipe is used on the most commonly used vented type of oiler. This stem pipe serves two purposes: (1) it sets the level via wing nuts; and (2) it breaks any meniscus that may form due to surface tension. Many users throw this stem pipe away, which could prevent feeding or cause a low oil level condition with only the set screw supporting the upper casting. State-of-the-art non-vented oilers offer easier installation methods with outside adjustment and visual verification of proper settings that can be done via a sight gauge.

Pressure differential (vented types only). . .
Airflow across equipment housings generated by fans, blowers and even the equipment motor can be sufficient to create pressure differential between the bearing housing and the oiler reservoir, thus causing the oiler to feed so much that it reaches an overfill condition. As discussed previously, equipment operating temperature changes also can create pressure differentials.

Pressure increases/decreases can be controlled by closing the housing through the use of non-vented oilers, replacement of vents with expansion chambers and proper seal selection.

Oiler location. . .
Location of a constant level oiler relative to shaft rotation can affect how the device dispenses. The recommended placement is on the side of the equipment facing the direction of the shaft rotation at the bottom when using the side mount. Oil is pushed up into the reservoir versus being pulled away—which can cause feeding and an overfill condition (Fig. 5).

0307_lubrication_img3Blocked or plugged fittings. . .
It is important to check the connection fitting between the oiler and the housing to verify that there is no blockage. When oil becomes oxidized or contaminated, it can plug this fitting. If this situation occurs, the device will not feed and the level can become low. It’s easy to check for blockages and/or plugs by removing the oiler during oil changes and looking at the fitting opening.

Improper filling methods. . .
When filling through the top of the equipment, knowing the required oil volume is necessary to achieve the preset level. If the oil quantity is known, then this method is considered to be a safe filling procedure. More times than not, however, the quantity is unknown and new oil is haphazardly filled through the top, using a sight gauge to determine the level. Unfortunately, this results in a high fluid level because of residual lubricant draining from the internal components such as a shaft or gear.

Proper filling can be achieved through the surge body when a sight gauge is present. The sight gauge provides a visual aid for achieving the predetermined fluid level in the sump. Filling without a sight gauge can cause overfilling of the oil sump and surge body. An indication of overfilling will become evident if fluid begins flowing from the surge body once the reservoir is replaced. To adjust for overfilling, drain the lubricant from the sump until the constant level oiler begins feeding, and reaches the preset level.

Excessive refilling of the reservoir also will have a negative effect on the oil level. Each time the reservoir is removed and replaced, a small amount of lubricant is added to the oil sump level. Over time, this chain of events will increase the fluid level. To minimize unnecessary filling, refill the reservoir only when it is half full or less.

In summary
Constant level oilers are an easy and effective method of maintaining proper oil level in equipment. Whether utilizing vented or nonvented types, proper application and installation is critical for ensuring optimum performance of the device and the equipment it protects.

“If it’s not broke, don’t fix it” may apply if oil change intervals of only eight months are acceptable in your plant. On the other hand, if extending oil change intervals is a goal of your lubrication program, standardizing on reliable seals, nonvented oilers, expansion chambers or other types of effective breathers is a best-practice approach you’ll want to seriously consider.

Rojean Thomas is senior technical consultant with Trico Corp., Pewaukee, WI. For more information, telephone: (262) 691-9336 ext. 200; or




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