Constant-level Oilers

Carolyn Boldt, Oil-Rite Corporation
Tags: automatic lubrication, bearing lubrication

A constant-level oiler is used to maintain the fluid level in a piece of equipment that naturally depletes fluid through use, wear, friction, misting or evaporation.

As oil is depleted in equipment, such as bearings, gearboxes, pillow blocks or pump housings due to its natural operation and the generation of heat from friction, the level of fluid changes. A constant-level oiler can be used to maintain optimum performance.

Constant Level Oiler Style CS
Figure 1. Style CS

The operation is based on the liquid seal principle: as fluid is depleted in the equipment, the liquid seal on the spout inside the constant-level oiler is broken. When this occurs, air enters into the oil reservoir from the air vent. This releases the fluid from the reservoir and allows it to flow into the equipment until the liquid seal reestablishes itself.

An automatic constant-level oiler can be used for antifriction, sleeve, roller, ball, tapered, spherical or slinger bearings involving excessive backpressure. Applications include fans, motors, blowers, gearboxes or other equipment where a constant level of fluid needs to be maintained.

Constant-level oilers are most useful in paper mills, cement plants, coal handling mills or industries with similarly dirty environments, because the sumps are opened less frequently.

Basic Design and Operation

In situations where pressure or a vacuum is generated in the sump, it is preferable to provide a vent line back to the equipment housing above the oil level to equalize the pressure.

By equalizing the pressure between the oiler and the equipment (Figure 1), the level is more accurately maintained, creating a closed-loop system. In the event that there is no place to pipe the air vent back to the equipment, a filtered vent plug at the reservoir can be used to prevent environmental contaminants from entering the system.

Figures 2 and 3 show the two fluid connection points, of which either point can be used for installation or drain. One is located on the side of the housing while the other one is on the bottom. These points will vary by design.

The liquid level in the oil reservoir is visible through a clear reservoir available in acrylic or glass and in various sizes. Maximum temperature rating is 160ºF for acrylic and 225ºF for glass.

The reservoir is mounted airtight on the base to prevent contaminants from entering. The reservoir is easily refilled through the cap on top port which will automatically shut off the flow while being refilled.

Operation will resume when the cap is returned and secured. It may be necessary to repeat the fill process until the fluid reaches the optimum level and no longer drains from the reservoir when the cap is installed.

Constant level oilers may be installed remotely or directly to the reservoir. A liquid level line is marked on the base of the constant-level oiler for ease in aligning to the proper oil level.

Figure 2. Style CS Adjustable

With certain equipment, high levels of oil can be carried to the upper portion of the housing during operation. Upon shutdown, this oil surges back to the reservoir establishing a high oil level. Should this amount of oil overtake the capacity of the reservoir, it will flow into the vent line (Style CS) and dissipate or in the case of those piped to the sump through the vent line, will run back to the housing.

A constant level oiler is installed with the oil level mark at the exact height at which the fluid in the housing is to be maintained. It should be level with short, rigid connections to avoid vibration.

The air vent is sometimes equipped with a filter when vented to the atmosphere. The air vent can also be connected back to the top of the gearbox or housing for a closed circuit.

Constant level oilers have been known to operate for years or even decades without maintenance. Degraded seals are the most likely cause of an operating problem.

The oiler can be tested by plugging the fluid outlets with pipe plugs, refilling the reservoir, tightening the cap and observing for leakage. The oil should seek and stay at the oil level mark over the course of several hours. If the seals are faulty, the oil will fill the viewing port and seep out the vent hole. In the case of faulty seals, it is advisable to replace the entire unit.

Figure 3. Style C


Proper installation of the oiler will assure maximum long-term operation. Key issues that must be addressed during installation include:

  1. Verify that the oil level is clearly marked on the base of the oiler. Mount the oiler by using side or bottom outlet at the desired level.

  2. The correct oil level is the lowest level at which the bearing operates properly, which should be at the middle of the element measured at the six o’clock position in the race.

  3. Lubricator must be level in all directions to function properly.

  4. All connecting pipes should be short, rigid and close to the housing to avoid vibration.

  5. Assure that the housing is filled at initial installation. Fill the housing through the oiler. Repeated filling may be necessary.

  6. Verify that the filler cap is tightly fastened. Removing the filler cap will shut off oil supply. Loose filler cap will cause leakage.

  7. The sump should be fitted with sight glass to confirm oiler operation.


A variety of table-top tests may be conducted to identify and resolve issues in the installation and maintenance of constant level oilers. A simple preventive maintenance inspection can be conducted as follows:

  1. Secure oiler so it is level in all directions.

  2. Plug outlets with pipe plugs.

  3. Remove fill cap and fill reservoir completely.

  4. Quickly screw fill cap back on the reservoir. Note: Closing the fill cap allows the check valve to open, thus allowing oil to fill the sight glass.

  5. When the fill cap is tightened securely, the oil in the sight glass should seek a level at or near the level mark shown in the casting.

  6. Leave oiler to sit for several hours. If it continues to hold oil at the initial level, the oiler is working properly. If oil appears to be filling the sight glass completely, and is seeping out of the vent hole, the seals in the unit are bad and the seals should be replaced.