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Using a Constant Level Lubricator With Turbulent Fluid

A brief description of how a constant level lubricator operates will help explain the effects of slinging.

Lubricant Bearing I Stock 572b9b4742ea8

Constant level lubricators are used with large bearings or gear boxes, where a constant presence of oil is required to lubricate moving components. The lubricator uses gravity to mimic the level of oil in the equipment housing and to replenish any lost oil.

The constant level lubricator is connected to the housing so that fluid flows freely between the two. When the level drops, the lubricator reacts by replenishing fluid until the level is restored. This happens in very small increments so that the oil is kept at a safe operating level for the equipment.

In some applications, a surge of oil into the lubricator is observed when the equipment is shut down. It is sometimes presumed that this excess is coming from the lubricator or that lubricator has overfilled the equipment housing. The more likely explanation is that the equipment is slinging oil. A brief description of how a constant level lubricator operates will help explain the effects of slinging.

Large bearings or gears rotate through oil at the bottom of the equipment housing. The bearing or gear manufacturer specifies the optimal amount of oil for the equipment housing. It is typically the amount necessary to cover the components at the lowest point of rotation. The oil is supplied at the time of start-up and the level is expected to remain relatively constant. It is not unusual for leaks to form between compartments or connection points.  If left undetected, a low oil level can lead to extensive equipment damage.

The lubricator operates using the liquid seal principle. It is analogous to turning a drinking glass upside down in dish water. If the opening of the glass is kept underwater, the liquid stays in the glass even when upside down and extending above the surface of the water. It’s not until the rim of the glass cracks the water surface that air is allowed to bubble into the glass and displace an equal amount of fluid.

Similarly, the reserve oil in a constant level lubricator is sealed off from contact with air and the resulting effects of gravity. The bottom of the reservoir is a spout and – like the rim of the glass in the dish water – it is below the fluid surface. We are likely to lift the drinking glass to observe the effect of gravity. A constant level lubricator remains stationary and it is the oil from the adjoining housing that drops, breaking the seal momentarily and sending air into the reservoir and oil into the housing (see Figure 1). This simple concept utilizes no moving parts and creates a reliable lubricating system that can last for decades.

Gravity is also key to determining the position of the lubricator. Two containers that share a connection between their contents and exposure to the atmosphere will fill to the same height, regardless of their capacity. So even though the bearing housing may hold a large amount of fluid in comparison to the adjoining lubricator, the lubricator will only accept oil to an equal height. The lubricator needs to be installed with relation to the surface height of the fluid in the housing (see Figure 2). This will position the spout from the reservoir just below the surface. When the fluid level drops, the replenishment action begins almost instantaneously.

Oil within the housing can slosh around and the oil height may fluctuate somewhat during operation. This usually does not present a problem. The oil will “find its level” and the spout will stay submerged until a measurable fluid drop.

There are some applications where there is more than a just a bit of sloshing. The oil may actually sling upward within the housing, causing an unequal distribution of oil along the inner walls. One wall may have a constant suspension of oil while oil is pulled away from the opposite wall (see Figure 3). This may be due to the size or shape of the components and the housing, the rotation speed, or the type oil being used. Under these conditions, the optimal fluid level may be more accurately described as the average between various surface points of the fluid.  Since the optimal level is neither the highest or the lowest point, the question becomes on what side to install the lubricator.

The lubricator should be installed on the high side of the fluid. This insures that the lubricator is not reacting to a false requirement for fluid.  Replenishment will still occur, although it may not be as instantaneous as when the fluid surface is calm.

If installed on the low side of the fluid, replenishment is likely to occur almost immediately upon start-up. Overfilling can potentially contribute to turbulence already in the housing. The fluid may reach higher highs and lower lows, creating a sustained need for replenishment from the lubricator.

Oil suspension also accounts for a surge in the constant level lubricator when the equipment is shut down. The lubricator is reflecting the higher level within the housing after all the oil has run off the components and accumulated at the bottom of the housing (see Figure 4). This oil is not coming from the reservoir on the constant level lubricator. The lubricator will continue to function correctly, despite this higher level reading. There is the potential for the surge to be large enough to reach the vent hole in the oiler. Venting the oiler back into the housing will prevent spills.

Constant level lubricators are extremely accurate and reliable. Select a unit that is sturdy and easy to refill. Install them with careful attention to the fluid level in the adjoining equipment and observe the simple way in which they interact. They are likely to outlast the equipment they service.

Kurt Rommelfaenger is the Marketing & Sales Manager for Oil-Rite Corporation. For more information, visit

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