The designs of many common machine mechanisms have not changed significantly in decades (centuries in some cases). This is especially true where lubrication is involved. Apparently, machine designers working for original equipment manufacturers (OEMs) are under the impression that lubrication has not been, or simply cannot be, improved. Sadly, there are new machines manufactured today based on this false premise. Conspicuous evidence of this is seen in many wet-sump oil systems found in pumps, gearing and bearings.

Good examples are the many non-circulating bearing and gear sumps that feed oil to frictional surfaces. These are wet-sump lubrication (WSL) systems that deliver lubricants using one of the following methods:

  • Flood Lubrication — Frictional surfaces are submerged in a bath of oil.
  • Slinger/Flinger Lubrication — Moving parts cup and toss oil to troughs and frictional zones.
  • Oil Ring/Collar Lubrication — Rotating rings and collars lift oil to the top of channels and grooves where oil is fed into bearings by gravity.
  • Splash Lubrication — Gear movement passing through an oil sump produces a splash or mist throughout the gearset to wet machine surfaces.
  • Climbing-Oil Lubrication — Gears rotating through a wet sump lift clinging oil into gear mesh zones. Some use paddle gears to help lift and transfer oil to loaded gears.


Figure 1. This illustration shows
the four basic types of oil
circulation systems.

All of these methods provide little more than basic lubricant supply. Today’s reliability organizations demand more than this, especially for machines that are prone to failure and/or have high machine criticality. One easy improvement that can be installed by many users is to retrofit an oil circulation unit to an existing wet-sump system. The benefits of circulation are not always fully understood by users and machine designers. This is one of the reasons why the added cost of oil circulation is dismissed as a necessary expense by both OEMs and users alike.

There are four basic types of oil circulation systems:

  1. Oil-feed Circulation — This is a wet-sump system that pumps oil from the sump through lines that direct lubricant to lubricated surfaces (cams, bearings, cylinders, gears, etc.). For instance, diesel engines use oil-feed circulation.
  2. Constant-level Circulation — This system uses a small external reservoir in addition to the machine’s wet sump. The wet sump supplies oil to the machine’s frictional surfaces by one of the WSL methods mentioned earlier.
  3. Off-line Circulation — This is a simple kidney-loop system used to provide off-line filtration and temperature control. The machine is lubricated by one of the WSL methods described previously.
  4. Hydraulic System — Hydraulic systems use pressurized flowing oil that enables machine movement and actuation. This is a totally flooded system, meaning frictional surfaces stay in constant contact with the circulating oil.

Hydraulic and oil-feed circulation are complex systems from a design and manufacturing standpoint. However, users can add constant-level and off-line circulation to most WSL machines retroactively to gain the numerous benefits of oil circulation.

Benefits of Constant-level and Off-line Circulation


Figure 2. This is an illustration of
a constant-level circulation (CLC) system.

As mentioned, many of the benefits of oil circulation are subtle or not easily understood. Nevertheless, they are significant in importance to machine reliability. Table 1 provides a breakdown of the advantages for using constant-level or off-line circulation when compared to conventional wet-sump lubrication alone. An “X” denotes whether the benefit applies to the designated circulation method. The column to the right estimates the importance of the benefit.

For a better understanding of the benefits associated with oil circulation, they are described in greater detail below:

Oil Level Control

Many machines require controlled and constant oil levels in the sump. These include some gearboxes, bearing housings and chain lubricators. Traditionally, these machines are manually fed with makeup oil to maintain the oil level. In other cases, external devices such as constant-level oilers drip oil from bottles to control the oil level. However, these are not circulating systems but rather oil-feed systems. A constant-level oil circulation unit is shown in Figures 2 and 3. The oil not only circulates but supplies makeup oil to the machine’s sump. Off-line circulation systems don’t have an external tank and thus cannot provide makeup fluid to control the oil level.


Figure 3. An example of a commercially available constant-level circulation system that can be retrofitted to a conventional wet-sump machine. (Ref. Dodge-Rockwell)

Contamination Control

It is difficult to maintain oil cleanliness and dryness without onboard filtration (and separators in certain cases). Filtration requires oil circulation, which is not available from wet-sump lubrication alone. The only alternative would be to use periodic portable filter carts to clean the oil and flush the sump. Onboard filtration is far superior and should be included in both constant-level and off-line circulation. Filtration offers the added benefit of being able to draw down contaminant levels even when the machine is at rest.

Live-Zone Oil Sampling

Obtaining a representative oil sample from non-circulating systems is always challenging. Circulation homogenizes the oil so that samples contain contaminants, wear debris and oil properties that are consistent with the current conditions of the machine and the oil. Oil analysis data is always more reliable from live-zone samples compared to those from static or non-circulating fluids.


Table 1. Benefits of using constant-level or off-line circulation

Oil Volume Benefits

A constant-level circulation system includes a small external reservoir (see Figure 2). This reservoir fluid is added to the circulation loop along with the pre-existing sump fluid. Filters and piping also add fluid to the circulating system. When, for example, you have twice as much fluid, you have twice as much additive (additives are sacrificial), half as much contaminant (per liter or gallon), half as much wear metal, half as much thermal damage to the oil and an overall lower oil temperature. The benefit from twice as much oil typically will translate into tripling the oil change interval (and all the hidden costs of an oil change). Keep in mind that a constant-level circulation system with an external reservoir can be used in place of an off-line circulation system to reap the benefits of greater oil volume.

Oil Cooler/Heater Option

Where needed, an oil cooler or heater can be installed in the circulating system.

Magnetic Plug Option

The periodic inspection of magnetic plugs to assess abnormal wear conditions is dramatically improved if the plugs are installed on circulating lines rather than sump drain ports.

Minimal Risk of Sediment and Free Water

Circulating oil interferes with settling zones that allow sludge and sediment to gather. Instead, these contaminants are trapped in the filter. Many wet-sump systems accumulate more sediment in the sump than oil, a condition that is nearly impossible to see. Sediment can be suddenly disturbed by machine movement, oil turbulence and oil drain-and-refill activities. This disturbed sediment, referred to as the fishbowl effect, can relocate to critical zones in the machine and induce sudden-death machine failure. Circulating oil also releases water vapor into the atmosphere more effectively. Conversely, simple wet-sump systems allow water to puddle on the sump floor over time.

Online Sensor Installation

Circulating oil is far superior to static oil when it comes to mounting online oil sensors. These optional sensors can monitor viscosity, wear debris, particle counts, moisture contamination and other fluid properties.

64% of lubrication professionals use sump systems that do not circulate oil, based on a recent poll at machinerylubrication.com

New Oil Filtration

When new oil is added to a wet sump, it typically is not filtered and usually comes from a dirty transfer container. When filters are installed on constant-level circulation systems, all oil entering the sump must pass through the filter (single-pass filtration). This ensures a greater degree of contamination control where there is uncertainty about new oil cleanliness. Off-line circulation systems can also be fitted with quick-connects and valves so that new oil is pumped into the sump through the filter mounted in the off-line loop.

What It All Means

Not all machines need oil circulation. However, many would benefit greatly if circulation was provided (see Table 1). In my opinion, the best candidates are machines that are configured with wet-sump lubrication and are prone to failure and/or are mission critical (high downtime and/or repair cost). Often these are the machines with the largest sump volume (e.g., 20 gallons or more).

Many filter companies and fluid power distributors can supply these products. High flow rates generally are not required, which reduces the hardware and installation costs. When you add it all up, the benefits can substantially outweigh the retrofit costs.