There are three opportunities to inspect the state of in-service grease. One is by disassembly (such as by removing the bearing cap), the second is by sampling the grease using a probe (ASTM D7718), and the third is by examining the purge discharge. The purge discharge is the grease that’s extruded from exhaust ports, seals and other openings during relubrication or machine operation.
Not all grease-lubricated machines have a purge stream, but many do. Machines (mostly bearings) that purge grease provide a valuable opportunity for inspection. The opportunity is significant because of the frequency and simplicity of the inspection. Machines that purge are generally “total loss” systems, meaning the grease is not recovered for reuse but instead is discharged to a catch-pan, trap, grease thief, exterior surface or straight to the floor (Figure 1).
A grease discharge trap (GDT) is a perfect inspection device. One version of the GDT uses a simple barb fitting that is installed in the purge port (also known as a drain port, vent port or exhaust port). A 1½-inch zip-lock plastic bag (of various lengths) is positioned on the barb side of the fitting using an O-ring (see photos on the right). Grease that purges out of the fitting goes straight into the bag for easy inspection, disposal and sampling.
Following are examples of machines that commonly have a purge stream:
In many cases, purging grease through a bearing is not recommended, although it is commonly practiced. The decision to purge or not to purge should not be trivialized. To understand this better, see the sidebar below about purge versus volume control methods for lubricating bearings.
Figure 1. In total loss systems, grease is discharged to a catch-pan (left), exterior surface (right) or straight to the floor. Source: OilDoc
Too often the opportunity to inspect grease discharge is dismissed largely due to ignorance. In fact, there is a story to tell from the condition and state of grease discharge. This relates both to the state of lubrication and the health of the machine. There is also information to be learned about the application of the grease, the relube frequency and the relube volume that can be assessed by inspecting grease discharge.
The discharge from bearings and other machine components is basically a sample of the grease condition as it exits (its terminal state). It carries out a historical account of the bearings. This includes debris from the bearing, contaminants the bearing was exposed to and degradation byproducts from the grease. The state of the discharge correlates to the quality and state of lubrication and ultimately the reliability of the bearing.
So what questions might the purge stream be able to answer? Take a look at the following list for examples:
Figure 2. This is an example of excessive lubrication.
Wrong or Mixed Grease - A wrong or mixed grease color can be observed in the discharge. An incorrect grease consistency might also be detected.
Degraded Grease - Evidence of oxidation (tar-like), thermal distress and/or dry, caky grease (oil loss) may be visible.
Contaminated Grease - Signs of water, corrosion, dirt or other impurities can be seen.
Inadequate Grease Volume or Frequency - This is shown from prematurely degraded and/or contaminated grease.
Excessive Grease Volume or Frequency - Large piles of grease discharge reveal problems (Figure 2).
Bearings are often lubricated using a grease gun until a fresh grease purge is observed. While there are many cases when this is best practice, there are an equal number of cases when it is not.
Anyone who lubricates bearings with a grease gun should understand the alternative methods and when each should be applied. Of course, the machine or component manufacturer should always be consulted.
Noria refers to the two options as the Fresh Grease Purge method and the Grease Purge and Volume method. These methods and target applications are described below:
The bearing is lubricated until fresh grease emerges from the purge port (vent) or shaft/seal interface, or back-pressure is encountered. When to use the FGP method:
The bearing is lubricated until a pre-established maximum volume of grease has been introduced, fresh grease emerges from the purge port (vent) or shaft/seal interface, or back-pressure is encountered. When to use the GPV method:
Cake-lock Conditions - The telltale sign of this condition is when the catch-pan only has oil. This means the thickener is binding up in the bearing.
Abnormal Wear Conditions - Visible evidence of wear debris is seen. Use a magnet to extract larger wear particles. Solvents can also be used to separate particles from the grease.
Figure 3. A grease thief (left) and a bellows-type grease discharge trap (right) can be connected
to a purge port.
Obstructed or Diverted Purge Path - The normal amount of grease discharge is not observed, meaning that grease is being diverted to another purge path.
Auto-Lube Malfunction or Neglected Grease Gun Relubrication - The normal amount of grease discharge is not seen, resulting in a potential starvation condition.
A quick, daily visual inspection is sometimes adequate. Look for abnormal grease discharge, color, consistency and location. Clean away the discharge so the amount of new discharge (since the last inspection) is easily recognized for inspection. Alternatively, use a simple grease discharge trap (see sidebar above). A discharge trap is a plastic bag, grease thief or bellows device connected to the purge port (Figure 3). Grease exhaust is held by the trap for later inspection, sampling and/or disposal.
Figure 4. A magnet placed under a gold pan or glass bowl can enable you to observe ferrous debris in a grease sample.
If the conditions of the discharge merit further inspection, consider the following:
|35%||of lubrication professionals never inspect the grease discharge from bearings and other machine components at their plant, based on a recent survey at machinerylubrication.com|
The routine inspection and analysis of grease discharge should be a part of the skill set of operators and technicians responsible for lubrication, maintenance and machine reliability. The discharge carries bits and pieces of potentially valuable information. This could range from a clean bill of health to the remnants of a building internal machine graveyard.