Recently, innovative approaches and novel tools were introduced to overcome these challenges and provide more routine and effective sampling of critical grease lubricated equipment. In a presentation at the OilDoc Conference in Rosenheim, Germany, in January, Wurzbach described the methods outlined in ASTM Standard D7718 that provide a way to take samples that accurately represent the condition of grease in the contact areas.

Lubricant Sampling

Wurzbach explained, Oil analysis is successful only when significant effort is made to obtain representative samples. This depends on evaluating machine operation, lubricant flows and circulation, the presence of filters, sumps and other particle separation effects in the system.

In an oil-lubricated machine, oil circulates throughout the machine, so a representative sample can be taken at numerous points through judiciously placed valves and fittings. Grease, however, does not circulate, posing a formidable challenge to obtaining a sample that accurately represents grease condition in the contact area.

Some early efforts at grease analysis identified methods similar to those used to take a sample of circulating oil. However, Wurzbach said, the methods cannot control the point at which the sample is taken. In addition, they risk filling part of the sampling tube with grease that does not represent the area close to the target bearings or gears, known as the live zone.

Wurzbach then explained initiatives undertaken by the Electric Power Research Institute and the Danish Wind Energy industry to overcome these issues. Based on these studies and industry demand for guidance in obtaining grease samples, the ASTM grease subcommittee assembled a working group that in 2011 produced ASTM D7718, Standard Practice for Obtaining In-Service Samples of Lubricating Grease.

The subcommittee was intent on ensuring that certain existing industry practices were included, said Wurzbach. It was noted that certain industries used tools such as syringes and plastic tubing to extract samples. Also, a longstanding and common use of grease analysis was applied in failure analysis, he added. The practice was written to address the special considerations of analyzing failed components for indications of contributing failure causes.

The standard also addresses the sampling containers, and includes many of the acceptable, if not optimal, commonly used devices such as plastic and glass oil sampling bottles and metal containers as well. Methods include use of spatulas, plastic tubing, syringes and other tools to access appropriate locations in the target machine, said Wurzbach.

While the subcommittee made an effort to include historically used methods, it noted the limitations of some of these methods. In this way, Wurzbach noted, good and representative samples are the primary goal, and users of the standard are aware that some methods may be more effective than others. Or some can give less than optimal results unless precautions are taken.

One precautions addresses the use of plastic tubing and a syringe to extract a sample from a machine enclosure. While this method can be quite effective at producing a sample, Wurzbach said, one note in the practice points out the non-Newtonian properties of greases and the potential challenges of inducing uniform grease flow using suction.

In addition, some concern has been raised that the use of tubing does not preclude the capture of unrepresentative grease samples from locations far from the target component. Thus, the standard includes the following statement Note that the proximity of the lubricating grease in the interior of the tubing to the bearing or gear cannot be guaranteed when using this method.

Among methods outlined in the standard are the active and passive techniques enabled by the use of the proprietary Grease Thief tool. While the standard does not endorse specific products, or preclude the use of others, Wurzbach explained, the Grease Thief has been shown to be an effective tool in the techniques outlined in several methods in the standard. Other effective products may be included in future revisions of the standard if brought to the attention of the subcommittee.

Walk through the Standard

ASTM D7718 sections include sample handling, an active sampling procedure for enclosed housings, an active sampling procedure for pillow (or pillar) block bearings and exposed bearings and gears, a passive sampling procedure and a procedure for sampling from failed components.

Important considerations that apply to all methods include cleanliness of sampling tools, sample homogeneity and the importance of uniformity and design of the sampling devices used, Wurzbach said. The importance of operator training and knowledge of equipment being sampled also has an impact on sample quality.

Active sampling is the process of inserting sampling devices to the contact surface inside a machine. The active grease-sampling device features a stinger probe, allowing it to be inserted into a gearbox, electric motor or other similar contained housing. The stinger probe allows the target surface to be located and the core of the sample to be precisely adjacent to the bearing or gear of interest, said Wurzbach. The procedure describes use of the probe, including the attachment of a handle that allows the remote actuation and coring of the sample from within the machine.

The plastic tubing method is another active sampling method. While [this method has] some limitations, it is in widespread use, and the standard addresses some critical considerations and techniques for getting the best possible sample by this method, Wurzbach noted. The procedure is consistent with instructions found on some company or laboratory websites.

Another section describes active sampling from pillow block bearings and other similar exposed bearings or open gears. This method generally relies on the use of a soft spatula to harvest the grease from exposed surfaces, and the use of a syringe to gather and transfer the grease to the primary sample container, Wurzbach said. ASTM D7718 emphasizes the importance of removing the outermost layer of grease prior to sampling because the exposed grease often captures environmental contaminants that are not representative of the component of interest.

Passive sampling is the collection of grease that naturally purges from the bearing upon relubrication and operation, using a device attached to the machine drain. The device captures the most recently purged grease within the body and protects it from contamination by the environment.

A key consideration for this method, said Wurzbach, is the patience that may be required in waiting for a sample. Unlike active methods that allow for immediate sample gathering, the passive method relies on accumulating a sufficient quantity of purged grease.

While relubrication introduces new grease that can displace the used portion, overgreasing must be avoided, he cautioned. It is never prudent to add grease for the sole purpose of purging the required sample quantity.

Therefore, normal relubrication frequency and quantities must be followed, and sufficient time must be allowed to produce the needed sample. This approach provides the added benefit of maintaining a purge path in the machine, Wurzbach pointed out.

Failure analysis of grease harvested from failed components helps identify the root causes contributing to component failure. These samples can be a strong complement to the observational and metallurgical analysis typically performed on failed bearings and other grease lubricated components, said Wurzbach, and can contribute to a more comprehensive picture of the factors involved in machine failure.

Conclusions

While grease analysis is not nearly as widespread as oil analysis, there is a growing demand for the analysis of grease lubricated components, Wurzbach stated. And some applications have proven that effective grease analysis provides significant cost benefit. Grease sampling has received new attention, and methods and tools have been developed to improve the consistency and quality of samples being submitted for analysis.

Wurzbach concluded by saying, ASTMs grease subcommittee has worked diligently to consider important factors to ensure the taking of quality samples, and new techniques and tools have become available through the development of this standard. Acceptance and use of this document will help ensure that the lubricant analysis industry continues to grow and improve the value of in-service grease testing.