ROSENHEIM, Germany – Oil analysis is widely accepted as an effective tool to help machine operators monitor equipment and avoid expensive breakdowns. Countless businesses regularly collect oil samples and have them analyzed for lubricant degradation and early clues of component wear and other potential problems.
Grease analysis, by comparison, is used far less. A number of industries have many grease-lubricated machines that theoretically could benefit, but, as an industry insider explained recently, there has long been a lack of good means of obtaining samples and practical ways to analyze them.
At last, however, the situation is improving. MRG Labs President Rich Wurzbach told the Oildoc conference here in January that industry has recently introduced tools – supported by a new ASTM method – that can extract worthwhile samples from various types of machines. In addition, there are now a number of tests capable of analyzing such samples for clues about equipment conditions.
With the use of specially designed sampling devices and availability of a new ASTM standard practice on grease sampling, trendable samples can be obtained from in-service components and provide a level of analysis not previously available, Wurzbach said.
Oil analysis is one of a number of practices that fall under the umbrella of condition monitoring and predictive maintenance. condition monitoring is a discipline that prescribes continual collection of data involving a machines operating conditions – its vibrations, the heat and sounds that it generates, variations in electrical current. The idea is to identify patterns that represent normal operating conditions. Then, when those patterns change – when vibrations increase, when an area becomes hotter than usual – the operator has an early clue that a part may be deteriorating.
This allows operators to schedule maintenance to investigate and address the problem. They may need to replace a component, but doing this at a scheduled time can be much less expensive than an unexpected breakdown. Condition monitoring may also provide information that helps prevent the problem from recurring in the future.
With oil analysis, operators are generally watching for one of three things:
changes in viscosity or the appearance of chemical compounds that would indicate the oil is degrading. When this happens, it is time to change the lubricant.
the appearance of contaminants, such as water or debris. This may indicate a leak or the need to install filters.
an upward trend in the presence of materials from which components are made. This often signals an increase in the rate of wear and that the component in question may be on its way to failing.
Theoretically, in-service grease should offer similar insights, but Wurzbach said it has been more difficult to obtain worthwhile grease samples. Lubricating oil, because of its flow characteristics, is relatively homogenous throughout a circulating system. As a result, operators can draw representative samples from properly located valves or retrofitted sampling fittings while machines are running, without infringing on operations. Obtaining sufficient quantities of oil to perform testing is generally achievable in this way.
By comparison, traditional grease tests require larger sample volumes than are practical in operating equipment, Wurzbach said. These traditional grease tests were mostly developed for new grease quality control and physical property analysis, where sample size is not a limitation. Moreover, because grease is less fluid than oil, its constituent makeup is likely to be less uniform. In some machines a meaningful sample – especially one that contains wear debris – may need to come from close to the wear surfaces, be they bearings or gears. As a result, extracting samples from machines with grease systems often require that the machine be not just halted, but also disassembled.
Historically, in-service grease samples from motors, valves and various bearing housings typically have required the equipment to be out of service and disassembled, Wurzbach said. A key factor is that a large volume of sample is needed to perform current analytical testing methodologies, and it is extremely difficult to obtain that representative sample from near the bearing while the component is still in service.
MRG Labs is based in York, Pennsylvania, United States, and offers reliability-centered maintenance services and products. Wurzbach said effective grease sampling tools have been introduced in recent years, with differences between them depending on the machine from which they are designed to extract. Some motors and other machines are designed so that during relubrication grease migrates from the contact surface to a drain. For such cases, operators can now use small, syringe-type cartridges that essentially can replace drain plugs. Acting passively, they are capable of capturing about one gram of representative grease, and because they are sealed, they do not run the risk of contamination from the ambient environment. MRG offers such a device, called the Grease Thief.
Machines that do not deliver grease to the drain require a more active solution. The challenge, Wurzbach said, is that the device must be able to get from the access [port] to the active lubrication location, near the bearing or gear mating area, and bypass the non-representative grease along the way. This requires the device to push grease out of the way in the space between the access hole and the lubricated surface, and then capture a small amount of grease close to the gear surface or bearing grease shear area.
MRG Labs suggests using the Grease Thief mounted on a t-shaped extension that the company also offers. The cartridge has a probe that extends from its end when the cartridge is closed. Starting with the probe extended, users reach the handle through an access port inside the machine to the wear surface. When the probe pushes against the gear or bearing, it causes the cartridge to move forward, and take a core sample. Standards writing organization ASTM recently adopted ASTM D7718 prescribing several methods for obtaining in-service grease samples, including how to use the Grease Thief to extract samples in this way.
Wurzbach discussed solutions for a few other applictions as well. Some machinery, such as wind turbines, has open gears or bearings with accessible grease, and in these cases samples are best obtained using a spatula to scoop some grease directly from the gear or bearing area, he said. The auto industry continues to make increasing use of grease-lubricated robots on assembly lines, and companies want grease analysis methods that can help them monitor these machines. Wurzbach recommended temporarily attaching syringes on Zerk grease fittings at grease reservoirs to collect samples.
Industry has also made some progress in the development of test equipment that can analyze small volumes of grease. Wurzbach cited a die extrusion method that can turn small volumes of grease into thin films on a plastic substrate. These substrates, or strips, are divided into several sections that can be tested for a number of parameters: changes in consistency, presence of wear particles, color, contamination and oxidation.
But Wurzbach maintained that there is room for more progress in this area.
The further development of repeatable analysis methods that utilize smaller quantities of grease will produce greater value, and encourage the sampling of greases from locations where reliability is important, he said.
Wurzbach warned that a variety of issues can invalidate information gained from grease analysis:
Grease in some machines can bypass the wear area on its way to drain and therefore samples will suggest a lack of wear particles when in fact they simply contain no information about wear. Thus it is important, Wurzbach said, to understand the dynamics of grease flow in a machine before taking samples.
Some reservoirs designed to purge old grease may not do so; besides creating a risk of over-lubrication, this may require individuals taking grease samples to actively obtain them from the wear area.
In some machines, wear particles can remain concentrated especially close to lubrication surfaces. In such cases, samples will offer little information unless taken from this area.
Changes in the condition of the grease – such as softening of the thickener or loss of oil – can mask changes in the data that maintenance personnel are observing.
But the bottom line, Wurzbach said, is that grease analysis has much to offer equipment operators, from fewer breakdowns to warnings of contamination.
For grease lubricated machines such as wind turbines, industrial robots and important manufacturing equipment, grease sampling and analysis can provide a critical role in their successful operation, he said.