Regreasing quantity (how much grease?) and regreasing frequency (how often do I grease?) are two of the most common questions when referring to the application of grease in industry.
Some applications, such as double sealed bearings in electric motors, do not require regreasing at-all, but most others should be re-lubricated periodically to replace grease that has deteriorated, leaked away, or become contaminated. In practice, these factors are extremely important to bearing performance, as both undergreasing and overgreasing can lead to major maintenance issues. Undergreasing can cause metal-to-metal contact and equipment wear, as a result of inadequate lubricant delivery in contact zones. Overgreasing can cause overheating, seal damage, energy losses and oil bleed due to grease churn.
Many more bearing failures are associated with overgreasing than undergreasing. In fact, a recent survey by Machinery Lubrication found that 70 percent of lubrication professionals believe that overgreasing is a problem in their plant. This likely results from common maintenance practices that involve frequent grease application to ensure the equipment doesn’t run dry, but do not consider the potential problems that can occur when grease builds up in systems.
Usually, equipment manufacturers will advise general guidelines for relubrication, and will be the best starting point for your regreasing strategy, but for additional background into the process, take a look at the calculations below.
Determining regreasing quantity is relatively straightforward and simply requires knowledge of the physical dimensions of the bearing in question. In particular, you will need to know the bearing outer diameter (OD) and width. These parameters can be found in the equipment manual or, if you know the bearing number, can be provided by the manufacturer (e.g. here for SKF). Then, to calculate, plug the values into these equations:
Quantity of grease (g) = Outer bearing diameter (mm) X bearing width (mm) X 0.005
Or, for those not using the metric system,
Quantity of grease (oz) = Outer bearing diameter (in) X bearing width (in) X 0.114
Regreasing frequency calculation is somewhat more complex than the calculation for regreasing quantity, and it largely depends on the operating conditions of the application. Factors that affect the regreasing frequency include: temperature, continuity of service, quantity of grease in housing, size and speed of bearing, vibration, exposure to contaminants, effectiveness of seals and the grease’s suitability for the particular service.
Bearing or equipment manufacturers recommend relubrication intervals based on operating conditions and type of grease. For example, typically, light to medium duty electric motors, that run continuously, will require at least annual relubrication. However, high temperatures, continuous service, high speeds and vibration are all factors that increase the required frequency of relubrication. A commonly used rule of thumb dictates that the relubrication interval should be reduced by half for every 10°C above the nominally recommended temperatures.
A common calculation for relubrication frequency, which incorporates the operating conditions, is:
· Bearing ID = bearing internal diameter in mm
· F1 bearing type = 1.0 for spherical or thrust bearing, 5.0 for cylindrical bearing, 10.0 for ball bearing
· F2 temperature = 1.0 for under 70°C, divide by two for every 10°C above 70°C (e.g. 0.25 for 90°C)
· F3 contamination = 0.1 to 1.0 depending on the level of contamination¯motor bearings normally 1.0
Using this example, for a spherical bearing application operating at 80oC, with shaft speed 1000 rpm, a bearing internal diameter of 100 mm and minimal contamination, the frequency at which an operator should regrease the bearing can be calculated as below;
= 500 hours
*Source: The above formula was originally referenced in the Practical Handbook of Machinery Lubrication: Second Edition. This blog post has added factors for temperature and contamination from ExxonMobil field experience.
Alternatively, operators can calculate regreasing frequencies based on graphical methods, which provide recommendations based on shaft speed and bearing type.
In addition to gaining a strong sense of the quantity of grease required and the frequency at which you should regrease a bearing, selecting an advanced grease for your application can also help you to control regreasing maintenance. For example, if you have a high temperature application that requires constant grease reapplication due to oxidation, oil bleed and equipment wear, consider replacing the grease in use with a polyurea based synthetic grease. This conversion can help to extend regreasing intervals, offering the opportunity to greatly decrease the quantity of grease required for operation, as compared to a standard mineral grease. Of course, remember to check the compatibilities of grease products before changing or mixing greases of different technologies.
Source; Conor Wilkinson, Industrial Field Marketing Advisor for the Nordics, UK & Ireland