Arguably the most common activity performed in lubrication is to grease bearings. This involves taking a grease gun filled with grease and pumping it into all the grease Zerks in the plant. It’s amazing how such a common task is also plagued with ways to make mistakes, such as overgreasing, undergreasing, overpressurizing, greasing too frequently, greasing infrequently, using the wrong viscosity, using the wrong thickener and consistency, mixing multiple greases, etc.
While all these greasing mistakes can be discussed in length, calculating the grease quantity and how frequently each bearing application needs to be greased is something that can be determined from the very beginning using known variables about the bearing’s operating conditions, environmental conditions and physical parameters.
The amount of grease during each relubrication procedure can usually be calculated by simply looking at a few bearing parameters. The SKF formula method is frequently used by multiplying the bearing’s outside diameter (in inches) with the total bearing’s width (in inches) or height (for thrust bearings). The product of these two parameters along with a constant (0.114, if inches are used for the other dimensions) will give you the grease quantity in ounces.
There are a few ways to calculate the relubrication frequency. Try Noria's bearing grease volume and frequency calculator. Some methods are simplified for a specific type of application. For general bearings, it’s best to take into consideration several more variables besides the operating and environmental conditions. These include:
Temperature– As the Arrhenius rate rule indicates, the higher the temperature, the quicker oil is going to oxidize. This can be taken into practice by shortening the relubrication frequency as higher temperatures are anticipated.
Contamination– Rolling-element bearings are prone to three-body abrasion due to their small film thickness (less than 1 micron). When contamination is present, early wear can result. The environmental contaminant types and the likelihood for contaminants to enter a bearing should be taken into consideration when defining the relubrication frequency. Even the average relative humidity can be a point of measure to indicate water contamination concerns.
Moisture – Whether bearings are in a moist indoor environment, dry-covered arid area, occasionally facing rain water or even exposed to washdowns, the water ingression opportunities need to be taken into consideration when defining the relubrication frequency.
Vibration – The velocity-peak vibration can be an indication of how much shock-loading a bearing is experiencing. The higher the vibration, the more you need to grease to help protect the bearing with fresh grease.
Position – A vertical bearing position will not hold onto grease in the lubrication zones as effectively as those positioned horizontally. In general, it’s advisable to grease more frequently when bearings are closer to a vertical position.
Bearing Type – The design of the bearing (ball, cylinder, tapered, spherical, etc.) will have a significant impact on the relubrication frequency. For example, ball bearings can allow more time between regrease applications than those of most other bearing designs.
Runtime – Running 24/7 versus sporadic use, or even how often there are starts and stops, will have an impact on how quickly the grease will degrade and how effectively the grease will stay in the key lubrication zones. Higher runtime typically will require a shorter relubrication frequency.
All the factors listed above are correction factors that should be considered along with the speed (RPM) and physical dimensions (bore diameter) in a formula to calculate the time until the next grease relubrication for a rolling-element bearing.
While these factors play a role in calculating the relubrication frequency, often the environment is too contaminated, the likelihood of contaminants entering the bearing is too high and the resulting frequency is not enough. In these cases, a purge procedure should be performed to push grease through the bearings more frequently.
Remember, filtration is to oil as purging is to grease. If the cost of using more grease is less than the risk of bearing failure, then purging grease might be the best option. Otherwise, a specified calculation to determine the amount of grease and the relubrication frequency will be best to help avoid one of the most frequent mistakes made in one of the most common lubrication practices.
This article was previously published in the Machinery Lubrication 2018 Conference Proceedings.