Arguably the most common activity performed in lubrication is greasing bearings. It’s amazing how such a common task is also plagued with ways to make mistakes, such as over-greasing, under-greasing, 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 at the foundation of getting greasing right.
Grease Volume Calculation
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) by 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.
Grease Frequency Calculation
There are a few ways to calculate the relubrication frequency such as this 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:
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.
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.
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.
Moisture – Whether bearings are in a moist indoor environment, dry-covered arid area, occasionally facing rainwater or even exposed to washdowns, the water ingression opportunities need to be taken into consideration when defining the relubrication frequency.
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.
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.
Using Ultrasound to Calculate Regrease Frequency and Intervals
The aforementioned calculations apply to manual greasing methods using a grease gun. Ultrasound, however, presents a way to calculate greasing volume and frequency without you doing the math. The OnTrak automatic regreasing system is a single-point lubricator that calculates the exact amount of grease needed. Once the sensors detect friction in a bearing, the user gets an alert via UE Insights, a cloud-based system. From there, the user can give the single-point lubricator permission to dispense the right amount of grease to reduce bearing friction. By using friction as a guide, you can regrease with precision and confidence.