To avoid wear and tear on your bearings and other components, it is essential to maintain a sufficiently thick lubricant film. All of the factors shown in Figure 1 affect the lubricant film thickness in your machines.
Figure 1: Factors affecting lubricant film thickness
In most cases, you cannot do much about factors such as static and dynamic load, preload, alignment, geometric quality of the housing and shaft, and geometric quality of the bearing. They result in the total load of the bearing.
The lubricant type and temperature will result in the viscosity which in turn, along with supply and velocity, contribute to the lubricant film thickness.
When you go on a speedboat, you only achieve full separation between the hull of the boat and the water when the speed of the boat reaches a certain point. This is what we want to achieve with the speed of our machines. The slower the machine, the more difficult it is to achieve full separation between the rolling elements and the raceways.
When evaluating the operating condition of the bearing using the SPM system, we measure the low rate of occurrence of high shocks and the high rate of occurrence of weaker shocks.
Figure 2: The SPM system
The main inputs for the system are the mean diameter, the RPM, and the bearing type. The system will provide a green, yellow, or red code as well as a lube number representing the lubricant condition in the bearing. And if there is bearing damage, you will get a condition number.
There are, of course, different types of bearings. Normally, you can type in the bearing number, and the software will find the bearing type. The norm number is calculated from the mean diameter and the RPM, and that will result in an accurate speed of the rolling element surface.
Figure 3: Norm number
The norm number will adjust the green, yellow, and red scales according to the speed and size of the bearing.
If you measure a bearing like that represented in Figure 4, you will find that there is full separation. The high rate of occurrence of low shock pulses will be quite low, and the low rate of occurrence of higher shocks will not be much higher. This is a small delta.
Figure 4: Code A
The software will give you a Code A, meaning all is well.
If you have poor lubrication, perhaps due to under-lubrication, excessive load, excessive belt tension, or wrongly mounted bearing, the delta will be the same as the previous case because the bearing is not badly damaged, but the low rate and high rate will both be higher.
Figure 5: Code B
The software will give you a Code B.
If your bearing is damaged, the low rate will usually rise quite a bit. This will result in a Code C. After a while, the low rate will increase even more. The software will give you a Code D for damage. The high rate is relatively high, and there is also a high delta between lor rate and high rate.
Figure 6: Codes C and D
In the odd-looking diagram in Figure 7, the X-axis represents the high rate. The small white square represents a measured value of 39, and the delta, which is on the Y-axis, is 4. As the readings for lubrication increase, the square will move to the right. If there is bearing damage, the square will move up in the evaluation box, resulting in condition numbers.
Figure 7: Evaluation box
Figure 8 shows how this looks in the Condmaster Ruby software. These measurements are from an alignment roll in a paper machine. The graph at the top shows RPM. This machine goes between 200 and 450 RPM.
Figure 8: Condmaster Ruby
In the middle graph, the blue dots represent the low rate. You can see that they follow the RPM. The red dots represent the high rate.
In the bottom graph, we have the lube rate. Although the RPM fluctuation is quite large, the lube rate remains steady because the lubrication condition is constant.
If we were to return to the evaluation box, the condition of this machine would remain in the green, with a high rate of 20 and a delta of 3. The green area represents Code A, the yellow area represents Code B, and the red area represents Codes C and D.
We can also use a simulation graph in a function called Lubemaster in which you can enter different types of oil and viscosity. You enter information about the lubricant, the temperature, and the RPM.
Figure 9: Lubemaster simulation graph
The X-axis represents Kappa (k), or the ratio of viscosity at operating temperature divided by the required viscosity for the bearing. The Y-axis represents the a23 factor, which is related to L10, or the bearing’s expected lifetime. The a23 factor takes into consideration the lube condition and the material of the bearing. So a 1 on the Y-axis means 100% of the bearing’s lifetime, or L10.
In this case, we are using a 220 centistoke mineral oil. The load is 5%. The bearing temperature is 65°. This will give us an a23 of 1.9, which is almost twice the calculated lifetime of the bearing.
However, if we increase the temperature to 120°, the a23 will decrease to 0.67. If you reduce the viscosity to 150, the bearing life decreases even more. It will continue to decrease if you remove the additives.
Conversely, if you reduce the temperature, you will increase the life of the bearing. You can adjust the viscosity and see what happens. This tool is extremely useful to check how changing the properties of your lubrication impacts the life of your bearings.
