Gear lubrication is crucial for maintaining efficient and durable gear systems by reducing friction, dissipating heat, and preventing wear and corrosion. Proper lubrication ensures smooth operation, extends component lifespan, and enhances system reliability.
At low temperatures, lubrication becomes even more important. Cold conditions can cause lubricants to thicken, reducing their ability to flow and protect gear surfaces. This can lead to increased friction, wear, and start-up issues. Specialized low-temperature lubricants are designed to remain fluid and effective, ensuring consistent protection and smooth operation even in cold environments.
Case description
In this case, the goal is to compare simulation results with experimental findings, focusing on lubrication at different temperatures. The base case is previously described. The details and the general case set-up can be read there.
The main difference lies in viscosity variations at different temperatures, affecting lubricant performance. We’ll compare two cases at high rotation speeds against each other and experimental data.
By analyzing these cases, we aim to assess how lubrication behaves under varying temperature conditions and validate simulation results against real-world performance. This comparison will provide insights into the effectiveness of lubricants across temperature ranges and help optimize gear system design for enhanced reliability and efficiency.
Case set-up
As said before the case uses the same geometry as the base case therefore only the differences to the base case are mentioned here. The two fluids properiers, which were used represent a lubricant ant 25°C and -40°C.
| Parameter | Case 1 (25°C) | Case 2 (-40°C) |
|---|---|---|
| Input speed | 1543 RPM | 1543 RPM |
| Liquid volume | 400 ml | 400 ml |
| Density fluid | 809.4 kg/m3 | 840.5 kg/m3 |
| Kinematic viscosity | 3.356E-5 m2/s | 3.247E-3 m2/s |
| Surface tension | 0.021 N/m | 0.021 N/m |
| Gravity | 9.81 m/s2 | 9.81 m/s2 |
| End Time | 9 s | 15 s |
Results
Case 1: 25 °C
Front view
In the front view perspective, it can be seen that all three bearings are supplied with lubricant through the channels leading to them: The left bearing is continuously filled (red circle) and the chamber in front of the bearing is similarly filled as in the experiment. The middle bearing (green circle) is also filled through the pool, but the simulation seems to overestimate the experimental results, leading to the right bearing (yellow circle) also being more filled in the simulation than in the experiment. Nonetheless, the channels are filled and supply the bearings in both cases.
Top view
In this perspective, it is clearly visible how the pool (large green circle) is continuously filled with fluid, from which the channels to the middle bearings are supplied (small green circles).
Side view
In the side view, the lubricant distribution on the casing surface can be compared with the experiment. It is noticeable that in the simulation, the fluid is spread significantly wider than in the experiment. In the experiment, the left half of the casing is only slightly wetted, and the clear red color cannot be seen; instead, one must look closely, indicating a much thinner fluid film than on the right side of the casing. This is not reflected in the simulation. However, when comparing Case 1 and Case 2, the trends are consistent: the heavily wetted area becomes significantly narrower in Case 2 than in Case 1, and this tendency can also be observed in the simulation.
Volume Flow Rate to the Bearings
In the diagram, the volume flow rates to the individual bearings are recorded over time. It can be seen that all volume flow rates are almost in a steady state, indicating that the simulation has run long enough. The volume flow to the left bearing pulsates the most, fluctuating between approximately 0.7 and 1.5 L/min – this pulsation can also be observed in the experiment. The second highest volume flow is to the middle bearing, at 0.6 L/min, followed by the volume flow to the right bearing, at 0.1 L/min.
Case 2: -40 °C
Front view
In the front view, it is clear that the fluid in comparison to Case 1 has a significantly higher viscosity. The left bearing (red circle) is also filled through the designated channel in this case, but if looked at the timestamp, it can be see that this takes considerably longer than in Case 1. Due to the high viscosity, significantly more fluid accumulates in the chamber in front of the bearing. This observation is consistent with the experiment. However, the fluid distribution within the chamber in the experiment is slightly different: in the simulation, the fluid spreads almost concentrically, whereas this is not observed in the experiment.
In the simulation, a periodic detachment of the lubricant can be observed in the right area of the simulation (yellow circle) Lubricant is flung upwards from the gear of the middle shaft, where it briefly adheres to the housing before falling onto the input shaft. This periodic behavior cannot be confirmed or denied by the experiment due to the video quality; one can only see that lubricant reaches this part of the gearbox not how.
Top view
In the top view, two things can be observed. First, both the experiment and the simulation show that the small pool (green circle) on the casing is not continuously filled. In both cases, a small amount of lubricant reaches this spot, but it is far from sufficient to fill the channels (small green circles) to the middle bearings.
The second observation in this view is the wetting of the casing due to the second shaft (yellow circle). Here, too, there is a consistency in the fluid distribution between the simulation and the experiment.
Side view
In the side view, the distribution of the lubricant on the casing can also be observed and compared. Here, too, it can be seen that the lubricant distribution is very similar to the experiment: due to the high viscosity, the fluid is not spread out and only wets the casing directly behind the gear, unlike in Case 1 where it covers a relatively large area.
Volume Flow Rate to the Bearings
From this diagram, it can be seen that the volume flow rate to the left bearing is on average slightly higher than in Case 1 and pulsates less, fluctuating between approximately 0.1 and 0.16 L/min. As previously observed, the channels to the other bearings are not supplied at all.
