I believe everyone has a question: if bearings are used in gearboxes with lubricating oil inside, what style of bearings should be used? Open, sealed, or both sealed and open?

In practical product applications, when disassembling gearboxes from different equipment and manufacturers, it can be found that the bearing sealing methods are also different. Today we will take a look at the considerations behind different choices and what to pay attention to when applying them.

However, it should be mentioned here that sealed bearings refer to contact rubber sealed bearings. Non contact rubber sealed bearings or bearings with metal dust covers are not within the scope of discussion, as these two forms of bearings are generally not used in gearboxes due to their protective effects.

Open type bearings:

Open type bearings, due to the lack of seals on both sides, have rolling elements that directly contact the lubricating oil inside the gearbox after installation. The bearings and gears share the same lubrication. The advantage is that the bearing has low friction resistance and good heat dissipation during operation, which can effectively reduce operating resistance or starting and operating torque. It is also suitable for medium to high speeds and occasions with high requirements for transmission efficiency.

But we know that bearings are more sensitive to the degree of contamination of lubricating media compared to gears. If the cleanliness of the lubricating oil in the gearbox is not high or impurities such as iron filings from gear meshing wear are mixed in, the bearings will quickly fail. Therefore, when using open bearings, there must be requirements for the cleanliness of the gearbox (such as not lower than 17/15/12 in ISO4406 standard), and if necessary, a filtration system needs to be added.

The lifespan calculation of open bearings involves selecting oil as the lubricating medium and then calculating based on the cleanliness of the oil.

Sealed bearings

Sealed bearings have good protective effects due to the contact rubber seals on both sides. However, unlike open bearings, they have high frictional resistance, slightly higher operating temperatures, and reduced efficiency of the entire gearbox. They are usually used for medium and low speeds.

Sealed bearings can effectively prevent impurities from entering the bearings inside the gearbox.

Due to the fact that the gearbox is not completely filled with lubricating oil, but only a small part, the lubrication of its gears relies on splashing or spraying. At this time, some bearings in the gearbox may not be completely immersed in the lubricating oil, and the lubrication amount may not be enough through splashing or other methods. In this case, sealed bearings can rely on the lubricating grease carried by themselves to meet the usage requirements.

In addition, if the gearbox is used in environments with high humidity and temperature differences, the water content of the lubricating oil inside the gearbox may increase over time, leading to emulsification and affecting the performance of the lubricating oil, increasing the wear of gears and bearings. Sealed bearings can reduce the impact of external lubricating oil on the bearings.

When calculating closed bearings, the lubricating medium is lubricating grease.

Some students may ask: Will the grease inside the sealed bearing be washed out by the lubricating oil inside the gearbox? This may exist, especially when the gearbox is not equipped with a vent plug and the internal working pressure is too high, or the temperature of the gearbox is too high, which affects the performance of the bearing sealing rubber, or there are vibration impact loads, etc. At this time, the sealing effect will be compromised, leading to oil mixing and flushing phenomena.

So when choosing lubricating oil for gearboxes, we need to consider compatibility with bearing grease.

Open and closed bearings on one side

This type of bearing is generally less commonly used. Usually used for the connection between the bearing hole and the outside as shown in the figure below, with the sealed side facing the outside and the unsealed side facing the inside of the gearbox.

The operating characteristics of bearings are between fully open and sealed. The open side can reduce operating resistance or starting and operating torque, while the sealed side effectively prevents pollutants from entering. When calculating the bearing life, the lubricating medium should be lubricating oil, and attention should also be paid to the cleanliness of the lubricating oil.