What characteristic is essential for lubricating slow-speed plane journal bearings?

The essential characteristic for lubricating slow-speed plane journal bearings is high viscosity oil. In slow-speed applications, such as those involving plane journal bearings, the lubricant must be able to effectively maintain a film between the bearing surfaces to reduce friction and wear. High viscosity oil helps to create a thicker film that can better support the load and reduce the chances of metal-to-metal contact, which can lead to premature wear or failure of the bearings.

High viscosity oils remain stable under pressure and have good shear stability, ensuring that they perform well over a range of temperatures and conditions typically experienced in slow-speed operations. This property is particularly important in applications where the bearing might experience variable loads or starting conditions, as the lubricant must protect the surfaces during both operation and while at rest.

In contrast, low viscosity oils or light oils would not provide adequate support under the heavier loads typical in slow-speed bearings, potentially increasing wear and reducing the bearing's lifespan. Similarly, while thick grease has its uses, it may not allow for sufficient lubrication in all scenarios where slow-speed plane journal bearings are used, particularly if the grease is unable to effectively flow and coat all the surfaces as needed in these applications.