When selecting a lubricant for bearings, what is the main distinction between grease and oil?

• A. Grease provides long-term lubrication with a lubricant film for slow or vertical motion; oil provides cooling and high-speed lubrication.
• B. Oil provides long-term lubrication with a film for slow or vertical motion; grease provides cooling and high-speed lubrication.
• C. Grease and oil have identical functions; the choice is cost.
• D. Grease is for high-temperature environments; oil is for low-temperature environments.

Answer: (A)

The key idea is how the lubricant behaves in the bearing. Grease is a thick, semi-solid film that stays in place once applied, so it provides long-lasting lubrication and a stable film even when the shaft is moving slowly or lies in a vertical orientation. That makes it well suited for slow or occasional motion, where you don’t want lubricant to drain away or require frequent reapplication. It also helps seal out contaminants.

Oil, on the other hand, is a fluid that can flow and circulate. This makes it excellent for cooling and for providing lubrication at higher speeds, because it can fill the bearing gaps and move heat away from the surfaces through convection and, if circulating, through the oil system. At high speeds, a thin oil film can form that reduces wear and efficiently dissipates heat.

So the best description is that grease gives long-term lubrication with a stable film for slow or vertical motion, while oil provides cooling and lubrication suitable for high-speed operation.