What is the difference between hydraulic fluid and lubricating oil in terms of function?

• A. Hydraulic fluid is water-based; lubricating oil is oil-based.
• B. Hydraulic fluid transmits power and can provide lubrication; lubricating oil primarily reduces friction and carries away heat.
• C. Lubricating oil transmits power; hydraulic fluid only cools.
• D. They are identical in function.

Answer: (B)

The key idea is what each fluid is mainly built to do. Hydraulic fluid is chosen because it can transmit pressure from the pump to the actuators, turning mechanical movement into controlled motion. It also provides some lubrication and heat removal, but these are secondary benefits compared to its power-transmitting role. Lubricating oil, on the other hand, is designed to minimize wear by reducing friction and to carry heat away from moving parts. It’s not optimized for transmitting high hydraulic pressures, so using it as a hydraulic fluid would degrade power transmission and could cause issues with seals and viscosity. So, hydraulic fluid’s primary function is to transmit power (with some lubrication/cooling), while lubricating oil’s primary function is lubrication and heat removal.