After choosing the best oil for your hydraulic system (1st Day), the next important decision is which oil viscosity grade to use. Viscosity is “a measure of a fluid’s resistance to flow. A thin watery fluid has a low viscosity; a thick syrupy oil has high viscosity.”1

Important for Pump Life

The correct viscosity is particularly important for the life of the pump. If the viscosity is too high, the pump will cavitate. Depending on the severity of the cavitation and sensitivity of the pump, this can destroy a pump in a matter of seconds up to a few hours.

If the viscosity is too low, then the pump and other components will wear out prematurely due to lack of lubrication.

The fluid viscosity also affects pump efficiency, as demonstrated in the graph2 below:

How Hydraulic Fluid Viscosity Affects Pump Efficiency2

Optimum Viscosity

This optimum viscosity range will vary a little for each pump type, but in general: “For optimal functionality of a hydraulic system, the kinematic viscosity should be 13 to 36 mm2/sec at an operating temperature of 50 to 65°C.”, as stated by Ing. R. van den Brink1. This is confirmed by the following graph from Bosch Rexroth3:

Optimum Fluid Viscosity Range for Bosch Rexroth A10VSO Pump3

This graph actually provides a lot more information, but we’ll get into that on another day. So based on the optimal temperature range of 50 to 65°C mentioned above, we can choose a VG32 or VG46 fluid. However, this should be checked based on actual operating temperature of the system and actual viscosity curve of the chosen fluid.

Caution

Your machine start-up temperature MUST also be considered in the fluid selection. More than just a few pumps have been destroyed on a cold start-up. The above graph tells us that the maximum viscosity for start-up of the pump is 1,000 mm2/sec. For the given VG32 curve, that’s approx. -10°C. Then you need to let the oil warm up to 0°C before running the pump at full speed.

Fluid Viscosity Affects Your ROI

In the best case, the wrong fluid viscosity only affects the efficiency of your system. That alone can reduce your ROI. In the worst case, the pump fails on a cold start-up. In addition to shutting down production, it’s a costly repair.

Questions or Comments?

Please leave any questions, or comments in the fields below, or contact me via burk@burkschmidt.com.

References:

  1. Technical Instruction – Filtration Technology, Ing. R. van den Brink, MK Publishing, First Edition: 2020
  2. White paper by Eaton Hydraulics on “Selecting Fluids for Hydraulic Pumps”, Aug. 2016
  3. Bosch Rexroth data sheet re-a92714_2015-09 for the A10VSO series 32 pump