Hydraulic Oil Pumpability and Pump Efficiency
by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

Hydraulic pump efficiency is technically regarded as a ratio of the actual flow rate output to the theoretical pump flow output. To the equipment operator, pump efficiency is generally looked at in regards to fuel or power consumption and system responsiveness. Pump design will govern pump efficiency, but a fluid’s pumpability is also a contributing factor which translates into energy efficiency for the operating machinery. This is especially relevant to outdoor mobile equipment where loads, working conditions and temperatures can vary widely. Pump efficiency is a critical factor in system responsiveness, power and fuel efficiency.

Pump efficiency loss is affected by mechanical energy lost due to fluid friction and power transmission lost as the result of internal fluid slippage within the pump. Fluid formulation reduces the impact of these negative influences by providing improved pumpability characteristics.

Proper viscosity within a hydraulic system is based on temperature, load and speed. Hydraulic fluid viscosity characteristics directly affect fluid friction and slippage in a pump. Maintaining an optimal viscosity range of a fluid across the range of operating temperature is fundamental to fluid pumpability and pump efficiency. Too high of a viscosity increases fluid friction within a pump, which makes the pump work harder. Higher viscosity will also increase fluid temperatures. Too low of a viscosity will increase slippage of the fluid in the pump. A low viscosity will also risk an insufficient fluid film for protecting moving parts.

In order for a quality hydraulic fluid to operate within required temperature ranges they are often formulated with viscosity improver additives to improve both high and low temperature pumping efficiency. Viscosity index improver (VII) additives can be subject to excessive shearing, which results in permanent viscosity loss of the fluid if the product is not formulated with a shear stable VII additive. This can reduce a hydraulic oil’s effectiveness at higher operating pressures and temperatures.

Synthetic hydraulic oils can deliver enhanced overall hydraulic pump efficiency. Synthetics provide low temperature pumpability at startup while maintaining viscosity requirements at higher operating temperatures. Hydraulic fluids formulated with shear stable viscosity index improvers or synthetic oils can improve hydraulic system response time and lower energy cost.

Another formulation factor for efficient power transmission is a high resistance to reduction in volume under pressure due to little or no compressibility (high bulk modulus). Compressibility of the system fluid should be as low as possible. Excessive aeration or air content due to system design or fluid formulation also increases the fluids compressibility and waste power.

Hydraulic fluid characteristics related to improved pumpability and pump efficiency include:

  • Ability to maintain a proper fluid film and load carrying capacity within operating temperatures
  • Low shear stress
  • Low compressibility
  • Low foaming characteristics
  • Ability to shed water readily
  • Good air release characteristics

ALS Tribology testing services can provide information on your in-service hydraulic fluid that helps monitor you fluid’s performance. Feel free to contact any of the laboratories within our global network for help in complementing your asset reliability practices, or contact us through www.alstribology.com.