Marginal Column

Oil under pressure


March 2016


Hydraulic fluids have a significant impact on the wear characteristics of hydraulic components, in particular pumps and motors. With the rapidly evolving and increasing demands on hydraulic systems, the standards do not always keep up. Therefore, Bosch Rexroth now offers a new evaluation method on the cutting edge of technology.

Why is fluid rating necessary?

In recent years, the performance of hydraulics has increased significantly. Pressures, temperatures and oil circulation rates have risen sharply. However, the valid minimum requirements in the relevant requirement standards DIN 51524, ISO 15380 and ISO 12922 do not reflect the resulting higher demands for hydraulic fluids. The result: With one fluid, considerable wear shows up after a relatively short operating time, with another there is no deterioration in its state even after 500 hours – even though both meet the same standard. Until now, machine manufacturers could only rely on the information provided by the fluid manufacturer and only find suitable liquids through the arduous and expensive trial-and-error method.

What is fluid rating?

Fluid rating is a new and standardized assessment procedure, by which Bosch Rexroth tests the behavior of fluids and their interactions with the core components of pumps and motors in real-life situations. In the process, the technical characteristics of the liquids are checked for plausibility and conformity with standards. If the fluid meets all of the requirements, it is put on the ‘fluid rating list’, which will become available to customers in the future.

What is the takeaway?

This service offers manufacturers of lubricants and additives the opportunity to have the performance of their hydraulic fluids rated independently and regardless of the application. Machine manufacturers and operators can thus improve operational reliability, and reduce the probability of failure of pumps and motors, which, in turn, reduces their maintenance and downtime costs.

Runtime 145 h
Runtime 510 h

Comparison image from the RFT-APU-CL pump test: 145 versus 510 hours of runtime.

How does the process work?

The fluids are subjected to high pressure in different cycles over many operating hours in selected Rexroth hydraulic components, both at high temperatures and at low viscosities. Specialists describe the interactions of fluid and components according to standardized evaluation methods. These include, for example, the wear behavior and the material compatibility. To obtain an exact evaluation, they measure the component weight and dimensional changes and visually inspect the surfaces. In addition, they analyze the condition of the fluid before, during and after the trial. Depending on the pump-motor combination and the fluid used, further analysis are conducted.

Classification of hydraulic fluids

Classification of hydraulic fluids

Example RFT-APU-CL (Rexroth fluid test – axial piston unit closed loop)


closed circuit such as in a hydrostatic drive


A4VG045EP hydraulic pump and A6VM060EP hydraulic motor


Test runs

Break-in test:

To achieve the same initial condition of the hydraulic components, the components are evened out by means of a break-in test.

Duration: 10 hours

Pressure: 250 bar

Speed: 2,000 rpm

Temperature: 60°C

Swivel cycle test:

Under high pressure and low viscosity, the pump and motor are swiveled in and out. The characteristics examined are shear strain behavior, wear protection properties and material compatibility, among others.

Duration: 300 hours

Pressure: 450 bar

Speed: 4,000 rpm

Temperature: 100°C

Corner power test:

Maximum load capacity (pressure and speed) of the components at low viscosity. The characteristics examined are shear strain behavior, wear protection properties and material compatibility, among others.

Duration: 200 hours

Pressure: 500 bar

Speed: 4,000 rpm

Temperature: 100°C

Rating criteria

1. Interaction fluid component

  • Measurement of component weight or dimensional change
  • Material compatibility
  • Visual inspection of components and component surfaces
  • Oil analysis (beginning, during, end of the test)

2. Statements about the long-term running behavior