Innerhalb der Hydraulikflüssigkeiten stellen schwerentflammbare Fluide eine Nischengruppe dar. Schwerentflammbare Hydraulikfluide werden vor allem im Bereich Bergbau, in der Stahlindustrie, in Druckgießmaschinen und in Hydraulik-Anwendungen in der Nähe von Metallschmelzen eingesetzt. Die wasserfreien, schwerentflammbaren Druckfluide der Gruppe HFDU sind in der Regel auf Basis von synthetischen Esterölen aufgebaut. Druck- und Temperaturbereich der Esteröle liegen im Bereich der Mineralöle. Hier gibt es jedoch gewisse Einschränkungen im Vergleich zu mineralölbasischen Druckfluiden.

Der Vortrag zeigt neueste Entwicklungen bei wasserfreien, schwerentflammbaren Hydraulik-flüssigkeiten. Es werden neue Kenntnisse hinsichtlich der Kupfer- bzw. Buntmetallverträglichkeit von fertig formulierten Fluiden vorgestellt. Außerdem werden die relevanten Schwerentflammbarkeitstests im Rahmen des Vortrages vorgestellt. Die technischen Besonderheiten von schwerentflammbaren Druckflüssigkeiten werden diskutiert. Neueste Entwicklungen aus dem Bereich von HFDU-Fluiden auf Esterbasis wie auch auf Polyglykolbasis werden im Rahmen des Vortrages vorgestellt.

1. New Rexroth fluid rating procedure
Over the past few years, it has become evident that fluids that just meet the DIN or ISO standards no longer satisfy all of the requirements of hydraulic applications under high load.
Bosch Rexroth defined a new fluid rating procedure that helps determine the suitability of hydraulic fluids across the wide range of Rexroth hydraulic equipment.

The goal of the fluid rating procedure is to minimize the risk of damage to Rexroth hydraulic equipment due to under-performing fluids.

The new Rexroth fluid rating procedure consists of two levels – Basic Level and Premium Level.

1.1. Basic Level
Amongst others Bosch Rexroth demands the fluid data of the manufacturer due to DIN / ISO requirements and further Bosch Rexroth requirements.
If the fluid meets all requirements, it will be listed on the Rexroth Basic Level List RDE 9024X.
(RDE 90240 for HLP/HVLP)

1.2. Premium Level
It is a precondition for the Premium Level to meet the Basic Level. The Premium Level contains specific fluid tests which show the suitability of the hydraulic fluid with defined Rexroth tests. According to the Rexroth components used the corresponding test has to be passed with respect to the oil category.
If the fluid meets all requirements, it will be listed on the Rexroth Premium Level List RDE 90245.

The “RFT-APU-CL” is one of the fluid tests.

2. New Rexroth fluid test “RFT-APU-CL”
(Rexroth Fluid Test Axial Piston Unit Closed Loop)
Over the last two years Bosch Rexroth designed a standard fluid test for axial piston units in closed loop applications.

2.1. Test components
The test unit is a hydrostatic transmission. A 45 cc swashplate pump and a 60 cc bent axis motor, both with variable displacement, are used as test components.

2.2. Test procedure
The test runs over 510 hrs in 3 different steps: 10 hrs break-in test, 300 hrs swivel operation of pump and motor and 200 hrs operation with constant speed and maximum pressure. Several oil samples are taken before, during and after the test.

2.3. Rating / test results
The rating of the fluid suitability is based on visual inspections and measurements of several components of the test unit before, during and after the test. Furthermore fluid analyses of the oil samples are done.

Will it Work?
Fluid Power and Functional Safety

Starting Position
Functional safety deals with the capability of a system to work reliable in case of emergency. Standards like the IEC 61508 have been written for electric, electronic and programmable electronic safety-related systems. But in real applications the actor is usually a mechanical valve. Therefore manufactures and operators of plants require so called SIL (=Safety Integrity Level) calculations for those mechanical components. There are some hints in the seven parts of the IEC 61508 which attest that the named methods can be used for mechanical components too.
This presentation shows what the world of fluid power has to observe when valves are evaluated and how the appliance is done by TÜV Rheinland.
Characteristic Numbers and their Interpretation
Several abbreviations exist to describe the ability of a component for its use in safety-related systems, like PFD, PFH, SIL, MTTFd, MTTR, MTBF, and many more. A focus will be held on the simple equation MTTFd = 1/λd (ISO 13849; MTTFd = Mean time to dangerous failure; λ = probability of failure). Based on the so-called bathtube curve different points of view of electrical and mechanical engineers are shown and the weak points of slack application of this method is demonstrated.
Electrical engineers act on the assumption that failure rate is constant. Effects of wear or ageing can be neglected at simple electrical components like transistors, capacitors or resistances. So failure rates can be taken out of data books.
Mechanical engineers have to deal with effects of wear and ageing. The failure rate depends on the time of use and especially the ambient conditions. It can be described as constant only in intervals. The simple equation MTTFd = 1/lλd suggest, that there is a clearly defined correlation between failures due to random effects on the one hand and wear and ageing progresses on the other hand. It is also shown that the meaning of a MTTFd in the range of more than 100 years can be compared to the so-called “hundred year flood”.


Experience of TÜV Rheinland in SIL Calculations for Mechanical Components
Since nearly 15 years TÜV Rheinland does SIL-calculations for mechanic components for safety related systems. We have evolved different types of approvals for different kinds of components and applications. Usually the approach will be done in three steps:
First we take a look at the components, the application and the documents from the customer. Hereby we can define operating mode, type of sub systems, test intervals and other definitions. A first estimation of the achieved SIL-capability is communicated with the customer.
In the second step a FMEA is done by us together with the customer. All possible failures are evaluated in a systematic way to reach a conclusion about the safe failure rate. By analyzing the technical documents and drawings we are able to proof the correctness of design and the capability of the manufacturer to produce safety relevant components.
The most important part from our point of view is experimental testing, because we do not trust in data books. This is the last but most extensive part of our approach. Depending on the SIL-capability which should be achieved the extent of tests is defined. In endurance tests the components have to proof their capability for their use in safety relevant applications.
An established way of analysis of all three steps and calculations leads to probabilities of failure and to a statement about the achieved SIL-capability. Our daily experience has shown that components which are made with systematic and approved manner usually fulfill all these requirements.
The shown procedure is approved over years and appreciated by our customers. They benefit from our experience and knowledge and the close collaboration which anyway results in improvements in the design.