Increased Efficiency of Displacer Units Through Optimal Wear-In

 

After the production of displacer units, the tribological contacts must be matched to each other, which results in slight changes in shape and surface. Up to now, there is only a short function check at the manufacturer's, the wear-in process then takes place uncontrolled at the customer's. The aim is to determine a wear-in strategy that allows the highest possible efficiency at low wear rates.

 
Benefit Procedure
Analysis of the tribological system Systematic investigations on the rotary tribometer
Test bench trials on selected tribological contacts Optimization of the individual contacts
Scaling of the results to displacer units typical for the market Determination of the reference condition
Derivation of an optimal inlet procedure Investigation of specific wear-in strategies

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Reduce Wear due to Optimum Wear-In Parameters

Axial piston machines in swash plate design are the most common pump design in hydraulic drive technology in the pressure range 250 to 400 bar. Before new units are delivered, a functional test of the units is usually carried out at the manufacturer's premises, which can be associated with a short wear-in procedure. The structure and formation of the tribological contacts, in particular the piston drum - control mirror, piston - bushing and piston sliding shoe - swash plate, is significantly influenced by the wear-in process. This initially leads to an increased particle load of the system and to changes in the system behaviour (e.g. efficiency, control behaviour, ...) during commissioning. The optimum selection of the inlet parameters is intended to change the system behaviour in a targeted manner.

 

Objective for the Wear-In Process of new Units

Wear Rate as a Function of the Wear-In Load for Diesel Engines Wear Rate as a Function of the Wear-In Load for Diesel Engines

The aim of this project is the development of a wear-in strategy with regard to an increase of the overall efficiency and a wear reduction for axial piston pumps. Systematic investigation of tribological effects on the basis of different levels of abstraction is planned. It starts with investigations on the disc-disc-tribometer, which is used to work out an optimal wear-in procedure. Subsequently, customary displacement units are compared with those that have been wear in according to a previously developed procedure. The basic knowledge gained can also be used for other displacer types and other branches of mechanical engineering.

At ifas, it was shown in a previous project that significant efficiency increases of up to 3% are possible, depending on the wear-in procedure. It was also shown that the short wear-in process after the functional test is far from complete and that the units continue to wear in at the user's premises during operation.

The wear-in behaviour is known from the field of automotive engineering, among others. For example, measurements from diesel engine development show that the wear-in strategy has a significant influence on the wear rate.

 

Tribological Basics

A tribological system generally consists of a basic body, a counter body and an intermediate material as well as an ambient medium. Friction and wear depend primarily on the load spectrum and the system structure.

Since the system structure is difficult to change due to the complex geometry, this approach should mainly be applied to the load collective. This collective is dependent on the task to be performed during the period of use. During an initial, short wear-in phase, however, the stress collective can be freely selected, which makes it possible to influence the tribological system in a certain way.

  Surface of a Tribologically Active Workpiece Surface of a Tribologically Active Workpiece

Activation of the Surfaces

In order to change the tribologically active surfaces an energy input is necessary. This can be done thermally, chemically or by mechanical stress. In this process, the metal surface is changed to a depth of about 400 nm. The structure of a typical metal surface is divided into adsorption layer, reaction layer, oxide layer and tribomutation layer. Underneath is the original substrate.

 

Disc-Disc Tribometer

Section Through the Disc-Disc Tribometer Section Through the Disc-Disc Tribometer

A disc-disc tribometer, which was set up as part of a previous project, is available for testing at ifas. This tribometer will be adapted to the new boundary conditions and supplemented with additional measurement technology. Principle construction is build by resting basic body (marked blue) and movable counter-body (marked red). Both bodies are located in a temperature-controlled chamber, which makes it possible to use different hydraulic fluids. By means of the temperature control it is possible to reproduce the real environmental conditions within a displacer unit. The force measuring device and a hydraulic ram to adjust the surface pressure are located above the shown section. Below the section there is a drive motor which drives the lower disc.

 

Acknowledgement

The IGF project 20083 N / 1 of the Forschungsvereinigung Forschungskuratorium Maschinenbau e.V. - FKM, Lyoner Straße 18, 60528 Frankfurt am Main was funded via the AiF as part of the Programme for the Promotion of Joint Industrial Research and Development (IGF) by the Federal Ministry of Economics and Energy based on a resolution of the German Bundestag.

  Copyright: AIF