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Analysis of a vane oil pump mechanism failure: Multibody, fluid-dynamic and validation


Fig. 1 - 3D model of vane oil pump
Fig 2 - Internal components

This article presents an activity carried out by the Calculation & Simulation and Testing departments in Pierburg. The subject of the analysis is a vane oil pump for engine lubrication. This product represents a new generation oil pump in the automotive industry due to the possibility to reduce the displacement at high engine speed for fuel saving consumption.
The aim of the article is to present the validation of a simulation model for the description of damaging behavior occurred in an aggressive durability test. This paper describes the evolution of both rigid body model and fluid model used for the calculation of internal loads which best fit the experimental evidences.

Product presentation
Variable displacement oil pumps contribute significantly to the fuel saving capability in the automotive industry. In comparison to conventional pumps, they have the possibility to optimize the oil flow according to engine demand, with a significant reduction of power absorption. The oil flow rate of a vane pump, such as that of a general volumetric pump, depends on its actual displacement, i.e. the difference between the maximum and minimum trapped volumes. This difference is a function of the pump eccentricity that is defined as the distance between the rotor axis (which is fixed) and the control ring axis. That’s why, in order to obtain the displacement variation in this kind of pumps, the control ring is made to slide or rotate into the housing.
The variable displacement oil pump considered in this study is driven by engine crankshaft (Figure 1) so the rotor is in axis with the crankshaft.

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Fig. 3 - Workflow

The analysis is focused on the contact between vane and control ring which is one of the most critical tribological couple in this kind of mechanisms. Calculations are based on a multidisciplinary approach. In fact the contact force between vane and control ring is calculated by means of multibody and CFD combined models. Physical parameters have been tuned until model validation is obtained, accordingly to experimental durability test.
The Scheme in Figure 3 shows the workflow of the entire activity. The aim of this study is to calculate the contact force between the vane and the control ring that best explains the wear observed on the internal track of the control ring after durability testing, since the contact force is not easy to evaluate at the beginning, before some key hydraulic values are defined (like presence of dissolved air in oil and time-constant free-to-dissolved). Through this method the failure can be explained and countermeasure can be taken.
Starting from a simplified multibody model and a simplified angular history of pressure, the target force is obtained and compared to wear signs. If they are not in accord, further iterative refinements of the model (multibody and/or fluid parameters) are needed until the target is matched.


Fig. 4 - Chamber pressure

In this activity, 3 refinement iteration loops were necessary to correspond with the experimental evidence. The main characteristics of the used models are summarized in Table 1.
The first model contains only the side pressure effect on the vanes and uses a nominal shape of pressure signal. Side pressure depends on the pressure in upstream and downstream chambers (Figure 2),that are alternatively equal to 0 when the chamber is connected to the inlet and equal to delivery pressure when the chamber is connected to the delivery side.
The second model is equal to the first, with the addition of the radial pressure effect on the top and the bottom of the vanes. The pressure on the top of the vane reply the pressure distribution into the gap between the vane and the control ring. The pressure on the bottom of the vane reply the pressure in the internal side of rotor.
The third model is equal to the second but the nominal pressure signal is substituted by the pressure signal calculated via CFD simulation. Different aeration levels are simulated (0.5, 4 and 7%).
The fourth model is equal to the third but the aeration level is fixed and a range of time constant for passage free air – dissolved air into oil (tF→D ) are simulated.
The contact force is compared to the worn profile of the control ring as appear in the end of endurance test. It is a consistent approach since, according to Archard formulation for adhesive wear, the worn volume is proportional to the contact force.


Fig. 5 - Multibody model

Fig. 6 - 1-D CFD model

Multibody model
A multibody model of the pump has been created. The model consists of several rigid bodies representing the internal components (see Figure 5). Inertia properties of the bodies have been obtained on the basis of 3D CAD drawings. Internal components are mounted with average clearances according to their tolerance range. The rotor motion has been imposed. The vane is able to slide radially during rotor revolution in a sort of “prismatic joint” and is subjected to mechanical and oil pressure loads. Mechanical interaction between rigid bodies has been guaranteed by solid contact elements; they consist on equivalent spring-damper elements which laws are based on overlap volume due to rigid bodies penetration.
The hydraulic pressure effect has been modelled by force elements applied on each vane as a function of the rotor angular position (Figure 4). Both side effect and radial effect are modelled as in Figure 2. Friction effects are included in the model.


Table 1 - Simulation models

CFD model
A 1-D CFD model of the pump has been created (Figure 6). The model represents the pump system with the typical strategy of 1-D lumped parameters approach (representations of fluid volumes, pipes, orifices).
The parts used for experimental test were measured and the same clearances were used in order to simulate exactly the pump that was tested. The model was built representing the geometry of each rotor chamber connecting the inlet and the delivery side of the pump and the most important leakages among rotor chambers and surrounding volumes. In all simulations, a proper model of oil aeration has been applied. Therefore the value of Bunsen coefficient was provided, time constant for passage free air – dissolved air into oil (tF→D), time constant for the opposite passage (tD→F) and the amount of free air initially present in oil. The values of parameters mostly affecting simulation results are given in Table 1.

Experimental tests
Experimental test on bench is carried out under controlled conditions. Speed, pressure, flow-rate, temperature are set according to product requirements and are sensor-monitored. In this test, the control ring of the pump is physically locked to the maximum eccentricity position to accelerate the wear process: this has been recognized as the most aggressive condition for wear. Eccentricity is defined as the distance between rotor axis and control ring axis and it is directly proportional to the pump displacement. Working conditions for durability tests are: oil SAE5W30 at 120°C, 6500 rpm pump speed.


Fig. 7 - Resulting contact force



Fig. 8 - Comparison

The contact force between vane and control ring has been calculated for a set of simulation models and for a set of physical parameters. Figure 7 shows the evolution of the resulting contact force according to model 1 up to 4.
In each simulation the contact force has been compared to a worn profile of the internal track of the control ring. Model 4 with 7% aeration and 10-5s time-constant free-to-dissolved is the combination that best explains the wear occurring in the durability test.
The Polar plot in Figure 8 shows the comparison between the calculated contact force and the worn profile of the internal track of the control ring. The blue line shows the wear track measured by means of a profilometer in Pierburg metrological department. The green line represents the polar plot of the contact force calculated in model 4 with 7% aeration level and time-constant free-to-dissolved (tF→D ) 10-5 s. A good agreement is found between the contact force and the worn shape of internal track of control ring.

In this activity, different simulation models are shown for the validation of the contact force between the vane and the control ring that best fits the wear observed on the control ring after the durability test. A good synchronization between calculated contact force and wear signs is finally found. In the end this calculation has been fundamental to explain the root cause of the failure and to operate the best selection of the material of the components.

Articolo pubblicato sulla Newsletter EnginSoft Anno 11 n°4

Andrea Barbetti, Matteo Gasperini, Fabio Guglielmo, Nicola Potenza, Raffaele Squarcini
Calculation & Simulation, Testing, R&D - Pierburg

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