Pär Marklund

A mixed lubrication model incorporating measured surface topography. Part 1: Theory of flow factors:

Abstract A mixed lubrication model that permits real three-dimensional surface topography as input is developed. The theory of computing flow factors within the model is presented, and with a following paper (Part 2) the method of measuring and adapting the surface roughness, and model validation through flow measurements and application to a bearing is shown. A contact mechanics model is used to calculate the elastoplastic displacement of a periodic topography signal. A method based on homogenization is used to calculate flow factors for all lubrication regimes. The flow factors are compared with the Patir and Cheng method. Results indicate that the two methods compare well for longitudinal roughness lay, but differ significantly for a cross-patterned surface roughness due to the more complete flow description of the current model.

Elastohydrodynamic lubrication friction mapping – the influence of lubricant, roughness, speed, and slide-to-roll ratio

A friction test is conducted in a Wedeven Associates Machine ball-on-disc test rig. The output from the test, friction coefficient versus entrainment speed and slide-to-roll ratio (SRR), is presented as a three-dimensional friction map. A number of parameters are varied while studying the friction coefficient; surface roughness, base oil viscosity, base oil type, and extreme pressure (EP) additive package. Entrainment speed, SRR, and oil temperature are also varied. The results show that the mapping is efficient in showing the different types of friction that may occur in an elasto-hydrodynamic lubrication contact. The results also show that the friction behaviour can be strongly influenced by changing surface roughness as well as base oil viscosity, base oil type, EP additive content, and operating temperature.

Wet clutch under limited slip conditions - simplified testing and simulation

Abstract When designing new wet clutch systems, simulations and laboratory tests are important to obtain a functional system. An engineering tool is here developed to simulate wet clutches working under boundary lubrication conditions. Boundary lubrication friction is measured by a pin-on-disc test, and cooling oil flow is simulated by solving the Reynolds equation. Depending on the generated heat, the temperature in the clutch pack changes during engagement, affecting the transferred torque. With the developed method it is possible to easily and quickly investigate the behaviour of wet clutches with a variety of friction materials and groove patterns. The model is validated in a wet clutch test rig and the predicted torque agrees well with experimental data.

Lubricant ageing effects on the friction characteristics of wet clutches

Abstract The friction characteristics and performance of wet clutches have been investigated by several authors. Studies have also been made to understand the frictional performance during the service life of the clutch system. However, most lifetime studies have been conducted for systems with paper-based friction material so that systems using sintered bronze friction material remain largely unexplored. To study the friction performance of how these systems can vary over time, the friction characteristics for a clutch system using lubricants aged in three different ways were compared. The effects on friction characteristics resulting from oxidation of the lubricant, reduced additive concentration, and ageing under real operating conditions in a wet clutch test rig were studied. The oxidation effects on friction characteristics were examined using a modified waterless turbine oil oxidation stability test on a fully formulated lubricant. Five oxidation time periods from 48 to 408 h were investigated. For each period of oxidation, a friction performance test was run using a pin-on-disc machine. The ageing carried out in a wet clutch test rig is a standard test of a wet clutch systems manufacturer which is used in order to verify that an oil-friction disc combination will last the full service life of the specific application. This test gives a realistic ageing process similar to that in a wet clutch in a field test. Under boundary-lubricated conditions, additives are vital to the performance of wet clutches. Therefore, the effect of reducing the additive concentration in the oil was also studied, in the range of 10 to 100 per cent of the original additive package used in the fully formulated wet clutch lubricant. Results showed that a general friction increase can be observed for oxidation, additive reduction, and test rig ageing. It was also concluded that different methods of simulating the wet clutch ageing process differ and cannot be directly correlated with each other.

Modelling and simulation of thermal effects in wet clutches operating under boundary lubrication conditions

Abstract Wet clutches are frequently used in the drive trains of many modern vehicles. The behaviour of the clutches influences the behaviour of the whole drive train and therefore of the whole vehicle. The design of the clutch is very important because it operates in cooperation with the other parts of the drive train. The clutch also often has to work in the lubricant present in the transmission. To optimize the clutch for an application, properties such as disc geometry, materials, friction disc surface, and engagement axial force can be varied when designing the clutch. Today, the design process involves much testing, which is expensive and time consuming. There are no good hand-book solutions or engineering tools available, hence the designer has to be very experienced and often use trial and error methods in order to end up with a working clutch for an application. A simulation model is developed in this article, which in combination with a simple measurement technique for measuring the boundary lubrication friction coefficient is used to estimate temperature and torque transfer for a wet clutch working under limited slip conditions. The developed simulation model can be used as a design tool for wet clutches. The approach developed in this article can be used to investigate torque behaviour for wet clutches that have not been designed and is, therefore, suitable to use when optimizing the performance of a new clutch. The model includes fluid dynamics, contact mechanics, and temperature computations in the fluid film between the friction disc and the separator disc. Temperature computations in the clutch discs are also included in the model. The fluid dynamics calculations use homogenized flow factors to enable simulations of flow on a coarser grid and still include all surface roughness effects. The temperature distribution in the film in the sliding interface is approximated as a polynomial of the second order. The heat transfer in the grooves of the friction discs is solved by means of an equilibrium equation that includes radial cooling flow effects because of centrifugal flows. The temperature in the friction disc and separator disc is obtained from the solution of the full three-dimensional energy equation in polar cylindrical coordinates. The model is validated by measurements made in a test rig and good agreement between measurements and simulations is obtained, both with regard to temperature and transfered torque. The use of this model can reduce the time needed to develop a limited slip wet clutch application since the systematic way of finding the optimal clutch design will be more efficient than the often used Edisonian trial and error approach.

The influence of contact time and event frequency on acoustic emission signals

Studies of acoustic emissions (AE) of rotating machine elements is often performed under a constant speed. There are few investigations on speed and contact time dependency of acoustic emission signals, even though some investigations have reported difficulties at elevated rotational speeds. Simplified experiments are, therefore, presented in this article to increase the understanding of the time dependency of acoustic emission signals. Hertzian impacts and tensile tests are used to study contact time, the time duration of an event and offset time, the time between events and accordingly the event frequency. The results of these model experiments indicate an indirect quadratic proportionality of acoustic emission amplitudes and contact time ( u max ∝ 1 / ( t c 2 ) ), as well as a proportional relationship between the root mean square and event frequency ( RMS ∝ f event ∝ 1 / t offset ). The relationship between contact time and the root mean square of acoustic emission signals is strongly dependent on the damping of the system.