Project 1B.2

Project / Leader

1B.2 - Prof. Ashlie Martini (Purdue)

Title

Surface Effects on Start-up Friction and Their Application to Compact Gerotor Motor Design

Statement of Project Goals

The objective of this project is to develop and validate a model for static friction to improve the start-up efficiency of hydraulic components. The resulting modeling tool will be the first experimentally validated start up friction model that incorporates surface characteristics and lubricant effects. At its conclusion, a successful project will result in a fundamental understanding of the relationship between characteristics of a component’s interfaces and the friction it must overcome at start up. The modeling tools and corresponding experimental test set up developed in the project will be used to evaluate existing and novel surfaces to improve start-up efficiency.

Project's Role in Support of the Strategic Plan

In the context of the CCEFP strategic plan, this project contributes to overcoming the transformational technical barrier of efficient components. Many hydraulic motors exhibit extremely poor start up efficiency, forcing designers to specify larger motors than necessary and that, in turn, makes the overall cost and weight of the machines greater. This project will provide an understanding of the physical mechanisms underlying static friction, which will lead to specific approaches for minimizing start up friction. This research is not only relevant in terms of the start-up efficiency of fluid power applications, it is fundamental in its focus on understanding static friction in line and point contacts from a tribological perspective.

Description and explanation of research approach

This project employs modelling and experimental methods. The novel start up friction model being developed incorporates material properties, scanned surface profiles, load and geometric characteristics as input.  Using this information, the model creates a discrete domain for which the gap and surface interference between surfaces is calculated.  The discrete convolution- fast Fourier transform algorithm is used to predict contact parameters of those elements (asperities) in the discretized domain.  Depending on the amount of deformation of the asperities, an elastic-plastic deformation simulation is used to predict the minimum tangential load required to start sliding. The individual effects of each discrete element are summed to calculate the total interfacial static friction coefficient.  A flow chart and pictorial representation of the model’s key features are shown in Figures 1 and 2. 

Figure 1: Static friction model flow chart

Figure 2: Pictorial representation of the model

The predictions from the numerical model are being validated using novel and versatile experimental methods. One method developed specifically this purpose, a high load and variable contact static friction apparatus, is shown in Figure 3. The device is designed to accurately characterize the conditions simulated in the model. It can be adjusted to fit different contact types (point, line or flat) and forces that represent those typically seen in components in hydraulic machinery.

(a)    (b)    (c)  

Figure 3: CAD model (a) and photos (b and c) of the high pressure, variable contact static friction experimental setup.

The test setup is designed for high loads and multiple types of contacts and so will provide a valuable and unique capability for characterizing static friction.  The high load conditions and lubrication regimes seen in fluid power components can be replicated using this test setup. In addition, it will complement work being done for other CCEFP projects.