CCEFP Researchers Develop Novel Test Apparatus to Measure Start-Up Friction of Hydraulic Motors

Achievement date: 

Researchers at the Center for Compact and Efficient Fluid Power (CCEFP), an NSF-funded Engineering Research Center (ERC) headquartered at the University of Minnesota, have developed a first-of-its-kind test apparatus to measure an important characteristic of hydraulic motors, their friction at start up.


The CCEFP test apparatus enables a fundamental understanding of the mechanisms underlying start-up friction so it can be predicted and modulated. Knowledge gained from the apparatus can be used to address the substantial inefficiencies of hydraulic motors at start-up that require hydraulic-system designers to specify larger motors to overcome start-up friction and make the overall weight and cost of the vehicle higher than otherwise necessary. Additionally, development and use of this test rig demonstrated the value of collaborative ERC programs; in this case, progress and full utilization of the test rig resulted from the efforts of researchers at three CCEFP institutions. Future plans for the test rig also illustrate this project’s value and reach in education.


Hydraulic motors are used in many applications because of their durability, serviceability, and high power in a compact package. However, they also tend to be very inefficient at start-up, primarily due to extremely high friction generated between their internal components as motion begins. This inefficiency requires larger motors than necessary along with attendant weight and cost penalties. The first step in addressing this limitation is to understand the start-up-friction mechanisms so designers can predict and modulate them in future designs.

This test rig (see accompanying figure) can reproduce interface conditions typical of hydraulic motors (i.e., high loads induced by fluid pressure, variable contact area shapes, and the presence of hydraulic fluid). The research team used the rig to study effects of surface patterning and hydraulic-fluid chemistry. Surface patterning was made possible through collaboration with researchers at the University of Illinois, Urbana-Champaign (UIUC). The UIUC team provided samples containing dimples, of varying size and surface distribution, whose effect on start-up friction was investigated using the test rig. The goal of this aspect of the project was to identify optimal combinations of surface features that could be used by designers to minimize start-up friction.

The test rig was also introduced to study the effect of hydraulic-fluid formulation. Although viscosity is a major factor in friction, it is thought to play a less significant role in start-up friction because maximum friction occurs before motion begins. The team studied hydraulic fluids with different chemical compositions but similar viscous properties provided by researchers at the Milwaukee School of Engineering.