Fluid Power Research Update: Rheological Design for Efficient Fluid Power

We’ve written before about the Center for Compact and Efficient Fluid Power (CCEFP)—the network of fluid power research laboratories, academic faculty, graduate and undergraduate students at seven universities—that is making a difference when it comes to preparing a better-educated workforce for the fluid power industry. The CCEFP has created a 500% increase in the number of fluid power focused advanced degrees awarded in the United States, with almost half of its graduates going on to work in the fluid power industry.

 

In addition to increasing interest in fluid power, the CCEFP has also been home to a number of research projects in fluid power. One such project involves investigating rheological design for efficient fluid power.

Rheological Design for Efficient Fluid Power

Jonathon K. Schuh,  Yong Hoon Lee,  Lakshmi Rao , James T. Allison, and Randy H. Ewoldt, University of Illinois at Urbana-Champaign

Friction between lubricated surfaces in fluid power systems decreases the efficiency of the system. By reducing the frictional loss, the efficiency of the system can be increased. One proposed method for decreasing the friction in fluid power systems is to use surface texturing on one of the surfaces. We have examined both symmetric and asymmetric texture profiles with Newtonian fluids in order to determine the effects of breaking symmetry on the friction reduction. The surface textures decrease viscous friction from the flat plate reference, independent of direction, and the symmetric texture decreases the viscous friction more than asymmetric textures. The symmetric texture also produces normal forces that are barely above the experimental limits, and the forces are direction independent. The asymmetric textures, however, are able to produce forces much larger than the experimental limits, and the sign of the forces produced depends on the direction of motion. The magnitude of the forces produced depends on the value of the asymmetry angle β, suggesting an optimal asymmetry angle β exists for producing normal forces with textures.

A numerical tool is needed to determine the optimal surface texture for decreasing friction in lubricated sliding contact. We have developed code for solving the Reynolds equation in cylindrical coordinates, and have validated the code against our experimental results. Using this code, we can determine the optimal β for decreasing friction with asymmetric surface textures.

The above analysis is only for Newtonian fluids; Non-Newtonian fluids can also assist in friction reduction by decreasing their viscosity and producing separating normal forces, leading to decreased friction within the fluid power system. We experimentally tested our symmetric and asymmetric surface textures with a Non-Newtonian lubricant to determine the friction reduction effects of changing the fluid and surface. The Non-Newtonian lubricant produces normal forces on its own due to viscoelastic effects, and the asymmetric surface textures produce normal forces above the viscoelastic response. These results also suggest that an optimal β exists for decreasing friction with Non-Newtonian fluids, and the friction reduction is greater with Non-Newtonian fluids than Newtonian.

Figure 1: Schematic of experimental setup (modified rotational rheometer) and the three types of textures tested (flat, symmetric, and asymmetric). FN is the measured normal force and M is the measured torque. The top plate rotates in both directions in order to determine directional-dependent effects.

Figure 2: Effective friction coefficient µ ∗ = M/R0 FN for all the surface textures at each of the gaps tested experimentally with both Newtonian and Non-Newtonian fluids. The results with the Non-Newtonian fluid are always lower than those with the Newtonian fluid, meaning that the combined performance of the surface textures and Non-Newtonian fluids is better than the surface textures alone.

Prof. Randy Ewoldt may be reached at ewoldt@illinois.edu.