Mixed Lubrication Regime

Tags Wiki: #mixed film lubrication , #mixed lubrication , #mixed regime , #Stribeck curve , #What is mixed lubrication .

TL;DR: The mixed lubrication regime occurs when a tribological contact is supported partly by lubricant film pressure and partly by asperity contact. It sits between boundary lubrication and full-film hydrodynamic or elastohydrodynamic lubrication, so friction and wear depend on both surface roughness and lubricant film formation.

Key takeaway: Mixed lubrication is the transition regime between boundary lubrication and full-film lubrication.
Key takeaway: Load, speed, viscosity, surface roughness and temperature determine how much of the contact is carried by the lubricant film.
Key takeaway: The Stribeck curve is the common way to explain the movement from boundary to mixed to hydrodynamic lubrication.
Key takeaway: Mixed lubrication is important in gears, bearings, seals, piston rings and machine elements that operate under changing speed or load.

What is the mixed lubrication regime?
Why mixed lubrication matters
Background
Definition
Friction in mixed lubrication
Mixed lubrication models
Research on mixed lubrication regime
FAQs

What is the mixed lubrication regime?

The mixed lubrication regime is a condition where the lubricant film is not thick enough to fully separate the surfaces. Part of the applied load is carried by hydrodynamic or elastohydrodynamic pressure in the lubricant, while the remaining load is carried by direct contact between surface asperities.

This is why mixed lubrication is often the most difficult regime to model and control. Engineers must consider fluid-film behavior, rough surface contact, lubricant viscosity, additives, surface texture, temperature and operating speed at the same time.

Why mixed lubrication matters

Mixed lubrication matters because many real machine contacts do not operate permanently in ideal full-film lubrication. Start-up, shut-down, high load, low speed, rough surfaces and lubricant starvation can all push a contact into mixed lubrication. In this regime, friction and wear can rise quickly if the lubricant, surface finish or operating conditions are not suitable.

Factor	Effect in mixed lubrication
Higher speed	Usually increases film thickness and can move the contact toward full-film lubrication.
Higher load	Reduces film separation and increases asperity contact.
Higher viscosity	Can improve film formation, but may increase drag and heat in some systems.
Surface roughness	Controls asperity interaction, local pressure peaks and wear risk.
Temperature	Changes lubricant viscosity and therefore film thickness.

Background:

The lubrication regimes at the contact interface in any mechanical components are very important to be studied because it determines the efficiency of the components. The type of regime determines the tribological properties of the surface. There are various parameters that affect the lubrication such as the applied load, viscosity of the lubricant, pressure generated at the interface, velocity of the lubricant films, etc. The changes in these parameters give rise to different lubrication regimes which in turn affect the frictional behaviors of the surface. A mixed lubrication regime is one of the important lubrication regimes that have to be understood to improve efficiency in many applications.

Fig-1 Schematic Stribeck curve showing coefficient of kinetic friction (µ) as a function of sliding velocity in the three different lubrication regimens [1].

Definition:

The lubrication arises when the surface interfaces are in between the contact of the asperities on the surface and full film lubrication. This lubrication regime is called mixed film lubrication because it falls in between boundary lubrication and hydrodynamic lubrication. The proper understanding of this regime is the combined knowledge of the boundary and hydrodynamic lubrication regimes. So it is important to have a comprehensive knowledge of those lubrication regimes to study the mixed lubrication regime [2].

Fig-2 Schematic of mixed lubrication [3]

Friction in mixed lubrication:

The main purpose of the lubricant is to reduce the friction at the interface of the surfaces in contact. In the case of the mixed lubrication regime, the prediction and reduction the friction is a great challenge because the friction varies with the operating parameters such as temperature and speed. The friction prediction is difficult in this regime because it lies at the extreme boundaries of the boundary lubrication regime and enters the full film Lubrication or hydrodynamic lubrication regimes [2].

Fig-3 Friction force comparison between the experiment and simulation [4]

Mixed lubrication models:

Many studies have been done on mixed lubrication models for both conformal and nonconformal contacts. However, the deterministic and stochastic models are studied mostly.

Stochastic model: In the case of the stochastic model the statistic parameters are used to study the effects of surface roughness on lubrication. But this model does not provide information on local pressure peak, local film thickness, and asperity deformation. The well-accepted analysis of this model is the combination of the average flow model and the asperity contact model [5]. On TriboNet, stochastic model based calculators are available for point and line contacts:

Stribeck Friction Calculator for Elliptical Contact: Online

Deterministic model: In this model, the solvers can be categorized as separate and unified approaches. In the separate approach, different equations are solved for the solid and lubricated regions. However, in the case of the unified approach, the solutions are obtained from both solid and fluid pressures through Reynolds equations. Also, the information on the asperity pressures and the performance of the heavy-loaded lubricated contacts can be analyzed [6].

Fig-4 CFD investigation of the reynolds flow of the solid obstacles [7]

Research on Mixed lubrication regime:

There are various studies have been conducted on mixed lubrication such as modeling the lubrication regime, improving the efficiency of the mechanical components in this regime, etc. This section mainly focuses on this area of research to understand mixed lubrication.

Mixed lubrication models: In the previous session we discussed the two approaches for modeling the mixed lubrication conditions, however, there are many more models that have been investigated by the researchers in studying mixed lubrication conditions. Various computer-based models are developed to study the effect of different parameters in mixed lubrication conditions. Wen-Zhong Wang et.al. studied the effect of various surface roughness parameters like skewness and Kurtosis. It was found that these parameters have a significant influence on contact area ratio, load ratio, maximum pressure, and temperature at the mixed lubrication regime [8]. The surface forces in mixed lubrication play an important role in achieving better lubrication since it affects the surface roughness parameters. Shuowen et.al., studied this effect using the unified mixed lubrication model and have identified that reducing the surface roughness improves the surface forces which helps in achieving super lubrication at low-speed conditions [9].
Increasing efficiency using different surface modification techniques: various surface modification techniques were implemented by the researchers to increase the efficiency of the mechanical components. In a study on the tribological behavior of the steel sliding pairs Daniel Braun et.al., implemented laser surface texturing for improving the frictional properties in mixed lubrication conditions. They found that the major parameters influencing the friction reduction were the dimple diameter of the textured and the operating temperature, where the reduction in temperature improved the COF for lower dimple diameters [10].

On TriboNet, 2 EHL Film thickness calculators are available: https://www.tribonet.org/?page_id=46078 https://www.tribonet.org/?page_id=45398

Reference:

[1] Porta, C., Sironi, C., Bodega, F. and Agostoni, E., 2016. Pleural Lubrication. Lubricants, 4(2), p.15.

[2] Spikes, H.A., 1997. Mixed lubrication—an overview. Lubrication Science, 9(3), pp.221-253.

[3] Zhang, Y., Kovalev, A. and Meng, Y., 2018. Combined effect of boundary layer formation and surface smoothing on friction and wear rate of lubricated point contacts during normal running-in processes. Friction, 6(3), pp.274-288.

[4] Cheng, D., Gu, L. and Sun, Y., 2022. Mixed Lubrication Modeling of Multi-Lip Reciprocating Seals Based on Elastohydrodynamic Lubrication Theory. Machines, 10(6), p.483.

[5] N. Patir and H. S. Cheng, 1978, “An average flow model for determining effects of three-dimensional roughness on partial hydrodynamic lubrication”, ASME, Journal of lubrication technology, Vol. (101) No. (1), pages 12–17.

[6] Chang, L., 1995, “Deterministic model for line-contact partial Elastohydrodynmic lubrication”, Tribology international, vol. (28), No. (2), pp.75-84.

[7] Patel, R., Khan, Z.A., Saeed, A. and Bakolas, V., 2022. CFD Investigation of Reynolds Flow around a Solid Obstacle. Lubricants, 10(7), p.150.

[8] Wang, W.Z., Chen, H., Hu, Y.Z. and Wang, H., 2006. Effect of surface roughness parameters on mixed lubrication characteristics. Tribology international, 39(6), pp.522-527.

[9] Zhang, S., Zhang, C., Hu, Y. and Ma, L., 2019. Numerical simulation of mixed lubrication considering surface forces. Tribology International, 140, p.105878.

[10] Braun, D., Greiner, C., Schneider, J. and Gumbsch, P., 2014. Efficiency of laser surface texturing in the reduction of friction under mixed lubrication. Tribology international, 77, pp.142-147.

FAQs

What is mixed lubrication?

Mixed lubrication is a lubrication condition where surfaces are partly separated by a lubricant film and partly in contact through surface asperities.

Where is mixed lubrication on the Stribeck curve?

On the Stribeck curve, mixed lubrication lies between boundary lubrication and hydrodynamic or full-film lubrication.

Why is mixed lubrication difficult to predict?

Mixed lubrication is difficult to predict because friction depends on both fluid-film pressure and rough surface contact, including local pressure peaks, asperity deformation and lubricant behavior.

Which machine components operate in mixed lubrication?

Gears, rolling bearings, journal bearings, seals, piston rings, cams and many sliding contacts can operate in mixed lubrication during start-up, high-load or low-speed conditions.

How can mixed lubrication be improved?

Mixed lubrication can be improved by selecting suitable lubricant viscosity and additives, optimizing surface roughness or texture, controlling load and speed, and ensuring adequate lubricant supply.