Development of Grease focusing on Improved

from September / October 2014 NLGI Spokesman

Development of Grease Development of Grease focusing on Improved Energy Efficiency focusing on Improved Energy Efficiency

Masamichi Yamamoto Junichi Imai Kyodo Yushi Co., LTD

1. Introduction

In modern times, where energy and resource saving strategies are becoming more important for environmental conservation, the control of air pollutants such as CO2 has been one of the most urgent global issues. Among other CO2 emission sources, plant facilities and automobiles are considered to be the two major contributors. In plant facilities, production machinery and equipment as well as motor-driven fans and pumps consume large amounts of electricity and power plants consequently emit significant amounts of CO2 to cover the electricity demand in the facilities. As for automobiles, there has been a downward trend in the amount of CO2 emissions since the 1990s thanks to the growth of the eco-car. Still, looking at CO2 emission sources by sectors, transportation sector is highly influential against the industrial, residential and energy sectors.

Accordingly, electric and automobile sectors are working on energy- and resource-saving strategies including a reduction of power consumption by improving the torque characteristics of an electric motor. To be more specific, component and material designs and lubricants are being reviewed for potential torquereducing effects, and grease in its unique “semi-fluid” state has attracted considerable attention being extensively studied to find out application-specific formulations.

This study describes a design concept and study case to develop a rolling element bearing grease with torque reducing effects.

2. Torque generating factor during rolling element bearing operation

The torque during rolling element bearing operation occurs accompanied by friction resistance which is classified roughly into three types: grease’s churning resistance, rolling-viscous resistance and seal resistance1).2). Greases are involved especially in the two torque-generating factors: resistance acting on the rolling contact surfaces of rolling elements in elastohydrodynamic lubrication (EHL) and lubricant’s churning resistance observed at any other site (Fig.1).

Considering that these two factors are, basically, dependent on grease base oil viscosity, a low viscosity base oil was conventionally used to decrease bearing torque. However, a low viscosity oil had limited effectiveness as it could only form a thin oil film and shortens the rolling fatigue life of the bearing.

Thus, a new low-torque technique without using a low viscosity base oil, or with using a low viscosity base oil not adversely affecting the rolling fatigue life has been of great interest to bearing manufacturers.

3. Design concept for low torque

3.1 Low torque by preventing metal-metal contact at low speed

Recently, greases in rolling contact at very low speeds were found to form a much thicker EHL film compared to oil lubrication. Endo et al. conducted an optical interferometric study (Fig.2) on EHL film formed by some typical greases. In their interferometric measurements(Fig.3), a base oil behaved according to the EHL theory in both low- and high-speed ranges. Whereas, greases approached asymptotically to the base oil at high speed but formed a thick EHL film at low speed. In particular, urea-thickened greases formed a thicker EHL film than lithium-thickened greases, and a horse shoe-shaped interference fringe typically observed in the EHL contact, suggested that the EHL effect was dominating at a low speed, being responsible for the increase in the film thickness 3)4). Their study implied that urea and some greases can form a robust EHL film even at very low speeds in the partial EHL regime and are applicable to a wide range of lubrication regimes. Subsequently, Doe et al. reported their study on an EHL film thickening effect at low speed, based on measurements of isolation voltage and frictional torque between inner and outer rings of deeply grooved ball bearings subjected to axial load (Fig.4). The results of isolation voltage measurements (Fig.5) showed that a base oil maintained stable electrical continuity below 100 rpm, the current flowed less easily at 200 rpm and was almost insulated at 600 rpm. By contrast, grease was not only insulated at high speed but also conducted almost no electricity at 1rpm. These results confirmed the EHL film thickening effect of grease in real rolling element bearings operating at low speeds (Fig.6) and agreed well with the findings of Endo’s study. In their frictional torque measurements (Fig.7), base oils formed an EHL film at high speed and a drop in resistance of the film with decreasing speed, decreased torque. At a lower speed, metal contacts occurred and increased torque. By contrast, greases had a low torque even at low speed, at which base oils had a high torque.

These results demonstrated that frictional torque of the bearing can be considerably decreased even under severe low speed and high load conditions, where metal contacts leading to high torque are likely to occur in oil lubrication, by the technique of selecting an appropriate thickener that will impart an EHL film thickening effect to a low viscosity oil-based grease5).

3.2 Low torque by reducing churning resistance

Figure 8 shows torque-generating factors for deeply grooved ball bearings operating at 8000rpm. The grease-related resistance makes up more than half of all torque-generating factors, specifically by grease churning resistance and rolling viscous resistance accounting for 45% and 16%, respectively.

Churning resistance of grease in bearings can be reduced by the following two approaches: decreasing viscous resistance, i.e., apparent viscosity of grease with respect to shear rate and optimizing the behavior of grease in bearings. The former can be done, when the same base oil is used, by reducing thickener content (Fig.9). That means, a grease needs to be softened or formulated with a thickener having an exceptional thickening performance. In the torque measurements for two greases of the same base oil viscosity and penetration but a different thickener (Fig.10), grease A having a low thickener, content had a low torque indicating grease churning resistance is significantly affected by the amount of thickener. As for the latter, it should be noted that a grease is usually packed to fill about 20-30% of bearing void and makes a channel which has a large impact on torque level. The behavior of grease in bearings can be divided into two distinct periods as shown in Fig.11: churning period, at which grease is present in all shear area while being churned and channeling period at which a certain portion of grease stays in a fixed position to be involved in lubrication and the other portion is eliminated making a channel. The impact of churning resistance is more substantial at the churning period than at the channeling period6). A thickener with short fibers is effective in inducing early channeling while long and tangled thickener fibers can stimulate churning (Fig.12). In the torque measurements for two greases of the same base oil viscosity but a different thickener (Fig.13), thickener I had a stable torque within a short time due to early initiation of channeling.

These results confirmed that frictional torque can be decreased by selecting an appropriate thickener to optimize the behavior of grease in bearings instead of using a low viscosity base oil.

3.3 Low torque by reducing rolling viscous resistance

In case of bearings with a contact angle like angular contact ball bearings and tapered roller bearings, toque is greatly affected by traction in grease-lubricated EHL contacts. There have been attempts to reduce traction focusing on “hardware” aspects, for instance by improving bearing internal factors. On the other hand, grease is considered to have much potential for further traction reduction. Under the same operating conditions, contribution of traction differs significantly depending on the molecular structure and formulation of lubricating oil (Fig.14). With lubricating oils of similar formulation and/or structure in the same series, viscosity at normal pressures has minimal impact because traction is associated most closely with a viscosity-pressure coefficient, α7). Therefore, naphthenic mineral oils having “bulky” branched molecular structure with a higher α have a higher traction coefficient than paraffinic mineral oils (Fig.15) 7). Oils with a lower α are effective in reducing traction, meaning that oils with a low traction coefficient such as poly-alpha-olefin can effectively decrease torque. The results of traction measurements showed that grease I formulated with flat structured poly-alpha-olefin had a lower traction coefficient than a mineral oil-based grease (Fig.16).

5. Summary

This report introduced a study case where a rolling element bearing grease having improved torque-reducing effect was developed without relying on the conventional low viscosity base oil technique. We successfully achieved low torque and long service life at the same time, instead of using a low viscosity base oil which often shortens bearing life, by selecting the best combination of base oil and thickener and identifying the most effective thickener (solid) content to ensure a sufficient EHL film formation and to optimize the grease behavior in bearing.

The developed bearing grease with improved torquereducing effect can be beneficial to the resource- and energy-saving strategies in various industries.

4. Study case – Rolling element bearing grease with torque-reducing effect

Table.1 summarizes formulation and general properties of a grease developed for use in automobile bearings. The development objective was an improved torque-reducing effect over the whole speed range. Based on the abovementioned findings, the grease was thickened with urea to prevent an increase in frictional torque at low speed by forming a thick EHL film to avoid metal contact. To ensure low torque at high speed, an urea thickener with exceptional thickening performance was used so as to decrease the solid content and apparent viscosity of the grease. A synthetic hydrocarbon oil was used as a base oil.

In apparent viscosity measurements using a rheometer (Fig.17), the developed grease had approx. 30% less apparent viscosity compared to a conventional grease (Fig.18) indicating that apparent viscosity is highly dependent on solid content in grease. In traction coefficient measurements (Fig.19), the developed poly-Įolefin based grease had a lower traction coefficient than mineral oil-based grease A, being in good agreement with the results with base oil alone as mentioned earlier.

In torque measurements (Fig.20), the developed grease with a low apparent viscosity and a low traction coefficient was proved to have a torque-reducing effect showing about a 50% reduction in running torque in relation to churning resistance and rolling viscous resistance.

References

1) Tedric A.Harris : Rolling Bearing Analysis Second

Edition (1984) 2) Rolling Bearing Engineering, edited by Editorial

Committee of Rolling Bearing Engineering, Yokendo (1978) 3) Endo et al. : Tribology Conference Proceedings 5(2008)181. 4) Don et al. : Tribologist, 57,8(2011)62. 5) Don et al. : Tribologist, 56,1(2011)24. 6) Hoshino : Junkatsu : 25 (1980) 547 7) Masayoshi Muraki : Junkatsu,33,11,(1988) 811