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Lubricants evolve to meet demanding bearing tasks
How must lubricants evolve to meet the challenges of rolling bearing operation in increasingly demanding applications? We asked the experts at Koyo about the direction of their research.
The rolling bearing is an important component which supports the rotational motion of machinery. But as machine design trends demand smaller size, lighter weight longer service life and increased efficiency, even more is being asked of these components.
Rolling bearings will continue to evolve to meet these needs, but so too must the lubricating grease that is used with them. Grease must become high performing and highly functional. Two key areas that contribute to the improved performance that is required are the use of additives and through the analysis of rheology and oil forming ability. Such research has been conducted by Koyo's parent company JTEKT.
Rolling bearings lubricated with grease can contribute much more to reduction of friction in machines, machine size, and machine weight than those lubricated with oil due to smaller agitation resistance of lubricant and simpler sealing equipment.
Rolling bearings are required to work across a range of temperatures, from low to high, sometimes at a high surface pressure where plastic deformation will occur in the bearing material. Bearings are also expected to support rotational motion - and sometimes swaying motion - lightly, quietly, smoothly and for a prolonged period over the duration of the product's life. Therefore, while bearing model selection is important, so too is selecting the appropriate grease.
For rolling bearings to be able to perform well, the grease used must be heat resistant, have good oxidation stability, work at low temperatures, have good load carrying capacity, low torque, high speed and acoustic properties, be resistant against corrosion and so on. These characteristics are determined by combining thickener to maintain the semi-solid state of the base oil, the main acting agent in lubricant, and various additives to improve each characteristic.
The contact portion between the rolling elements of rolling bearings and the race has a high surface pressure of several GPa, and also performs rolling motion at a high speed. If the grease in this contact portion contains solid particles, the rolling bearing will begin to vibrate, which will create abnormal noise.
Solids inside the grease include lumps of thickener, which is a component of grease, and other foreign matter. Thickener lumps are dispersed ﬁnely in roll mill treatment when grease is manufactured, however roll mill treatment increases the cost of grease as it is time consuming. Various efforts such as using a filter on the base material and manufacturing in clean rooms are taken to prevent foreign matter entering the grease and help improve acoustic properties.
Extreme-pressure additivesIn demanding applications, involving high temperatures and/or high loads, an improving agent and extreme-pressure additive are added to reduce the friction and wear on rolling bearings, extend service life, and more.
Looking at friction reduction and wear prevention, it was found that characteristics varied depending on the extreme-pressure additive used, or which combinations were used. Importantly, it was recognised that friction and wear characteristics could be improved when additives were used together rather than independently. For example, tests showed that antimony oxide and sulfide were effective for friction reduction while zinc sulfide was effective for preventing wear.
The likes of lead and chloride compounds have long been abandoned as extreme-pressure additives on environmental grounds, but there may well be further restrictions on additives in the future, so research into even safer additives is important.
Bismuth is one likely candidate, and JTEKT research has confirmed that bismuth dithiocarbamate has excellent friction and wear characteristics. When added to grease, it offers similar friction and wear characteristics to antimony dithiocarbamate. In grease lubrication, the lubrication state is affected significantly by the rheological characteristic of the grease itself, as well as conditions of use such as temperature, load and rotational speed. So it is important to understand the rheological characteristic of the grease. This is even more important given the current trend to lubricate with smaller amounts of packed grease - a measure that aims to reduce the friction caused by agitation resistance of the grease and the need to reduce grease consumption.
One of the concerns of reducing the amount of packed grease is that it will not supply sufficient lubrication to the contact portions. A thorough understanding of the rheological characteristic in this lubrication-deprived state is needed to increase its effectiveness and so improve the performance of rolling bearings. JTEKT has undertaken a number of studies focusing on worked penetration. It was discovered that it takes a long time before the grease seizes when penetration is high, and a correlation exists between grease fluidity to the friction surface and the shear viscosity and yield stress of the grease reserve at the entrance to the contact portion.
Furthermore, the respective impact of thickener types and base oil kinetic viscosity on the rheological characteristic was studied and it was discovered that, even if the penetration was the same, the grease fluidity would differ depending on the thickener type. Further, by adjusting the thickener type, the grease film can be maintained for an extended period of time.
It is predicted that machinery will continue to be made lighter and smaller, and will operate at increasingly higher speeds, making the environment in which rolling bearings are used even more severe. At the same time, the demand for rolling bearings with low torque and longer service life will increase. Inevitably, environmental considerations will become more acute. Research into, and a thorough understanding of, new greases and additives, as well as the rheological characteristics of these grease products, will be required to address these ever more stringent requirements.
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