Oil is preferable to grease in those cases, in which grease is not suitable for technical or economic reasons. At higher operating temperatures usually oil is preferred. These higher temperatures could be a result of high rotational speeds, high loads or high surrounding temperature. We choose for oil lubrication when at grease lubrication the lubrication intervals would be become too short, when the machine with the bearing is oil lubricated or where the heat from the bearing must be removed. Both grease and the oil used for the lubrication of bearings shoud be well resistant to oxidation, prevent evaporation and also prevent corrosion.
Types of oil
Slogans as “machine oil” and “lathe oil” are still used, but have no longer meaning as trade names. Instead, the different oils are referred to as lubricating oils and classified as mineral, animal or vegetable oil. The most commonly used lubricating oils for roller bearings and gears are mineral oils. These are refined types from crude oil; these can be paraffin wax or naphthenic or combinations of the two.
- Mineral oil: in most cases, mineral oils of good quality are the most suitable lubricating oils for bearings and gears. Pure mineral oil does not contain instable components such as nitrogen; oxygen-and sulfur-containing components and acids might affect the lifespan of a bearing negatively. The most commonly used oils today are highly refined paraffinic wax oils.
- Synthetic oil: synthetic oils are used only for special applications and predominantly at temperatures above 90° C or at very low temperatures. The most important synthetic oils are discussed later.
- Animal and vegetable oil: animal and vegetable oils can in general not be used to lubricate bearings and gears, because the quality can decline after a short time as a result of axid-shaped. However, in special cases, so-called compound oils, i.e. mineral oils with a maximum of 10% animal or vegetable oil, may be used. These oils are more commonly applied in the food processing industries. Follow the recommendations of the supplier when using these oils.
Types of synthetic oils
- Di-ester oils: di-ester oils have a low viscosity and are generally used in instrument bearings. They have excellent properties in the range of temperature over between -50 and +120° C and often offer excellent protection against corrosion. Because the viscosity of di-ester oils are less affected by temperature than those of mineral oils, di-ester oils are quite popular in the air and space industry particularly for use in turbo jet engines and helicopter gearboxes.
- PAOs (poly-Alpha olefins): these are synthetic hydrocarbons (SHC-oils) that actually can be described as mineral oils manufactured by man. They are similar to plastics and rubbers. They exist not so long and have properties that are very similar to that of di-ester oils regarding applications at high rotational speeds. They may, however, endure higher temperatures (-20 to + 160°C) and higher loads.
- Silicone oils: silicone oils are used in instrument bearings and other lightly loaded bearings, in the range of temperature over between -70 and +200° C. The lubricating and corrosion resistant properties of these oils are limited. Fluor silicone oils have properties that are superior to those of other silicone oils.
- Fluoriced oils: these oils, also called poly fluor alkiline ethers, have a good stability against oxidation and can be used at very high loads.
- Poly glycols: this group of oils is mainly used in cases where the operating temperature is above 90° C. Examples of applications include bearings in the wet area of paper machines and bearings for plastic manglers. Poly glycols are highly resistant to oxidation. They can be ten times longer lasting as mineral oils. Poly glycols do not thicken and do not form carbon depositions. Their density is greater than one so water floats on top of the oil. However, the water can be dispersed in the oil when firmly stirring. This oil should not be applied to all general markets.
The most common additions are antioxidants, anti-corrosive agents, anti-foaming agents, anti-wear agents and additions for use at very high pressure (EP).
- Antioxidants: oils that are used at high temperatures and are in contact with the air, oxidize. Substances are formed, which can change the viscosity and can cause corrosion. Antioxidants improve the stability of the oil against oxidizing with a factor of 10 or more.
- Anti-corrosive agents: there are two types of additions that provide protection against corrosion: water dissoluble additions, such as sodium nitrate, and oil dissoluble additions, such as agents based on zinc.
- Anti-wear agents: many additions reduce the wear that can occur as a result of metallic contact when the lubricating film is broken. These additions, mostly called anti-wear agents (AW), form a layer at the surface of the metal that protects against wear. The layer prevents direct contact between the metal contact surfaces, even in the extremely thin molecular layer at the metal surface.
- Anti-foaming agents: when oil starts foaming, the load-bearing capacity becomes less. When oil is foaming strongly, it can run from the bearing. This loss of oil can adversely affect the efficiency of the lubrication. By adding anti-foaming agents, the surface tension is reduced and bubbles in the lubricating oil will burst as soon as they reach the surface of the oil bath.
- Active EP-additives: the most common EP-additives contain substances as phosphorus, chlorine or sulfur. We don’t know exactly how they work but they create a chemical connection with the metal. The subsequently created substance at the metal surface is less strong than the metal itself and easier to remove. Thus it is prevented, that the metal surfaces touch each other and stick to each other. For heavily loaded applications, for example bearings in rolling mills, it was custom to recommend those greases that contain EP-additives, since these additives increase the load capacity of the lubricating film. Originally the majority of EP-additives were lead connections and practice proved that these ingredients could increase the lifetime of bearings in cases where lubrication was critical. On the basis of environmental considerations many manufacturers of lubricants have, however, replaced the lead connections by other substances. Some of these new substances, however, appear to affect bearing steel, what in some cases resulted in extremely shortening of the life span. Choosing an EP-lubricant must therefore be done carefully and the manufacturer should provide the certainty, that the used EP-additives will cause no damage. In those cases where it is known that the lubricant works well, it has to be checked whether the composition is unchanged.
- Fixed additives: fixed additions, such as molybdenum disulphide, can also improve the lubrication properties. The particles need to be around 0.2 μm in size; at that size, the particles keep floating in the oil. Larger or smaller particles will secrete themselves.
The effect of temperature
Mineral oils based on paraffin wax, result in significantly poorer performance at low temperatures than other types. This is because paraffin (waxes) from oil start to crystallize and clot. The oils can, however, be deprived of their paraffin components to improve their behaviour at low temperatures. At temperatures above 90° C mineral oils can oxidize quickly. A guideline is that the lifetime of a mineral oil is 30 years at 30° C, 15 years at 40° C and so on – i.e. the lifetime of the oil is halved for every 10° C temperature rise. At 100° C the lifetime will be about three months. Use a synthetic oil at temperatures above 100° C.
Lubricating oil choice
Oil is chosen, based on the viscosity required to give adequate lubrication at the prevailing operating conditions. The viscosity depends on the temperature. This is reduced if the temperature rises and increases as the temperature drops. It is therefore important not only to know the viscosity at 40° C, but also the viscosity at the operating temperature. The temperature dependence of the viscosity is called the “viscosity index” (VI). A high viscosity index means a low temperature dependence. The greater the variation in temperature, the more important it is that the viscosity index is high. For rolling a viscosity index of 85 or higher should be maintained. The lifetime of a bearing can be extended, when the chosen oil has a slightly higher viscosity than necessary at the operating temperature. A higher viscosity also means an increase in operating temperature. So there is a limit to the extent to which the lubrication can be improved in this way. See chart.