Additive Containing Oils
Denis Summers-Smith discusses the selection of lubricating oils for industrial machines.
The tribological contacts in machines, where forces have to be transmitted across relatively moving surfaces, are rightly regarded as mechanical ‘fuses’. Numerous surveys have identified that problems with such components are a major factor in machine reliability and the need for maintenance.
These contacts have to be lubricated. The primary aim in lubrication is to separate the load transmitting surfaces by a fluid film that not only reduces the frictional losses at the contact, but can eliminate wear and the likelihood of breakdown. Selection of the correct lubricant is clearly a key issue. Engineers tend to regard lubricants as ‘chemicals’ and thus beyond their scope. This should not be the case. There is no reason why we should not regard the lubricant as one of the engineering materials involved – though we have to recognise that chemical reactions between the lubricant and the tribological surfaces also play an important part.
In most applications separation is achieved by the relative movement of the contacting surfaces (hydrodynamic lubrication). This is a function of the geometry of the contact, the relative sliding velocity and the lubricant viscosity. Separating film thicknesses are small: about 25¼m in the case of conformal contacts (journal and thrust bearings, etc), but only about 1¼:m in the non-conformal (concentrated) contacts that occur between the rolling elements and the tracks in rolling bearings and between gear teeth. The only lubricant property that is relevant in ensuring the development of a separating lubricant film in both these circumstances is its viscosity at the operating temperature. It is difficult to determine this temperature, but this is not a problem in practice as tribological components tend to adjust so that they operate at the same viscosity, irrespective of the viscosity grade used (viscosity grade is the viscosity in cSt at 40oC). It is this feature that allows one oil to be used in systems incorporating a complex of lubricated components. It also allows considerable rationalisation of the number of lubricants required on any one site. The ISO Industrial Lubricant Classification has 18 viscosity grades covering the range two to 1500cSt at 40oC; it is seldom that more than 5-7 of these are required in any one factory. In addition to the choice of a suitable viscosity, the lubricant must also protect the surfaces before the separating lubricant film is generated. This is the condition of boundary lubrication, where satisfactory performance depends on the formation of an adsorbed film of contacting surfaces. This is a matter of chemistry.
Mineral oils derived from petroleum are available in a wide range of viscosity grades, but are also excellent boundary lubricants for the materials used in tribological components. They also possess a number of other desirable properties: extensive liquid range, chemical inertness, thermal stability, corrosion protection for steel, non-toxicity. Certain of these properties can be enhanced by the incorporation of small amounts of additives, such as anti-oxidants, corrosion inhibitors and load-carrying additives. Additive-containing oils are more expensive than straight oils, but this does not mean they are better lubricants, only that they are different. While there is no doubt of their effectiveness in certain cases, the additives can also cause failures. This particularly applies to oils containing load-carrying additives in large systems where the additive can react in unexpected ways.
The understanding of the operation of tribological components and possible undesirable additive interactions in large systems leads to the following recommendations:
Rationalise the lubricant stock by taking advantage of the tolerance of tribological components to the viscosity grade of the oil used. (Rationalisation can give not only economies of ordering and stocking but, by limiting the number of lubricants on site, can reduce the risk of the wrong lubricant being used.)
Use straight mineral oils wherever possible – this applies particularly to small, self-contained systems where the criterion for oil change tends to be contamination rather than lubricant performance.
Use double-inhibited oils (oils containing anti-oxidant and corrosion inhibitors) for circulation systems where the enhanced stability will give extended oil life with sump temperatures up to 60-70oC.
Restrict the use of oils with load-carrying additives (extreme pressure, EP, and anti-wear, AW, oils) to situations where contact conditions are so severe that it is difficult to ensure the presence of a separating oil film under all operating conditions. Table 1 gives guidance where oils with these additives should be considered.
Where load-carrying additives are required, check to ensure that they will not cause problems in other parts of the system: anti-wear (AW) additives tend to hydrolyse in the presence of water forming sludges that can block fine clearances; extreme pressure (EP) additives can attack copper alloys (brass cages in rolling bearings, copper-based bearing alloys) and break down at temperatures above 60 Deg C forming sludges that can cause failures.
This article by Denis Summers-Smith was first published in the November 1999 edition of Plant & Works Magazine.