Hydrodynamic Lubrication

The modern period of lubrication began with the work of Osborne Reynolds (1842-1912). Reynold's research was concerned with rotating shafts rotating in bearings and cases. When a lubricant was applied to the shaft, Reynolds found that a rotating shaft pulled a converging wedge of lubricant between the shaft and the bearing. He also noted that as the shaft gained velocity, the liquid flowed between the two surfaces at a greater rate. This, because the lubricant is viscous, produces a liquid pressure in the lubricant wedgethat is sufficient to keep the two surfaces separated. Under ideal conditions, Reynolds showed that this liquid pressure was great enough to keep the two bodies from having any contact and that the only friction is the system was the viscous resistance of the lubricant.

There are four essential elements in hydrodynamic lubrication the first two are obvious, a liquid (hydro-) and relative motion (-dynamic). The other two are the viscous properties of the liquid, and the geometry of the surfaces between which the convergent wedge of fluid is produced. When considering hydrodynamic lubrication we must be very careful about how we treat the viscous properties of our lubricant. Since the only friction present in a hydrodynamic lubrication system is the friction of the lubricant itself, it would make since to have a less viscous fluid in order to minimize friction: the less viscous a liquid the lower the friction. Too low of a viscosity jeapordizes our system though. We have to be very careful that the distance between the two surfaces is greater than the largest surface defect. The distance between the two surfaces decreases with higher loads on the bearing, less viscous fluids, and lower speeds.

The surface geometry is also very important. The surfaces have to be such that a converging wedge of fluid can develop between the surfaces, allowing the hydrodynamic pressure of the lubricant to support the load of the shaft or moving surface. This is obtained in a number of ways, a common design other than the shaft and beraing configuration is the tilted pad bearing, where a tilted pad skims over a sheet of fluid.

Hydrodynamic lubrication is an excellent method of lubrication since it is possible to achieve coefficients of friction as low as 0.001 (m=0.001), and there is no wear between the moving parts. Special attention must be paid to the heating of the lubricant by the frictional force since viscosity is temperature dependent. One method of accomplishing this is to cycle the lubricant through a cooling resevoir in order to maintain the desired viscosity of the fluid. Another way of handling the heat dissapation is to use commercially available additives to decrease the viscosity's temperature dependence.

We also have to pay special attention to the extremes of motion, when using hydrodynamic lubrication: starting and stopping. When thesurfaces are at rest with respect to each other, or at very low speeds, the distance of separation is theoretically zero.


Aerodynamic Lubrication

Aerodynamic lubrication is a recent extension of the theory of hydrodynamic lubrication. The theory is the same, in that a converging wedge of high pressure is formed between two surfaces, supporting one surface from coming in to contact with the other surface. Extra attention has to be paid to the system though. Instead of using a thin fluid lubricant, a gas of 1,000 times less viscosity is used. The distance of separation is minute, reqiuring close to perfectly smooth surfaces. Besides the surfaces having to be virtually free of defects, using aerodynamic lubrication requires very high speeds, and low loads.

The benefits of aerodynamic lubrication are obvious. there is no need to cool the air that is the lubricant because heat build up is minimal, no waste, and air is cheap. The amount of precision required to allow this method to work properly is tremendous, making the applicability of aerodynamic lubrication limited.


Elasto-hydrodynamic Lubrication

In our earlier discussion of hydrodynamic lubrication we considered a rigid shaft inside of a rigid bearing. We have to consider the more realistic case of non-rigid bodies in contact. Consider a shaft of metal resting on a sheet of rubber as shown below.

With lubrication of some sort, lets use the generic case,oil, the pressures of the hydrodynamic film will complicate the picture. The pressure of the hydrodynamic film will exert pressure on the deformable medium and the shaft, lifting it upward. The shape of the rubber will be changed as suggested below. Actually however, the shaft will be shifted to one side depending on its direction of spin.

Lets extend this model to include all deformable medium, even metal gear teeth. Because metal is very hard it has a very high threshold of elastic deormation, often as high as 100,000 psi in gear teeth. When these two deformable mediums come together, it is theoretically possible to capture a hydrodynamic film between the two faces such that there is never actually any metallic contact occuring. In this case of extreme pressure the viscosity of the oil film may increase as much as 10,000x, behaving virtually like a solid between the two surfaces. This explains why may mechanisms are able to operate under much harsher conditions than would classically be expected. There is a catch though, this type of elasto-hydrodynamic lubrication works only when the thin film is on the order of 10-40 millionths of an inch! This requires the surfaces to be extremely smooth and carefully aligned, a machinist's nightmare. This is what keeps elasto-hydrodynamic lubrication from becoming an everyday lubrication answer. It is however, very aplicable and neccessary in high pressure situations.

A paradox arrises though. Why, if the harder a liquid is pressed the more like a solid it becomes, would lubricants ever fail under these enormous pressures? The answer isn't at all clear or accesible yet. It is believed though that tremendous heat generated through atomic level interactions fractures the lubricant.


Boundary Lubrication

Boundary lubricationis the most common type of luricatio in day-to-day usage because it finds its applicability where hydrodynamic and elasto-hydrodynamic lubrication fails, relatively slow speeds, high contact pressures, and with less than perfectly smooth surfaces. As running conditions become more severe such as with rough surfaces, and high contact pressures, wear becomes a severe problem to the system. to combat this wear it is neccesary to prepare the surface of the metal accordingly. Taking basic mineral oil as the basis of lubrication, which in most cases it is, it is possible to create a lubricant that forms a surface film over the surfaces, strongly athering to the surface. These films are often only one or two molecules thick but they provide enough of a protection to prevent metal to metal contact. This type of boundary protection is known as boundary lubrication.

A brief explanation of what needs to be added to basic mineral oil in order to create an effective boundary lubricant. Generally, the best additives are active organic compounds with long long chain molecules and active end groups. These compounds bind tightly and intricately with each other, forming a film that builds up on the surface of the metal, itself binding strongly to the metal. This results in a thin film that is very diffulcult to penetrate. When two surfaces, each covered with a boundary layer, come in contact with each other they tend to slide along their outermost surfaces, with the actual faces of the surfaces rarely making contact with each other. Liquids are rarely good boundary lubricants. The best boundary lubricants are solids with long chains of high inter-chain attraction, low shear resistence so as to slip easily, and a high temperature tolerence. The boundary lubricant should also, obviosly, be able to maintain a strong attachment to the surfaces under high temperatures and load pressures.

The most common boundary lubricants are probably greases. Greases are so widely used because they have the most desirable properties of a boundary lubricant. They not only shear easily, they flow. They also dissipate heat easily, form a protective barrier for the surfaces, preventing dust, dirt, and corrosive agents from harming the surfaces.