Wednesday, 29 June 2016 11:18

How Does Oil Work? Part One...

How does oil work? Part One.
By John S. Evans, B.Sc

Friction is a ubiquitous part of our daily lives. Walking, writing, opening doors and drawers and driving to work all involve energy being expended in order to overcome friction.

Although friction is generally thought of as a negative mechanical characteristic (approximately 20% of a car's energy is spent in overcoming friction) it must be remembered that without it we would not be able to walk and the brakes on our cars would not work.

Friction is usually seen as a mechanical problem and the force necessary to overcome it has long interested engineers. When man invented the wheel it enabled him to move weights around with far less expenditure of energy than if they were dragged from A to B. What he had in fact done was to replace sliding friction with rolling friction which has a much lower value.

This blog looks at the role of oil in reducing the harmful forces of friction.

Friction
An understanding of friction will be helpful as a starting point. Friction can be defined as the resistance encountered when one body moves relative to another body with which it is in contact.

The laws and coefficient of friction
The basic laws of friction were 'sensed' by Leonardo da Vinci but were not studied scientifically until the 18th century by Coulomb and Amontons, which led to the definition of three laws:

The first law states that the force of friction that exists between two surfaces is directly proportional to the perpendicular pressure between them. In order to slide a metal cube of mass M across a table, a tangential force, F, must be applied. If M is doubled then F must also be doubled to get M to move.

The second law states that the force of friction is independent of the surface area of M in contact with the surface over which it is being made to slide. If M were to have an oblong cross section, it would not matter whether the larger or smaller surface area was placed on the table; the force of friction, or the force required to move both blocks, would remain the same. Diagram 1 illustrates this point.

Diag1

The third law says that the force of friction is dependent on the nature and state of the surfaces.

Smooth surfaces generate less friction than rough surfaces. The first law can be restated as the ratio between F and M for a particular system is constant and F/M is known as the coefficient of friction. This coefficient can be easily measured by using gravity to make a metal block slide across a surface. Although the mass can be weighed and the force to get it to move easily determined, inertia causes problems.

Newton's first law of motion can be interpreted as saying that bodies at rest tend to stay that way unless acted upon. This means that the initial energy required to get the metal block to start moving (overcome static friction) is higher than the energy required to keep it moving (overcome dynamic riction). Once the initial energy has been expended to overcome the static friction, the coefficient of friction settles down to a constant value equal to the dynamic friction. Thus frictional drag is lower once a body is in motion.

Causes of friction
Surfaces that appear smooth and shiny to the naked eye will show peaks and furrows when examined with a microscope or even a magnifying glass. This does not mean that the component has been  poorly machined but that components manufactured to prescribed tolerances may still have fairly rough surfaces at microscopic level (see Diagram 2).

Diag2

When two surfaces are brought into contact, it is the tips of the peaks that actually touch, meaning that large areas of the surfaces never come into contact. For example, the contact area of two flat steel surfaces subjected to a pressure of 0.5 kgcm-2 is about 1/40 000th of the apparent surface area. The size of the contact area not only depends on the surface area and roughness but also on the load acting on the two surfaces. The peaks where contact actually takes place are called asperities.

Because it is the asperities that touch and because they make up a very small surface area, very high pressures and temperatures are achieved which can cause metal deformation and cold welding to take place. When the surfaces slide over each other, microscopic welds are produced between peaks which, due to the relative motion, are then torn apart. The laws of friction apply to one clean metal surface sliding over another. The laws break down when thin metal films or oxidised surfaces are considered. They also do not apply to very hard surfaces (eg. a diamond) or very elastic surfaces (eg. rubber) where the contact area no longer depends on pressure.

Friction and wear
Whenever friction is overcome, the dislocation of surface material generates heat and this frictional heat can be highly destructive to metal surfaces and cause wear to take place.

Additionally, when there is solid friction (as opposed to fluid friction) wear will also take place. Material is lost due to the cutting action of opposing asperities and to the shearing of microscopic welds. In extreme cases the combination of high frictional temperatures, welding and shearing can cause complete seizure of moving parts.

The harmful effects of friction cannot be overemphasised. The job of the engineer and particularly the lubrication engineer is to control friction: to increase it where it is needed and to decrease it where it will cause damage. Lubrication reduces friction by replacing solid friction with fluid friction.

Sliding and rolling friction
When one body slides over another, the force of resistance encountered at the points of contact is known as sliding friction. If a ball or cylinder were to roll over a metal surface, the relative velocity of the points of contact are actually zero and this results in rolling friction. It should, however, be noted that rolling friction is always accompanied by some sliding friction.

Two main types of rolling friction exist. The first is where large tangential forces are experienced as in the case of a car wheel in contact with the road which generates considerable sliding forces. The second is where minimal tangential forces are present as is the case with a ball or roller bearing. This is sometimes known as free rolling.

Studies show that elastic deformation of the roller and the surface occurs and this gives rise to the resistance to motion (friction) that is encountered. The energy returned to the system when the deformation returns to normal is less than the energy required to cause the deformation. This excess energy is lost as frictional heat. Tests show that rolling friction is not influenced by the presence or absence of a lubricant. However, lubrication is still important in this situation because the elastic deformation of the surfaces introduces sliding friction which can be reduced by the introduction of a lubricant to convert solid friction into fluid friction.

Lubrication
The friction that exists between two bodies in relative motion involves an appreciable energy loss which needs to be minimised. This is achieved by feeding lubricants between these sliding surfaces which replace solid friction with fluid friction which can be far smaller. A lubricant must possess two basic properties: some degree of fluidity to spread over the surfaces and adhesive power to allow the fluid to remain in place during motion. Oils are particularly well suited to this job.

When an oil is fed in between two moving surfaces it causes them to separate and in doing so eliminates the solid friction that exists between them. Unfortunately viscous drag (fluid friction) exists in the oil so that friction can never be totally eliminated but it can be greatly reduced. The oil can cause the surfaces to separate in a number of ways and, when this is not completely possible, friction is still kept to a manageable level.

The lubrication regimes that will be considered here are: boundary lubrication, mixed film lubrication and hydrodynamic lubrication which includes elastohydrodynamic lubrication (EHD).

Practical Analogy

More to follow in part two, alternatively click here to download the pdf.

 

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