As we’ve previously seen, the two chief functions which lubricating fluids perform, be they synthetic or oil based, are to conduct heat away from machining processes, and to reduce friction between surfaces within a machining process. Although these two functions are related, they do typically require different chemical solutions and the fluids achieve their results through different means.
Lubricants to reduce friction
Firstly, we need to understand exactly what friction is. Friction is the resistance of the relative motion between two contacting surfaces, in this case within machinery. A piston and a piston chamber, to use a classic industrial example, will move against each other constantly over hours of processing. If there is friction between the piston and chamber, more force is required to force the piston into the chamber, and as such more energy is required for the process. Equally, the constant frenetic contact between the surfaces could cause wear and tear to machine parts, causing damage and requiring resources to be spent on repairs. If we consider friction to be the resistance between the two surfaces, then lubricants reduce friction by eliminating this resistance. The friction between lubricant and surface is far smaller than that between the two surfaces.
Understanding how lubricants work at a molecular level can help us in understanding how they reduce friction. If we look at other fluids we can see more clearly why oil has its lubricant qualities. Water for example, is a polar compound. This means that it has positive and negatively charged particles, which are attracted towards each other. This attraction causes the water molecules to stick to each other, rather than moving past each other. Oil on the other hand, doesn’t have this charge. This allows for molecules to move past each other without charge to cause traction.
Also contributing to this quality is the way in which the oil molecules are flat in their structure, this shape is known as ‘planar’. As such, oils form in layers created by forces known as London Dispersion forces. Layers of molecules don’t bond together because of the lack of vertical projections from these layers, and as such they can easily slide over each other. This is another factor is how lubricants work.
Rather than the resistance created as two surfaces move against each other and cause friction, the lubricant’s layers easily slide over each other.
Fluids to Reduce Heat
Reducing friction in a machining process is one way to reduce the amount of heat produced in a process. However, to further reduce the amount of heat around a machining process, which can warp machinery, further developments to lubricating and cooling fluids are ideal for carrying heat away from these processes.
Both oil and water based fluids are used for coolants, although oil has a higher boiling point than water, meaning it puts less stress on the system. Water has the advantage of being lower in cost, which means that unless oil is required for specific purposes, water based fluids will be used for economic reasons.
Much of the success of cooling fluids is in the method of their application. A high flow system moves fluid very quickly through a process. This means more fluid passes the process within a specific time frame. In theory this allows for greater transfer of heat from the machinery to the fluid. However, if fluids which are in themselves poorer performing are used in combination with a high flow system as a means for compensating for their less effective heat transfer capacity then this can be potentially problematic. In the event of a catastrophic system failure, or even a reduction in the performance of flow system, then machinery can overheat if the coolant itself isn’t of a high enough conductive standard.