Putting the simple back into viscosity
Written by John Sander
Viscosity Considerations
Various conditions must be considered when specifying the proper viscosity of a lubricant for a given application. These conditions include the operating temperature, the speed at which the specific part is moving, and the load placed upon the part. One other consideration is whether or not the lubricant can be contained so that it remains present to lubricate the intended moving parts.
Temperature
The viscosity of a lubricant changes with temperature – in almost all cases, as the temperature increases, the viscosity decreases; and – conversely – as the temperature decreases, the viscosity increases. To select the proper lubricant for a given application, the viscosity of the fluid must be high enough that it provides an adequate lubricating film, but not so high that friction within the lubrication film is excessive. Therefore, when a piece of equipment must be started or operated at either temperature extreme – hot or cold – the proper viscosity must be considered.
Speed
The speed at which a piece of equipment operates must also be considered when specifying the proper lubricant viscosity. In high-speed equipment, a high-viscosity lubricant will not flow well in the contact zones and will channel out by fast-moving elements of the equipment. On the other hand, low-viscosity lubricant would have too low a viscosity to properly lubricate slow-moving equipment, because it would run right out of the contact zone.
Load
Equipment loads must also be considered when selecting the proper lubricant viscosity. Under a heavy load, the lubricant film is squeezed or compressed. Therefore, a higher viscosity lubricant is needed. The higher the viscosity, the more film strength the lubricant will generally possess. In addition, the load can be either a continuous or shock load. A continuous load is a steady load that is maintained while the equipment is operational, while a shock load is a pounding or non-steady load. Under shock-load conditions, a low-viscosity lubricant would not possess enough film strength to stay in place, whereas a high-viscosity lubricant could stay in place and act like a cushion in the contact area.
Containability
In some applications in which a fluid lubricant would leak out, a grease might be recommended. However, it is still important to consider both the base fluid viscosity and the NLGI grade when selecting the proper lubricant. If the lubricant's viscosity or consistency is too high, it might not flow where it is needed and the lack of lubricant – a condition known as lubricant starvation – would lead to metal-to-metal contact. This would cause wear that could ultimately result in equipment failure. The same thing could happen with a lubricant with too low a viscosity or consistency, because it might not stay in the area where it is needed.
