Simply stated, viscosity is defined as the internal resistance of a fluid to flow. That doesn’t sound too difficult, does it? Unfortunately, new temperature, speed and pressure demands on lubricating fluids have changed over the years, resulting in several different measurements and classifications being created to describe lubricant viscosity.
Some examples are SUS, cSt, cP, ISO, SAE engine, SAE gear and AGMA; it’s enough to make a person’s head spin.
As mentioned above, viscosity is a physical measurement of a fluid’s internal resistance to flow. Assume that a lubricating fluid is compressed between two flat plates, creating a film between the plates. Force is required to make the plates move, or overcome the fluid’s film friction. This force is known as dynamic viscosity. Dynamic viscosity is a measurement of a lubricant’s internal friction and is usually reported in units called poise (P) or centipoise (1 P = 100 cP). A common tool used to measure dynamic viscosity is the Brookfield viscometer, which employs a rotating spindle that experiences torque as it rotates against fluid friction.
A more familiar viscosity term is kinematic viscosity, which takes into account the fluid density as a quotient of the fluid’s dynamic viscosity and is usually reported in stokes (St) or centistokes (1 St = 100 cSt). The kinematic viscosity is determined by using a capillary viscometer in which a fixed volume of fluid is passed through a small orifice at a controlled temperature under the influence of gravity.
Grease Viscosity (Consistency)
Grease viscosity, traditionally called consistency, cannot be measured using the tests noted above. However, it is still relevant for selection of the correct grease for a specific application. Greases are fluid lubricants enhanced with a thickener to make them semi- solid. They usually are used in applications where a liquid lubricant would run out. Greases are sold by consistency grade, which in this case will be used synonymously with viscosity grade.
Grease consistency is measured using the cone penetration test. The National Lubricating Grease Institute (NLGI) created grade ranges for greases that have become the industry standard. These ranges characterize the flow properties of greases.
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.
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.
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.
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.
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.
With the SDT LUBExpert you have the ability to check on-condition lubrication and machine health using ultrasound.
Simply stated, viscosity is defined as the internal resistance of a fluid to flow, but it is probably the most important property of a lubricant. It can affect how the lubricant will function in a piece of equipment. If the wrong lubricant viscosity is selected for an application, the chances for equipment failure are dramatically increased.
Fortunately, organizations like ASTM, SAE, AGMA, ISO and others have created standards for lubricant viscosity and consistency that are to be used as guidelines when selecting the proper lubricant.
The best rule is to always check the original equipment manufacturer’s manual for lubricant viscosity recommendations. If the OEM makes no recommendations, the next step is to consider the operating speed, temperature and load of the application to be lubricated. Another suggestion is to contact lubricant manufacturers for recommendations; they often can provide technical support for proper fluid or grease selection.
After making a lubricating product selection, it is important to closely monitor the equipment to ensure the right choice was made. If possible, visually observe the moving parts to verify that a sufficient lubricant film is present to protect them. If not, listen for any unusual load grinding, chattering or squalling noises, which often are indications of metal-to-metal contact.
Thank you John Sander with Lubrication Engineers for this educational and informative article!
Lubrication by Diana Pereda