Understanding Shear Stability

The single most important aspect of a lubricant is its viscosity.

For any lubricant to fulfill its intended role the lubricant must maintain viscosity and its resistance to shear is one vital property which is also a measure of the quality of the oil.

Definition

Shear Stability is a measure of the resistance of an oil to change in viscosity caused by the oil being subjected to mechanical shear stress.

Most multigrade lubricants, including engine oils, high Viscosity Index (HVI), hydraulic fluids and certain gear oils are formulated with viscosity modifiers in order to increase the viscosity index (VI). This ensures the lubricant can provide sufficient film thickness to protect the equipment or engine from wear. Viscosity modifiers (VM) are heat sensitive, long chain, high molecular weight polymers that are added to oil to produce better Viscosity Index characteristics. When a lubricant undergoes shear thinning, the viscosity of the fluid can be lowered and they vary inversely with the rate of shear to which they have been subjected. As the Viscosity Index Improver itself is also subjected to shear, both temporary and permanent loss of viscosity can occur.

Temporary shear occurs when the polymeric structures align themselves in the direction of the stress or flow. This alignment generates less resistance and allows a reduction in fluid viscosity. When the stress is removed, the molecules return to their random arrangement and the temporary loss in viscosity is reinstated.

Permanent shear occurs when the shear stress ruptures the long chain molecules and conversion into shorter, lower molecular weight molecules occurs. These permanently compromised lighter molecules offer less resistance to flow, which, in essence, is the definition of viscosity.

While all Viscosity Index Improvers are subjected to shear stress, the quality of VI improvers vary in their capability to resist both temporary and permanent viscosity loss.

The Shear Stability of Viscosity Index Improvers are evaluated using standard mechanical stress or shear testing laboratory equipment where the VI improved lubricants are measured before and after being subjected to the tests.

There are three recognised test methods to which the formulations can be subjected to. They are the Bosch Injector, the Sonic Shear and KRL (tapered roller bearing) test.

1. The Bosch Injector is associated with many methods differing mainly by the number of cycles the lubricant is passed through the injector. This test is designed to mechanically shear the lubricant. The lubricant is sheared when it is passed through a pintle orifice (injector) at a pressure of 13 to 18 MPa, spraying and atomizing the oil in each pass. It is considered the least severe shearing method of the three discussed here. This method is typically used for engine oils and hydraulic fluids.
2. The sonic shear test irradiates the lubricant in a sonic oscillator for a set period of time. This test is often used in the hydraulic fluid industry for specifications. It has also been successfully applied to transmission fluids and tractor fluids. The shearing mechanism in this testing is cavitation of the fluid. This mechanism differs from the mechanical shear tests, but there is a good correlation when comparing the same VM additive or additives in the same or similar chemical family.
3. The KRL shear test uses a tapered rolling bearing in a cup fitted to a four ball instrument. Load is applied to the bearings as they are rotated at a certain RPM for a specified length of time. The test is typically run for 20 hours. KRL is considered to be one of the most severe shear tests and is used for driveline fluids and gear lubricants.

With each of the shear degradation methods, the viscosity is analysed before and after the permanent shearing of the fluid. The fluid can be analysed with ASTM D445 Kinematic Viscosity to determine the Permanent Shear Stability Index, per ASTM D6022. The fluid viscosity can also be analysed under high temperature, high shear (HTHS) conditions using the Tapered Bearing Simulator (TBS) and a low shear rotational viscometer.

The Permanent Shear Stability (SSI) Index – ASTM D6022 rate the results. The lower the value, the more resistant is the VI improver to mechanical stress or shear.

High quality multigrade engine oils, hydraulic fluids and certain gear oils are formulated by the use of VI improvers with low Shear Stability Index (SSI ) polymers.

Comparisons of different lubricants for shear stability show that small changes or differences on the SSI Index indicate a significant drop in performance (all other things being equal).

An important footnote to this information.

PM Lubrication (preventative maintenance) research concurs with an article written by Tudor M Ratiu in his article entitled “ Understanding Shear Stability”. Find below an excerpt of the piece he wrote regarding this critical aspect of lubrication.

Quote: “The High Temperature/High Shear Test simulates shearing conditions at elevated temperatures. The viscosity of the oil is measured at l50⁰A’C under shearing forces, and results are reported in centipoise (cP). The higher the result, the greater the level of protection offered by the oil. A temperature of 150⁰A’C is necessary because bearings and other components require the greatest protection during high-temperature operation which is easy to achieve by many engines used in moderate to severe conditions. Remember that the oil’s temperature is a lot of time higher than your water/glycol’s temperature and represents an average of all hot spots in an engine. The hottest spots are a minimum of 20⁰A’C above the average oil temperature you see on your gauge (presuming you have one). Most of today’s engines run an average temperature of around 95⁰A’C give or take I0⁰A’C). These hot spots are also the ones where most of the oil goes and they determine the (resistance to) oil pressure given the engine lubrication system is of the force-feed type.

We think that the oil grades as defined by the J300 document are really not as corelated to oil pressure protection, fuel economy as the HT/ HS viscosity and therefore consider that kinematic viscosity at l00⁰A’C (as in J300) is outdated and can cause partial miss-information. However kinematic viscosity remains a critical indicator for an oil and is best kept at the lowest level (for the desired HT/HS value) to minimize windage losses and maximize engine cooling. We call oils with high HT/HS viscosity percentage compared to the kinematic viscosity as robust and very shear stable. High Temperature High Shear Viscosity of engine lubricants is a fluid property that relates to the viscosity requirements of an operating engine. The regions in an engine where HTHSV are of particular importance are crankshaft bearings, camshaft bearings, and piston ring – cylinder wall contact. The temporary shear rate in the bearings is on the order of 106 s-1 while piston ring – cylinder wall contact shear rates are believed to be greater than 107 s-1. ASTM and SAE archives contain many papers on the significance of HTHSV to engine operation, shear rate and temperature. It should be noted thar the relationship between HTHSV and engine performance have a dependency on engine design. Some of these design criteria are oil flow rate, bearing clearances, and finish of contact surfaces.

To truly move down to a lighter grade for fuel economy and horse-power or a higher grade for more protection (also well-worn engines, or ones built with looser bearing clearances, require heavier oil to maintain pressure at lower RPM) we recommend changing to an oil with the right HT/HS viscosity regardless of the SAE J300 I00AoC kinematic viscosity grade. We can say that around 70% of times a higher grade (lubricant) also means a higher HT/HS viscosity and vice-versa.” End of quote.