Machinery and the lubricants that protect them are often exposed to a wide variety of ambient and operating temperatures. As a result, viscosity is the most critical characteristic of synthetic lubricants, as previously discussed (link to previous article). The viscosity of a lubricant relates to its ability to perform at varying temperature and work under extremely loads.
Prime examples are hydraulic fluids used in industrial and mobile equipment applications that operate in a wide range of environments and temperatures. It is not difficult to find a lubricant to meet these requirements, but there can be performance concerns if the fluid is not properly formulated. Fluids with a wide operating temperature range, are often formulated with special viscosity additives to improve both high and low-temperature viscometrics, these additives are subject to forces that can shear the molecular structure of a lubricant.
A lubricant’s ability to withstand shear, its shear stability, is an important consideration when selecting a synthetic lubricant. Once a lubricant shears, it quickly loses viscosity, leading to a lower temperature operating window. This results in accelerated equipment wear, excessive oil consumption, increased downtime, money spent on maintenance and repairs and, ultimately, equipment failure.
Rick Russo, Product Technical Advisor at ExxonMobil provides the following example;
‘When selecting a lubricant for a specific pump, in which the Original Equipment Manufacturer (OEM) recommends the use of an ISO VG 46 hydraulic fluid for operations at a given high temperature, a reliability engineer selects a shear unstable high VI hydraulic fluid. Within one day’s time in operation, the shear unstable ISO VG 46 fluid can degrade to an ISO VG 32 grade, and in less than two days, the same fluid could drop below the ISO VG 32 grade.
Further, the high temperatures within the system, common in late-day operations, can contribute to lower viscosity and result in “afternoon fade,” whereby the volumetric efficiency of the system pumps diminishes, reducing volume output from these pumps.’
To prevent a lubricant shearing, it must be formulated with shear stabilizing additives and base stocks. There are three commonly used methods to determine the shear stability of a high viscosity fluid.
- DIN 51382 (The Bosch Injector Test)
This is considered to be the least severe method. The lubricant is run through 250 cycles at 2550 psi, after which the change in viscosity is measured.
- ASTM D5621 (The Sonic Shear method)
This method shears a sample of hydraulic fluid in a sonic oscillator for 40 minutes and the change in viscosity is once again measured.
- CEC L45-A-99 (The KRL Tapered Roller Bearing test)
This test is fast becoming the global test of choice for original equipment manufacturers (OEM’s), as it is considered to be the most severe and offers the best correlation to actual field performance. The test lubricant is run in a fitted tapered roller bearing for 20 hours under a designed load. Before and after viscosities are then compared and a percentage viscosity loss is calculated.
This table demonstrates a 12.6% reduction in viscosity shear loss from a shear unstable lubricant to a shear stable lubricant tested in the CEC L45-A-99 test.
While laboratory tests provide useful data on shear stability of hydraulic fluids, another method is to measure shear stability in an operating hydraulic pump. Testing in a hydraulic pump replicates the forces and conditions encountered in everyday use.
This graph demonstrates shear stable and unstable lubricants under hydraulic pump test conditions. The shear unstable high VI hydraulic fluid suffered a nearly 30 percent viscosity loss in just two days, a more shear stable formulation retains consistent viscosity for the duration of the test.
When selecting a lubricant it is important to not only take into account the viscosity index of a lubricant but also its shear stability. Well formulated, shear stable, high viscosity index lubricants have been tested to maintain optimum performance through a wide temperature operating window.
MobilTM synthetic hydraulic lubricants not only provide shear stability but also are developed to a high viscosity index.
For more information regarding the implementation of the correct lubricants to improve your operational performance and environmental impact contact WP’s expert team of lubricant engineers on 0800 980 6172.