Do other fluid properties matter?
Other fluid properties do not effect hydraulic pump efficiency.
A typical technical data sheet will also offer information about additional fluid performance features, including: oxidation stability, anti-foam and demulsibility tendencies, anti-corrosion, antiwear, flash point, pour point, etc.. Products that deliver MEHF performance should also provide good performance in these areas to ensure long equipment life and consistent operation, please consult your fluid supplier for this additional data.
Which MEHF viscosity grade do I need?
Please consult your fluid supplier and/or OEM for recommendations on the correct MEHF viscosity grade that is best for your equipment. Fluid must meet the limits for maximum cold temperature viscosity at start-up, as well as minimum high temperature viscosity at peak operating temperature. Climate and oil system cooling/heating capabilities will determine the choice of MEHF viscosity grade.
An excellent reference for viscosity grade selection is NFPA Recommended Practice T2.13.13-2002. For a printable .pdf file of the above information, please click here. |
| There are additional tests which may be reported in a TDS depending on the special use the fluid is intended for. Other important information regarding product performance, often including OEM approvals, can be found in the text of the TDS instead of in the table.
The MEHF Technical Data Sheet:
What distinguishes an MEHF fluid from other premium hydraulic fluids is how the viscosity of the fluid responds to temperature changes and mechanical stress while in use. Here are some of the factors listed in a TDS that deals with viscosity. ISO Grade:
The ISO grade tells the user how thick the fluid is. The thickness is measured by the viscosity of the fluid (in units called centistokes, or cSt) at a temperature of 40°C (or 104°F). The reported ISO grade is simply the viscosity of the fluid in cSt at 40°C. The higher the ISO grade, the thicker the fluid.
In principal, fluids could be blended to any desired viscosity. In practice, the industry blends to certain viscosity targets (typically 32, 46 and 68 cSt for hydraulic fluids) that cover the viscosity range of interest for most hydraulic equipment. Keep in mind that the ISO system allows the viscosity to vary by plus/minus 10%; thus an ISO 46 might have a viscosity at 40°C that ranges from a low of 41.4 to a high of 51.2 cSt.
On a practical level, most hydraulic fluids spend little time at 40°C (104°F), but typically experience a wide daily range of temperatures from start-up to operating temperature, and even these can change with the weather, how hard the equipment is working and the season. The property that measures temperature effects on fluid viscosity is the viscosity index. Viscosity Index:
The viscosity index (VI) is a dimensionless number that is a measure of how much the fluid viscosity changes with temperature. The higher the number, the less the viscosity changes with temperature.
Keep in mind that all fluids will become less viscous (thinner) with increasing temperature. However, the viscosity of a hydraulic fluid with a higher VI will not change as much as that of a fluid with a lower VI. This resistance to temperature changes has important real life consequences as can be seen in the following list:
1. A high VI can allow a fluid to be used year round, eliminating seasonal change-outs between a summer and a winter fluid.
2. A high VI fluid will lead to cooler operating temperatures, avoiding unscheduled shut-downs due to overheating.
3. A high VI fluid will allow efficient and smooth operation at higher temperatures, and permit start-up at lower temperatures. This increases the temperature operating range of hydraulic equipment.
4. An MEHF fluid will provide all the above advantages, but because it has an especially high VI it also offers improved power and efficiency over standard hydraulic fluids which results in fuel cost savings and/or increased power and productivity. There are many good high VI hydraulic fluids with a VI ranging between 140 and 149 that can provide the first three benefits. However, only MEHF fluids with a VI of 150 or more provide the full energy savings and/or productivity gains characteristic of this fluid. Fluids with a VI of less than 150 will provide only a fraction of the energy savings and/or productivity gains provided by MEHF, and that fraction will decrease the further below 150 the VI is. Viscosity and Viscosity Index Stability:
Another important factor in fluid performance is viscosity and VI stability under operating conditions. High VI fluids can be made with high VI oils (such as expensive synthetic oils) and/or by adding polymers called Viscosity Index Improvers to the formulation. Viscosity Index Improvers are a common and well-tested technology first used to made multi-grade engine oils in the 1940s. They are still used for this purpose as well as to make high VI oils for a wide range of other applications, including automotive transmission fluids such as Dexron and Mercon, manual transmission gear oils and high VI hydraulic fluids.
Modern hydraulic systems apply great force to the hydraulic fluid. The base oil and most other additives will not be affected by this force, but in some circumstances the viscosity index improver might be. In the worst case, the forces will break (shear) the VI improver into smaller pieces, resulting in a decrease of fluid viscosity and VI. Thus the benefits of a high VI fluid could be lost in operation. With modern technology, VI improvers that are resistant to breaking (shearing) are commercially available, and an MEHF fluid requires that only these shear-resistant VI improvers be used. Fluid manufactures can provide information on the shear stability of their products under high pressure conditions typical in modern equiptment. The 40 minute sonic shear test (ASTM D 5621) is an idustry standard test that is recomended for this purpose. A fluid meeting the MEHF performance level will retain a specified level of viscosity in sevice after break-in (or after the sonic shear test) and will maintain a higher level of in-service viscosity compared to a standard monograde or high VI oil. HIgher in-service viscosity results in abetter volumetric efficiency. This minimum level of viscosity varies according to the ISO grade and is documented above. Locating an MEHF:
Take the short sample Technical Data Sheet above and discuss it with your supplier. It lists the three basic properties to look for when seeking an MEHF. If a fluid satisfies these values, then it is an MEHF. Of course, your particular application might require other performance features such as electrical resistance, or additional approvals for maintaining warranty coverage, etc., which you should also look for when selecting a fluid.
The MEHF concept is a new one, and there are only a few fluids currently (October 2005) available that have a VI greater than 150. However, premium hydraulic fluids with a VI between 140 and 149 are widely available and will provide some of the MEHF benefits (low-temperature start-up, cooler high temperature operation and elimination of seasonal oil changes). Over time, we expect that MEHF fluids will become more common and easier to find. |
