Hydraulics & Pneumatics March 2022

HYDRAULICS Effective equipment maintenance generates savings, as 83% of operators recognise. 1 For hydraulic mining and construction equipment working in cold climates, good equipment maintenance has traditionally meant accepting the cost of switching between summer and winter fluid grades. This is because a conventional single fluid is too viscous to protect equipment against seizure, damage and downtime in winter start-up disrupted schedules should this lead to failure. Conventional single-fluid solutions The most important property of a hydraulic fluid, as far as lubrication of the system is concerned, is its viscosity. Hydraulic fluid viscosity requirements are complicated by the fact that viscosity changes with pressure and temperature. The viscosity index (VI) of an oil is a The oil must remain viscous enough to act as an effective lubricant at the highest operating temperatures but not become so viscous at low temperatures that it does not flow freely and makes starting from cold difficult. Most hydraulic fluids have a VI of about 100 but, where a very wide range of operating temperatures is encountered, an oil with a VI of 150 or more may be required (Figure 2). VI improvers widen the operating temperature range of hydraulic fluids (Figure 3). These large, long-chain molecules are tightly coiled when cold but expand as they warm up and eventually stick together to increase the fluid’s viscosity with temperature (Figure 4). However, they can be susceptible to shearing when forced over sharp edges and through tight orifices and vanes, which leads to high-temperature viscosity loss, also known as shear loss, and reduced fluid life. A new, all-season fluid technology Shell has developed a one-fluid solution for all seasons: Shell Tellus S4 VK. This product changes the game by using Shell’s proprietary gas-to-liquid (GTL) technology to give it an inherently high VI and contains VI improvers designed to resist shearing. The resulting fluid has a 220+ VI for a wide operating temperature range (Figure 2) and long fluid life as indicated by its turbine-oil-stability-test (TOST) life of up to 8,000 hours. It also has a higher flashpoint ( ≥ 190°C) than arctic grades and so provides increased safety. Shell Tellus S4 VK is suitable for use in extreme cold. The ISO viscosity grade 32 fluid is pumpable to 30°C, has a 50°C pour point and Brookfield viscosity values (ASTM D2983) at 30°C that remain below the maximum equipment manufacturer threshold for safe pump start-up (Figure 5), i.e., it does not become too viscous until below 30°C. The same fluid has a kinematic viscosity at 87°C above the minimum threshold for safe pump operation, meaning the fluid protects pumps and components at high and low temperatures and so offers all-season protection. High-VI fluids offering wide operating temperature ranges are prone to losing their high- temperature kinematic viscosity as their long-chain A fluid for all seasons? A new hydraulic fluid for all seasons could potentially cut maintenance costs and overall total cost of ownership by up to 10% compared with a two-fluid, seasonal solution. Ralf Ortlepp of Shell Lubricant Solutions explains how. conditions and not viscous enough to guard against wear at high operating and ambient temperatures (Figure 1). This cost includes fluid disposal, changeover labour and downtime. But there is also the risk of operating with the wrong fluid during unseasonal conditions and the associated unplanned costs plus Fig. 1: The two-fluid solution for seasonal hydraulic system operation. Fig. 2: Minimum temperature range (depending on application) of Shell Tellus V-grade fluids and Shell Tellus S2 MX, all ISO viscosity grade 32. measure of the extent to which its viscosity changes with temperature. An oil that has a high VI shows less viscosity variation with temperature than an oil with a low VI. The VI of a hydraulic oil must be high enough to ensure that the oil functions effectively over the full range of operating temperatures of the system. 22 HYDRAULICS & PNEUMATICS March 2022 www.hpmag.co.uk