The higher VI of synthetic lubricants actually allows us to pursue further energy savings. If we use a viscosity grade lower than designed, but with a higher VI, we could target to have the same oil film at the operating temperature but increase the amount of savings at lower temperatures. In addition, if we reduce the oil temperature, which typically occurs when switching from a mineral oil-based lubricant to a synthetic, the viscosity of the oil goes up - perhaps allowing us to drop the viscosity grade even further!
Figure 1. Gearbox losses
The first step to saving energy is having the correct oil film at the operating conditions, not too thick to introduce energy losses in shearing the oil but not too thin to initiate surface contact, leading to higher friction and wear. Doing that is difficult in practice because, even with a gearbox running at a steady speed and load, different internal components run at different speeds with different lubricant viscosity requirements. Even more difficult are gears with variable speeds and loads such as those found in vehicle transmissions.
As the viscosity of the gear oil will change with temperature, we have to try and select a lubricant that will provide a suitable protective oil film under normal operating conditions. Assuming the correct viscosity grade (VG) is used for the equipment, the only way to improve energy efficiency (without any change in hardware) is to switch to a lubricant with a higher viscosity index (VI) so that changes in oil viscosity with temperature are reduced.
Typically, that means using synthetic base stocks which have a higher VI than mineral base stocks. Using a lubricant of the same viscosity grade but with higher VI means as the temperature falls, the oil does not thicken as much, reducing churning and pumping losses. However, at the operating temperature the viscosity may be higher than required, leading to higher shear losses. In general, a simple change of oil from mineral to fully synthetic PAO with the same ISO VG will usually drop the oil temperature by between 5-8°C, a clear indication of a reduction in energy losses. The lower traction coefficient of PAO based lubricants, a measure of the energy required to shear the oil film in the EHD load zone, enables this temperature drop (see figure 2).
Depending on the type of equipment, measurable energy savings of between 1% and 7% [3] can be obtained when switching to a synthetic lubricant. Due to the continuous sliding action typically found in worm gears, the efficiency of these mechanisms can be poor, and the use of synthetic lubricants with low traction coefficients can offer higher savings, even up to 30% [4]
As a rule of thumb, synthetic lubricants offer at least 3 times the oil life of an equivalent mineral oil based lubricant [5] because of their improved oxidative and thermal stability. For remotely operated equipment such as mining conveyor belts, wind turbine gearboxes and combined heat and power engines, extended drain intervals can offer considerable benefits. In addition to reducing the cost of maintenance, oil, filters and waste oil disposal, synthetic lubricants also help increase productivity through reduced machine downtime. Extended oil life offers additional energy savings by avoiding the need to mobilise maintenance resources; what is the energy cost of shipping lubricants and maintenance teams out to a wind turbine at sea? It is much better to change the oil every 3-4 years than every year!
For more information on the Energy Outlook visit
www.exxonmobil.com/energyoutlook
For more information on synthetic base stocks visit
www.exxonmobilchemical.com/synthetics
Sandy Reid-Peters, CEng MiMeche Marketing Technical Support Engineer ExxonMobil Chemical Ltd
Sources
[1] The Society of Tribologists & Lubrication Engineers – Basics of friction.
http://www.stle.org/resources/lubelearn/friction/default.aspx [Accessed 03/03/15] [2] Theo Mang, Kirsten Bobzin, and Thorsten Bartels, Industrial Tribology: Tribosystems, Friction, Wear and Surface Engineering, Lubrication, Chapter 1.2 The importance of Tribology
[3] ed. Leslie R. Rudnick and Ronald L. Shubkin, Synthetic lubricants and high performance functional fluids, 2nd edition, Table 20, p31 - Industrial Gear Oil Applications. Marcel Dekker Inc. New York. [4] Energy & general cost savings from the use of synthetic food grade lubricants.
http://www.fuchslubricants.com/Content/downloads/16550_ 15599_Cassida%20Cost%20Saving%20Flyer%20-%20Web.pdf [Accessed 28/10/14]
[5] ed. Leslie R. Rudnick and Ronald L. Shubkin Synthetic lubricants and high performance functional fluids, 2nd edition, Figure 6, p429 - Service life depending on temperature for different synthetic oils containing inhibitors. Marcel Dekker Inc. New York. (From Neale, M.J.,ed. Tribology Handbook, Butterworth, London 1973.)
LINK
www.exxonmobilchemical.com/synthetics
Figure 2. PAO offers a lower traction than mineral basestocks to help reduce energy losses in EHD contacts.
In part 3 - see how these global energy trends affect lubricant demands.
LUBE MAGAZINE NO.127 JUNE 2015
33
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