The roller equipment in sugar cane processing operate in aggressive environments to crush and extract juice out of high loads of harvested sugar cane sticks. To ensure effective and smooth operations, the journal bearings in the equipment require a tenacious lubricating film to protect against wear and corrosion, seal out any ingress of juice/ water and debris like bagass. The lubricant should be hydrolytically and oxidatively stable as well as offer good anti-wear performance. The ability of a lubricant to maintain bearing temperature, provide environmental acceptability, food grade certified and be efficient in terms of lubricant consumption would further complement the application.
Polyalkylene Glycols (PAGs) offers an excellent choice to meet these demands as high performance synthetic lubricants. Their properties confer numerous benefits for heavy duty applications. The Dow Chemical Company owns unique PAG lubricant technology for lubricating equipment used in the sugar cane process. As a replacement for mineral oil and other common types of lubricant, the PAG lubricant represents a new alternative for sugar cane customers to improve lubrication and reduce bearing temperature.
Avelyn Lim is a Technical Service and Development Specialist from Dow Chemical, Industrial Solutions. Based in Singapore, she provides technical service and handles application development projects in the business segment of performance lubricants (automotive and industrial lubricants, of which include the base components and additive materials) in the South East Asia Region.
Friction and wear control
The abundance of oxygen or ether groups along the polymer backbone of the PAG provides chemical affinity towards the metal surface and can lead to thick elasto-hydrodynamic (EHD) films for surface protection against friction and wear. Figure 1 shows the traction data of two PAGs and two common bearing lubricant based on Polyisobutene (PIB) technology. The PAGs exhibit significantly lower friction coefficients than PIB over various slide-to-roll ratios (SRR). Four ball scar testing (under ASTM method D2783, at 30 min, 80kg) of PAG based lubricant also showed low wear results of less than 1mm. Better friction and wear control allows reduced energy consumption.
High viscosity and viscosity index
A good practical choice of PAG for this application is a high molecular weight PAG with a relatively high kinematic viscosity (above 12,000 cSt at 40°C) and viscosity index (above 200). The high viscosity index allows efficient lubrication over wide operating temperature range. PAGs can exhibit viscosity index values more than 400, larger than any lubricant available in the market as a superior and high yield product for bearing application in sugar mills. This also brings about mitigating loss and leaks through intersections of the bearings, reducing the lubricant consumption per MT of sugar cane processed and therefore generating economic savings for the mills. Figure 2 shows that PAG based lubricant (UCON™ SL-17,000) exhibits higher and more stable viscosity over temperature compared to other lubricants.
PAG is not susceptible to hydrolysis, and therefore remains chemically stable if contaminated with sugar cane juice/ water. It also exhibits a superior volumetric heat capacity, therefore generates lower bearing temperature and increasing energy efficiency of operation. Figure 3 illustrates a field trial test result done in a Brazil sugar mill, comparing a PAG and another common bearing lubricant, asphaltic oil. As shown, the bearing temperature using the PAG lubricant is significantly lower than asphaltic oil by a difference of 11°C. Significant reduction of lubricant consumption per MT of sugar cane processed was observed in the PAG lubricant compared to asphaltic oil.
In the trial the PAG remained clear and sludge free after one cycle run in the equipment (see Figure 4). Mineral oils and asphaltic oils on the other hand, form heavy deposits and residue upon degradation and significantly impact the lubrication and seal performance. Figure 5 shows the deposit formed from using asphaltic oils in the Brazil sugar mill and the heavily pitted and scratched metal surfaces of the shaft of the crushing rollers. The PAG is easy to clean off with water as it is water soluble, while oil based fluids are not so easy to clean moreover with the sticky solid degradation residues formed during use.
PAG based formulated lubricant UCON™ SL-17,000 eliminates toxicity concerns in case of incidental leakage; it meets both the U.S. Food and Drug Administration (FDA) compliance (21CFR178.3570) and obtained National Sanitation Foundation (NSF) H-1 accreditation (registration no. 143230), so its intention for use in food and beverage processing is considered to be safe for incidental food contact at micro levels. Asphaltic oils on the other hand is not food grade certified.
The field trial was arranged and carried out in Brazil in a sugar mill in 2010 to investigate and consider replacing asphaltic oil in their milling system with a PAG based lubricant. After one season (3 to 4 months) of continuous sugar cane crushing with UCON™ SL-17,000, there was no sludge or deposit formation in the bearings or on the equipment surfaces. The average bearing temperature was 37˚C which is much less than 46˚C when running with asphaltic oils. Lubricant consumption was significantly reduced to 2.5g/MT with UCON™ SL-17,000 from 9g/MT with asphaltic oils (see Figure 3).
The conversion from asphaltic oil or other oils to UCON™ SL-17,000 PAG lubricant does not require any change or adjustment of the lubrication system and direct top-up is adequate. To convert from grease to UCON™ SL-17,000 PAG lubricant, the bearing set-up or system with good bearing seal is recommended to minimize leakage loss and to re-circulate lubricant back to the bearings. The central feeding system for grease is suitable for a liquid lubricant.
PAG based lubricant (UCON™ SL-17,000) is lightly formulated with corrosion inhibitor/ antioxidant to meet the demands as a high performance bearing lubricant for sugar cane mills, alongside with the aforementioned performance and properties. The low friction and intrinsic heat capacity properties are leading contributions to lowered bearing temperatures, higher production efficiency and energy conservation, possibly eliminating the need for external cooling system. Food grade accreditation brings about safety for use.
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