Equipment & Technology

Picking the right industrial lubricant

Steve Davis, Senior Bulk Handling Expert at Advisian, discusses the topic of lubrication and why some greases may be inadequate for certain applications.

Steve Davis, Senior Bulk Handling Expert at Advisian, discusses the topic of lubrication and why some greases may be inadequate for certain applications.

Lubricants are generally considered oil or grease. There are other solid lubricants such as graphite and molybdenum disulphide for particular applications, and sometimes included in grease and solid bearings. Many other liquids, including water, can be used as a lubricant. This article focussed broadly on the basics relating to oils and greases, as these are the most used in the bulk materials industry in my experience.

So, why do we use a lubricant? Fundamentally the use is to reduce the effects of friction. This is done by creating a film between components that are moving in relation to each other. The smoothest finishes in the industry are quite rough on a microscopic scale, and the moving surfaces look like the sketch in Figure 1.

Figure 1: On a microscopic scale, even the smoothest of finishes are quite rough. A lubricant film separates components and reduces friction.

The most common lubricant is oil. This can be mineral, semi or full synthetic. All oils are mixed with a selection of chemical additives depending on the application and brand. These include detergents, corrosion inhibition, antioxidant, alkalis, anti-wear, viscosity modifiers, extreme pressure, dispersants, anti-foam, and many others. Additives improve oil life, provide multigrades and provide specific benefits for each application.

The most important parameter in oil is kinematic viscosity, measured in centistokes, cSt. The scale is exponential, and we purchase ISO grades. The ISO grade indicates the viscosity at 40°C. Viscosity changes with temperature, decreasing as temperatures increase. For example, an oil with 68 cSt viscosity at 40°C will reduce to about 8 cSt at 100°C. Viscosity is the main indication of the capacity of the oil to form a lubricating film. Hydraulic oils tend to be lower viscosity, typically ISO 32 to 68. Gearbox oils have higher viscosity, typically ISO 220 to 460 or more. Overall ISO range is generally from ISO 10 to ISO 1500, but can be higher or lower.

The three inputs that determine oil viscosity selection are load on the lubricated components, speed of relative movement and operating temperature. Correct selection is complex, but typically higher speeds need lower viscosity oils, heavy loads and higher temperature need higher viscosity oils. It is relatively common to have lightly loaded slow running bearings where the standard lubricant can’t form a film and failure occurs.

Automotive applications generally use SAE grade oils, and these viscosities are measured at 100°C. The oils are the same, but additives are for automotive application. An SAE 30 oil is comparable with an ISO 100 oil. We often see multigrade oils in automotive use. These include viscosity modifying additives that will see SAE 10W-40. The 10W, for winter is to give easier cold starting and economy with the oil gradually becoming equivalent to SAE 40 at operating temperature in the engine. Transmission oils are typically higher viscosity. I use 85W-140 in my differentials. SAE “S” and “C” codes are mostly about additive packs, which vary with supplier. All are supposed to comply with the relevant testing that matches the vehicles recommended minimum.

Regardless of type, oil function is to provide a film to separate the rough sliding /rotating surface. In most applications the relative movement of the surfaces is sufficient to pull oil into the gap between the surfaces and general a hydro-dynamic film, if the correct oil has been selected and is kept clean and changed when required. All oil companies will provide guidance on these aspects.

Greases have a ‘soap’ base that acts as a sponge to retain lubricating oil. They are used where oil alone would not remain in place or where oil circulation and sealing would be complex. Typical grease application is a rolling element bearing in an idler roll or in a plummer block housing, common on conveyor pulleys. The grease carries the oil and acts as a seal and carries dirt when purged. The oil is the lubricant.

Greases can also be used for open gear lubrication, typically loaded with many solid lubricants and other things. There are many good products but they are not always universal in application. Then there is ‘blackjack’ which is usually solvent diluted bitumen or similar to give stickiness and loaded with solids and additives. These give wide ranging outcomes in both consumption rates and lubricating ability.

Various grease soaps are available, lithium and lithium complex are the most common, but calcium, sodium, aluminium, and others are available for specific use. Soaps have an ‘equivalent’ to viscosity in the National Lubricating Grease Institute (NLGI) grade. This runs from a very liquid 000 through 00, 0 1, 2, 3 up to 6. This grade is tested at 25°C. Table 1 and 2 give context.

In the resource and other industries, we have standardised on LGEP2 for across site purposes. Lithium Grease with Extreme Pressure additives and NLGI grade 2. No mention of the oil viscosity in the description. In some locations this one grease is used for every application as it is the only one available. Every oil company supplies a version of this grease, and it is a good general-purpose grease. We pump it into bearing across site and never give a second thought to whether it is appropriate for the duty.

LGEP2 is water resistant, but not to salt water. It is relatively stable at temperatures up to 130°C and under shear. I have seen LGEP2 with base oil viscosity of ISO 460, ISO 680, and ISO 1000. I have no doubt there are others outside this range. Is the grease on your site appropriate for those slow heavily loaded overland conveyor pulley bearings, or the trunnion rolls on car dumpers? Are you seeing bearing failures from overload, metal to metal contact, seizure? Perhaps the grease is incorrect? One article that I have read suggests that up to 80 per cent of bearing failures are due to incorrect lubrication. First call should be to a bearing specialist. They have software that will model the application and pass judgement.

We all use many conveyor idlers. When purchasing, apart from attention to sealing, balance, weight and all the good things we can get these days, do we consider all the load conditions and the lubricant grease requirement that would suit them? From the lowest loaded wing roll in the smallest conveyor to the highest loaded centre roll?

The discussion that led to this article was about some large bearings that would be running at slow speed and high load, and likely to be above 60°C operating. Could the bearings work with LGEP2? The answer is yes, provided the oil viscosity was at ISO 1000 grade. Relubrication was expected to be twice a year. Site grease had ISO 460. To use the grease with lower viscosity oil, we could theoretically consider a larger bearing or multiple bearings, but the rolling elements on a large bearing would rotate at a slower speed and there was a limit on available space. We could have installed oil lubricated bearings, but this adds cost and complexity and takes more space. I would keep a special grease gun and a special reminder to relubricate and make sure that the regrease fittings are unique. Maybe a sign that says do not relubricate?

Perhaps two or three greases are needed on site. It’s easy enough to manage, we seem to be able to manage different oils. There are several sizes and types of grease nipple (zerk) available. Designate one for each grease and have separate guns and pumps. Use different size and type of cannisters where automated.

Don’t confuse incorrect selection of lubricant with incorrect use. If oils are not monitored, filtered, and changed when needed they will fail. If oil leaks it can’t lubricate. If there is too much or too little grease in a bearing its life will be reduced. One of the most common issues I see is the use of cheap and useless breathers on gearboxes and hydraulic reservoirs. Even if a good breather filter is fitted, it does not last forever and will not work if buried in spillage.

If grease is dirty when pumped in, or the fitting is dirty, contamination will get in and damage bearings. Grease seals are meant to keep grease in and dirt out, but they can do neither if damaged or buried in dirt. Regreasable systems need real attention to the grease path through the equipment. New grease must get into the bearing and flush old grease out through the seals. I have a concern with plummer blocks that have greased seals. If the bearing is not flushed at the same time as the seal, does dirty grease from the seal get pushed into the bearing?

Oils can be mixed, but not on a random basis. If you must do this, make sure that the two oils are compatible before use. More than likely it will be the additives that are incompatible but beware mixing mineral oils with synthetics and synthetics with synthetics.

Grease is more complicated, as the oils in greases may be compatible but the soaps not or vice versa, or both. There are many conflicting opinions on the internet, so don’t take a chance. American Society for Testing and Materials Standard D6185, Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases provides guidelines for testing for determining grease mixing compatibility. Who provides the equipment used for relubrication on your site? Could there be residue of a different lubricant or be dirty or did they bring the wrong lubricant with them and use it anyway?

Always check out before pumping something new into a bearing or gearbox. What is the potential cost if a pulley bearing or gearbox collapses? It could be more than just financial and production.

The lubrication industry is huge, and the knowledge base includes most circumstances. I have found that OEMs, bearing suppliers and oil companies all have good resources and generally will assist. We also have many specialised organisations that monitor oil, instigate forensic analysis and the like.

Would a little more attention to lubrication and assessment of failed components lead to improved reliability and up time? 

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