Choosing the Right Lubricant

Pumps are becoming more expensive to produce and maintain, making it more costly and inefficient to replace parts when things go wrong. With some pumps costing thousands of dollars-not including associated downtime costs-engineers need to ensure their pump systems are properly maintained. Choosing a suitable lubricant has become increasingly important as one of the least expensive and most reliable ways of helping to maintain parts as long as possible. In particular, hydraulic systems rely on consistent pump performance. When anything relating to the hydraulic system is modified-temperature, location, pressure, oil thickness, or internal surface roughness-lubricant performance can be affected. Increased pressure and temperature can drastically affect the lubricant viscosity, and changes to viscosity can affect the way the pump operates. Altering viscosity can also change the hydraulic system's efficiency. It is important that plant engineers work with their lubricant provider to make calculations about what will happen to the lubricant when modifications are made. What factors should engineers consider when choosing a lubricant? Engineers must consider the lubricant part of the entire hydraulic system as well as the needs in the application. Is fire resistance needed in the event of a high-pressure hose leak so fluid will not propagate a flame? Is there the potential for contact with food such that a food-grade product would be required? Would there be potential damage to the environment in the event of a hose rupturing? The lubricant's properties, as well as the specific application, must be considered when choosing a pump lubricant.

Consider the Chemical Make-Up Versus the Application

Lubricants are made of base fluids and additives-both of which play vital roles in determining how the lubricant will perform in service. The base fluids commonly used include mineral oils, water-based fluids or phosphate esters for fire resistance, biodegradable fluids-such as rape seed and synthetic esters for environmentally adapted performance-or specially selected hydrocarbons (synthetic or mineral) offering food-grade compliance. Additive systems are chosen to offer prolonged and consistent performance and equipment protection. For example, some oils contain an additive system incorporating the zinc-based anti-wear additive called ZnDTP and other additives to provide pump protection and oil service life over a wide temperature range. Hydraulic fluids for use in environmentally sensitive areas, or as part of a companies' commitment to ISO 14001 compliance, require not only biodegradable but also non-eco-toxic additive and base fluid combinations for minimal impact to the environment in the event of a spill. For applications where there is the possibility of incidental food contact, only selected base oils and additives that are NSF registered are permissible.

Watch for Contamination

The risk of contamination in the field (for example, water, air and dust) and how it can affect the lubricant are important factors. Water from the atmosphere or moisture from the process can contaminate lubricants, so it is important to test the water effects on the product (a test known as hydrolytic stability). Water can also have a big effect on oil filterability, and a range of "filterability" tests exist to reveal contamination. In addition to contamination from water or dust, some operators can accidentally pour the wrong fluid into the lubricant section-engine oil, transmission fluid or even chemicals. Dirt and other contamination in lubricants can be a problem when running pumps outdoors, and extra care should be taken to keep systems clean. Specific pump systems can be affected more than others by lubrication contamination. Anything involving fine tolerances can be drastically affected by contamination of the lubricant. For example, any contamination of the hydraulic fluid in the pistons of an aircraft simulator can result in jerkiness, making it difficult for the user to imagine that he or she is in an aircraft.

Design the Ideal System with the Lubricant in Mind

How a lubricant acts can vary widely depending on application, location, temperature and pressure. To improve pump reliability over a longer period, the choice of lubricant should be considered at the design stage. Working with lubricant manufacturers at the design stage helps parts and lubricants work longer and more effectively. Outdoor pumps need special attention because of fluctuating temperatures and atmosphere. It is easy to test pumps indoors, because of the easily regulated parameters; however, pumps used in applications outdoors may experience extreme temperatures and conditions. For example, in an open cast mining application in Siberia, it takes six weeks for supplies to be delivered, so the engineers had to take care that the lubricant chosen was correct and would require little attention in use. Previously, engineers at this site used two grades of lubricant-for summer and winter-to cope with the extreme temperature ranges. However, contamination could occur when the lubricants were changed, downtime was necessary to swap the grades and the operators might accidentally use the wrong lubricant at the wrong time. Further, two lubricants had to be sent down the lengthy supply chain to Siberia. The answer was a new lubricant that could be used year round. The specified lubricant had to pour at minus 30-deg C but also have specified viscosities at minus 50-deg C and 80-deg C due to cold start and hot operation. The lubricant had to cover all these temperatures, as well as meet other requirements.

Match Lubricants to Demands

Pump engineers want systems to be energy efficient and cost effective within an ever-smaller footprint. They need to be aware of environmental standards regarding the pollution of water by pump lubricants. Engineers should stay informed of all the lubricant changes as they continue to evolve. Lubricants are designed to match engineers' requirements for much longer-lasting fluids. Lubricants cannot be a single-feature hydraulic fluid-an Olympic sprinter specializing in only one performance feature. They have to be decathletes. Falling down on one property could be a disaster. Engineers should consider any features necessary for their application to ensure that the lubricant meets all the requirements. Pumps & Systems, March 2008