Grease – Filling the Void


Last January, an engineer related a story about a piece of equipment with brass bushings that required grease. The maintenance people were not happy about the mess that the grease made, so they decided to stop greasing the machine.

That corrected the housekeeping issue, but it wasnt long before they sought out the engineer to show him gold flecks all over the machine. When they confessed what they had done, he told them that they would now have to fill the void with grease because the bushing clearances had increased from 20 microns to 150 microns. Nothing but more grease would fill the void. Hence, the title of this article.

Throughout my territory, I have witnessed such a lack of basic grease knowledge at customer sites that equipment reliability is sometimes compromised. So, its good to review the basics to fill the knowledge void about grease.

Three Cs of Grease

The single most important characteristic of grease is viscosity. Base oil viscosity provides lubrication for an application. Base oil that is too heavy can increase heat, reduce power efficiency and decrease equipment life. Alternatively, base oil that is too light may not be able to carry the load, leading to premature wear and possibly failure. With this mind, three key factors to consider when selecting grease are composition, consistency and compatibility.

Composition: While there are many definitions of grease, the sponge analogy remains a good one. Grease is like a sponge filled with water where the sponge is the thickener, and the water is the oil formulated with performance additives. A more technical definition for grease is a lubricant consisting of a fluid containing property-enhancing additives that has been thickened to the required consistency.

Thickeners are more than the sponge for the lubricating oil. They also impart certain characteristics such as pumpability, operating temperature range, load carrying ability, shear stability, water resistance and dropping point. As examples, calcium and aluminum offer excellent water resistance. Disadvantages of aluminum thickeners include poor mechanical stability and poor compatibility that can lead to increased grease consumption and the inability to convert to another thickener type without a great deal of difficulty. Lithium formulated with calcium improves water resistance, offers better shear stability and a wider operating temperature range.

Clay greases are chemically inert, forming effective barriers against process chemicals. Clay thickened greases are not very compatible with other types of greases. Additionally, these greases are more prone to oil separation in central systems when under constant pressure.

Consistency: Consistency refers to grease stiffness, which can range from very fluid (like oil) to almost solid (like a waxy brick). Consistency depends on the ratio of thickener to oil. More thickener creates stiffer grease; less thickener makes softer, more fluid grease. The correct consistency is critical for the grease to stay in the equipment and allow the oil to lubricate.

The National Lubricating Grease Institute classifications range from triple zero (000) to 6. NLGI numbers are useful to determine if the product can be expected to behave as a solid body at room temperature or if it will flow. Consistency is not an indication of performance characteristics beyond stiffness.

Greases with a consistency of NLGI 0 or softer are fluid at room temperature and are not likely to exhibit good sealing properties. NLGI 5 or stiffer greases primarily exhibit properties of a solid when at rest. A high NLGI number is no guarantee of excellent sealing, shear stability or load carrying capacity.

Compatibility: To avoid problems, review the type of thickeners in use around the plant. Store incompatible thickeners separately. Use dedicated grease guns for manual greasing, and clearly identify their specific use.

Storage & Handling

Sealed drums of grease are still capable of breathing in moisture. Best practices for storage and handling include storing drums under cover in a dry atmosphere. Grease should never be stored outdoors.

Grease naturally bleeds oil. The oil lubricates the application; therefore oil bleed is desirable. Store tubes of grease standing upright with the caps on top. If grease bleeds during storage, oil puddles on the shelves are a nuisance. Bleed rate can be controlled through proper storage and usage.

Consider implementing a color coding system for each lube family that includes different symbols for every ISO viscosity grade. Tag equipment clearly as to which lubricants to use. Label totes and tanks to avoid contamination. Labeling is especially important for specialty greases like those used in electrical motors and high-speed couplings.

In many plants, the electrical department is responsible for greasing electric motors, and the grease is stored in a separate place. This practice ensures motors are not contaminated with the wrong grease. Coupling grease presents a similar challenge and should be clearly identified and segregated.

Proper Greasing Techniques

A discussion of central grease systems and automatic lubricators is beyond the scope this article, though a few tips are included for consideration.

Manual grease guns should be calibrated to ensure the proper amount of grease is added to the component being lubricated. As an illustration, a plant manager once noticed that bearings in critical equipment were failing prematurely. He reviewed how to properly grease the bearings with his technicians. Then he observed what they were actually doing. He noticed one pumping the bearing full of grease while talking on the phone. He approached and asked, How many shots of grease did you pump into that bearing? When the technician could not answer, he was told to clean out the bearing and start again.

The plant manager went one step further. He had the technicians meet him with their grease guns. He supplied a scale and cups and told them to put 10 shots of grease into a cup. When they weighed the grease in each cup, the measurements were different. They could visually see the difference and understand the importance of paying attention. They do a much better job lubricating machinery with grease now.

Central grease systems may operate differently than designed if grease thickener types are not considered. The best approach to ensure equipment protection and pumpability is to consult the grease supplier and the automatic equipment supplier. Dont compromise equipment reliability to compensate for an inadequate central grease system.

At one steel mill, the central system had over 90 meters of piping from the bulk grease tank to the various lube points on the line. The plant located the bulk grease tank indoors, which helped protect against temperature swings. The pipe diameter was only 25 millimeters (1 inch), which was less than optimum. As a result, grease selection was severely limited to a product that would pump through that small diameter for that distance in winter. A 40-mm (1.5-inch) diameter pipe would have increased the number of grease choices available and extended bearing life.

When installing new systems, locate totes or bulk tanks indoors, and consider using a minimum 40-mm pipe if pumping lengths exceed 60 meters because grease pumpability is dramatically reduced in winter.

Single-Point Lubrication

Single-point lubricators (SPLs) are often a cost-effective addition to a grease program. Electromechanical single-point lubricators are widely used and provide optimal lubrication of greased machinery components such as rolling element bearings. They are a reliable alternative to manual lubrication, are superior to gas or chemical operated lubricators, and are the best solution where central systems are not practical. SPLs are widely used to lubricate pumps, electric motors, conveyors, fans and blowers.

Electromechanical drive units typically last 3 to 5 years. They offer variable settings so grease cartridges last up to a year, providing precise injections of small quantities of grease over time. Safety can also be improved by remote mounting SPLs away from dangerous operating conditions.

Some SPLs operate with enough pressure to be mounted 30 meters away. This feature makes it convenient to locate grease points at a lower height, eliminating the need for safety harnesses. Grease points can also be shielded from heat sources or moved away from rotating equipment.

SPLs eliminate the manual greasing issues of over or under lubrication. The units alleviate lack of labor resources and allow personnel to focus on more urgent maintenance tasks. They are an effective seal against the introduction of contaminants as well.

Common Applications

Three applications account for approximately 80 percent of industrial grease used: general purpose, electric motor and coupling greases.

General Purpose Grease – Most plants have one multipurpose grease used almost everywhere in the facility. Lithium thickeners are commonly selected as multipurpose grease for their compatibility with a wide range of materials and other lubricants. While not every plant can get by with a single product, many can use a lithium complex synthetic grease with an ISO 220 polyalphaolefin base oil and an NLGI 2 consistency.

Can this be recommended for everyone? Certainly not, but if a one grease solution sounds appealing, consider a full synthetic base oil. Greases formulated with synthetic oil cost a little more per kilogram but offer a wider operating temperature range, reduce friction on start-up and reduce inventory. One grease also eliminates the possibility of cross-contamination.

Electric Motor Grease – The right viscosity is important for motors. Grease formulated with an ISO grade base oil higher than the OEM recommendation will consume excessive energy and elevate bearing temperatures. Grease life is reduced by half for every 15 degrees a motor operates above 70 degrees C.

I have seen motors greased with base oil viscosities as much as six times higher than the OEM specification, plus the grease contained extreme pressure additives. The plant did not realize that their desire to consolidate to one grease negatively impacted equipment performance.

If multiple greases are used in electric motors around the facility, compatibility is of concern. If lithium grease is filled on top of urea grease, the greases may soften and lose shear stability. This largely depends on the type of urea thickener used. Check with the lubricant supplier to verify compatibility. Some customers will tape a new cartridge of grease to a motor going to their rebuilder. They require the used cartridge to be taped to the motor when it is returned. This practice can be effective when the rebuilder carries many greases to satisfy multiple industrial motor specifications.

Over greasing and under greasing are two common causes of premature motor failure. Single point lubricators are an effective way to deliver OEM specified quantities and frequencies to remove the guesswork from electric motors lubrication.

High-Speed Coupling Grease – Greases not formulated for high-speed couplings cannot resist separation when subjected to high centrifugal forces. Under high-speed rotation, the base oil and additives can separate from the thickener, resulting in leakage. Thickener then accumulates in the gear teeth, and this build-up can cause premature bearing failures. Thus, an essential property of coupling grease is the ability to resist centrifugal separation, which requires a special blend of chemistry.

High-speed coupling grease is formulated with a high-viscosity base oil and tackifier to keep the grease in place and prevent separation in the ASTM D 4425, High Speed Centrifugal Test. This test develops g-forces in excess of 36,000 at 15,000 rpm.

In one plant, lube technicians used coupling grease for general plant maintenance. The extremely tacky grease, formulated with ISO 3200 base oil, was used in applications requiring a grease with an ISO 220 base oil. To prevent problems in the future, this customer chose to lock up the high-speed coupling grease, granting sole access to the lube techs responsible for couplings.

The proper selection and application of greases can improve reliability and extend equipment life. OEM manuals are a good resource for determining equipment needs. If the manual is out of date or equipment modifications have been made, consult a reputable lubrication engineer, often accessible through the local lubricant supplier.