GREASE—WHAT IS IT & WHY IS IT USED?

Barry Taylor, ILM Services

Why are Greases Used?

The reason for the use of greases in preference to fluid lubricants is due to the requirement of the lubricant to maintain its original position in a mechanism, especially where opportunities for frequent re-lubrication may be limited or economically unjustified.

This requirement may be due to the physical configuration of the mechanism, the type of motion, the type of sealing, or to the need for the lubricant to perform all or part of any sealing function in the prevention of lubricant loss or the entrance of contaminants.

Because of their essential solid nature, greases do not perform the cooling and cleaning function associated with the use of a fluid lubricant. With these exceptions, greases are expected to accomplish all other functions of fluid lubricants.

A satisfactory grease for a given application is expected to:

1. Provide adequate lubrication to reduce friction and harmful wear of bearing components
2. Protect against corrosion
3. Act as a seal to prevent entry of dirt and water
4. Resist leakage, dripping, or undesirable throw off from the lubricant surfaces
5. Resist objectionable change in structure or consistency with mechanical working (in the bearing) in prolonged service
6. Not stiffen excessively to cause undue resistance to motion in cold weather
7. Have suitable physical characteristics for the method of application
8. Be compatible with elastomer seals and other material of construction in the lubricated portion of the mechanism
9. Tolerate some degree of contamination, such as moisture, without loss of significant characteristics

While this statement is concerned primarily with the use of lubricating greases in automotive equipment, the same considerations apply to the use of grease in most other applications.

What is Grease?

Fundamentally greases consist of two parts, one being the lubricating base oil and the other being the thickener, sometimes referred to as the carrier.

Thickener

Basically the thickener is a soap. Some thickeners are made with metallic compounds such as aluminium, others are made with lithium, lithium complex, barium, clay, etc. Some of the earliest thickeners were even made with calcium.

The job of the thickener is to carry and hold the lubricating base oil in place, allowing the oil to perform under extreme conditions for prolonged periods of time.

In some cases thickeners will vary to suit particular applications, these variances relate to the compounds used in the thickener, or to their NLGI rating when addressing the lubrication requirements of such mechanisms as rolling elements (bearings), pin and bush type (earth moving equipment), electric motors, labyrinth sealing, open gears, etc.

It is very important to use the right thickener as some products can have adverse effects on equipment and may not be compatible with other products. The quality of the thickener will also reflect its ability to hold the base oil and to resist bleed off. Bleed off occurs when the base oil separates from the thickener (not good), often seen as oil settling on the top of grease within the drum, or as a hard soapy residue left within inadequately serviced bearings.

Some thickeners appear tacky, this tackiness comes from a product (tacifier) that is mixed into the grease when manufactured. Tacifiers are used to assist in the stability of the grease, holding it in position more efficiently.

Tackier greases are generally used in extreme applications such as earthmoving equipment, wet environments, labyrinth seals or mechanisms in which frequent lubrication may not be possible.

Latter technologies have seen thickeners developed that hold the base oils extremely well under adverse conditions, some thickener compounds actually act as a lubricant within their own right, such as Calcium Sulphonate.

The most common thickener used in the mining industry today is generally Lithium Complex as it superseded straight Lithium sometime ago. These thickeners are used due to their ability to work extremely well in a wide variety of grease applications, even where temperatures range from -0 to +200 degrees. The quality of the thickener will always affect the performance of the grease.

Examples of some Common Thickeners

Lithium Complex, Calcium Sulphonate, Calcium Complex, Calcium, Aluminium Complex, Lithium, Clay, Sodium, Barium, Polyurea.

NLGI Rating

The NLGI (National Lubricating Grease Institute) rating generally refers to the consistency or the pump-ability of the thickener.

NLGI ratings vary from 000 being a liquid type consistency to 6.000 which is almost a solid. The most common greases found in the mining industry that are used on standard types of applications are generally NLGI 1 or 2.

The NLGI rating of any thickener is very important when addressing the lubrication requirements of plant or equipment. It may vary due to the application method or to the requirements of the internal mechanism of the equipment. It is recommended that you seek advice when addressing the lubrication requirements of any plant or equipment.

Base Oils

The other component that makes up grease is the base oil. The base oil is encapsulated within the thickener and varies dependent on the required application i.e. ISO VG 100, 150, 220, 320, 680, etc.

The higher the number the higher the viscosity or thickness of the oil. Low viscosity oils are generally used in greases designed for high speed applications and reduce the chance of ball skid in rolling element bearings.

Higher or heavier viscosity base oils are generally used in high load slow moving applications, like earthmoving equipment or heavy sliding and shearing environments.

Most common greases found in the market place generally use base oils that vary between ISO VG 100 to ISO VG 270, and whilst these products service well for a wide range of applications they may not necessarily suit equipment such as earthmoving or pin and bush types of applications where a heavier base oil is required to create a more durable boundary lubricant.

Additives

Other than the anti wear additives in the base oils, some greases are manufactured with compounds such as Molybdenum Disulphide (Moly) and PTFE (Polytetrafluorethylene), more commonly known as Teflon.

Compounds such as these are used to assist in reducing the surface tension within the mechanism by filling the asperities within both apposing surfaces creating a residual lubrication film that is far more durable than oil alone.

Whilst these compounds are classed as solids they do have a very low co efficiency of friction (very slippery), and in some cases can dramatically reduce friction and wear.

Most manufacturers of greases when recommending products for earthmoving or heavy slow moving types of applications would recommend greases with either heavier base oils or products that contain additives such as Moly or PTFE’s. The benefits of using such additives in particular applications are well documented and in some cases manufacturers of equipment such as Caterpillar even recommend the use of greases that contain these additives.

Some other advantages of using additives such as Moly or PTFE is that the grease manufacturers can use lighter base oils in heavy grease applications. This becomes a financial benefit as the manufacturers can streamline the number of different greases they need to make by merely adding these additives to the product when addressing heavy application.

Application

The application of the correct grease is very important, and the methods and systems used should never dictate to the type of grease being applied.

For example, if your plant or equipment requires grease with heavier base oils or a product that contains an additive such as Moly or PTFE, then that is the product that should be used. It is very important to understand that the lubrication requirements of any plant or equipment should never be compromised by the application method. Simply put, if the system cannot pump or apply the right grease, then do not use that system, as the consequences of premature plant failure could become very costly.

Over the years ILM has seen many systems installed to plant and equipment which, whilst very efficient in delivering grease, struggled to deliver the correct grease. ILM would recommend any plant owners being introduced to new lubrication systems, first ask the salesman if his system can apply the right product for your equipment. If he cannot tell you what type of grease your plant requires, or recommends you use his grease in his system, be wary.

Whilst ILM apposes the use of some automated lubrication systems on particular equipment, our experience with them is well known, as to-date ILM has installed almost 90% of all lubrication systems within the three Brown Coal Mines in Latrobe Valley, and in some cases has had to personally design systems so as to ensure that they can deliver the correct grease.

Whilst these products are possibly some of the best on today’s market for these types of applications, some of them are not suitable to be applied through standard type automated lubrication systems, as they either contain high levels of Moly or have heavy thickeners equal to or higher than NLGI 2.

It would be up to the supplier of the lubrication system to ensure that it can appropriately deliver the greases. If they are unable to do that, or claim that the types of greases are detrimental to their equipment, ILM would suggest you do not use that suppliers system.

If any CMPA member requires assistance with lubrication specifications, to either streamline or improve their plants efficiency, or would just like to ask a question about grease in general, they are more than welcome to contact Barry at ILM on 0413 881 890 for free advice. Phone calls are cheap, correct information is priceless.

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