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Grease Primer


MotoTribology

Grease is a tool used so universally around the world but remains somewhat of a mystery for many of the people who use it. Every motorcycle needs grease at some point and there are several different areas where it is applied. Although grease manufacturing is a somewhat complex process it is somewhat seen as an art and each company’s methods and exact formulas are different. This results in sometimes subtle but sometimes drastic differences in products.

The basic formula for grease is this: base oil, thickener and additives.

  • Base oils can be anything from petroleum to synthetic to plant based oils.
  • There are several different commonly used thickeners. The most common types are lithium, aluminum and calcium sulfonate.
  • The additives used are dependent on the type of grease and the purpose of the grease.

Base Oil:

The base oil composition of a grease will impart a few crucial properties:

Grease-Base-Oil.png?w=404

Table 1: Basic base oil comparison

The reason synthetics are less versatile than non-synthetic base oils is because of synthetic types likes silicone and poly-alyklene glycol (PAG). These types of oil are usually only meant for very specific industrial applications and are unsuitable in many others, so care is needed when selecting grease to avoid these types of synthetics in many instances. With that same reason in mind, additive selection is also more limited with these alternative synthetic options. However, plant based oils are still generally less versatile due to the temperature constraints they are typically limited by to.

Thickener:

Different thickener types have different performance attributes distinct to each type. Here are the most common types of base grease thickeners used for multipurpose motorcycle greases.

Lithium offers a good water resistance, heat tolerance and mechanical stability. It is currently the least expensive type of grease to make so it is very prevalent in the marketplace. The drawbacks of it compared to other types are that it is not completely waterproof and will accept moisture over time.

Aluminum is practically waterproof but is more expensive to manufacture than lithium grease. It offers high temperature stability, but is slightly less mechanically stable than other types.

Calcium sulfonate has excellent high temperature, low temperature and inherent properties that allow it to use fewer additives to obtain certain performance levels. Its water resistance is excellent and it is often compatible with other greases. The big drawback to calcium sulfonate is the price. It is typically much more expensive than either of the other two types to produce.

A grease complex is a variation of the standard base grease that is possible to make with lithium and aluminum. Aluminum and lithium complex greases exhibit higher temperature limits and better mechanical stability than their uncomplexed counterparts.

All three of the grease types listed are often compatible with one another up to around 25% contamination with one type and 75% of the other. Beyond that 3:1 ratio though, incompatibilities are more common and certain properties may be sacrificed if mixed.

There are many other types of thickener types I haven’t mentioned but those are rarely, if ever, used for the types of greases commonly used for motorcycle.

Additives:

Common additives for greases include: anti-oxidation, anti-corrosion, anti-wear and extreme pressure(EP) additives. Additional types are certainly used, but those are going to be found in a lot of greases with perhaps the exception of extreme pressure additives if the grease is not labeled as EP grease.

One last fairly universal additive is dye. Most greases are dyed some color and many people believe these colors mean something. Let me be absolutely clear here so there is no confusion; THE COLOR DOES NOT MATTER. The colors are arbitrary and chosen by the manufacturer for aesthetics and nothing else. They may have their own standards and reasons for why they color certain grease a certain way, but it is not to conform to any industry standard.

Grease Applications & Properties:

Grease has some advantages over oil in certain applications. It can be applied in open areas without a sump or reservoir. It forms a significantly stronger physical barrier on a surface making it more suitable in extreme applications. It can utilize solid lubricants more effectively than liquids can.

There are basically two types of greases commonly used in most motorcycles. They are assembly grease used during engine building or repairs and multipurpose greases for everything else. Multipurpose greases are usually good for bearings, axles, pivots and really any grease point on a bike. Assembly lubricants usually contain a high level of solid lubricants and provide lubrication to machine internals that are normally lubricated by oil or special applications that require a high content of solid additives.

The purpose of assembly lubricants is to provide lubrication on parts that have never been exposed to engine or gear oil yet, so when the bike is started for the first time after maintenance; those parts have some protection before the regular lubricant begins circulating. These assembly greases are usually washed away by the oil and are removed from the system during subsequent oil changes. Another application for these products are areas such as final drives where a high content of solid additives can be beneficial for surface protection.

Most grease points on motorcycles are fairly low load compared to more extreme grease applications in commercial applications. This means specialized grease is rarely needed and a single multipurpose grease is usually able to serve all of those grease points. They go into places that are open and exposed, high load or in places that oil films cannot be maintained. Bearings, axles and chassis linkages are common applications for these greases. They generally will provide extreme pressure protection and decent anti-wear protection. Because they form a physical barrier against water and oxygen, corrosion protection is inherently high, but this is also often boosted further by additives to protect against rust and corrosion. They should maintain a physical barrier to keep out moisture and dirt from these applications that would self destruct very quickly if contaminated. Grease does all of this through both physical and chemical means and there are a few key points to consider when choosing the right grease for your application.

First and foremost is the grease consistency or hardness. This property for grease is just as important as the viscosity is for oil. Using an incorrect grease consistency can quickly result in part failure and under-lubrication.

Grease is categorized into different grades by the National Lubricating Grease Institute (NLGI) scale based on a grease’s penetration test result. Grease penetration is a measurement of the depth at which a calibrated metal cone will penetrate into the surface of grease when dropped form a standard height. Penetration is represented in decimillimeters (tenths of a millimeter), and the penetration is often taken under two different conditions: worked and unworked.

An unworked grease is fresh from the container and has never been used. A worked grease is one that has been put through mechanical stress to simulate usage. The purpose is to indicate the stability of the grease with regards to its consistency. Working grease is a standard process that involves a piston churning the grease a standard number of times using an instrument known as a grease worker. The standard method uses a plunger with 60 holes in it and it is pushed a pulled a total of 60 times in 1 minute.

Grease-worker-v2.png?w=455

Figure 1: Mechanical Grease Worker (please imagine there are 60 holes in the piston face)

After that minute, the grease is considered worked and can now be tested for NLGI consistency. The grades identify significant differences in the hardness or softness of greases. The simplest way I find to describe them is to compare them to common foods.

NLGI-Chart-v2.png?w=390

Table 2: NLGI grades and consistencies.

Another important property of grease is the base oil viscosity. During the manufacture of base grease, the ingredients of the thickener are mixed with oil. When the grease reaction takes place, that oil becomes part of that grease. Typically the higher viscosity the oil, the more heavy duty application it can withstand. However higher viscosity base oils usually limit the low temperature performance, so for general purpose grease, a base oil blend balanced for moderately high and low temperature performance is preferable.

Assembly grease typically contains a high level of solid lubricant meant to withstand high pressure and remain in place in the absence of the regular lubricant that would normally protect the surface. The reason this regular lubricant needs replacing is usually because the machine is rebuilt and hasn’t had the oil circulation system running yet. These greases don’t need to have a very long usable lifespan since they are designed to be used up fairly quickly, washed away by the oil and removed by either a filter or through the next oil change. Therefore, anti-oxidation and long term stability are not key features for assembly lubricants. However, another application for assembly greases comes from the typical high level of solid lubricants. Since these solid lubricants will resist extreme loads there are applications in some bikes that call for a grease like this such as final drive shaft gears.

Grease application is an aspect that a lot of people have difficulty with as well. I often see comments implying to just pump in as much grease as a bearing can hold and that is how much it should use. That advice is almost universally bad. Over-packing a bearing can lead to some very bad failures. Alternatively too little grease is also a problem for more obvious reasons; under-lubrication and all that goes with it being the biggest of them. You can read about the pitfalls of these mistakes and how to avoid them here.

So I hope that gives you a good basic starting point to look at greases and you are now armed with the knowledge to at least ask the right questions when trying to choose between different brands of grease.

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