The basics of grease
Grease consists of three parts: a base oil, a thickener, and an additive.
The official definition provided by ASTM (American Society for Testing and Materials) defines a grease as being a “solid to semifluid product of dispersion of a thickening agent in a liquid lubricant”. Other ingredients conferring special properties (additives) may be included. The purpose of grease is for it to keep in contact with moving surfaces and to lubricate them without risk of leakage due to gravity. So ordinary grease and oil are not interchangeable. The advantages of a grease are a reduced frequency of application, it acts as a sealant against contaminants, it is preferable under extreme conditions and solid additives can be used (non-soluble such as graphite) unlike oils.
The base oil of a grease (the fluid) is a mineral oil in the majority of greases produced. On the other hand, under certain extreme conditions of temperature and pressure, a synthetic base oil may be preferable.
The Thickener is a soap-based or metallic soap product that is the result of a reaction between a metal hydroxide and a fatty acid. Under a microscope, the thickener looks like fibers that sneak through the base and additive. In other words, the thickener acts as a sponge and holds the components of the grease together. The thickener can be considered complex if more than one fat is used in the recipe which aims to improve the properties of the grease. Finally, the thickener may also be of organic and/or inorganic (non-soap based) nature, for example: polyurea, petrolatum, and bentonite.
The additive can represent up to 10 to 15% of the formulated grease. Additives play many important roles in a lubricating grease. These particularly include the improvement of existing desirable properties, the removal of existing undesirable properties and the formation of new properties. The most popular additives are intended to act as oxidation and rust inhibitors, anti-wear agents, agents against extreme pressure, and waterproofing.
Important characteristics of grease
The dropping point is the temperature at which a grease becomes fluid enough to drip. The dropping point indicates the upper limit of temperature at which a grease retains its structure, not the maximum operating temperature of the grease. Under normal conditions, a grease should be used at a maximum of 40 degrees Celsius below its dropping point.
Pumpability is the ability of grease to be pumped through a system. In other words, pumpability is the ease with which grease under pressure can flow through lines, nozzles and connections of system.
Water resistance is the ability of a grease to resist the effects of water without impairing its primary function of lubricating.
The consistency of “the grease depends on the type and amount of thickener used and the viscosity of the base oil. The consistency of a grease is its resistance to deformation by an applied force. The measure of consistency is called penetration. Penetration depends on whether the consistency has been altered by handling or working” [1]. ASTM Methods D217 and D1403 measure penetration by leaving a cone of a given weight in the grease for five seconds at a standard temperature of 25°C (77°F). “The depth, in tenths of a millimeter, at which the cone sinks into the grease is defined as penetration” [1]. The following table shows the classification of fats according to their level of penetration:
Oxidation stability is the ability of a grease to resist a chemical reaction with oxygen. The reaction of grease with oxygen produces deposits that result in slow operation, increased wear and reduced clearance between parts. Prolonged exposure to high temperatures promotes this chemical reaction in greases.
High temperatures are more harmful to greases than oils. Grease, by its nature, can not dissipate heat by convection like a flowing oil. As a result, without the possibility of heat transfer, excessive temperatures cause accelerated oxidation or even carbonization as the grease hardens or crusts. It is therefore important to look at the technical data of greases such as the flash point, boiling point, operating temperature, and drop point.
Similarly, low temperatures can affect the apparent viscosity of greases. Pumpability suffers and machine operation may become impossible due to power requirements.
Apparent viscosity is a characteristic of non-Newtonian fluids. That is, the viscosity is dependent on the shear rate. In the field of lubrication, the most common unit for grease viscosity is the centistoke (cSt). Simply put, the higher the viscosity of a fluid, the more the fluid resists movement.
Practical tables:
Grease is the key element to your lubrication system.
The choice of grease type is particularly important for the proper function of the system and will have direct impact on its efficiency.
Characteristics
After analyzing the working conditions of your automatic greasing system, here are the recommendations concerning the choice of the grease to use.
Mixing different types of grease may damage your system.
Before changing your grease, make sure the new grease is compatible with the current grease.
Procedures to bleed your system
In order to avoid mixing greases, it is strongly advised to bleed your system. The Alltech Group proposes you a grease replacement procedures in order to avoid damaging your system.