Grease Versus Oil

You probably know that oil goes in the car engine and grease is used to pack the wheel bearings. But what is it about these two similar, yet fundamentally different lubricants, that make them ideally suited for certain tasks but not others? Read on to find out.

Grease

Grease is a lubricant that consists of a base oil, a thickener that keeps the grease in a semisolid state, and various performance additives. The base oils are usually either vegetable or mineral oil (petroleum) and the thickeners are soaps, such as lithium, calcium, or aluminum soap. Grease is a pseudo-plastic fluid: it has a high initial viscosity or thickness but as stress is applied, the viscosity decreases, approaching that of the base oil.

Oil

Oil is a substance that is liquid at normal ambient temperatures. A signature characteristic of oil is that it does not mix with water due to differences in molecular polarity but it may mix with other oils and solvents. In addition to lubrication, oil is used for a diverse variety of applications including cosmetics, skin conditioners, fertilizers, fuel, wood finishes, paints, plastics, and medicines.

Types of Grease

There are many types of grease, including ones made from animal fat. For example, white grease is made from inedible hog fat while yellow grease includes the darker parts of the hog. Brown grease is made from beef and mutton fats as well as hog fats. Other animal-based greases include bone grease, fleshing grease, hide grease, and garbage grease.

Greases can also be made from mineral oil, synthetic oils, or a combination of each. Common thickening agents are soaps, inorganic gels, or various organic substances. Additives are used to inhibit oxidation and corrosion, modify viscosity, and reduce wear. Silicone greases are made from synthetic oils, offer varying degrees of water resistance, and function over a relatively wide temperature range.

Types of Oil

The most common types of oils are petroleum-based oils and vegetable oils. Other types include volatile essential oils and synthetic oils. Common petroleum-based oils, aka mineral oils, are made by refining crude oil to to separate out the long-chain oil hydrocarbons from the other hydrocarbons such as those used to produce gasoline and diesel fuel. These oils are used to lubricate auto engines and other machinery. Other petroleum distillates are used for cell culturing, cosmetics, food preparation, industrial cooling systems, and food-safe wood finishes.

Vegetable oils are lipids made from plants. They may or may not be edible depending on the plant source and production method. This category includes castor oil, coconut oil, corn oil, olive oil, palm oil, soybean oil, and peanut oil. Vegetable oils are used for baking, candles, perfumes, skin products, medicines, pet food additives, and various industrial uses.

When to Use Grease

Grease has a higher viscosity or thickness than oil, making it a better choice for applications where liquid lubricants such as oil would not stay in place or where a continuous supply of oil cannot be maintained. These applications include automobile chassis, wheel bearings, and gears. Because grease tends to stay in place, it helps prevent debris from entering, resists washing out when exposed to water, and provides decent rust protection. Grease reduces lubricant loss and lubrication frequency; this is very advantageous in situations where access is difficult such as sealed bearings on heavy equipment.

When to Use Oil

A big advantage of oil is that it can function as both a lubricating agent and a cooling agent. For example, in an automobile engine, the engine oil lubricates the bearings and other components while keeping the engine temperature within an acceptable range for efficient operation. The oil is able to transfer the concentrated heat to a larger volume of oil which is then pumped through heat exchangers for cooling. Oil is commonly used as a cooling agent in electrical transformers.

Because oil flows better than grease, it is also used in situations where it is difficult to apply grease. An example is a door hinge - it's a simple matter to squeeze in a few drops of oil versus removing the hinge pin and spreading grease on it.