A grease gun is a special tool used to inject lubricating grease, under pressure, into motors or other moving parts of machines. Grease guns, like the caulk guns they resemble, usually operate with a disposable grease cartridge, although some grease guns are filled manually. The different types of grease guns are characterized by the source of energy for creating the pressure that forces the grease out of the gun and into the motor being lubricated. Some popular types of grease guns are trigger-operated, lever-operated, pneumatic, and battery.
There are three main components of a grease gun — a metal cylinder that contains the grease itself; the propulsion control, which is either a trigger or a lever; and the grease delivery device, either a flexible hose or a rigid metal tube, both of which are fitted with a special nozzle called a "coupler." The grease cylinder is usually from 15 to 18 inches (38 – 46 cm) long, and the delivery device, whether rigid tube or flexible hose, can be anywhere from 12 to 36 inches (30.5 – 91.5 cm) long. An electric grease gun will include a regular power cord, and a pneumatic grease gun will have an attachment for an air hose. A type of grease gun that has gained in popularity is the cordless grease gun, which incorporates a battery pack that can be as large as the grease cylinder itself.
A grease gun is designed to handle grease that's often very thick, sometimes approaching the consistency of fudge. It must transmit that grease only to those parts of an engine that require such a heavy lubricant, without contaminating other parts of the engine and without permitting the grease itself to become contaminated. This is accomplished by the installation of a special fitting, variously called a grease fitting, a grease nipple, or a Zerk fitting. The fitting is installed in the motor's casing and has a spring-loaded protective cover that prevents contamination from entering and grease from escaping.
Correct operation of a grease gun requires both proper training of the operator and proper calibration of the tool itself. The operator needs to know the amount of grease required by the part being lubricated, and the amount of grease that will be discharged with every pump of the trigger or lever, because overlubrication can result in damage. The operator presses the nozzle of the grease delivery tube or hose against the Zerk fitting with enough pressure to open the spring-loaded cover and holds it in place while pumping the trigger or lever as many times as necessary to discharge to appropriate amount of grease. When done, the coupling is removed from the fitting, the cap snaps back in place, and no further attention is necessary.