A grease gun is a simple yet effective tool designed to apply lubricant, usually grease, to moving parts of machinery. It’s essential for reducing friction and wear in bearings, joints, and other components. Grease guns come in various types: manual (lever or pistol grip), pneumatic, and electric, but their fundamental working principle remains the same.

### Basic Components and Working Principle:
1. **Grease Reservoir:** Grease is stored in a cylindrical cartridge or directly loaded into a chamber within the gun. The grease must be kept pressurized and ready for delivery.

2. **Delivery Mechanism:** Grease guns use different delivery mechanisms to push grease from the reservoir into the hose and through the nozzle, where it's applied to the desired location. Here's how each type operates:

   - **Lever-operated grease gun:** The user manually operates a lever to create pressure. As the lever is pulled and released, it activates a piston within the gun. When pulled, the piston retracts, drawing grease from the reservoir. Releasing the lever pushes the piston forward, forcing grease through the nozzle.
   
   - **Pistol grip grease gun:** This design allows for one-handed operation. Squeezing the trigger activates a Air-Operated Grease Guns spring-loaded piston mechanism. As the trigger is squeezed, the piston is pushed forward, delivering grease to the nozzle.

   - **Pneumatic grease gun:** This type uses compressed air to pressurize the grease. A trigger controls the airflow, which moves a piston or diaphragm to push grease from the reservoir through the nozzle. Pneumatic grease guns are ideal for heavy-duty or continuous lubrication tasks.

   - **Electric grease gun:** These use a motor-driven mechanism to force grease out of the nozzle. The motor powers a piston or gear system that pressurizes the grease and delivers it with consistent force. This type is commonly used in industrial or large-scale applications where constant lubrication is necessary.

3. **Grease Hose and Nozzle:** Once pressurized, grease flows through a hose attached to the main body of the gun. The hose leads to a nozzle (usually fitted with a coupler). The most common coupler is a "zerk fitting," which connects securely to a corresponding grease fitting on the machinery, allowing grease to flow into the intended component without leaking.

### Pressure Relief and Safety:
A pressure-relief valve or vent is typically included to prevent over-pressurization of the system, which could damage the tool or the machinery being lubricated.

grease guns rely on a mechanism (lever, trigger, air, or electric motor) to pressurize grease and deliver it through a hose and nozzle to the parts needing lubrication. This ensures precise application and prevents waste or contamination.