Hydraulic cylinders and pneumatic cylinders differ mainly in working medium, working principle, applicable occasions and performance characteristics. The following are their main differences:

1. Working medium
Hydraulic cylinder: The working medium is liquid (usually hydraulic oil), which is incompressible and can withstand higher pressure.
Pneumatic cylinder: The working medium is gas (usually compressed air), which is compressible and suitable for low-pressure and high-speed applications.

2. Working principle
Hydraulic cylinder: The driving force is generated by the pressure of the liquid, and the liquid pushes the piston to move inside the cylinder.
Pneumatic cylinder: The piston is pushed by the pressure of compressed air, and the compressibility of the gas causes the thrust to be relatively small.

3. Pressure to bear
Hydraulic cylinder: It can withstand higher working pressure. Usually, the working pressure of the hydraulic system can reach tens of MPa (megapascals), which enables the hydraulic cylinder to provide greater thrust.
Pneumatic cylinder: The working pressure is low, usually not exceeding 1~1.5 MPa, suitable for occasions that require less thrust or have low pressure requirements.

4. Thrust size
Hydraulic cylinder: Due to the incompressibility of liquid, hydraulic cylinder can generate very large thrust, which is suitable for occasions requiring high force, such as mechanical equipment, construction machinery, etc.
Pneumatic cylinder: Since gas is compressible, the thrust generated by pneumatic cylinder is relatively small, which is suitable for light load and high speed working environment.

5. Applications
Hydraulic cylinders: widely used in heavy machinery, engineering equipment (such as excavators, cranes), industrial automation and other occasions that require large torque and high pressure.
Pneumatic cylinders: usually used in light load, high speed, accurate control, such as automated production lines, pneumatic equipment, tool clamping systems, etc.

6. Structure and maintenance
Hydraulic cylinders: The structure is relatively complex, requiring well-sealed oil seals, and the hydraulic oil needs to be replaced regularly to keep the system clean and effective. The hydraulic system has high requirements for temperature and oil quality.
Pneumatic cylinders: Relatively simple structure, the pneumatic system is relatively clean, and there are no pollutants in the air, but the humidity of the air and the quality of compressed air have a certain impact on the operation of the pneumatic cylinder.

7. Speed and response
Hydraulic cylinders: The response speed is slow, but it can work stably and provide greater thrust.
Pneumatic cylinders: Due to the compressibility of gas, pneumatic cylinders can provide faster movement speeds and are suitable for fast-moving applications.
Summary: Hydraulic cylinders are suitable for high pressure, high thrust, and heavy load applications, and are usually used in engineering machinery and industrial equipment; pneumatic cylinders are suitable for low pressure, low thrust, and light load applications that require fast response, and are commonly used in automated production and pneumatic control systems. Both have their own advantages, and the choice of engineering cylinder manufacturers should be based on specific work requirements.