The core structure of a hydraulic oil cylinder can be divided into five main components. 
1. Cylinder barrel
Function: The cylinder barrel is the main body of the hydraulic oil cylinder. It forms the internal pressure chamber and bears the main working pressure.
Characteristics:
Typically made of high-strength seamless steel tubes.
The inner bore surface requires precision machining, such as honing or rolling, to achieve high dimensional accuracy, geometric precision (e.g., straightness, roundness), and very low surface roughness. This ensures the reliability and service life of the sealing components.
It must have sufficient strength and stiffness to withstand system pressure without bursting or excessive expansion.

2. Piston assembly
Piston:
Installed inside the cylinder barrel, it divides the internal space into two sealed chambers (rod side and blind side).
It moves back and forth in a straight line under the push of hydraulic oil.
The piston is equipped with piston seals (e.g., Glyd rings, Step seals, U-cups) to prevent leakage of pressure oil between the two chambers (internal leakage).
Piston Rod:
One end is connected to the piston, and the other end extends outside the cylinder barrel to connect to the load mechanism.
It transmits the thrust force of the piston to the external mechanical device.
The surface is usually treated with hard chrome plating to achieve high hardness, smoothness, and corrosion resistance, resisting wear, scratches, and corrosion.
It must have sufficient strength to withstand push and pull loads without breaking or bending.

3. End caps
Cap end (blind end): Seals one end of the cylinder barrel and usually includes an oil port.
Rod end (gland end):
Also known as the gland, it is installed at the other end where the piston rod extends.
Its functions include:
Support and guidance: Provides precise support and guidance for the piston rod, preventing misalignment and wear.
Sealing Installation: Houses the rod seals (preventing oil leakage) and the wiper seal (preventing external dust and contaminants from entering the cylinder).
The end caps are typically connected to the cylinder barrel via threads, cartridge keys, or tie rods.

4. Sealing system
This is one of the most critical components ensuring the proper operation of the hydraulic oil cylinder. It mainly includes:
Piston seals: Prevent high-pressure oil from leaking between the two sides of the piston (internal leakage), ensuring the efficiency and precise control of the cylinder's output force.
Rod seals: The primary seal preventing pressurized oil inside the cylinder from leaking outward along the piston rod (external leakage).
Wiper seal (dust seal): Located at the outermost side, it removes dust, dirt, and other contaminants adhering to the piston rod when it retracts, protecting the internal seals and hydraulic system.
Guide rings / wear rings: Usually made of wear-resistant materials (e.g., plastic, metal), installed on the piston and gland to support the piston and piston rod, prevent direct metal-to-metal contact, and reduce friction and wear.
Static seals: Such as O-rings, used for sealing fixed interfaces like end caps and the cylinder barrel.

5. Cushioning device
Function: Prevents severe mechanical impact between the piston and end caps when the piston reaches the end of its stroke, avoiding noise, vibration, and damage.
Principle: At the end of the stroke, the piston enters a special cushion sleeve or throttle hole, gradually reducing the flow area of the outlet. This creates back pressure in the return chamber, allowing the piston to decelerate smoothly until it stops.
Other Accessories
Bleed Valve: Usually located at the highest points at both ends of the cylinder, used to remove air entrapped in the hydraulic system, preventing "creeping" motion and vibration.
Mounting Styles: Such as clevis, pivot, flange, and foot mounts, used to connect the cylinder to the machine base.