Design steps and common fault problems of hydraulic presses

Hydraulic cylinders are everywhere around us. We see them so often in our daily lives that we might not even realize it if we're not paying close attention: they're found in excavators, trucks, forklifts, tractors, aerial work platforms, mining equipment—you name it. A hydraulic cylinder is one of the four main components of a hydraulic system, a technology in which a fluid (most commonly hydraulic oil) is used to move energy from a motor to an actuator: the most common being a hydraulic cylinder.

A hydraulic cylinder is part of a machine's hydraulic system. Simply put, a hydraulic cylinder is a hydraulic actuator that generates linear motion by converting hydraulic energy back into mechanical motion.

Hydraulic Cylinder Design Steps

1. Understand the hydraulic cylinder's motion characteristics and determine the desired cylinder design form. All design begins with a requirement. The desired product performance becomes the standard requirement that the subsequent design must meet. The same is true for cylinder design. Before designing the cylinder, it is also necessary to understand the application function requirements and realize the required functions in the later design. There are many types of hydraulic cylinders, including piston type, plunger type, and telescopic sleeve type. According to the movement form, they can be divided into reciprocating linear type and swing type. According to the function, they can be divided into double-acting type and single-acting cylinder. Therefore, before determining what type of cylinder to use, you must understand how you want the cylinder to operate and determine the appropriate hydraulic cylinder type based on the set movement form and characteristics.

2. Further understand the operating conditions of the hydraulic cylinder.

(1) The working conditions of the hydraulic cylinder, such as temperature, ambient humidity, etc., are used to determine the corrosion resistance and dustproof level of the hydraulic cylinder.

(2) The output, load condition, stroke size, working system, etc. required by the hydraulic cylinder are used to determine the size of the piston and piston rod of the hydraulic cylinder and the ultimate strength verification and fatigue life calculation of the hydraulic cylinder. (3) The working pressure and flow selected by the hydraulic system; assist in determining important dimensions such as the hydraulic cylinder piston and piston rod.

3. Select the rated pressure of the hydraulic system. Calculate the cross-sectional area of the hydraulic cylinder piston based on the required cylinder output of the main engine, and round it according to the national standard series.

4. After selecting the materials for the main components, calculate the wall thickness of the hydraulic cylinder barrel and the diameter of the hydraulic piston rod based on the required cylinder output and material strength.

5. Determine the hydraulic cylinder structure and the connection method for the front and rear end caps based on the connection interface with the main engine and the installation space. Determine the sealing method and design of the hydraulic cylinder seal based on the hydraulic oil pressure, the operating temperature range of the hydraulic cylinder, and the presence of dust.

7. Design the hydraulic cushioning system appropriately based on the operating load and control conditions of the hydraulic cylinder. A proper cushioning design can reduce impact loads and prevent premature damage to the hydraulic cylinder.

8. For slender parts, a buckling strength analysis is required, and the buckling strength of the piston rod is calculated when the piston rod is fully extended to verify whether buckling failure will occur.

9. If the hydraulic cylinder is subjected to radial forces during operation, it is necessary to verify whether the piston rod will contact the end caps under the radial forces. 10. Design an appropriate anti-corrosion coating based on the operating environment to protect the hydraulic cylinder from corrosion during extended operation.

11. Draw component and assembly drawings and prepare corresponding technical documentation.

12. Produce samples according to the drawings and conduct experimental verification. The design process is considered complete only when experimental verification confirms that the design requirements are met.

Common Problems and Repairs of Hydraulic Cylinders

External leakage refers to the leakage of oil from various loose seals to the atmosphere outside the hydraulic cylinder. The most common external leakage is from the following three places:

(1) Oil leakage at the sealing part between the hydraulic cylinder sleeve and the cylinder head (or guide sleeve) (Solution: Replace the new O-ring);

(2) Oil leakage at the relative movement between the piston rod and the guide sleeve (Solution: If the piston rod is damaged, it can be cleaned with gasoline. After drying, apply metal glue to the damaged part, and then use the piston rod oil seal to move back and forth on the piston rod to scrape off the excess glue. Wait until the glue is completely cured before putting it into use. If the guide sleeve is worn, a guide sleeve with a slightly smaller inner diameter can be processed for replacement);

(3) Oil leakage caused by loose sealing of the hydraulic cylinder pipe joint (Solution: In addition to checking the sealing condition of the sealing ring, you should also check whether the joint is correctly assembled, whether it is securely tightened, and whether there are any scratches on the contact surface. Replace or repair it if necessary)

Internal leakage of the hydraulic cylinder refers to the leakage of oil from the high-pressure chamber to the low-pressure chamber through various gaps inside the hydraulic cylinder. Internal leakage is difficult to detect and can only be determined by observing the system's operating conditions, such as insufficient thrust, reduced speed, unstable operation, or increased oil temperature. Internal leakage in hydraulic cylinders generally occurs in two locations: 

(1) The static seal between the piston rod and the piston (solution: install an O-ring on the sealing surface of both);

(2) The dynamic seal between the inner wall of the cylinder liner and the piston (solution: when internal leakage is discovered, all mating parts should be strictly inspected first. The cylinder liner is often repaired by boring the inner hole and then fitting a piston with a larger diameter);