As an important actuator of the hydraulic system, the accurate selection of hydraulic cylinders is directly related to the compatibility with the equipment and the performance of the equipment. In different industrial scenarios and working conditions, from precision machine tool processing to heavy-loaded engineering machinery operations, the requirements for hydraulic cylinders vary significantly. Improper selection may lead to inefficient equipment operation, frequent failures and even safety hazards. Therefore, a scientific and systematic selection method is essential. This article will deeply analyze the key elements and steps of hydraulic cylinder selection, and provide you with a comprehensive and practical selection guide to help you choose the most suitable hydraulic cylinder under various working conditions.

01. Determine the system pressure P

The choice of pressure depends on the load size (i.e. F) and the type of equipment. It is also necessary to consider the limitations of the assembly space, economic conditions and component supply of the actuator.

When the load is constant, the working pressure is low, and the structural size of the actuator must be increased. For some equipment, the size is limited, which is not economical from the perspective of material consumption; on the contrary, if the pressure is too high, the material, sealing and manufacturing accuracy of components such as pumps, cylinders and valves will also be very high, which will inevitably increase the cost of the equipment. Generally speaking, for equipment with fixed size that is not too limited, the pressure can be selected lower, and the pressure of mobile machinery should be selected higher.

For specific selection, please refer to the following table:

Select the design pressure of the hydraulic cylinder according to the load:

Select the design pressure of the hydraulic actuator according to the host type:

02. Preliminary selection of cylinder diameter D/rod diameter d

After selecting the design pressure, that is, P is known, and the load size F is also known, then the formula is used to obtain the force area S, and then the cylinder diameter of the oil cylinder is calculated based on the force area;

Thrust F1 = A1×P1×β Pulling F2 = A2×P2×β

A1: Push side piston pressure area cm², A1 = π/4D² = 0.785D²

A2: Pull side piston pressure area cm², A2 = π/4 (D² – d²) = 0.785 (D² – d²)

D: Hydraulic cylinder inner diameter, that is, piston diameter cm

d: Piston rod diameter cm

P1: Push side action pressure kgf/cm²

P2: Pull side action pressure kgf/cm²

β: Load rate

Note: 1. The actual output of the hydraulic cylinder is lower than the theoretical output. 2. The load rate β value is 80% when the inertia force is small and 60% when the inertia force is large.

Example: Assume that the hydraulic cylinder output is 1000 kg and the actuating pressure is 70kgf/cm². What is the inner diameter of the hydraulic cylinder?

Answer: Output F = 1000kg, actuating pressure P = 70kgf/cm², load rate β = 0.8, F1 = A1×P1×β, A1 = F1/(P1×β) = 1000/ (70×0.8)= 17.86cm², A1 = π/4D² = 0.785D², so D² = 17.86/0.785 = 22.75cm², D = √22.75 = 4.8cm = 48mm, so the inner diameter of the cylinder is 50mm;

You can also refer to the following table to select:

According to the selection principle:

1.Select the design pressure of the hydraulic actuator according to the host type, generally ≤21Mpa;

2. The cylinder barrel tube bore diameter should be small to reduce costs;

3. Choose the standard size of the barrel tube, remember the formula: P = 4F/π D²

Then choose the rod diameter d

P ≤10Mpa, d = 0.5D

P = 12.5 ~ 20Mpa, d = 0.56D

P > 20Mpa, d = 0.71D

03. Select Stroke S

According to the requirements of the overall design of the equipment or device system, determine the installation method and stroke S. The specific determination principles are as follows:

1. Stroke S = actual maximum working stroke Smax + stroke abundant margin △S;

Stroke abundant margin △S = stroke margin △S1 + stroke margin △S2 + stroke margin △S3.

2. Principles for determining stroke abundant margin △S

Under general conditions, comprehensive consideration should be given to: the stroke margin △S1 required for the manufacturing error of the system structure installation size, the stroke margin △S2 that may be required at the start of the stroke when the hydraulic cylinder is actually working, and the stroke margin △S3 that may be required at the end of the stroke (note that when the hydraulic cylinder has a buffer function requirement: the size of the stroke margin △S will have a direct impact on the buffer function, and it is recommended to reduce the stroke abundant margin △S as much as possible);

2.For hydraulic cylinders with long strokes (exceeding the longest stroke allowed by each series of this product sample) or specific working conditions, the stability of the hydraulic cylinder needs to be checked according to its specific working conditions (load characteristics, installation method, etc.).

4. Hydraulic cylinders with ultra-short stroke (exceeding the shortest stroke allowed by certain installation methods of each series of this product).

04. Select the installation method

The installation method of the cylinder refers to the form in which the cylinder is connected to the equipment. After the installation method is determined, the installation size is determined. Principles for determining the installation method:

1. Flange installation (end flange, middle flange, tail flange)

Suitable for fixed installation during the operation of the hydraulic cylinder, where the force and the support center are on the same axis; there are three installation options: end, middle or tail. How to choose depends on whether the main force acting on the load causes compression (push) stress or tensile (pull) stress on the piston rod. Generally, the compression (push) stress is installed by the tail and middle flanges, and the tensile (pull) stress is installed by the end and middle flanges. The determination of the end, middle or tail flange installation needs to be combined with the overall structural design requirements of the system and the bending stability of the hydraulic cylinder under the long-stroke compression (push) force condition.

2. Hinge installation

It is divided into tail single (double) earring installation and end, middle or tail ear shaft installation. It is suitable for the working condition that the force of the hydraulic cylinder during operation makes the machine component moved in it move along the same motion plane in a curved motion path; when driving the machine component to perform angle operation, the force to achieve the torque is proportional to the angle of the lever arm of the machine connecting rod mechanism and the force generated by the hinge installation.

a) Tail earring installation (tail single ear, tail double ear, welded single ear, welded double ear)

Tail single earring installation is the most commonly used installation method in hinge installation conditions. It is suitable for the piston rod end to move along the same motion plane in a curved motion during operation. The piston rod will move along a path with a temperature of no more than ±3°C on both sides of the actual motion plane or the single earring installation condition required by the structural design; at this time, the tail and rod end spherical bearings can be used for installation, but attention should be paid to the pressure load allowed by the spherical bearing installation.

The tail double earring installation is suitable for the working condition that the piston rod end has a curved motion path along the same motion plane during operation; it can be used at any angle in the same motion plane. In the long stroke thrust working condition, the lateral load caused by the “folding force” of the cylinder must be fully considered to cause the longitudinal bending of the piston rod.

b) End, middle or tail trunnion installation

The middle fixed trunnion installation is the most commonly used installation method for trunnion installation. The position of the trunnion can be arranged to balance the weight of the cylinder body or at any position between the end and the tail to meet the needs of various uses. The trunnion pin is designed only for shear load and should not be subjected to bending stress. It should be installed with a rigid mounting support seat with the same length as the trunnion and a support bearing. During installation, the support shaft should be as close as possible to the end face of the trunnion shoulder to minimize the bending stress.

The tail trunnion installation is similar to the tail double earring installation condition, and the selection method is the same as above.

End trunnion installation is suitable for hydraulic cylinders with smaller rod diameters than cylinders with hinged fulcrums at the tail or middle positions. The influence of the overhanging weight of the hydraulic cylinder must be considered for cylinders with long stroke end trunnion installation. To ensure the effective load bearing of the supporting shaft, it is recommended that the stroke of the hydraulic cylinder installed in this way be controlled within 5 times the cylinder diameter.

3.Foot type installation (front and rear foot, left and right foot, welding foot)

Suitable for fixed installation during the working process of the hydraulic cylinder. Its installation plane is not in the same plane as the central axis of the cylinder. Therefore, when the hydraulic cylinder applies force to the load, the foot-mounted cylinder will generate a flipping moment. If the hydraulic cylinder is not well fixed to the component it is installed on or the load is not properly guided, the flipping moment will generate a large lateral load on the piston rod. When choosing this type of installation, the installed component must be well positioned, tightened and the load properly guided. There are two installation methods: end and side foot type installation.

05. Choice of end buffer

The following working conditions should consider choosing two-end buffer or one-end buffer:

1. When the hydraulic cylinder piston runs throughout the entire stroke and its reciprocating speed is greater than 100mm/s, two-end buffer should be selected.

2. When the hydraulic cylinder piston has a one-way reciprocating speed greater than 100mm/s and runs to the end of the stroke, one-end or two-end buffer should be selected.

3. Other specific working conditions.

06. Selection of oil port type and diameter

1. Oil port type:

Internal thread type, flange type and other special types. The selection is determined by the connection method of the connecting pipeline in the system.

2. Oil port diameter selection principle:

When the medium flow rate in the connecting pipeline between the system and the hydraulic cylinder is known, the medium flow rate through the oil port is generally not more than 5m/s. At the same time, pay attention to the flow rate ratio factor to determine the oil port diameter.

07.Hydraulic cylinder with valve

1. Maintaining pressure: Sliding valve type reversing valves have gap leakage and can only maintain pressure for a short time. When there is a requirement to maintain pressure, a hydraulically controlled one-way valve can be added to the oil circuit, and the tightness of the cone valve closure can be used to maintain the pressure of the oil circuit for a long time.

2. “Support” of the hydraulic cylinder: In a vertical hydraulic cylinder, due to the leakage of the sliding valve and the pipe, the piston and the piston rod may slide down under the gravity of the piston and the piston rod. Connecting the hydraulically controlled one-way valve to the oil circuit of the lower chamber of the hydraulic cylinder can prevent the movable parts such as the hydraulic cylinder piston and the slider from sliding down.

3. Locking the hydraulic cylinder: When the reversing valve is in the middle position, the two hydraulically controlled one-way valves are closed, which can tightly seal the oil in the two chambers of the hydraulic cylinder. At this time, the piston cannot move due to external force.

08. Specific working conditions for condition selection

1. Working medium

The normal medium is mineral oil. Other media must pay attention to their effects on the sealing system, material properties of various components and other conditions.

32# and 46# anti-wear hydraulic oil are recommended. The most suitable oil temperature is 20~55℃. When the temperature is lower than 15℃ or greater than 70℃, it is prohibited to operate. To adjust the oil temperature, it can be heated or cooled in advance. Hydraulic oil should be replaced once every 1-6 months, and the oil tank should be cleaned to remove dirt and dust. Hydraulic transmission is most afraid of oil deterioration and dirt, otherwise dust will stick to the oil suction filter, causing increased noise and reducing the life of the oil pump, so the oil should always be kept clean.

2. Ambient or medium temperature

The normal working medium temperature is -20°C to +80°C. If it exceeds this working temperature, it must be paid attention to its effects on the sealing system, material properties of various components and cooling system settings.

3. High operating accuracy

For servo or other hydraulic cylinders with low starting pressure requirements such as medium and high pressure, attention must be paid to its impact on the sealing system, material properties of each component and detailed design.

4. Zero leakage

For hydraulic cylinders with specific pressure maintenance requirements, attention must be paid to its impact on the sealing system, material properties of each component and other conditions.

5. Working pressure and speed, working conditions such as:

a) Medium and low pressure system, piston reciprocating speed ≥70-80mm/s

b) Medium and high pressure, high pressure system, piston reciprocating speed ≥100-120mm/s, attention must be paid to the impact on the sealing system, material properties of each component, connection structure and matching accuracy.

6. High-frequency vibration working environment:

Attention must be paid to its impact on factors such as material properties of each component, connection structure and detailed design.

7. Low temperature icing or contaminated working environment, working conditions such as:

a. High dust environment;

b. Water spray, acid mist or salt mist environment.

Attention must be paid to its impact on the sealing system, material properties of each component, surface treatment of the piston rod and protection of the product.

09. Selection of seal quality

There are specific working conditions and specified quality requirements as mentioned above. The consequences of failure of the sealing system of the hydraulic cylinder are serious (such as affecting safety, difficult to replace, large economic losses, etc.). For special requirements such as the sealing system of the hydraulic cylinder for export, it is recommended that the professional engineers of the manufacturer recommend the use of well-known sealing qualities with good interchangeability and easy procurement based on the working conditions.

10.Other feature selections

1. Exhaust valve

According to the working position of the hydraulic cylinder, it is normally set at the highest point where the air in the two end chambers finally accumulates. After the air is exhausted, it can prevent creeping, protect the seal, and slow down the deterioration of the oil.

2. Leakage oil port

In a working environment where oil leakage is strictly prohibited, due to the long stroke of the hydraulic cylinder or certain working conditions, the oil accumulates behind the dust ring during its round-trip work. To prevent leakage after long-term work, a leakage port must be set at the location where the oil accumulates.

 

Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

Mobile/Whatsapp:+86 15902166721

HYDRAULIC CYLINDER and CNC parts specialist

JW GROUP is an integrated steel product group, products include: hydraulic cylinder, hydraulic cylinder spare parts, drilling forging parts, pneumatic actuator-scotch yoke, CNC machinery parts etc.