Hydraulic Cylinders - From Basic Working Principles to Advanced Technical Applications
Hydraulic Cylinders - From Basic Working Principles to Advanced Technical Applications
Hydraulic Cylinders - From Basic Working Principles to Advanced Technical Applications
- Introduction
Definition and Importance of Hydraulic Cylinder: Introduction as actuators that convert fluid power (hydraulic energy) into linear mechanical motion (work). Its critical role in industry (construction, machinery, mobile equipment, etc.).
Historical Context: A brief summary of Pascal's Principle and its impact on the development of hydraulics.
Purpose of the Article: To delve into the fundamental components, working principles, types, critical design parameters, and calculations of hydraulic cylinders with technical details.
- Basic Working Principle and Components
2.1. Pascal's Principle (Technical Basis)
Formulation: Equal transmission of pressure applied to the liquid in all directions in a closed system ().
Generation of Force: Explanation of how pressure applied to a small area is converted into a large force over a large area.
2.2. Detailed Examination of Main Components
Cylinder Body (Barrel): Material selection (usually seamless steel pipes, such as), inner surface roughness (value), and the importance of the honing process.
Piston: The main part that takes the pressure. The task of creating two different surface areas (piston area and ring area) in double-acting cylinders.
Piston Rod: The part that transfers the movement out. Material selection (, widespread use).
Surface Coatings: Technical imperatives of hard chrome plating (wear, corrosion resistance) and alternative plating methods (e.g., thermal spraying).
Cylinder Heads (Front and Rear Cover): The task of housing the connection and sealing elements.
Sealing Elements (Seals):
Seal Types: Piston seal, rod seal, wiper seal.
Material Selection: Selection based on operating temperature, pressure, fluid type (NBR, FKM, PTFE) criteria.
- Types of Rollers and Their Applications
3.1. According to the Mode of Operation
Single-Acting Cylinders: Pressurized fluid generates force in one direction, with return achieved by spring or load.
Double-Acting Cylinders: Force generation with pressure in both directions. The most commonly used type.
3.2. Structural Types
Telehanded Cylinders: Nested sections for high stroke/aspect ratios. Application areas (dump trucks).
Tandem Cylinders: Obtaining higher force with two separate pistons in the same arm (Preferred when pressure increase or diameter increase is limited).
Rotary Rollers: Types that provide angular motion instead of linear motion (Toothed or winged).
Padded Cylinders: Mechanism to prevent impact by slowing down piston speed at the end of the stroke. Narrowing of the cross-section with cushioning bushing and journal.
- Basic Hydraulic Calculations (Critical Technical Section)
Hydraulic Cylinder Force Calculation
- Basic Principle and Formula
The formula underlying the force calculation is:
: The force produced by the cylinder (usually Newton (N) or Kilogram Force (kgf)/ Ton).
: The working pressure applied to the hydraulic system (usually Pascal (Pa), Bar or ).
: The piston surface area on which pressure acts (usually or ).
- Thrust Force (Piston Output) Calculation
In the forward (pushing) movement of the piston, the pressurized fluid acts on the entire piston surface.
- Domain ()
The area used to calculate thrust is the exact piston area, which is determined by the cylinder inner diameter ().
: The inner diameter of the cylinder (Piston diameter).
- Thrust Force Formula
- Pulling Force (Piston Return) Calculation
In the back (pull) movement of the piston, the pressurized fluid acts on the ring area between the piston and the piston rod. The area occupied by the piston rod () must be excluded from the calculation.
- Domain ()
The area used to calculate the pulling force is the difference between the full area of the piston and the area of the piston rod.
: The inner diameter of the cylinder.
: The diameter of the piston rod.
- Tensile Force Formula
Result: It is always less than because of the piston rod. So, the thrust force of a hydraulic cylinder is always greater than the pulling force.
- Calculation Units and Sample Application
The most commonly used units in industrial hydraulics are Bar and square centimeter ().
Unit Conversion (Practical Engineering Calculations)
1 Bar is roughly equal to or .
The practical formula to find the force in Kilograms of Force (kgf) or Tons:
Example Calculation (Thrust)
Data:
Cylinder Inner Diameter ():
Working Pressure ():
Thrust Area Calculation:
Force Calculation:
Note: In actual practice, this theoretical force calculated due to frictional losses due to sealing elements and fluid viscosity will be slightly higher than the net force that the cylinder will produce (Generally 95%-98% efficiency is accepted).
4.3. Power and Yield Calculations
Hydraulic Power ():
Mechanical Strength ():
Performance: Explanation of losses due to friction and leaks.
- Buckling Analysis of Cylinder Rod (Critical Design Section)
- The Concept and Purpose of Sprain
Buckling: When the axial compressive load applied on a column (in this case, the piston rod) exceeds a certain critical value, the material suddenly deforms (bends) laterally before it reaches its yield stress. This means a structural failure for the cylinder.
Purpose of the Analysis: To verify that the working load is always lower than the calculated critical buckling load (including the safety factor).
- Euler's Buckling Formula
It is the standard formula that calculates the buckling load for long and thin columns:
Detailed Explanation of Formula Ingredients
|
Symbol |
Definition |
Unit |
Notes |
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Critical Buckling Load |
The maximum axial load at which the column will begin to buck. |
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End Linkage Coefficient |
Unitless |
It is determined by the way the rod and cylinder are connected (see Table of Materials). Part 3). |
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Modulus of Elasticity (Young's Modulus) |
or |
The stiffness value of the rod material (e.g. steel). (). |
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Moment of Inertia (Area Moment of Inertia) |
Stiffness of the rod section with respect to geometry. For circular section: |
(: Rod diameter) |
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Effective Buckling Length |
It is found by the formula. Buckling length coefficient () and Free Stroke Length (). |
- Critical Parameter: End Connection Coefficient ()
How the piston rod is connected to the cylinder and machine chassis greatly affects its resistance to buckling. This is expressed by the coefficient:
|
Connection Type (End Conditions) |
Sprain Length Coefficient () |
Buckling Coefficient () |
Typical Application |
|
Articulated - Articulated (Pin - Pin) |
The most flexible. (For example, hook connections) |
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Fixed - Pin |
Widely used mobile equipment. |
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Fixed - Fixed |
The most rigid. (Only in very sensitive assemblies) |
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|
Free - Fixed |
The most critical situation. (For example, a cylinder connected only from the base) |
- Buckling Safety Factor -
The theoretical critical buckling load is . However, designs should always fall under this burden.
Factor of Safety (SF): Selected depending on the risk and sensitivity of the application.
Typical SF Values:
Fixed, low-risk applications:
Mobile, impact and vibrating applications:
- Analysis Steps (Application Summary)
Determine Application Conditions: The longest stroke (), the highest compressive force () and the end connection shape of the cylinder are determined.
Select Coefficients: The coefficient is selected according to the connection type.
Determine Material Parameters: The value of the rod material is found.
Calculate Moment of Inertia (): It is calculated using the piston rod diameter ().
Calculate Critical Buckling Load (): All values are placed in Euler's formula.
Checking and Verification: The relationship between the roller's maximum operating force () and the critical load is checked:
If the calculated safety factor is less than the required safety factor, the rod diameter () must be increased or a more rigid connection type (a choice that increases the coefficient) must be made.
Maintenance, Fault Detection and Life
How to Maintain a Hydraulic Cylinder?
Hydraulic cylinder maintenance is critical for extending the reliability, efficiency, and lifespan of the system. The maintenance process should be examined under two main headings as Preventive Maintenance (Periodic Controls) and Corrective Maintenance (Fault Repair).
Here are the detailed maintenance procedures and technical checkpoints of hydraulic cylinders:
- Preventive Maintenance (Periodic Checks)
Preventive maintenance aims to identify and address faults before they occur.
- Management of Hydraulic Fluid Quality
The vast majority of hydraulic system failures are caused by fluid contamination. The cylinders work directly with clean fluid.
Oil Level and Color Check (Daily/Weekly): Ensure that the oil level does not fall below the minimum level and that there is no abnormal darkening (overheating/oxidation) or whitening (water mixing) in color.
Oil Analysis (Periodic - Every 6 Months / 1000 Hours): Critical parameters of the oil should be examined in a laboratory environment:
Particulate Level: Compliance with ISO 4406 cleanliness code (Generally expected to be 20/18/15 or better).
Viscosity: Compliance with the specified value at operating temperature.
Water Content: Control of the percentage of moisture in the oil (Water causes corrosion and swelling of the seals).
Acidity (TAN): The level of chemical degradation of the oil.
Filter Condition: Filtering the system prevents particles entering the cylinder. Filter contamination indicators should be monitored regularly and filters should be replaced according to manufacturer recommendations.
- Cylinder Exterior and Mechanical Controls
Piston Rod Control:
Visual Inspection: The rod surface should be checked for scratches, pitting, rust or chrome plating peeling. These damages damage the wiper and piston seals, causing internal/external leaks.
Cleaning: Removing dust, mud and impurities from the rod surface (especially the front of the wiper seal) prevents contamination from entering the cylinder.
Sealing Elements (Seals) Control:
Observed Leaks: Oil leaks around the rod or at the cover joints indicate seal damage or mounting looseness. If there is an accumulation of fat deposits, it should be addressed immediately.
Dust Seal: The dust seal is checked for tears or hardening; a damaged wiper will allow contamination to enter.
Connection and Mounting Check: Checking the joints, bushings, stud bolts, and flange connections where the cylinder connects to the frame for looseness. Loose connections put a lateral load (lateral force) on the cylinder, causing premature wear of seals and bearing elements (Side Load Problem).
- Operation Performance Check
Speed and Stability Control: The roller's movement is monitored if it is slow, erratic (jerky), or choppy. This can be a sign of air entrapment, cavitation, or internal leakage in the system.
Noise and Vibration: Abnormal noises (rattling, buzzing) or excessive vibration during operation may indicate bearing wear or an air problem.
Thermal Analysis (Thermal Imaging): Overheating of the cylinder body and joints could indicate high friction (internal leakage) or misalignment.
- Corrective Maintenance and Repair (Service Procedure)
When leaks or loss of performance are detected, the cylinder is serviced.
- Secure De-Assembly
Pressure Relief: Ensure that the system is completely stopped and all hydraulic pressure is reset.
Disassembly: As the cylinder is disassembled from the machine, proper capping is performed to minimize oil leakage. The cylinder is cleaned before it is disassembled.
- Inspection and Repair of Critical Parts
Rod and Sleeve (Pipe) Damage:
Bent rods are usually replaced.
Surface scratches and pitting are removed by re-chroming and grinding or honing.
Replacement of Sealing Elements:
All seals, seals and O-rings (piston, rod and bearing) are removed and replaced with new ones of the appropriate material (NBR, FKM, PTFE) and correct size/profile. During seal replacement, it is essential that all components inside the cylinder are clean.
Bearing Elements: Guide bushings and bearing elements are checked for wear and ovality and replaced if necessary.
- Cleaning and Assembly
Thorough Cleaning: All metal parts (inside the barrel, piston, caps) are washed with solvent-based cleaners and rinsed with clean oil. There should be no metal shavings, dirt, or old felt residue inside.
Installation: When installing new seals, care is taken to ensure that they are not damaged. It is reassembled in accordance with the mounting torque values of the cylinder manufacturer.
- Testing and Commissioning
Bleeding: Once the cylinder is connected to the system, the air trapped during operation is expelled from the system through the discharge ports or by slowly running the piston several times during the full stroke. The presence of air leads to spongy operation and cavitation damage.
Leak and Pressure Testing: The cylinder is operated at low and then rated working pressure to check for any leaks or abnormal movements.
Hydraulic Fluid Quality: The impact of ISO 4406 cleanliness classification on cylinder life. The importance of effective filtration.
Seal Wear: Effects of high speed/temperature, axial disturbance and fluid contamination on seal life.
Rod Damages: The impact of chrome plating degradation and corrosion (pitting) on system reliability.
- Conclusion and Future Outlook
Summary: The indispensability of hydraulic cylinders' flexibility and power density in industrial solutions.
Future Trends: Smart hydraulic cylinders (integrated sensors, position feedback), energy efficiency, and lighter material usage.