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I. What is the low-temperature performance of a hydraulic cylinder?
The low-temperature performance of a hydraulic cylinder refers to its ability to start normally, operate smoothly, and maintain its sealing performance and mechanical integrity in low-temperature environments (typically below 0°C, especially below -20°C). Low-temperature environments have a series of unique effects on the hydraulic cylinder and its medium (hydraulic oil), posing serious challenges to its performance.
Key impacts of low temperatures on hydraulic cylinders and performance requirements:
1. Impact on hydraulic oil (the most significant challenge)
Performance requirement: Hydraulic oil with good fluidity at low temperatures must be used (low pour point, high viscosity index).
Failure mechanisms:
Increased viscosity: This is the most direct impact. The oil thickens, like syrup, and its fluidity decreases sharply.
Loss of fluidity/solidification: When the temperature drops below the oil’s “pour point,” the oil will completely lose its fluidity and cannot be pumped.
2. Effects on Seals
Performance Requirements: Sealing materials must maintain elasticity and flexibility at low temperatures.
Failure Mechanisms:
Loss of Elasticity and Hardening: Rubber or polyurethane seals harden, shrink, and lose elasticity at low temperatures. This can lead to:
Sharply Increased Starting Friction: The seal cannot deform smoothly, potentially causing the piston rod or piston to “stick,” making starting difficult.
Seal Failure: The hardened seal cannot conform tightly to the sliding surface, resulting in internal and external leakage. Under extremely cold conditions, the seal may even become brittle and fracture.
3. Effects on Mechanical Structure and Materials
Performance Requirements: Metal components such as the cylinder and piston rod must be able to withstand changes in material properties and thermal stress at low temperatures.
Failure Mechanisms:
Increased Material Brittleness: Some metal materials (especially low-carbon steel) experience a decrease in toughness at low temperatures, becoming more susceptible to brittle fracture, particularly under impact loads.
Changes in Clearance: Due to the different thermal expansion coefficients of different materials, the clearance between the piston and cylinder, and between the piston rod and guide bushing, will change at low temperatures. This may result in tighter clearances (increasing friction) or looser clearances (increasing the risk of leakage).
Risk of freezing: If there is moisture in the system, it may freeze internally, blocking oil passages or small holes, or even causing parts to crack due to expansion.
The low-temperature performance of a hydraulic cylinder is fundamentally determined by its ability to start smoothly and operate stably under harsh conditions of high oil viscosity, hardened seals, and brittle materials. A hydraulic cylinder with good low-temperature performance requires targeted optimization in oil selection, seal formulation, structural clearance design, and material selection.
II. Measures to be Taken for Low-Temperature Requirements
1. Measures Regarding Sealing Materials
1.1 Selection of Special Low-Temperature Sealing Materials:
Silicone rubber and fluorosilicone rubber: Possess excellent low-temperature elasticity (working temperature can be as low as below -60°C), but have poor wear resistance, and are often used for static sealing or low-pressure dynamic sealing.
Ethylene propylene diene monomer (EPDM) rubber: Has good cold resistance (below -50°C), but poor resistance to mineral oil, and is suitable for phosphate ester hydraulic oil or water-glycol systems.
Polyurethane: Through formula modification, some polyurethane seals can maintain good mechanical properties at -50°C and have excellent wear resistance.
Filled polytetrafluoroethylene (PTFE) composite materials: PTFE itself has excellent low-temperature performance. Seals such as U-cups and O-rings made from modified filled PTFE are the preferred dynamic sealing solution for ultra-low temperature hydraulic cylinders.
1.2. Optimizing Seal Structure Design:
Adopting a low-pressure start design: For example, using PTFE composite seals with elastomers, or employing low-friction, low-starting pressure sealing forms (such as certain types of U-rings).
Adding anti-extrusion rings: Installing low-temperature resistant anti-extrusion rings (such as PEEK or filled PTFE) on the high-pressure side to prevent hardened seals from being squeezed into the gap under high pressure at low temperatures.
2. Measures to be taken regarding Mechanical Structure and Materials
2.1. Material Selection and Treatment:
• Selecting low-temperature materials for critical parts: For critical components subjected to impact loads, consider using alloy steels with good low-temperature toughness (such as certain nickel-chromium-molybdenum steels), which have a low brittle transition temperature.
• Surface treatment: Applying a highly wear-resistant and corrosion-resistant coating to the piston rod to ensure good surface hardness and smoothness even at low temperatures, reducing friction with the seals.
2.2. Optimizing Low-Temperature Fit Tolerances:
Based on thermal expansion coefficient calculations: Accurately calculate the material contraction/expansion between the piston and cylinder, and between the piston rod and guide sleeve, at the lowest and highest operating temperatures. Redesign the optimal fit tolerances at room temperature to prevent jamming due to excessive tightness or leakage due to excessive looseness at low temperatures.
3. Measures to be taken regarding Hydraulic Oil
3.1. Selecting Low-Temperature Hydraulic Oil
High viscosity index (VI) hydraulic oil: Viscosity changes less with temperature, and viscosity increases more slowly at low temperatures.
Low pour point/freezing point hydraulic oil: The pour point must be at least 10-15°C below the lowest operating ambient temperature of the equipment.
Synthetic hydraulic oil: Such as polyalphaolefin (PAO) synthetic oil, alkylbenzene synthetic oil, etc., which have extremely low pour points (down to -60°C or below), excellent low-temperature fluidity and high-temperature stability, making them the preferred choice for extreme low-temperature environments.
3.2. Equipped with a fluid temperature management system:
Oil tank heater: An electric heater is installed in the oil tank to preheat the fluid to a safe viscosity range (usually above 10-20°C) before starting the system.
Heat Exchanger and Circulation Loop: The system fluid is continuously kept warm using engine coolant or an independent heating circuit.
Piping and Component Insulation: Exposed hydraulic lines, valve blocks, and hydraulic cylinders are insulated to reduce heat loss.
III. How to Test the Low-Temperature Performance of Hydraulic Cylinders?
The test aims to simulate the working conditions of the hydraulic cylinder in a cold environment and verify its reliability during startup and operation. Similar to high-temperature testing, it is divided into laboratory testing and field testing.
A. Laboratory Simulation Testing (Precise and Controllable), commonly used for product development and quality certification.
Low-Temperature Starting and Running Test
Test Principle: The hydraulic cylinder and its fluid are thoroughly cooled in a low-temperature environment, and then the force (pressure) required for starting and running is tested, and the smoothness of movement is observed.
Test Steps:
1. Pre-treatment: Fill the clean hydraulic cylinder with the specified low-temperature hydraulic oil, and then place the entire assembly into an environmental test chamber (low-temperature chamber).
2. Temperature Stabilization: Lower the temperature of the test chamber to the target low temperature (e.g., -20°C, -40°C, -54°C), and maintain it for a sufficiently long time (usually ≥8 hours) to ensure that the hydraulic fluid and all parts inside the hydraulic cylinder reach this temperature.
3. Testing Starting Friction:
Outside the low-temperature chamber, connect the hydraulic power source through piping (the oil source should use the same type of low-temperature oil, or heating should be used to ensure that the oil temperature does not affect the test).
Slowly apply pressure to one chamber of the hydraulic cylinder while monitoring with a pressure sensor.
Record the pressure value at the moment the piston rod is about to start moving, which is the starting pressure. This pressure value directly reflects the sealing friction and fluid viscous resistance at low temperatures.
4. Testing for smooth operation:
Allow the hydraulic cylinder to perform low-speed reciprocating motion at low temperatures.
Observe: Whether severe “crawling” (jerky movement) occurs. This is usually caused by a large difference between static and dynamic friction, combined with poor fluid flow.
5. Monitoring for leaks: During and after the entire test process, check for external leaks at the piston rod seal and other areas due to shrinkage and hardening of the seals.
B. On-site Working Condition Testing and Monitoring (Practical Verification)
For equipment used in cold regions, the following methods can be used for evaluation:
1. Cold Soak and Start-up Test:
Method: The equipment is left outdoors overnight (or for a sufficient period) and then started directly in the early morning when the temperature is coldest.
Observation: Check for smooth start-up, stable operation, unusual noises, and leaks.
2. Operating Status Monitoring:
Method: Monitor the system pressure during the initial operation of the equipment. Abnormally high starting pressure indicates poor low-temperature performance.
Using an infrared thermometer: Scan the surface of the hydraulic cylinder to understand its actual working temperature and compare it with the ambient temperature.
The core of laboratory testing for the low-temperature performance of hydraulic cylinders is to quantify the “low-temperature starting pressure” and observe the “low-temperature operating stability”; while on-site testing focuses more on the start-up capability and reliability under actual harsh working conditions.
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