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1.3.8 Technical requirements for sealing devices
The previous articles in this series discussed the Technical requirements for design and manufacturing of hydraulic cylinders-Barrel tube, Technical requirements for hydraulic cylinders-Piston and Piston rod. Technical requirements for Cylinder head and cylinder bottom. Technical requirements for guide sleeves. Today this article will explain Technical requirements for sealing devices.
One meaning of hydraulic cylinder sealing refers to the hydraulic cylinder sealing device. These sealing devices are crucial components of a hydraulic cylinder, used to seal all reciprocating motion areas (dynamic seals) and connections (static seals). Hydraulic cylinder seals generally include piston seals, piston rod seals, seals between cylinder (barrel) assemblies, seals between piston and piston rod assemblies, and seals at oil ports. Hydraulic cylinder sealing devices typically also include piston rod dust protection (sealing) and piston and piston rod guidance and support.
Another meaning of hydraulic cylinder sealing is relative to hydraulic cylinder leakage, describing a phenomenon or state opposite to leakage.
Hydraulic cylinder leakage refers to the phenomenon where the hydraulic working medium overflows the cavity boundary, flowing from the high-pressure side to the low-pressure side. Leakage is further divided into internal leakage and external leakage.
In current hydraulic cylinder standards, hydraulic cylinder sealing technology requirements are a crucial component, and hydraulic cylinder design and manufacturing must meet these requirements. Although the wording of the hydraulic cylinder sealing technology requirements in the standards listed below varies, they primarily concern the static and dynamic sealing performance of hydraulic cylinders.
Under specified conditions, the pressure resistance of hydraulic cylinder seals includes high-pressure and low-pressure resistance, durability, and other performance characteristics related to hydraulic cylinder seals, such as starting pressure and minimum speed. These must generally be guaranteed in the design and manufacturing of hydraulic cylinders.
1.3.8.1 General Technical Requirements for Hydraulic Cylinder Seals
(1) Requirements for sealing in standard “Technical Conditions for Marine Reciprocating Hydraulic Cylinders”
① The hydraulic cylinder should function normally when the ambient temperature is -25℃ to +65℃.
Note: Normal operation (state) refers to the hydraulic cylinder operating under specified working conditions where all performance parameters (values) change within a predetermined range.
② The hydraulic cylinder should not experience jamming when the working medium temperature is -15℃.
③ The hydraulic cylinder should not leak at any mating surfaces when the working medium temperature is +70℃.
④ The seals in the hydraulic cylinder should be resistant to high temperatures, corrosion, aging, and hydrolysis, and have good sealing performance, meeting both the requirements for oil sealing and the requirements of the marine air environment.
⑤ The hydraulic cylinder should not leak at the seals or welds when subjected to 1.5 times the nominal pressure.
⑥ The internal leakage of a double-acting piston hydraulic cylinder should not exceed the specified value.
⑦ There should be no external leakage at any seal or during movement of the hydraulic cylinder.
⑧ For a double-acting piston hydraulic cylinder, after 50,000 full-stroke reversal cycles, there should be no dripping leakage from the piston rod.
⑨ After 50,000 reversal cycles, for every 100m movement of the piston in a double-acting piston hydraulic cylinder, the external leakage should not exceed 0.01mL when the piston diameter d ≤ 50mm; and should not exceed 0.0002dmL when the piston rod diameter d > 50mm.
⑩ For a plunger-type hydraulic cylinder, after 25,000 full-stroke reversal cycles, there should be no dripping leakage at the plunger rod.
⑪After 25,000 reversing cycles of a plunger-type hydraulic cylinder, the external leakage should not exceed 0.01 mL per 100 m movement of the plunger when the plunger rod diameter d ≤ 50 mm; and should not exceed 0.0002 d mL when the plunger rod diameter d > 50 mm.
⑫When the hydraulic cylinder inner diameter D ≤ 200 mm, the minimum stable speed of the hydraulic cylinder is 4 mm/s; when the hydraulic cylinder inner diameter D > 200 mm, the minimum speed is 5 mm/s.
(2) Sealing requirements in standard “Hydraulic Cylinders for Metallurgical Equipment (PN≤25MPa)”
① This standard applies to hydraulic cylinders for metallurgical equipment with a nominal pressure PN≤25MPa and an ambient temperature of -20～+80℃.
② When the nominal pressure is applied to one side of the piston, the internal leakage on the other side of the piston should not exceed the specified value. When the stroke is greater than 1m, the internal leakage at the middle position of the stroke must also be measured.
③ (External leakage) When the piston rod travels 100m, the total oil leakage at the piston rod dust seal should not exceed 0.002d mL. No oil leakage is allowed at other parts.
(3) Technical requirements for sealing in standard “Technical Conditions for Agricultural Double-Acting Hydraulic Cylinders”
① This standard applies to agricultural double-acting hydraulic cylinders (hereinafter referred to as cylinders) with a rated pressure not exceeding 20 MPa.
② The recommended oil for testing is N100D tractor transmission and hydraulic dual-purpose oil or mineral oil with equivalent viscosity. The kinematic viscosity of the oil at 40℃ should be 90–110 mm2/s. The kinematic viscosity at 65℃ should be 25–35 mm/s, or 25–35 mm2/s.
③ During the trial run, the piston movement should be uniform, without any abnormal phenomena such as crawling or external leakage.
④ In the pressure resistance test, the piston is positioned at both ends of the cylinder, and oil is supplied to the cavity to maintain an oil pressure of 1.5 times the working pressure (test pressure) for 2 minutes. There should be no external leakage, damage to mechanical parts, or permanent deformation.
⑤ In the internal leakage test, the piston movement caused by internal leakage should not exceed 1 mm within 10 minutes at the test pressure.
⑥ In the external leakage test, the leakage at the piston rod should not exceed 0.008 d mL after the piston moves 100 m. [d is the piston rod diameter, in millimeters (mm)]. No other parts must leak oil.
⑦ In the high-temperature performance test, the hydraulic cylinder should operate normally at an oil temperature of 90–95℃, with oil leakage at the piston rod not exceeding twice the value specified in the external leakage test above, and no external leakage at other parts.
⑧ In the low-temperature performance test, the hydraulic cylinder should operate normally at an ambient temperature of -25 to -20℃.
⑨ In the durability test, after the hydraulic cylinder pressure test, the internal and external oil leakage should not exceed 2.5 times the values specified in the internal and external leakage tests above. No parts should show signs of damage.
(4) Sealing Requirements in standard “Hydraulic Cylinders”
① This standard applies to single- and double-acting hydraulic cylinders with a nominal pressure below 31.5 MPa, using hydraulic oil or other mineral oils with equivalent performance as the working medium.
② Generally, the ambient temperature for hydraulic cylinder operation should be within the range of -20 to +50℃, and the working medium temperature should be within the range of -20 to +80℃.
③ The internal leakage of a double-acting hydraulic cylinder shall not exceed the specified value.
④ The internal leakage of a piston-type single-acting hydraulic cylinder shall not exceed the specified value.
⑤ For a double-acting hydraulic cylinder, after 50,000 reversing cycles (based on a stroke ≤ 500 mm), the external leakage at the piston rod shall not be dripping.
⑥ For a double-acting hydraulic cylinder, after 50,000 reversing cycles (based on a stroke ≤ 500 mm), the external leakage at the piston rod shall not exceed the specified value.
⑦ For a piston-type single-acting hydraulic cylinder (stroke ≤ 500mm), after 40,000 reversing cycles, there should be no dripping leakage at the piston rod.
⑧ For a piston-type single-acting hydraulic cylinder (stroke ≤ 500mm), after 40,000 reversing cycles, the external leakage at the piston rod must not exceed the specified value.
⑨ For a plunger-type single-acting hydraulic cylinder (stroke ≤ 500mm), after 25,000 reversing cycles, there should be no dripping leakage at the plunger.
⑩ For a plunger-type single-acting hydraulic cylinder (stroke ≤ 500mm), after 25,000 reversing cycles, the external leakage at the piston rod must not exceed the specified value.
⑪ For multi-stage sleeve-type single/double-acting hydraulic cylinders (stroke ≤ 500mm), after 16,000 reversing cycles, there should be no dripping leakage at the sleeve.
⑫ For multi-stage sleeve-type single/double-acting hydraulic cylinders (stroke ≤ 500mm), after 16,000 reversing cycles, the external leakage at the sleeve must not exceed the specified value.
⑬ There should be no oil leakage at the piston rod seal. (Leakage under low pressure) At the end of the test, the oil film on the piston rod should not be sufficient to form oil droplets or oil rings.
⑭ There should be no oil leakage at all static seals and welds.
⑮ There should be no oil leakage from the throttling and/or buffering elements installed on the hydraulic cylinder.
⑯ After the durability test, the increase in internal leakage should not exceed twice the specified value.
⑰ The test oil should be compatible with the sealing material of the hydraulic cylinder under test.
⑱ Under rated pressure, input 90°C working oil into the hydraulic cylinder under test and run it in a reciprocating motion for 1 hour. It should meet the performance requirements agreed upon with the user.
(5) Sealing requirements in standard “Large Hydraulic Cylinders”
① This standard applies to large hydraulic cylinders with an inner diameter of not less than 630 mm. Mineral oil, fire-resistant oil, water-glycol, and phosphate ester working media can be selected as needed.
② The internal leakage of the hydraulic cylinder should not exceed the specified value.
③ When the piston rod of the hydraulic cylinder stops at both ends, it should be held at the nominal working pressure for 30 minutes without any external leakage.
④ When the hydraulic cylinder is subjected to pressure resistance test, 1.5 times the nominal working pressure is applied to the working chamber, and the pressure is held for 10 seconds during the factory test. There should be no external leakage.
⑤ The test oil should be compatible with the sealing material of the hydraulic cylinder.
⑥ Under the lowest starting pressure, make the hydraulic cylinder reciprocate more than 3 times in its full stroke, and stay at the end of the stroke for at least 10 seconds each time. The oil film on the piston rod is not enough to form oil droplets or oil rings, and there is no oil leakage at all static seals and welds.
(6) Sealing requirements in standard “Hydraulic Cylinders for Marine Hatch Covers”:
① When the inner diameter D of the hydraulic cylinder is ≤200mm, the minimum stable speed of the hydraulic cylinder is 8mm/s; when the inner diameter D>200mm, the minimum temperature velocity of the hydraulic cylinder is 10mm/s.
② The internal leakage of the hydraulic cylinder should not exceed the specified value.
③ The pressure for the hydraulic cylinder pressure test is 1.5 times the nominal pressure, and there should be no leakage within the pressure holding time (5min).
④ There should be no external leakage at any static or dynamic seal when the cylinder is stationary.
⑤ After 10,000 reversing cycles at the piston rod dynamic seal, there should be no dripping external leakage. For every 100mm movement: when the piston diameter d ≤ 50mm, the external leakage should not exceed 0.05mL/min; when the piston rod diameter d > 50mm, the external leakage should not exceed 0.001dmL/min.
⑥ Under nominal pressure, there should be no malfunctions after 5000 continuous reciprocating cycles.
(7) Sealing Requirements in standard “Technical Conditions for Hydraulic Cylinders for Dump Trucks”
① This standard applies to single-acting piston hydraulic cylinders, double-acting single-piston rod hydraulic cylinders, single-acting plunger hydraulic cylinders, single-acting telescopic sleeve hydraulic cylinders, and final-stage double-acting telescopic sleeve hydraulic cylinders (hereinafter referred to as hydraulic cylinders) used in dump truck lifting systems that use hydraulic oil as the working medium.
② During internal leakage, external leakage, and pressure resistance tests, the hydraulic cylinder must not have external leakage.
③ The internal leakage of the hydraulic cylinder should not exceed the specified value (under rated pressure, pressure held for 30 seconds).
④ Under rated pressure, the hydraulic cylinder can complete 50,000 full-stroke reciprocating cycles or 50 km full-stroke reciprocating cycles. Before a hydraulic cylinder completes 10,000 full-stroke reciprocating motions or 10 km of full-stroke reciprocating motion, there must be no external leakage. Thereafter, for every 100 reciprocating motions or 100 m of full-stroke reciprocating motion, for hydraulic cylinders with piston rod, plunger, and sleeve diameters less than or equal to 50 mm, the external leakage should be less than or equal to 0.1 mL; for hydraulic cylinders with piston rod, plunger, and sleeve diameters greater than 50 mm, the external leakage should be less than or equal to 0.002 d mL (d is the diameter in mm).
(8) Sealing Requirements in standard “Technical Conditions for Hydraulic Cylinders for Mining Machinery”
① This standard applies to hydraulic cylinders used in mining machinery that use hydraulic oil as the working medium and have a rated pressure not exceeding 31.5 MPa.
② The internal leakage of the hydraulic cylinder should not exceed the specified value.
③ There should be no external leakage except at the piston rod.
④ There should be no external leakage when the piston rod is stationary.
⑤ After 50,000 piston reversals, there should be no dripping external leakage at the piston rod.
⑥ After 50,000 reversals, for a qualified hydraulic cylinder, for every 200m movement of the piston (for a first-class hydraulic cylinder, for every 100m movement of the piston): when the piston rod diameter d ≤ 50mm, the external leakage qv ≤ 0.05mL; when the piston rod diameter d > 50mm, the external leakage qv < 0.001dmL.
⑦ Reliability or durability quality grading. When the piston stroke L < 500 mm, the cumulative stroke ≥ 100 km; when the piston stroke L ≥ 500 mm, the cumulative reversing cycles ≥ 200,000 times, it is considered a qualified product. When the piston stroke L < 500 mm, the cumulative stroke ≥ 150 km; when the piston stroke L ≥ 500 mm, the cumulative reversing cycles ≥ 300,000 times, it is considered a first-class product.
⑧ Pressure Resistance Test Method and Technical Requirements. The piston of the hydraulic cylinder under test shall be stopped at both ends of its stroke (without contacting the cylinder head). When the rated pressure is less than or equal to 16 MPa, adjust the relief valve to make the pressure in the working chamber 1.5 times the rated pressure; when the rated pressure is greater than 16 MPa, adjust the relief valve 2 to make the pressure in the working chamber 1.25 times the rated pressure, and maintain the pressure for 5 minutes. No abnormal phenomena shall occur during the pressure resistance test.
(9) Technical requirements for sealing in standard “Servo Hydraulic Cylinders Part 1: Technical Conditions”
① This part applies to double-acting or single-acting servo hydraulic cylinders that use hydraulic oil or other mineral oils with equivalent performance as the working medium.
② The internal leakage of single-acting and double-acting servo hydraulic cylinders with an inner diameter of 40–500 mm shall not exceed the specified value under rated working pressure.
③ For double-acting or single-acting servo hydraulic cylinders with an inner diameter greater than 500 mm, the internal leakage shall be below 0.8 MPa after applying the rated working pressure to the rodless chamber, opening the oil port of the rod chamber, and holding the pressure for 5 minutes.
④ Except for the piston rod (plunger rod), there must be no leakage at any other part.
⑤ When the piston rod (plunger rod) is stationary, there must be no leakage at any other part.
⑥ For a double-acting servo hydraulic cylinder, after 50,000 full-stroke piston reversal cycles, there should be no dripping leakage at the piston rod. After 50,000 reversal cycles, for every 100m movement of the piston, when the piston rod diameter d ≤ 50mm, the external leakage qv ≤ 0.05mL; when the piston rod diameter d > 50mm, the external leakage qv ≤ 0.001dmL.
⑦ For a single-acting piston-type servo hydraulic cylinder, after 40,000 full-stroke piston reversal cycles, there should be no dripping leakage at the piston rod. After 40,000 reversal cycles, for every 80m movement of the piston, when the piston rod diameter d ≤ 50mm, the external leakage qv ≤ 0.05mL; when the piston rod diameter d > 50mm, the external leakage q ≤ 0.001dmL.
⑧ Single-acting servo hydraulic cylinder with plunger: 25,000 reversing cycles throughout the plunger’s full stroke, with no dripping external leakage at the plunger rod. After 25,000 reversing cycles, for every 65m plunger movement, when the plunger diameter d ≤ 50mm, the external leakage q ≤ 0.05mL; when the plunger rod diameter d > 50mm, the external leakage qv ≤ 0.001dmL.
⑨ Durability.
a. Double-acting servo hydraulic cylinder: When piston stroke L ≤ 500mm, cumulative stroke ≥ 100km; when piston stroke L > 500mm, cumulative reversing cycles N ≥ 200,000.
b. Single-acting servo cylinder with piston: When piston stroke L ≤ 500mm, cumulative stroke ≥ 100km; when piston stroke L > 500mm, cumulative reversing cycles N ≥ 200,000.
c. For single-acting plunger-type servo cylinders, when the plunger stroke L ≤ 500 mm, the cumulative stroke ≥ 75 km; when the plunger stroke L > 500 mm, the cumulative reversing times N ≥ 150,000 times.
After the durability test, the increase in internal leakage must not exceed twice the specified value, and parts should not exhibit abnormal wear or other types of damage.
⑩ The cylinder body of the servo hydraulic cylinder should be able to withstand a pressure of 1.5 times the nominal pressure. During pressure holding for 5 minutes, there should be no external leakage, deformation of parts, or damage.
1.3.8.2 General Technical Requirements for the Quality of Sealing Products in Hydraulic Cylinders
(1) Appearance Quality
Under natural conditions, when the sealing product is observed under appropriate lighting with a 2x magnifying glass, the surface should not have defects exceeding the allowable limits, or other surface defects such as cracks, breaks, bubbles, or impurities.
The working surface of the rubber sealing ring should be flat and smooth, and should not have pores, impurities, cracks, bubbles, scratches, or axial flow marks.
The working surface of the fabric-reinforced rubber sealing ring should not have broken threads, exposed fabric, delamination, bubbles, impurities, or unevenness. For fabric-reinforced rubber sealing rings with the parting surface on the working surface, the edge height, width, and repair depth should not exceed 0.2mm. Unevenness of the fabric layer is allowed at the corners.
①The appearance quality of O-ring rubber seals for hydraulic and pneumatic applications should comply with the relevant provisions in global standard.
② The appearance quality requirements for reciprocating rubber seals and their pressure rings, support rings, and retaining rings shall comply with the relevant provisions of global standard.
③ The appearance quality of polyurethane seals may refer to the relevant provisions of global standard.
④ Some terms may refer to the terms and definitions specified in global standard.
(2) Dimensions and Tolerances
The measurement of the dimensions of rubber seal specimens shall be carried out in accordance with the relevant provisions of global standard “General Procedures for Specimen Preparation and Conditioning in Physical Testing Methods for Rubber”.
Rubber and plastic seals for hydraulic cylinders shall be selected according to the following standards:
Note: Based on the definition of “rubber sealing products” in global standard, please define “rubber and plastic sealing products” for hydraulic cylinders as follows: Rubber and plastic components used to prevent fluid leakage from the hydraulic cylinder sealing device and to prevent dust, sediment, and air (for high vacuum) from entering the sealing device.
① Global standard “O-rings for Hydraulic and Pneumatic Use – Part 1: Dimensional Series and Tolerances”.
The corresponding groove standard is: “O-rings for Hydraulic and Pneumatic Use – Groove Dimensions”.
② Global standard “Reciprocating Rubber Seal Structural Dimension Series – Part 1: One-Way Seal Rubber Seals”. The corresponding groove standard is: “Dimensions and Tolerances of Dynamic Seals for Hydraulic Cylinder Pistons and Piston Rods”.
The grooves specified in global standard “Dimensional Series and Tolerances of Narrow Section Dynamic Seal Grooves for Hydraulic Cylinder Pistons and Piston Rods” currently lack suitable sealing rings.
③ Global standard “Structural Dimensional Series of Reciprocating Rubber Seals Part 2: Bidirectional Sealing Rubber Seals”. The corresponding groove standard is “Types, Dimensions and Tolerances of Seal Grooves with Support Rings for Hydraulic Cylinder Pistons”.
④ Global standard “Structural Dimensional Series of Reciprocating Rubber Seals Part 3: Rubber Dustproof Seals”. The corresponding groove standard is “Groove Types, Dimensions and Tolerances of Dustproof Rings for Hydraulic Cylinder Piston Rods”.
⑤ Global standard “Dimensional Series and Tolerances of Coaxial Seals for Dynamic Seal Devices of Hydraulic Cylinder Pistons and Piston Rods”. The corresponding groove standard is: “Dimensional Series and Tolerances of Mounting Grooves for Dynamic Sealing Devices of Hydraulic Cylinder Pistons and Piston Rods”.
⑥ Global standard “Dimensional Series and Tolerances of Support Rings for Dynamic Sealing Devices of Hydraulic Cylinder Pistons and Piston Rods”.
The corresponding groove standards are: “Dimensional Series and Tolerances of Mounting Grooves for Support Rings for Dynamic Sealing Devices of Hydraulic Cylinder Pistons and Piston Rods”, “Yx-type Sealing Rings for Holes”, “Yx-type Sealing Rings for Shafts”, and “Combination Sealing Gaskets”.
Global standard, “Tolerances of Rubber Products—Part 1: Dimensional Tolerances,” applies to products made of vulcanized rubber and thermoplastic rubber, but not to precision annular seals. However, standard specifies that “the dimensional tolerances of the seals shall meet the requirements of grade M1.”
(3) Hardness
Whether using a Shore hardness tester or a portable international hardness tester, the hardness of rubber is measured by the combined effect of indentation depth on the rubber surface, which is used to represent the hardness measurement result. International hardness (IRHD) is a measure of rubber hardness, derived from the indentation depth of a sample under specified conditions using a given indenter.
Although correction values for the conversion between Shore hardness and IRHD have been established for some rubbers and compounds, it is now not recommended to directly convert Shore hardness (Shore A, Shore D, Shore AO, Shore AM) values to IRHD values.
The hardness of vulcanized rubber and thermoplastic rubber can be determined by the methods specified in standard “Test method for indentation hardness of vulcanized rubber or thermoplastic rubber – Part 1: Shore hardness tester method (Shore hardness)” or standard “Test method for indentation hardness of vulcanized rubber or thermoplastic rubber – Part 2: Portable rubber international hardness tester method” and standard “Determination of hardness of vulcanized rubber or thermoplastic rubber (10～100 IRHD)”. Standard stipulates that “micro hardness testers used for O-ring hardness determination shall comply with the relevant provisions of global standards.”
The microscopic hardness test method specified in global standard is essentially a scaled-down conventional test method, applicable to rubbers with hardness in the range of 35–85 IRHD (and also applicable to rubbers with hardness in the range of 30–95 IRHD) and a sample thickness of less than 4 mm.
The hardness of polyurethane sealing rings specified in standard is as follows:
① At 23℃, the hardness value of the monomer sealing ring should be 90±5 Shore A.
② At 23℃, the hardness value of the outer ring of the composite sealing ring should be greater than 90 Shore A.
③ At 23℃, the hardness value of the inner ring of the composite sealing ring should be greater than 70 Shore A.
(4) Tensile Strength
Tensile strength is the maximum tensile stress during the stretching process of a specimen to fracture. Dumbbell-shaped specimens are preferred for determining tensile strength. The tensile strength of vulcanized rubber and thermoplastic rubber can be determined using the method specified in standard, “Determination of Tensile Stress-Strain Properties of Vulcanized Rubber or Thermoplastic Rubber.” The principle is as follows: A dumbbell-shaped specimen is stretched on a tensile testing machine with a moving clamp or pulley moving at a constant speed, and the maximum force during the continuous stretching process is recorded as required.
The tensile strength of polyurethane sealing rings specified in standard is as follows:
① At 23℃, the outer ring tensile strength of both monomeric and composite sealing rings should be greater than 35 MPa.
② At 23℃, the inner ring tensile strength of composite sealing rings should be greater than 16 MPa.
(5) Elongation at Break
Elongation at break is the percentage elongation of the specimen at fracture. Under the following conditions, the elongation at break of a ring-shaped specimen can be approximately the same as that of a dumbbell-shaped specimen:
① The elongation of the ring-shaped specimen is calculated as a percentage of the initial inner circumference.
② If the “calendering effect” is significant, the dumbbell-shaped specimen is cut with its length direction perpendicular to the calendering direction.
The elongation at break of vulcanized rubber and thermoplastic rubber can be determined using the method specified in standard “Determination of Tensile Stress-Strain Properties of Vulcanized Rubber or Thermoplastic Rubber”. The principle is as follows: A dumbbell-shaped or ring-shaped standard specimen is stretched on a tensile testing machine with a moving clamp or pulley moving at a constant speed, and the elongation at break is recorded as required.
The elongation at break of polyurethane O-rings specified in standard is:
① At 23℃, the elongation at break of the monomer O-ring should be greater than 400%.
② At 23℃, the elongation at break of the outer ring of the composite sealing ring should be greater than 350%.
③ At 23℃, the elongation at break of the inner ring of the composite sealing ring should be greater than 260%.
(6) Compression Permanent Deformation
Rubber inevitably undergoes physical and chemical changes when compressed. When the compressive force is removed, these changes prevent the rubber from returning to its original state, thus resulting in permanent deformation. The magnitude of compression permanent deformation depends on the temperature and time of compression, as well as the temperature and time of recovery. At high temperatures, chemical changes are the main cause of compression permanent deformation in rubber. Compression permanent deformation is measured after the compression force applied to the sample is removed and the height is recovered at a standard temperature. At low temperatures, changes caused by glass hardening and crystallization are dominant. These effects disappear when the temperature rises. Therefore, the test height must be measured at the test temperature.
In The test principles given in standard are divided into room temperature and high temperature test principles and low temperature test principles:
Room temperature and high temperature test principle: Under standard laboratory temperature, a sample of known height is compressed to a specified height according to the required compression ratio. Under specified temperature conditions, the compression is maintained for a certain time. Then, under standard temperature conditions, the compression is removed, and the sample is allowed to recover in a free state for a specified time. The height of the sample is then measured.
Low-Temperature Test Principle: Under standard laboratory temperature, a specimen of known height is compressed to a specified height according to the required compression ratio. At a specified low-temperature test temperature, this compression is maintained for a certain time. Then, the compression is removed at the same low temperature, allowing the specimen to recover in a free state. The specimen height is measured at regular intervals at low temperature, resulting in a logarithmic curve of specimen height versus time. This curve is used to evaluate the compression set characteristics of the specimen.
① Room Temperature Compression Set. The test is conducted at room temperature, with a test temperature of (23±2)℃.
The room temperature compression set of polyurethane O-rings specified in standard is:
a. The compression set of a single-unit O-ring should be less than 25%.
b. The outer ring compression set of the composite sealing ring should be less than 30%.
② High-temperature compression set. For testing under high-temperature conditions, the test temperature can be (100±1)℃, (125±2)℃, (150±2)℃, (175±1)℃, (200±2)℃, etc.
The high-temperature compression set of polyurethane sealing rings specified in standard is as follows:
a. The compression set of the monolithic sealing ring should be less than 45%.
b. The outer ring compression set of the composite sealing ring should be less than 50%.
c. The inner ring compression set of the composite sealing ring should be less than 30%.
(7) Liquid Resistance
The effects of liquids on vulcanized rubber or rubber generally result in the following:
① Liquid is absorbed by the rubber.
② Soluble components in the rubber are extracted.
③ A chemical reaction occurs with the rubber.
Usually, the amount absorbed (①) is greater than the amount extracted (②), leading to an increase in rubber volume; this phenomenon is called “swelling.” The tensile strength, elongation at break, and force of rubber that has absorbed liquid will change significantly. Because plasticizers and preservatives in rubber are easily extracted in volatile liquids, the physical and chemical properties of the rubber will also change significantly after drying. Therefore, it is important to determine the properties of rubber after drying or further drying.
Standard, “Test Methods for Liquid Resistance of Vulcanized Rubber or Rubber,” specifies the evaluation of the effect of liquid on rubber by testing the changes in its properties before and after liquid drying during the test.
Standard, “Test Methods for O-ring Rubber Seals,” provides methods for calculating the percentage change in mass and volume.
The water resistance requirements for polyurethane seals specified in standard are as follows:
The water resistance (over 8 weeks) of the outer ring of polyurethane, polypropylene, and composite seals should meet the following requirements:
① Thickness change decreases by less than 9%.
② Elongation strength change decreases by less than 18%.
③ Elongation at break change decreases by less than 9%.
④ Volume change is less than 6%.
⑤ Mass change is less than 6%.
(8) Hot Air Aging Performance
Accelerated aging and fatigue testing of vulcanized rubber or rubber under normal pressure and high temperature air involves aging the vulcanized rubber in air at high temperature and atmospheric pressure and measuring its properties. A comparative test is then conducted on the properties of the unaged vulcanized rubber. Commonly measured physical properties include tensile strength, stress at a given elongation, elongation at break, and thickness.
Standard, “Test Methods for Rubber Seals,” “Accelerated Aging and Fatigue Testing of Vulcanized Rubber or Rubber in Hot Air.”
Standard specifies the following performance requirements for the outer rings of polyurethane seals, polypropylene seals, and composite seals after aging:
They must meet the following requirements:
① Hardness change decreases by less than 8%.
② Tensile strength change decreases by less than 10%.
③ The change in elongation at break should decrease by less than 12%.
Standard specifies that the inner rings of polyurethane and composite sealing rings, after aging, should meet the following performance requirements:
① The change in hardness should decrease by less than 8%.
② The change in tensile strength should decrease by less than 10%.
③ The change in elongation at break should decrease by less than 30%.
(9) Low Temperature Performance
Standard, “Determination of Low Temperature Properties of Rubber – Temperature Rebound Method (TR Test),” specifies a method for determining the temperature rebound performance of stretched rubber. The principle is as follows: the rubber is appropriately stretched, and then cooled to a sufficiently low potential at which no rebound occurs when the tensile force is removed. After removing the tensile force, the rubber is uniformly heated. The potential at the specified rebound rate is measured.
Standard, “Test Method for Circular Rubber Seals”.
Standard, “Rubber, Determination of Compression Set at Room Temperature, High Temperature and Low Temperature.” Standard specifies that the inner and outer rings of polyurethane, polypropylene, and composite sealing rings, after low-temperature treatment, should meet the following performance requirements:
① Hardness decreases by less than 8%.
② Tensile strength decreases by less than 10%.
③ Elongation at break decreases by less than 12%.
However, there is a lack of standard basis for requiring the determination of the above three performance items.
(10) Reliability
The reliability of a seal (ring) refers to its ability to maintain its sealing performance under specified conditions and within a specified time. Reliability is determined by a combination of factors, including design, manufacturing, use, and maintenance; therefore, reliability is a comprehensive performance indicator.
The term reliability sometimes also broadly encompasses general performance, indicating availability (effectiveness) and durability. The reliability of seals specified in JB/T 10205-2010 mainly includes pressure resistance (including pressure resistance) and durability; please refer to standard “Hydraulic Cylinders” for details.
Standard specifies that the reliability of polyurethane seals should meet the requirement of 21,000 cycles of testing.
However, without “specified conditions,” the specified durability tests are generally not repeatable or comparable. Therefore, it is appropriate to measure the reliability of seals according to the relevant provisions in standard.
(11) Operating Temperature Range
Standard “Hydraulic Cylinders” stipulates that “Under normal circumstances, the operating ambient temperature of a hydraulic cylinder should be within the range of -20~+50℃, and the working medium temperature should be within the range of -20~+80℃.” It also stipulates that when the product has high-temperature requirements, “Under controlled pressure, 90℃ working oil is input into the hydraulic cylinder under test, and the entire stroke is repeated for 1 hour; it should meet the high-temperature requirements of the hydraulic cylinder agreed upon by both parties.”
Standard “Reciprocating Rubber Seal Materials” stipulates that: “The reciprocating rubber seal materials specified in this standard are divided into two categories, A and B. Category A is nitrile rubber material, divided into three functional grades and five types of rubber compounds, with an operating temperature range of -30~+100℃; Category B is cast polyurethane rubber material, divided into four functional grades and four types of rubber compounds, with an operating temperature range of -40~+80℃.”
In standard specifies that polyurethane sealing rings should be able to withstand temperatures ranging from -20°C to +60°C.
(12) Sealing Pressure Range
Sealing pressure refers to the pressure that the sealing ring withstands from the sealing medium during operation. Generally speaking, the sealing pressure of a sealing ring is closely related to the sealing material, structure, sealing medium and temperature, vibration type, dimensions and tolerances, clearance on one side, surface quality of mating parts, and relative motion speed. Therefore, the sealing pressure or sealing pressure range must meet certain conditions to be determined.
Standard “Hydraulic and Pneumatic Rubber Seals – Part 1: Dimensional Series and Tolerances” and standard “Support Dimensions of Hydraulic and Pneumatic Rubber Seals” do not specify the sealing pressure and range. Standard “Hydraulic Transmission Connections with Metric Connections and O-ring Seals – Ports and Stud Ends – Part 1: Ports” specifies: “The maximum working pressure applicable to the ports specified in this part is 63 MPa. The allowable working pressure should be determined based on factors such as port size, material, structure, operating conditions, and application.”
In standard, “Reciprocating Rubber Seal Rings – Structural Dimensions Series Part 1: Unidirectional Sealing Rubber Seals,” specifies working pressure ranges for Y-shaped rubber seals (0-25 MPa), lace-shaped rubber seals (0-50 MPa), and V-shaped combination seals (0-60 MPa).
Standard, “Reciprocating Rubber Seal Rings – Structural Dimensions Series Part 2: Bidirectional Sealing Rubber Seals,” specifies working pressure ranges for drum-shaped rubber seals (0.10-70 MPa) and mountain-shaped rubber seals (0-25 MPa).
Although standard, “Reciprocating Rubber Seal Rings – Structural Dimensions Series Part 3: Rubber Dustproof Seals,” specifies that C-type dustproof seals have an auxiliary sealing function, it does not specify the sealing pressure.
According to standard, “Dimensional Series and Tolerances of Coaxial Seals for Dynamic Sealing Devices of Hydraulic Cylinder Pistons and Piston Rods,” stipulates: “This standard applies to the seals of reciprocating hydraulic cylinder pistons and piston rods (plungers) that use hydraulic oil as the working medium, with a pressure ≤40MPa, a speed ≤5m/s, and a temperature range of -40~+200℃.”
Standards, “Yx-type Seals for Holes,” and, “Yx-type Seals for Shafts,” stipulate: “This standard applies to Yx-type seals for holes (shafts) that provide sealing when using air or mineral oil as the medium, with a temperature of -40(-20)~+80℃ and a working pressure p≤31.5MPa.”
In standard, “Combination Sealing Gaskets,” specifies that “this standard only specifies combination gaskets for welding, ferrule, flared pipe joints, and screw plug seals, with a nominal pressure of 40 MPa and an operating temperature of -25 to +80℃.”
Standard, “Rotary Shaft Lip Seals with Elastomer Sealing Elements—Part 1: Basic Dimensions and Tolerances,” specifies that “this part applies to rotary lip seals with shaft diameters of 6 to 400 mm and mating cavities of 16 to 440 mm, but not to rotary shaft lip seals with pressures exceeding 0.05 MPa.”
Standard specifies the sealing pressure range for polyurethane sealing rings: the sealing pressure range for bidirectional polyurethane sealing rings is 2 to 60 MPa; the sealing pressure range for unidirectional polyurethane sealing rings is 2 to 40 MPa.
(13) Other Performance Requirements
Standard “Test Methods for O-ring Rubber Seals” stipulates: “This standard specifies the test methods for solid flexible rubber seals, including dimensional measurement, power, compression performance, hot air aging, power deformation, compression set, corrosion test, liquid resistance, density, compressibility, low temperature test, and compression stress aging.” However, this standard does not provide performance indicators for the seals.
The standard, “Technical Conditions for Polyurethane Sealing Rings for Coal Mine Columns and Jacks,” stipulates: “This standard specifies the terms and definitions, sealing lever dimensions, requirements, test methods, inspection rules, marking, packaging, transportation, and storage of polyurethane sealing rings for coal mine columns and jacks. This standard applies to polyurethane sealing rings for coal mine columns and jacks using high-water-content hydraulic oil (including emulsions) as the working medium.” However, this standard does not provide specific test methods for the sealing ring performance; it only cites the following documents in the “Normative References”:
① Standard “Determination of Tensile Stress Hardness Properties of Rubber or Rubber”.
② Standard “Test Methods for Indentation Hardness of Rubber or Rubber – Part 1: Shore Hardness Tester Method (Shore Hardness)”.
③ Standard “Test Methods for Fluid Resistance of Rubber or Rubber”.
④ Standard “Rubber or Rubber – Accelerated Aging in Hot Air and Rubber Tests”.
⑤ Standard “Tolerances of Rubber Products – Part 1: Dimensional Tolerances”.
⑥ Standard “Rubber, Rubber – Determination of Compression Set at Normal, High and Low Temperatures”.
These standards generally specify “rubber” prepared according to relevant standards, not the actual sealing ring.
Besides the above standards, there are other standards for testing the performance of sealing components (rings), such as rubber properties and the solubility expansion index of metal adhesion, which can further determine the performance of sealing rings.
★★★Review of previous selected articles
- Classification and Structure of Hydraulic Cylinders
- Hydraulic Cylinder Parameters and Parameter Calculation
- Hydraulic Cylinder Parameter Calculation Example
- Technical requirements for design and manufacturing of hydraulic cylinders-Barrel tube
- Technical requirements for Piston and Piston rod
- Technical requirements for Cylinder head and cylinder bottom
- Technical requirements for guide sleeves
- Several Design Methods for Flow Regeneration Using Differential Circuits
- Design, calculation and processing points of hydraulic cylinders: recommended collection
- Piston rod that is too smooth or too rough will affect the sealing life of the hydraulic cylinder