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1.3.1 Technical Requirements for Hydraulic Cylinders
The previous article in this series discussed the Hydraulic Cylinder Parameter Calculation Example; today this article will explain Technical requirements for design and manufacturing of hydraulic cylinders-Barrel tube, this help it easier to better understand the hydraulic cylinder barrel tube.
(1) Applicability
① Anti-instability. To avoid bending or instability of the piston rod of the hydraulic cylinder at any position, attention should be paid to the stroke length, load and installation type of the cylinder.
② Structural design. The design of the hydraulic cylinder should take into account the predetermined maximum load and pressure peak.
③ Installation rating. When determining all rated loads of the hydraulic cylinder, its installation type should be considered.
Note: The rated pressure of the hydraulic cylinder only reflects the pressure bearing capacity of the cylinder body, but does not reflect the force transmission capacity of the installation structure.
④ Load caused by limit. When the hydraulic cylinder is used as a limiter, the size of the hydraulic cylinder and its installation type should be determined according to the maximum load caused by the restricted part.
⑤ Anti-shock and vibration. Any components and accessories installed on or connected to the hydraulic cylinder should be installed or connected to prevent loosening caused by shock and vibration during use.
⑥ Accidental pressurization. Measures should be taken in the hydraulic system to prevent the pressure from accidentally increasing beyond the rated pressure due to the difference in effective piston area.
(2) Installation and adjustment
The best installation method for hydraulic cylinders is to make the reaction generated by the load act along the center line of the hydraulic cylinder. The installation of hydraulic cylinders should minimize the following situations:
Excessive deformation of the hydraulic cylinder structure due to load thrust or tension; lateral or bending loads; rotation speed of the articulated installation type (which may force the use of continuous external lubrication).
① Installation position. The mounting surface should not deform the hydraulic cylinder and should leave a margin for thermal expansion. The installation position of the hydraulic cylinder should be easily accessible to facilitate maintenance, adjustment of the buffer device and replacement of the entire set of components.
② Installation fasteners. The selection and installation of fasteners for the installation of hydraulic cylinders and their accessories should be able to withstand all foreseeable forces. Hydraulic cylinders mounted on tripods may apply shear forces to their mounting bolts. If shear loads are involved, it is advisable to consider using hydraulic cylinders with shear load-bearing mechanisms. The installation fasteners should be sufficient to withstand overturning moments.
(3) Buffers and deceleration devices
When using internal buffers, the design of the hydraulic cylinder should take into account the effect of pressure increase caused by load deceleration.
(4) Adjustable stroke end stop
Measures should be taken to prevent the external or internal adjustable stroke end stop from loosening.
(5) Piston stroke
If the stroke length (including tolerance) is not specified in the relevant standards, it should be specified according to the application of the hydraulic system.
Note: For the tolerance of the stroke length, refer to global standard, or “maximum stroke” or “limit stroke” can be added to the basic parameters of the hydraulic cylinder to accommodate cylinders with variable stroke length, such as “adjustable stroke hydraulic cylinders”.
(6) Piston rod
① Material, surface treatment and protection. Appropriate piston rod material and surface treatment should be selected to minimize wear, corrosion and foreseeable collision damage. The piston rod should be protected from foreseeable damage such as indentation, scratches and corrosion, and a protective cover can be used.
② Assembly For assembly, piston rods with threaded ends shall be designed to allow the application of opposing forces with a wrench, see ISO 4395. The piston shall be securely fixed to the piston rod.
(7) Maintenance of sealing devices and wearing parts
Sealing devices and other wearing parts that are scheduled for maintenance should be easily replaced.
(8) Single-acting hydraulic cylinders
Single-acting piston hydraulic cylinders should be designed with a vent port and set in an appropriate position to avoid danger to personnel caused by the ejected oil spray.
(9) Replacement
Integral hydraulic cylinders are not desirable, but when they are used, parts that may wear should be replaceable.
(10) Gas discharge
① Vent position. When installing hydraulic cylinders on fixed industrial machinery, they should be able to automatically vent or provide easily accessible external vent ports. When installing, the vent port of the hydraulic cylinder should be in the highest position. When these requirements cannot be met, relevant maintenance and use information should be provided.
② Exhaust port. Hydraulic cylinders with an air chamber should be designed or equipped with an exhaust port to avoid danger. The hydraulic cylinder should be able to discharge air without danger using the exhaust port.
(11) Sealing performance
The seal should meet the technical requirements of the working medium and working conditions.
At 1.5 times or 1.25 times the nominal pressure, there should be no external leakage at all joint surfaces including the sliding surface (at rest).
At the nominal (or rated) pressure, the internal leakage and external leakage at the piston rod of the hydraulic cylinder should meet the standard requirements.
The seal of the hydraulic cylinder should be resistant to high temperature, corrosion, aging, and hydrolysis, and have good sealing performance, which can meet the requirements of both oil sealing and the environment (such as marine air).
①For hydraulic cylinders not used for weight lifting, the seal is recommended to adopt a sealing structure of support ring plus dynamic seal, and the support material is recommended to be filled with bronze powder tetrafluoroethylene or reinforced polyformaldehyde with long molecular chain.
②For hydraulic cylinders used for weight lifting, for the oil chamber where oil leakage will cause the weight to fall, the dynamic seal should adopt rubber sandwiched fabric V-shaped seal ring.
③For some hydraulic cylinders, the internal leakage can be calibrated (calculated) by the settlement amount.
(12) Assembly requirements
① Components should use qualified parts and purchased parts, and be assembled in accordance with the provisions and requirements of relevant product standards or technical documents. Any deformed, damaged or rusted parts and purchased parts cannot be used for assembly.
② After 100 non-destructive testing, the cylinder body should meet the global standard requirements; the weld strength should not be lower than the strength index of the parent material, and the weld quality should meet the requirements of standard; for hydraulic cylinders with hinged shafts, the hinged shaft should be integrally forged and should meet the requirements after 100 non-destructive testing.
③ Sharp edges on machined parts, except for the edges of seal grooves, not shown on the working drawing, should be removed. Parts should be deburred and thoroughly cleaned before assembly. They should be free of any contaminants such as iron filings, burrs, and fibrous impurities.
④ During assembly, cotton yarn, paper, or other fibrous materials should not be used to wipe the cylinder cavity, mating surfaces, or inlet and outlet passages.
⑤ Defective seals and seals that have exceeded their effective service life should not be used during assembly. Dust rings and seals must not be scratched, twisted, curled, or loose.
⑥ After passing the factory test, the exposed oil ports of the hydraulic cylinder should be covered with dust caps, the exposed threads of the piston rod and other connection points should be protected with protective sleeves, and the moving parts should be coated with rust-proof grease.
⑦ A clear and permanent mark or label should be placed in an appropriate and conspicuous location on the hydraulic cylinder, or a prefabricated label should be fixed in the position specified in the drawing. The label should be clear, correct, and flat.
(13) Appearance requirements
① The outer surface of the hydraulic cylinder should not have defects such as folds and obvious waves, cracks, burrs, bumps, scratches, rust, etc.
② The exposed surface should be treated with anti-corrosion (rust) by using a coating, passivation layer, paint layer, etc. The coating should be free of defects such as cracks, peeling, shedding or cavitation; the surface should be derusted or descaled before painting, and there should be no rust pits.
③ When painting, anti-rust paint should be applied first, followed by topcoat. The coating should be uniform, consistent in color, and smooth and smooth. Putty should not be applied before spraying.
(14) Safety Technical Requirements
The design and manufacture of hydraulic cylinders must provide basic safety guarantees for users within the specified service life. Any hydraulic machinery may be dangerous during adjustment, use and maintenance, so users can only reduce or eliminate the danger by adjusting, using and maintaining it according to the safety technical conditions or requirements of the hydraulic machinery.
① If necessary, a risk assessment should be conducted during the design of hydraulic cylinders used in special occasions.
② Measures to reduce risks should be taken during design and manufacture, such as reliable anti-loosening measures for various fasteners.
③ For dangers that cannot be avoided in the design and manufacture of hydraulic cylinders, such as unexpected travel of the slider (driven by the hydraulic cylinder) and accidental drop of its own weight, the main manufacturer should take safety protection measures, including risk (hazard) warnings.
④ For more specific safety technical requirements (conditions), please refer to the relevant standards and comply with them.
1.3.2 Technical requirements for cylinder barrel tube
The cylinder body is the main body of a hydraulic cylinder. In a broad sense, a hydraulic cylinder body refers to all the pressure-bearing components that form this special, sealed pressure vessel, encompassing nearly all components of a hydraulic cylinder. In a narrower sense, a hydraulic cylinder body refers to the hollow, pressure-bearing part within which the piston and/or piston rod reciprocate relative to each other. A cylinder body can be closed at one end or open at both ends. A tubular cylinder body that is open at both ends is called a cylinder barrel tube.
The cylinder body is one of the key components of a hydraulic cylinder and must be manufactured from specified materials and meet specific technical requirements.
The cylinder body described in this article is a cylinder body in the narrower sense. The inner bore of the cylinder body is circular, but not necessarily tubular. Its end faces must be perpendicular to the centerline (axis) of the inner bore.
For various cylinder body (barrel) structural types, please refer to the drawings in subsequent serialized articles.
Note: It is more appropriate to understand the hydraulic cylinder body (barrel) as a metal pressure-bearing shell. For details, please refer to standard “Fatigue pressure test of metal pressure-bearing shells in hydraulic transmission – Part 1: Test method.”
1.3.2.1 General
The hydraulic cylinder body (barrel) should have sufficient strength, rigidity, plasticity, and impact toughness. For cylinder bodies (barrels) requiring subsequent welding of the cylinder bottom (end cover), the material (whether the same steel or a different steel as the cylinder bottom) must exhibit good weldability. The weld strength should not be lower than the strength index of the parent material, and the weld quality should meet standard. The hydraulic cylinder oil port boss (tube) and the cylinder bottom (end cover) should be welded simultaneously.
For hydraulic cylinders requiring wear or corrosion resistance within the cylinder body (barrel), a lining structure made of a suitable material may be installed within the cylinder bore.
Hydraulic cylinder bodies (barrels) of the same model produced by the same manufacturer must be interchangeable.
For hydraulic cylinders used in special applications, for example, a risk assessment of the cylinder body (barrel) should be conducted during design.
1.3.2.2 Materials
For hydraulic cylinders with product standards or those with specified engine standards, the cylinder body or barrel should be selected in accordance with the relevant standards. The term “cylinder body” refers broadly to the hydraulic cylinder body.
The mechanical properties of the material used to manufacture the cylinder body (barrel) should generally have a yield strength of no less than 280 MPa. Commonly used materials are as follows:
① High-quality carbon structural steel grades, such as #20, #30, #35, #45, 20Mn, and 25Mn.
② Alloy structural steel grades, such as 20MnMo, 20MnMoNb, 27SiMn, 30CrMo, 40Cr, and 42CrMo.
③ Low-alloy, high-strength structural steel grades, such as Q345B.
④ Stainless steel grades, such as 12Cr18Ni9.
⑤ Cast carbon steel grades, such as ZG270-500 and ZG310-570.
⑥ Ductile iron grades, such as QT500-7, QT550-3, and QT600-3.
During the hydraulic cylinder pressure test, ensure that the cylinder body (barrel) does not exhibit permanent deformation. Under rated static pressure, no failure modes specified in standard “Test Methods for Rated Fatigue Pressure and Rated Static Pressure of Pressure Vessels of Hydraulic Components,” should occur.
For hydraulic cylinders operating in ambient temperatures below -50°C, the cylinder body (barrel) material must be quenched and tempered #35, #45 steel, or low-temperature steel.
For the mechanical properties of commonly used cylinder body (barrel) materials, see Table 1-7.
Table 1-7 Mechanical Properties of Commonly Used Cylinder (Barrel) Materials
Note: Since the mechanical properties of 20 steel are relatively low and it cannot be quenched or tempered, and the lower limit of the operating temperature of 20 steel pipe is only 0℃, it should generally be used as little as possible.
1.3.2.3 Heat treatment
Castings and forgings used in cylinder body (barrel) manufacturing should eliminate internal stresses through heat treatment or other stress-reducing methods. Forged steel (cast steel), high-quality carbon structural steel, and alloy structural steel used in cylinder body (barrel) manufacturing should be quenched and tempered before or after rough machining. The mechanical properties of cylinder body (barrels) manufactured from quenched and tempered 35, 45, 40Cr, and 42CrMo steel forgings are shown in Table 1-8.
Note: Heat treatment of cylinder body (barrels) can be expressed as follows: Quenching and Tempering should be performed before or after rough machining. Cylinder blocks that comply with global standard should be delivered as agreed upon between the supplier and the purchaser. The supplier should provide special instructions on the heat treatment status of the delivered cylinder blocks upon request.
Table 1-8 Mechanical properties of carbon and alloy structural steel forgings
For cylinder body (barrels) that use welded connections to cylinder bottoms (end covers) , heat treatment cannot be used to eliminate internal stress. Instead, methods such as vibration aging and static pressure stretching can be used to eliminate internal stress, and the methods used should be subject to process verification.
1.3.2.4 Geometric dimensions and geometric tolerances
(1) Basic dimensions
The basic dimensions of the cylinder body include the cylinder bore, the position of the cylinder body inner hole, the cylinder body shape, the cylinder body (inner hole) length and the oil port size;
For a tubular cylinder barrel with an inner hole concentric with the outer circle, the basic dimensions of the cylinder barrel include the cylinder bore, the cylinder outer diameter or the cylinder wall thickness (cylinder wall thickness is preferred) and the cylinder length.
(2) Cylinder bore
① The cylinder bore should preferably use the recommended dimensions shown in Table 1-9.
Table 1-9 Recommended cylinder bore dimensions mm
Note: Dimensions in parentheses are non-preferred.
For large hydraulic cylinders with an inner diameter of not less than 630 mm, the inner diameter should preferably be the recommended size shown in Table 1-10.
Table 1-10 Recommended inner diameter of large hydraulic cylinders mm
Note: The so-called “large hydraulic cylinder” is not defined in the current standard. It is debatable whether a hydraulic cylinder with a cylinder diameter D ≥ 630mm is a large hydraulic cylinder or a large hydraulic cylinder.
② The cylinder bore tolerance should be H8. For medium- and low-pressure or long cylinders, H9 or H10 may also be used.
The upper limit deviation (ES) values for holes H8, H9, and H10 are shown in Table 1-11.
Table 1-11 Upper limit deviation (ES) values for holes H8, H9, H10 um
(3) Cylinder barrel tube outer diameter
The allowable deviation of the cylinder barrel tube outer diameter shall not exceed ±0.5% of the nominal size of the cylinder outer diameter.
(4) Cylinder wall thickness
The cylinder wall thickness shall be based on the strength calculation results. While ensuring sufficient safety margin, the closest recommended value in Table 1-12 shall be preferred.
Table 1-12 Recommended cylinder wall thickness mm
(5) Cylinder wall thickness deviation
The cylinder wall thickness deviation shall comply with the requirements of Table 1-13.
Table 1-13 Permissible deviation of cylinder wall thickness mm
(6) Cylinder Length
The inner bore length of the cylinder body (barrel) must meet the hydraulic cylinder stroke requirements. The length deviation should comply with the requirements of Table 1-14 and refer to the cylinder stroke length tolerance (see Table 1-15).
Table 1-14 Permissible deviation of cylinder length mm
Table 1-15 Cylinder stroke length tolerance mm
(7) Geometric tolerances
During design, the inner bore axis of the cylinder body (barrel) is generally determined as the reference element.
① Inner bore roundness. The inner bore roundness of the cylinder body (barrel) is divided into four grades, and its tolerance value is expressed as a percentage less than the inner diameter tolerance value. The corresponding relationship is as follows: Grade A – 50% Grade – 60% Grade – 70% Grade – 80%
Or the roundness tolerance of the inner bore of the cylinder body (barrel) may be required to be no less than Grade 8 in standard.
② Inner bore axis straightness. The inner bore axis straightness of the cylinder barrel is divided into four grades: Grade A – 0.06/1000; Grade B – 0.20/1000; Grade C – 0.50/1000; Grade D – 1.00/1000. The corresponding values can be referred to Table 1-16.
Note: 0.06/1000 means the straightness tolerance is φ0.06mm over a length of 1000mm.
③ Inner bore surface straightness. Alternatively, the straightness tolerance of any 100mm of the inner bore surface may be required to be no less than Grade 7 in standard. Straightness tolerance value are shown in Table 1-16.
④ Inner bore surface parallelism relative to the inner bore line. The inner bore cylindricity error is composed of inner bore roundness, inner bore axis straightness, and inner bore surface parallelism relative to the inner bore line. The inner bore surface parallelism tolerance should be no less than Grade 8 in standard. Parallelism tolerance values are shown in Table 1-16.
⑤ Inner bore cylindricity. Depending on functional requirements, the cylindricity tolerance value of the cylinder bore may be specified separately, especially when the cylindricity tolerance value is required to be less than the combined result of its components. The cylinder bore cylindricity tolerance should be no less than Grade 8 in standard. Cylindricity tolerance values are shown in Table 1-16.
⑥ Cylinder (barrel) end face perpendicularity. The cylinder (barrel) end faces must be perpendicular to the inner bore axis (centerline). The perpendicularity tolerance between the cylinder flange end face and the cylinder bore axis must not be lower than Grade 7 in standard. The axial circular runout tolerance of the cylinder flange end face must not be lower than Grade 8 in standard. Perpendicularity and circular runout tolerances are shown in Table 1-16.
⑦ Trunnion perpendicularity and position. When the cylinder (barrel) is equipped with a fixed trunnion (front, middle, or rear end), the perpendicularity tolerance of the trunnion centerline to the cylinder centerline must not be lower than Grade 9 in standard. Perpendicularity tolerance values are shown in Table 1-16. The distance between the trunnion centerline and the cylinder centerline must not exceed 0.03mm.
Table 1-16 Geometric tolerance values μm
Note: The parallelism of the inner hole surface relative to the element line is line-to-line parallelism, and the reference line is another element line opposite to the measured element line.
⑧ When the cylinder body (barrel) is connected to the cylinder bottom (head) or end cover by thread, the thread should be a fine-pitch ordinary thread M with a grade 6 precision.
1.3.2.5 Mechanical properties
The mechanical properties of the cylinder barrel made entirely by machining should not be lower than the mechanical properties requirements specified in the standard of the material used.
The cold-drawn cylinder is affected by the material and processing technology, and its material mechanical properties are determined by the supplier and the buyer.
1.3.2.6 Surface quality
(1) Inner hole surface
① The surface roughness value of the inner hole should generally not exceed Ra0.4μm, and can also be selected from Table 1-17 according to design requirements.
② The inner hole surface is smooth and should not have visually visible defects such as shrinkage holes, inclusions (slag), white spots, ripples, scratches, bumps, pits, cracks, scars, warping and rust.
Table 1-17 Inner hole surface roughness μm
(2) External surface
The exterior surface should be free of visible defects such as shrinkage cavities, inclusions (slag), folds, ripples, cracks, scratches, bumps, and pull marks caused by built-up edge on the outer die during cold drawing.
Sharp edges on the cylinder body (barrel), except for the edges of sealing grooves, not shown on the working drawing, should be removed.
The exterior surface of the cylinder body (barrel) should be treated with rust prevention. Corrosion protection can also be achieved through plating, passivation, or paint. Before painting, the exterior surface should be free of scale and rust pits. When painting, apply anti-rust paint first, followed by topcoat. The paint layer should be free of defects such as scabs.
1.3.2.7 Solidity
For cylinder bodies made of forgings, 100° flaw detection shall be performed. After nondestructive testing, the cylinder body shall meet the requirements.
The welds between the cylinder bottom (head) and the cylinder body shall be subjected to 100° flaw detection. The quality shall meet the requirements of global standard.
Solidity testing shall be conducted by eddy current testing according to Acceptance Level A in GB/T 7735-2004, or by magnetic flux leakage testing according to Acceptance Level L4 in GB/T 12606.
Under the pressure test pressure, there shall be no leakage on the outer surface or welds of the cylinder body (barrel).
1.3.2.8 Other Requirements
① The lead-in chamfer for installing the seal should be determined according to the requirements of the selected seal, but the intersection with the inner bore must be rounded. Grooves, keyways, and flow channel intersections that the seal must pass through during installation should be rounded or chamfered and deburred.
② It is generally believed that the finishing process for the cylinder barrel (body) should be compatible with the sealing material of the seal (ring). After honing the inner bore of the cylinder barrel (body), it should be polished. Rolled bores, including a combined boring-rolling (scraping and rolling) process using a two- or three-edged tool, can also be polished.
Note: For further information on the selection of rolled and honing processes, please pay attention to the subsequent updated articles in our newsletter.
③ The inner bore of the cylinder barrel (barrel) can also undergo internal surface treatment, including hard chrome plating, but must be polished or ground after plating.
④ The strength and rigidity of the cylinder barrel (barrel) should be verified as necessary.
