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1.3.3 Technical requirements for pistons
The previous articles in this series discussed the Hydraulic Cylinder Parameters and Parameter Calculation; Hydraulic Cylinder Parameter Calculation Example; Technical requirements for design and manufacturing of hydraulic cylinders-Barrel tube, today this article will explain Technical requirements for design and manufacturing of hydraulic cylinders-Piston and Piston rod.
Standard defines the term “piston” as a cylinder component that moves within a cylinder bore and transmits mechanical force and motion by the action of a fluid under pressure. Typically, the piston seals the cylinder bore (diameter) with a seal, dividing the bore (diameter) into two chambers.
1.3.3.1 Structural Type
Hydraulic cylinder pistons should have sufficient strength and guide length. Pistons that require welding to the piston rod must be made of materials with good weldability.
The sealing groove structure on the piston determines whether the piston is integral or modular. The piston’s sealing system (combination of seals, guides, and other components) must be rational, reliable, and have a long service life. The connection between the piston and the piston rod must have a reliable connection structure (including locking measures), sufficient strength, and be easy to disassemble and assemble.
For various piston structures, please refer to the drawings in subsequent articles in this newsletter.
Pistons in hydraulic cylinders of the same model produced by the same manufacturer must be interchangeable.
1.3.3.2 Materials
Common materials used to manufacture pistons are as follows:
① Gray cast iron grades, such as HT200, HT250, and HT300.
② Ductile iron grades, such as QT400-15, QT400-18, and QT450-10.
③ Carbon structural steel grades, such as Q235 and Q275.
④ High-quality carbon structural steel grades, such as 20, 30, 35, and 45.
⑤ Alloy structural steel grades, such as 40Cr.
⑥ Low-alloy high-strength structural steel grades, such as Q345.
⑦ Others, such as aluminum alloys, composite materials, and plastics.
During the hydraulic cylinder pressure test, ensure that the piston does not exhibit permanent deformation, including collapse. Under rated static pressure, none of the failure modes specified in standard “Test Methods for Rated Fatigue Pressure and Rated Static Pressure of Pressure Vessels of Hydraulic Components,” shall occur in the tested pressure chamber.
The yield strength of the piston material for large hydraulic cylinders shall be no less than 280 MPa.
Steel should not be used as the piston material for pistons with clearances of H8/f8 or H9/f9 or smaller between the cylinder bore and the piston outer diameter.
Note: In addition to the meaning defined in standard, the term “crush” in the above-mentioned application also includes the failure mode caused by compressive stress between contacting parts, which is different from the failure mode caused by compressive stress in high-pressure mechanisms. “Crush” has the same meaning in the rest of this document.
1.3.3.3 Heat Treatment
Standard “Hydraulic Cylinders for Marine Hatch Covers,” requires that the piston material be quenched and tempered 45 steel.
After being welded to the piston rod, the piston should be heat treated or other stress-reducing methods should be used to eliminate internal stress.
1.3.3.4 Geometric Dimensions and Geometric Tolerances
During design, the piston bore axis (or piston rod axis) is generally determined as the reference element.
(1) Basic Dimensions
The basic dimensions of the piston include the piston (nominal) outer diameter, the piston outer diameter, the piston thickness, the seal, the guide and the connection dimensions.
(2) Piston (nominal) outer diameter
① The piston (nominal) outer diameter should preferably use the recommended size in Table 1-18.
Table 1-18 Recommended piston (nominal) outer diameter mm
Note: Dimensions in parentheses are non-preferred.
② The outer diameter tolerance of the piston that slides directly against the inner bore of the cylinder body (barrel) should be f8 or f9. The matching options are H8/f8 or H9/f9.
③The standard specifies the pistonsnbsp;matching outer diameter. The pistons (nominal) outer diameter D and matching piston outer diameter D1nbsp;specified in standardnbsp;Types, Dimensions, and Tolerances of Seal Grooves with Support Rings for Hydraulic Cylinder Pistons, are shown in Table 1-19.
Table 1-19 Piston outer diameter dimensions mm
Note: Dimensions in parentheses are non-preferred.
The D dimension in Table 1-19 is equal to the standard seal groove outer diameter (cylinder inner diameter); the D1 dimension is equal to the standard piston mating diameter.
④Piston outer diameter: The outer diameter, tolerance, geometric tolerance, surface roughness, etc. of the piston shall be selected in accordance with relevant standards and the requirements of the selected seal sample.
(3) Piston thickness
The piston thickness is determined by the guide length and the sealing structure (sealing system), and is generally 0.6 to 1.0 times the (nominal) outer diameter of the piston.
(4) Roundness tolerance
The roundness tolerance of the outer surface of the piston with a fit of H8/f8 or H9/f9 shall be in accordance with Grade 7 as specified in global standard.
(5) Cylindricity tolerance
The cylindricity tolerance of the outer surface of the piston with a fit of H8/f8 or H9/f9 shall be in accordance with Grade 8 as specified in global standard.
(6) Coaxiality tolerance
The coaxiality tolerance of the outer surface of the piston (matching) with the inner bore axis (or piston rod axis) shall not be less than Grade 8 specified in global standard; the coaxiality tolerance of the outer surface of the piston with a fit of H8/f8 or H9/f9 with the inner bore axis (or piston rod axis) shall be Grade 7 specified in global standard.
(7) Perpendicularity tolerance
The perpendicularity tolerance of the piston end face with the axis shall not be less than Grade 7 specified in global standard; the radial runout tolerance of the piston end face with the axis shall not be less than Grade 8 specified in global standard.
1.3.3.5 Surface Quality
(1) External Surface
① The external surface roughness of the piston with a combination of H8/f8 or H9/f9 is generally not greater than Ra0.8μm, and can also be selected from Table 1-20 according to design requirements.
② The surface should not have visual defects such as shrinkage holes, inclusions (slag), white spots, ripples, scratches, bumps, pits, cracks, scars, peeling and rust.
Sharp edges on the piston, except for the edges of the sealing groove, should be removed if not shown on the working drawing.
Table 1-20 Surface roughness μm
(2) End face
The surface roughness of the piston end face should generally not exceed Ra3.2μm, and can also be selected from Table 1-20 according to design requirements; however, the surface roughness of the end face selected as the inspection benchmark should generally not exceed Ra0.8μm.
1.3.4 Technical Requirements for Piston Rods
“Piston rod” as a cylinder component coaxially connected in parallel with the piston, transmitting mechanical force and motion from the piston.
Because there is no piston in a plunger cylinder, pressure acts directly on the piston rod. Therefore, it is more accurate to define the piston rod as the cylinder component that transmits mechanical force and motion.
1.3.4.1 General
Hydraulic cylinder piston rods should have sufficient strength, rigidity, and impact toughness. For combined piston rods requiring welding (hollow or piston-to-piston rod welded together), the material must have good weldability. The weld strength should not be lower than the strength index of the parent material, and the weld quality should meet Grade II as specified in global standard.
The connection between the piston rod and piston must have a reliable connection structure (including locking measures), sufficient connection strength, and be easy to disassemble and assemble (or, alternatively, the piston should be securely fixed to the piston).
The piston rod and the buffer plunger in a fixed buffer device should ideally be integrally formed.
The piston rod’s structural design must enhance longitudinal bending strength and stability under compression.
Piston rods with externally or internally threaded ends should be provided with standard wrench flats, wrench holes, or slots. These flats may be omitted if the piston rod is too small to accommodate the required flats.
Welded closed hollow piston rods must include a vent hole at the outer connection end of the piston rod.
For various piston rod structural types, please refer to the drawings in subsequent articles to this newsletter.
Piston rods of the same model and manufactured by the same manufacturer must be interchangeable.
1.3.4.2 Materials
The mechanical properties of the material used to manufacture the piston rod should generally include a yield strength of not less than 280 MPa. For piston rods with a chromium coating, the tensile strength should be greater than or equal to 345 MPa. Commonly used materials are as follows:
① High-quality carbon structural steel grades, such as 35, 45, and 50. ② Alloy structural steel grades, such as 27SiMn, 30CrMo, 30CrMnSiA, 35CrMo, 40Cr, and 42CrMo.
② Stainless steel grades, such as 12Cr18Ni9 and 14Cr17Ni2.
③ Cast carbon steel grades, such as ZG270-500 and ZG310-570.
④Others. Materials suitable for special operating conditions include forged aluminum, machined bronze, malleable cast iron, and chilled cast iron.
For the mechanical properties of commonly used piston rod materials, see Tables 1-7 and 1-8.
1.3.4.3 Heat Treatment
Piston rods should generally be quenched and tempered after rough machining. For structural steel, see Table 1-8 or Table 1-21 for tempered hardness. For piston rods subject only to unidirectional loads, low nominal pressures, small cylinder bores, or short strokes, quenching and tempering is not necessary.
The piston rod’s outer diameter sliding surface is preferably case hardened. For structural steel, see Table 1-21 for case hardening hardness. Tempering is required after case hardening.
Under certain conditions, normalizing or normalizing combined with tempering can be used in place of quenching and tempering.
For welded, combined piston rods (hollow or welded piston and piston rod), internal stresses should be eliminated by heat treatment or other stress-reducing methods.
For piston rods made of 45 steel, the quenched and tempered hardness should generally be 241-286 HRC. After quenching and tempering the sliding surface, the hardness should be 42-45 HRC, and should be relatively uniform.
Table 1-21 Reference values for hardness of structural steel for piston rods
The surface hardness of the piston rod should be relatively uniform. The hardness difference can be found in Table 1-22.
Table 1-22 Hardness difference
1.3.4.4 Geometric dimensions and geometric tolerances
(1) Basic dimensions
The basic dimensions of the piston rod include the piston rod outer diameter, piston rod length (sliding surface length or guide surface length), piston rod thread type and size (end connection type and size), piston connection type and size, and buffer plunger type and size.
(2) Piston rod outer diameter
① The piston rod outer diameter d shall comply with the provisions of global standard, see Table 1-23.
Table 1-23 Piston rod outer diameter mm
The outer diameter of the piston rod of large hydraulic cylinders shall comply with the requirements of Table 1-24.
Table 1-24 Large hydraulic cylinder piston rod outer diameter mm
② Piston rod outer diameter tolerance. The outer diameter dimension tolerance of the piston rod guide surface shall not be less than f8.
For hydraulic cylinders with special requirements, H8/h7 can also be used for the piston rod and guide sleeve, but the guide sleeve material cannot be steel.
The limit deviations of shaft f7, f8, and h7 are shown in Table 1-25.
(3) Piston rod thread type and size
The piston rod thread refers to the external connection thread of the hydraulic cylinder piston rod.
The standard specifies three types of piston rod threads: internal thread (Figure 1-1), shoulderless external thread (Figure 1-2), and shouldered external thread (Figure 1-3).
Table 1-25 Limit deviations of shafts f7, f8, and h7 μm
The thread type and size of the piston rod shall comply with the provisions of global standard, and the thread size of the piston rod shall comply with the provisions of Table 1-26.
Table 1-26 Piston rod thread size mm
Note: 1. Thread length L for internal threads refers to the minimum dimension; for external threads, it refers to the maximum dimension.
2. When a lock nut is required, use the long thread length.
The connecting thread on the piston rod should be a fine-pitch ordinary thread M with grade 6 accuracy.
(4)Geometric tolerances
① The roundness tolerance of the piston rod guide surface (before electroplating) shall not be less than Grade 8 specified in global standard; the roundness tolerance of the fine processing (polishing or grinding) after electroplating shall not be less than Grade 9 specified in global standard.
② The straightness tolerance of the element line of the piston rod guide surface shall not be less than Grade 8 specified in global standard.
③ The cylindricity tolerance of the piston rod guide surface shall be Grade 8 specified in global standard.
④ The perpendicularity tolerance of the end face used for piston installation to the piston rod axis shall be selected according to Grade 7 specified in global standard.
⑤ The radial runout tolerance of the piston rod guide surface relative to the axis of the cylinder on which the piston is mounted shall not be less than Grade 7 as specified in global standard; or the coaxiality tolerance shall not be less than Grade 7 as specified in global standard.
⑥ The radial runout tolerance of the buffer plunger relative to the axis of the cylinder (or guide surface) on which the piston is mounted shall not be less than Grade 7 as specified in global standard.
⑦ When there is a connecting pin hole at the end of the piston rod, the dimensional tolerance of the hole diameter shall be H11 as specified in global standard; the perpendicularity tolerance of the pin hole axis relative to the piston rod axis shall not be less than Grade 9 as specified in global standard; the distance between the trunnion centerline and the cylinder centerline shall not exceed 0.03mm.
⑧The coaxiality tolerance between the guide surface of the piston rod and the matching surface of the guide sleeve shall not be lower than Grade 8 specified in global standard.
1.3.4.5 Surface Quality
① The sliding surfaces of the piston rod and guide sleeve should be hard chrome plated. The thickness of the chrome coating (after single-sided polishing or grinding) should generally be between 0.03 and 0.05 mm. The hardness of the chrome coating should be between 800 and 1000 HV. Finishing should be performed after plating. The coating must be smooth, fine, uniform, and dense, free of any defects such as peeling, flaking, or blistering. Under the designed maximum load, the chrome coating must be free of cracks. The exposed portion of the piston rod at the end of the cylinder’s return stroke should also be hard chrome plated.
In addition to hard chrome plating, the piston rod’s outer surface can also be coated with an electroless nickel-phosphorus alloy.
For piston rods with special requirements, thermally sprayed alloys or ceramic coatings may be used.
The outer surface of the piston rod without hard chrome plating should be smooth and free of visible defects such as shrinkage cavities, inclusions (slag), white spots, ripples, scratches, bumps, pits, cracks, scars, warping, and rust.
Sharp edges on the piston rod, except for the ridges in the sealing grooves on the integrated piston and piston rod structure, not shown on the working drawing, should be removed.
② The sliding surface roughness of the piston rod and guide sleeve should generally not exceed Ra0.4μm. Alternatively, the roughness value can be selected from Table 1-27 based on design requirements.
Table 1-27 Surface roughness μm
1.3.4.6 Other Requirements
① The lead-in chamfer for installing seals should be determined according to the requirements of the selected seals, but the intersection with the outer diameter must be rounded.
② The finished piston rod should retain an intact center hole.
③ For piston rod connections where the slider (driven by a hydraulic cylinder) poses a risk of accidental drop, a risk assessment should be conducted during design and an expected service life should be specified. Users must promptly replace the piston rod upon reaching the expected service life.
④ For plunger cylinders and other equipment where the piston rod may become ejected (disconnected) during testing, installation, commissioning, operation, or maintenance, a travel limit stop device should be installed. If the stop device is ineffective, released, or removed, pressurization must not be applied to the working chambers of the hydraulic cylinder to prevent any hazards arising from cylinder failure.
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