English 
Spanish 
1.3.5 Technical requirements for cylinder heads
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. Today this article will explain Technical requirements for cylinder head and cylinder bottom.
1.3.5.1 Structural Type
The term “cylinder head” here specifically refers to the rod-end end cap (cylinder head) of a hydraulic cylinder.
The cylinder head can be integral with the guide sleeve or a separate structure (pressure cap). It can also be classified as either sealed or non-sealed, depending on whether there are seals between the cylinder head, cylinder body (barrel), piston rod, and guide sleeve.
The connection between the cylinder head and cylinder body (barrel) must have a reliable structure (including locking measures), sufficient strength, and be easy to assemble and disassemble.
Common methods for connecting the cylinder head and cylinder body (barrel) include flange connections, internal (external) threaded connections, internal (external) keyed connections, and tie rod connections. Because easy assembly and disassembly of the cylinder head is generally required, welding is not typically used.
Due to the different locations of hydraulic cylinder oil ports on the cylinder head, there are axial and radial ports.
For a list of various cylinder head structural types, please refer to the drawings in further articles in this newsletters.
Integral cylinder heads for hydraulic cylinders of the same model, manufactured by the same manufacturer, must be interchangeable.
1.3.5.2 Materials
Common materials for one-piece cylinder heads are as follows:
① Carbon structural steel grades, such as Q235 and Q275.
② High-quality carbon structural steel grades, such as #20, #30, #35, and #45.
③ Alloy structural steel grades, such as 27SiMn, 30CrMo, and 40Cr.
④ Low-alloy high-strength structural steel grades, such as Q345.
⑤ Stainless steel grades, such as 12Cr18Ni9 and 14Cr17Ni2.
⑥ Cast carbon steel grades, such as ZG270-500 and ZG310-570.
⑦ Gray cast iron grades, such as HT200, HT250, and HT300.
⑧ Ductile iron grades, such as QT400-15, QT400-18, and QT450-10.
⑨ Others. Such as bimetallic, (die) cast aluminum, cast copper, etc.
During the hydraulic cylinder pressure test, ensure that the cylinder head does not exhibit permanent deformation, including collapse. Under rated static pressure, no failure modes of the tested pressure chamber specified in standard, “Test Methods for Rated Fatigue Pressure and Rated Static Pressure of Pressure Chambers of Hydraulic Components,” should occur.
The yield strength of the cylinder head material for large hydraulic cylinders should be no less than 280 MPa.
For cylinder head materials with H8/f7, H8/h7, H8/f8, or H9/f9 inner bore and piston rod outer diameter combinations, steel cannot be used.
1.3.5.3 Heat treatment
The cylinder head should generally be quenched and tempered after rough machining, especially forged steel and cast steel that have not been normalized or annealed. They should be heat treated after rough machining, but high-temperature tempering is required after quenching.
For hydraulic cylinder heads with small bore diameter, low nominal pressure, good operating conditions, or cylinder heads made of non-quenched and tempered steel, heat treatment is not required.
1.3.5.4 Geometric dimensions and geometric tolerances
During design, the axis of the cylinder head inner hole is generally determined as the standard element.
(1) Basic dimensions
The basic dimensions of the integral structure flange connection sealing cylinder head include the cylinder head inner diameter, outer diameter, (guide) length, seal, guide and flange dimensions.
(2) Cylinder head inner diameter
① The cylinder head (nominal) inner diameter should preferably use the recommended values shown in Table 1-28.
Table 1-28 Recommended cylinder head (nominal) inner diameter dimensions mm
② The inner diameter tolerance of the cylinder head portion corresponding to the guide sleeve shall not be less than H9 in global standard; H8 is generally selected.
(3) Cylinder head outer diameter
① The outer diameter of the cylinder head (the portion that fits the cylinder body (barrel)) should preferably use the recommended values shown in Table 1-29.
Table 1-29 Recommended outer diameter of the cylinder head mm
Note: Dimensions in parentheses are non-preferred.
② The outer diameter tolerance of the matching part of the cylinder head and the cylinder body (barrel) is generally selected from f7; however, for some special-purpose hydraulic cylinders or hydraulic cylinders with special requirements, the matching of the cylinder head and the cylinder body (barrel) can be selected between H7/k6 or H8/k7 to H8/g7.
(4) Flange size
The outer diameter dimensions of the cylinder head flange of standard hydraulic cylinders for metallurgical equipment (PN≤25MPa) are shown in Table 1-30.
Table 1-30 Outer diameter dimensions of cylinder head flange of hydraulic cylinders for metallurgical equipment (PN≤25MPa) mm
The circumferential distribution of the hydraulic cylinder’s oil inlet and outlet ports, mounting bolts, exhaust valve, gland, and bracket is shown in global standard of Hydraulic Cylinder Design Drawings.
The outer diameter dimensions of the cylinder head flange for standard hydraulic cylinders for metallurgical equipment (PN ≤ 16 MPa) are shown in Table 1-31.
Table 1-31: Outer Diameter Dimensions of the Cylinder Head Flange for Hydraulic Cylinders for Metallurgical Equipment (PN ≤ 16 MPa) mm
The circumferential distribution of the hydraulic cylinder’s oil inlet and outlet holes, fixing bolts, exhaust valve, gland, and bracket can be found in global standard of Hydraulic Cylinder Drawing Design.
(5) Geometric tolerances
① The roundness tolerance of the cylinder head inner hole shall not be lower than Grade 7 specified in global standard.
② The cylindricity tolerance of the cylinder head inner hole shall not be lower than Grade 8 specified in global standard.
③ The cylindricity of the mating portion between the cylinder head and the cylinder body (barrel) shall not be lower than Grade 8 specified in global standard.
④ The coaxiality tolerance of the outer surface of the cylinder head [the mating portion with the cylinder body (barrel)] to the axis of the cylinder head inner hole (equivalent to the guide sleeve) shall not be lower than Grade 7 specified in global standard.
⑤ The perpendicularity tolerance of the end face (flange) of the cylinder head against the gland or the cylinder body and the end face installed in the rod cavity of the hydraulic cylinder to the axis of the cylinder head inner hole shall not be lower than Grade 7 specified in global standard.
1.3.5.5 Surface Quality
(1) Inner Hole Surface
① The inner hole surface roughness value should not exceed Ra1.6μm, generally selected as Ra0.8μm, or can be selected from Table 1-32 according to design requirements.
②The inner hole surface should be smooth and should not have visual defects such as shrinkage holes, inclusions (slag), white spots, ripples, scratches, bumps, pits, cracks, scars, peeling and rust.
Table 1-32 Inner hole surface roughness μm
(2) End face
The surface roughness of the end face of the cylinder head installed in the rod cavity should not exceed Ra1.6μm; however, the surface roughness of the end face selected as the inspection benchmark should generally not exceed Ra0.8μm.
(3) External surface
The external surface should not have visually visible defects such as shrinkage holes, inclusions (slag), folds, ripples, cracks, scratches, bumps, and pull marks caused by the built-up edge of the outer mold during cold drawing.
The sharp edges on the cylinder head, except for the edges of the sealing groove, which are not shown in the working drawing, should be removed.
The external surface of the cylinder head should be treated with anti-rust treatment, and can also be anti-corrosion by plating, passivation layer, paint layer, etc. The external surface should be free of oxide scale and rust pits before painting. When painting, anti-rust paint should be applied first, followed by topcoat, and the paint layer should not have defects such as scars.
1.3.6 Technical requirements for cylinder bottom
1.3.6.1 General
The hydraulic cylinder base should have sufficient strength, rigidity, and impact toughness. For bases that will be later welded to the cylinder body (barrel), the material must exhibit good weldability. The weld strength should not be lower than the parent material’s strength index, and the weld quality should meet Level II as specified in global standard.
The welds between the cylinder bottom and the cylinder body (barrel) must undergo 100° flaw detection according to the method specified in global standard. The weld quality must meet the Class I requirements specified in global standard.
The connection between the cylinder bottom and the cylinder body (barrel) must have a reliable connection structure (including locking measures and sufficient connection strength).
The cylinder bottoms (cylinder bodies) of the same model hydraulic cylinders produced by the same manufacturer must be interchangeable.
1.3.6.2 Structural Type
The term “cylinder bottom” here specifically refers to the rodless end cap (end) of the hydraulic cylinder.
Depending on the hydraulic cylinder mounting type, the cylinder bottom is integrated with the rear end mounting brackets (single or double) and/or round (square or rectangular) flanges.
Except for the buffer plunger, the hydraulic cylinder’s cushioning device is generally located on the cylinder bottom.
Due to the different locations of the hydraulic cylinder oil ports on the cylinder bottom, there are axial and radial ports.
Besides the cylinder base being an integral structure with the cylinder body (barrel), other common connection methods for the cylinder base and the cylinder body (barrel) include flanges, internal (external) threads, tie rods, and welding. Welding is the most common connection (fixation) method, i.e., a welded cylinder base that utilizes a lock-bottom butt weld.
Cylinders (barrels) with welded bases are generally referred to as closed or cylindrical cylinders.
The inner end surface of welded or forged cylinder bases is often flat, but other concave shapes include elliptical, dished, spherical, and hemispherical. Cylinder bases with axial oil inlet and outlet ports or filling valve mounting holes in the center of the base are called perforated bases.
For a variety of cylinder base structures, please refer to the drawings in newsletters follow-up articles.
1.3.6.3 Materials
The yield strength of the material used to manufacture the cylinder bottom should be 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 27SiMn, 30CrMo, 40Cr, and 42CrMo.
③ Low-alloy high-strength structural steel grades, such as Q345.
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 bottom does not experience permanent deformation, including collapse. Under the rated static pressure, any failure mode of the tested pressure chamber specified in standard “Test method for rated fatigue pressure and rated static pressure of pressure chambers of hydraulic components” shall not occur.
1.3.6.4 Heat Treatment
Castings and forgings used in cylinder bottoms should be heat treated or other stress-reducing methods to eliminate internal stresses.
Cylinder bottoms should generally be quenched and tempered after rough machining. Forged and cast steels that have not been normalized or annealed should be heat treated after rough machining. However, high-temperature tempering must be performed after quenching.
For hydraulic cylinder bottoms with small bores, low nominal pressures, good operating conditions, or those made of non-quenched and tempered steel, heat treatment is not necessary.
1.3.6.5 Geometric dimensions and geometric tolerances
During design, the axis of the buffer hole is generally determined as a reference element.
(1) Basic dimensions
The basic dimensions of the welded cylinder bottom include the cylinder bottom stop diameter, stop height, outer diameter, cylinder bottom thickness and connection dimensions; if a buffer device (such as a buffer cavity hole) is designed on the cylinder bottom, it shall be specified separately.
(2) Stop diameter
①The stop diameter shall be selected according to the inner diameter of the cylinder (body) to be installed.
②The stop diameter dimension tolerance shall generally be selected according to js7 specified in global standard; if a buffer cavity hole is designed on the cylinder bottom, the fit between the cylinder bottom and the cylinder (barrel) can be selected as H7/k6 or H8/k7.
(3) Stop height
The stop height of the welded cylinder bottom shall be able to keep the piston seal at least 20 mm away from the butt weld.
(4) Cylinder bottom thickness
The cylinder bottom thickness can generally be selected as 1.2 to 1.3 times the cylinder wall thickness.
Note: For calculation of cast iron, cast steel and concave cylinder bottom thickness.
(5) Geometric tolerances
① The cylindricity of the stopper (where the cylinder bottom and cylinder body meet) shall not be lower than Grade 8 as specified in global standard.
② The coaxiality tolerance of the stopper to the axis of the buffer hole shall not be lower than Grade 7 as specified in global standard.
③ The perpendicularity tolerance of the end face where the cylinder bottom and cylinder body meet and the axis of the cylinder bottom shall not be lower than Grade 7 as specified in global standard.
④ The perpendicularity tolerance of the threaded oil port sealing surface to the threaded diameter shall not be lower than Grade 6 as specified in global standard; it may also be selected according to global standard.
⑤ The perpendicularity tolerance of the pin hole axis to the axis of the cylinder body (barrel) shall not be lower than Grade 9 as specified in global standard.
1.3.6.6 Surface Quality
The exterior surface shall be free of visible defects such as shrinkage cavities, inclusions (slag), folds, ripples, cracks, scratches, bumps, and rust.
Sharp edges on the cylinder bottom, except for the edges of the sealing grooves, not shown in the working drawings, shall be removed.
The exterior surface of the cylinder bottom shall be treated with an anti-rust treatment. Corrosion protection may also be achieved by coating, passivation, or paint. The exterior surface shall be free of scale and rust pits before painting. When painting, anti-rust paint shall be applied first, followed by topcoat. The paint layer shall be free of defects such as scabs.
1.3.6.7 Solidity
For cylinder bottoms made of forgings, 100° flaw detection shall be performed. After nondestructive testing of the cylinder body, the quality shall meet the requirements of Grade I specified in global standard.
The welds between the cylinder bottom and the cylinder body (barrel) shall be subjected to 100° flaw detection according to the method specified in global standard. The quality shall meet the requirements of Grade I specified in global standard.
Solidity testing shall be conducted by eddy current testing according to acceptance level A in global standard, or by magnetic flux leakage testing according to acceptance level L4 in global standard.
Under the pressure test pressure, there shall be no leakage on the outer surface of the cylinder bottom or in the welds.
★★★Review of previous selected articles:
- Design, calculation and processing points of hydraulic cylinders: recommended collection
- 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
- Disassembly methods and precautions of hydraulic cylinder
- Difference between single-acting cylinder and double-acting cylinder. How to choose?
- Why is the hydraulic cylinder still leaking oil after replacing the sealing ring
- How to select a hydraulic cylinder? Here are the precise selection steps!