The cylinder barrel must have sufficient strength and impact toughness, and welded cylinder barrels must also have good weldability. Annealed cold-drawn or hot-rolled seamless steel pipes can be used, with materials such as 20, 35, 45, and 27SiMn. Cylinder barrels operating at temperatures below -50°C must be made of 35 or 45 steel, which has been quenched and tempered.
① The cylinder barrel inner diameter should be H7 or H8 grade, with a surface roughness Ra generally between 0.16 and 0.32 μm. Grinding is required. The inner diameter roundness, taper, and cylindricity must not exceed half the inner diameter tolerance.
② Heat Treatment: Quenched and tempered, hardness HB ≥ 241-285.
③ The cylinder barrel straightness tolerance must not exceed 0.03 mm over a 500 mm length.
④ The perpendicularity of the cylinder end face to the cylinder diameter must not exceed 0.04mm over a 100mm diameter.
⑤ The deviation of the axis of the piercing hole at the bottom of the cylinder from the cylinder diameter must not exceed 0.03mm, and the perpendicularity to the cylinder diameter must not exceed 0.1mm over a 100mm length.
⑥ The threaded oil port on the cylinder must be free of burrs and flash, and the chamfer length must not exceed 0.5mm. The inner bore of the cylinder should be chamfered at a 15° angle to prevent damage to the seal during assembly.
⑦ To prevent corrosion and rust and to increase service life, the inner surface of the cylinder may be chrome-plated and then ground or polished.
The piston rod and guide sleeve are generally H8/h7 or H8/f7. The coaxiality tolerance between the piston mounting journal and the outer diameter must not exceed 0.01mm, and the perpendicularity tolerance between the piston mounting shoulder end face and the piston rod axis must not exceed 0.04mm/100mm.
The piston rod's outer diameter roughness (Ra) is generally 1-3 μm (favoring oil film formation). The piston rod surface is chrome-plated to a thickness of 3-5 μm and then polished or ground to a surface hardness of 52-62 HRC. For harsh operating conditions with a high probability of collision, high-frequency quenching is required before chrome plating.
The piston rod and guide sleeve are mated to an H8/f7 tolerance, and the piston is mated to an H8/h8 tolerance. The piston rod's outer surface straightness should not exceed 0.03 mm over a length of 500 mm. The piston rod should have sufficient strength and rigidity (see Chapter 2 for calculations).
When the piston rod is fully extended, the distance from the midpoint of the piston support surface to the midpoint of the guide sleeve sliding surface is called the minimum guide length. If the guide length is too short, the initial deflection of the cylinder caused by clearance will increase, affecting the cylinder's operating performance and stability. For general hydraulic cylinders, the minimum guide length should meet the following requirements:
H≥
(where L is the maximum working stroke; D is the cylinder diameter, both in meters)
The guide sleeve's sliding surface length A: When the cylinder diameter is less than 80mm, A = (0.6-1)D; when the cylinder diameter is greater than 80mm, A = (0.6-1)d.
Metal guide sleeves are generally made of bronze, gray cast iron, ductile iron, or oxidized cast iron, which have low friction coefficients and good wear resistance. The fit between the guide sleeve inner diameter and the piston rod outer diameter is usually H9/f9. The coaxiality tolerance between the outer diameter and the inner bore is no more than 0.03mm, and the roundness and cylindricity tolerances are no more than half the diameter tolerance.
The connection between the piston and the piston rod must have a reliable locking mechanism, and a static seal must be provided between the piston and the piston rod. Pistons without guide rings are made of high-strength cast iron HT200-300 or ductile iron. Pistons with guide rings are made of high-quality carbon steel 20, 35, or 45. Tin bronze can be used for the outer ring of assembled pistons. The piston width is generally 0.6 to 1 times the cylinder diameter. The piston outer diameter is generally F9. The coaxiality tolerance of the outer diameter to the inner bore shall not exceed 0.04 mm/100 mm. The roundness and cylindricity of the outer surface shall generally not exceed half the outer diameter tolerance.
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