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Today I will share with you the technical requirements of the main structure of the hydraulic cylinder.
01. Barrel tube
1. Sufficient strength to withstand the maximum operating pressure and short-term dynamic test pressure for extended periods without permanent deformation.
2. Sufficient rigidity to withstand piston lateral forces and installation reaction forces without bending.
3. The inner surface, piston rod seal, and guide ring must withstand long-term operation with minimal wear due to friction. Dimensional and geometric tolerances must be sufficient to ensure the sealing performance of the piston seal.
4. Cylinder barrels that require welding must also exhibit good weldability to prevent cracking or excessive deformation after flanges or pipe fittings are welded on.
02. Cylinder head
1. The cylinder head inner bore dimensional tolerance is generally H8, with a roughness of no less than 0.8µm.
2. The cylindricity error between the cylinder head inner bore and the outer diameter (D) of the stopper must not exceed half the diameter tolerance. The axis runout must not exceed 0.04mm over a diameter of 100mm.
03. Piston
1. The piston width is generally 0.6 to 1.0 times the piston outer diameter, but it also depends on the type and number of seals and the groove dimensions of the guide ring. Sometimes, the piston width can be determined in conjunction with the layout of the spacer ring.
2. The basic deviation of the piston outer diameter is generally f, g, or h. Rubber-sealed pistons can have tolerances of 7, 8, or 9, piston ring seals use grades 6 and 7, gap seals use grade 6, and leather seals use grades 8, 9, or 10. The cylinder barrel tube and piston generally have a clearance fit based on the basic bore. When the piston uses a rubber seal, the cylinder barrel tube inner bore can adopt tolerances of H8 or H9, forming a clearance fit with the piston of H8/f7, H8/f8, H8/g8, H8/h7, H8/h8, H9/g8, H9/h8, or H9/h9.
3. The piston inner bore tolerance is generally H7, forming a transition fit of H7/g6 with the piston rod diameter. The coaxiality tolerance of the outer diameter to the inner hole is no more than 0.02mm, the perpendicularity tolerance of the end face to the axis is no more than 0.04mm/10mm, and the roundness and cylindricity of the outer surface are generally no more than half of the outer diameter tolerance. The surface roughness varies depending on the structural form. Generally, the surface roughness of the outer diameter and inner hole of the piston can be taken as Ra = 0.4 ~ 0.8um.
04. Piston rod
1. The piston rod slides in the guide sleeve, typically using an H8/H7 fit. Too tight will result in excessive friction; too loose will easily cause seizure and unilateral wear. The cylindricity and roundness tolerances should be greater than half the diameter tolerance.
2. The coaxiality tolerance between the piston shaft diameter and the outer diameter should be no greater than 0.01mm. This ensures the coaxiality of the piston rod outer diameter with the piston outer diameter to prevent seizure between the piston and the cylinder barrel, or between the piston rod and the guide sleeve. The perpendicularity tolerance between the piston shoulder end face and the piston rod axis should be no greater than 0.04mm/100mm to ensure that the piston is installed without skew.
3. The outer diameter roughness Ra value of the piston rod is generally 0.1-0.3µm. If the surface is too smooth, an oil film will not form, impeding lubrication. To improve wear resistance and rust resistance, the piston rod surface should be chrome-plated with a thickness of 0.03-0.05mm, then polished and ground.
4. For harsh operating conditions and high collision potential, the working surface should be high-frequency quenched before chrome plating. Ceramic can also be added if corrosion resistance is required or the environment is harsh. Surface treatment is not required for low loads (such as low speeds and low operating pressures) and good lubrication conditions.
5. The retaining ring groove, threads, and buffer plunger on the inner end of the piston rod must also be concentric with the axis. The buffer plunger, in particular, should ideally be integral with the piston rod. The retaining ring groove should have a dynamic fit tolerance, while the threads should have a tight fit.
05. Piston rod guide sleeve
The fit between the outer circle of the guide sleeve and the inner hole of the end cover is mostly H8/f7, and the fit between the inner hole and the outer circle of the piston rod is mostly H9/f9. The coaxiality tolerance between the outer circle and the inner hole is not greater than 0.03mm, and the roundness and cylindricity tolerances are not greater than half of the diameter tolerance. The annular oil groove and straight oil groove in the inner hole should be shallow and wide to ensure good lubrication.