Piston rod processing technology
As a core component of actuators such as hydraulic cylinders and pneumatic cylinders, the quality of piston rod machining directly impacts the equipment’s operating accuracy, sealing performance, and service life. Piston rods typically withstand axial and radial loads, as well as frequent reciprocating motion. Therefore, they require high dimensional accuracy, good surface finish, and sufficient strength and wear resistance. Piston rod machining is complex and involves multiple steps, including blank selection, rough machining, heat treatment, finishing, and surface treatment. Each step requires strict control of process parameters to ensure the final product meets design requirements.
The selection of the blank is the first step in piston rod processing, directly impacting the difficulty and cost of subsequent processing. Piston rod blanks are commonly made from high-quality carbon structural steels or alloy structural steels, such as 45 steel and 40Cr, which offer excellent mechanical and processability. Depending on the size and batch size of the piston rod, the blank can be made from hot-rolled round steel or forged parts. For larger diameter and longer piston rods, forging blanks are typically used. Forging eliminates internal defects such as porosity and pores, improving the material’s density and mechanical properties. For small and medium-sized piston rods, hot-rolled round steel blanks can be used to reduce production costs. The blank diameter should be determined based on the finished product dimensions and machining allowance. Generally, an 8-15mm machining allowance is allowed to ensure that subsequent processing can remove surface defects and scale.
Rough machining is a crucial step in piston rod processing. Its primary purpose is to remove most of the blank’s machining excess, laying the foundation for subsequent finishing. Rough machining operations include turning the outer diameter, facing the end, and drilling the center hole. When turning the outer diameter, the appropriate lathe should be selected based on the blank’s diameter and length, and high cutting parameters should be used to maximize efficiency. For longer piston rods, a steady rest or steady rest should be used to prevent bending and deformation during machining. End machining requires ensuring perpendicularity to the axis. When drilling the center hole, a suitable center drill should be used to ensure accurate positioning and depth, providing a reliable reference for subsequent center positioning. After rough machining, the workpiece should be aged to eliminate internal stresses and prevent deformation during subsequent machining. Aging can be either natural or artificial. Artificial aging is typically used for piston rods requiring higher precision. The workpiece is heated to 500-600°C, held at that temperature for 4-6 hours, and then slowly cooled.
Heat treatment is a key process for improving the mechanical properties of piston rods. It enhances the material’s hardness, strength, and toughness to meet the requirements of the piston rod’s performance. For 45 steel piston rods, quenching and tempering—that is, quenching followed by high-temperature tempering—is typically used to achieve excellent overall mechanical properties, achieving a hardness of 220-250 HBW. For piston rods made of alloy structural steels such as 40Cr, quenching and low-temperature tempering can be used to improve surface hardness and wear resistance, achieving a hardness of 35-40 HRC. During heat treatment, the heating temperature, holding time, and cooling rate must be strictly controlled to avoid defects such as overheating, overburning, and quench cracking. After heat treatment, the piston rod should undergo hardness testing and metallographic analysis to ensure that its performance meets design requirements. Any deformation after heat treatment should be straightened using pressure straightening or flame straightening to maintain a straightness within 0.1 mm/m.
Finishing is a critical process for ensuring the dimensional accuracy and surface quality of piston rods. It primarily includes precision turning, grinding, and polishing. Precision turning must be performed on a high-precision lathe using carbide or ceramic tools, selecting appropriate cutting parameters to maintain the outer diameter within the tolerances specified in the drawing. After finishing, the piston rod’s surface roughness should reach Ra1.6-3.2μm. Grinding is an important means of improving the dimensional accuracy and surface quality of piston rods. This process is typically performed on a cylindrical grinder. The high-speed rotation of the grinding wheel removes a small amount of machining allowance, achieving an outer diameter accuracy of IT6-IT7 and a surface roughness of Ra0.4-0.8μm. Areas requiring sealing also require precision grinding to ensure a surface roughness no greater than Ra0.2μm. Polishing primarily removes surface marks left by grinding, further improves surface finish, and reduces wear on seals. Cloth wheels or grinding wheels are typically used for polishing, achieving a surface roughness of Ra0.1-0.2μm.
Surface treatment is the final step in piston rod processing, aiming to improve the rod’s wear resistance, corrosion resistance, and sealing properties. Common surface treatment methods include chrome plating, nitriding, and phosphating. Chrome plating is the most common surface treatment. It forms a hard chrome coating on the rod surface through electrolysis. The coating is typically 0.05-0.15mm thick and has a hardness of 800-1000 HV, offering excellent wear and corrosion resistance. After chrome plating, honing or polishing is required to remove burrs and micropores from the coating and improve surface quality. Nitriding is suitable for piston rods requiring high wear resistance. By heating the workpiece in a nitrogen atmosphere, nitrogen atoms penetrate the surface to form a nitride layer, enhancing surface hardness and wear resistance. Phosphating primarily improves lubricity and corrosion resistance and is suitable for use in specialized environments. After surface treatment, the rod undergoes final inspection, including dimensional accuracy, surface roughness, coating thickness, and hardness, to ensure that all indicators meet design requirements.
