Winding of convex coil springs
A convex coil spring is a non-cylindrical spring whose effective coil diameter gradually increases from both ends to the center, forming a drum-like shape. It features strong load-bearing capacity, uniform stiffness, and excellent cushioning properties. It is widely used in automotive suspensions, mechanical equipment, and shock absorbers. Coiling convex coil springs is more difficult than winding ordinary cylindrical springs, requiring precise control of the spring’s diameter, pitch, and free length. Specialized spring winding machines, combined with molds, are commonly used to achieve variable diameter winding. Mastering the parameter design, winding process, and quality inspection of convex coil springs is key to ensuring their mechanical properties.
The design parameters for a convex coil spring include material diameter (d), maximum outer diameter (Dmax), minimum outer diameter (Dmin), free length (H0), number of effective turns (n), pitch (p), and direction of rotation (left-hand or right-hand). These parameters are determined based on the load requirements and installation space. Spring steels such as 65Mn and 50CrVA are typically used, with diameters d ranging from 5 to 30 mm. After quenching and tempering, the hardness reaches 40 to 45 HRC, ensuring sufficient elastic limit and fatigue strength. The difference between the maximum outer diameter (Dmax) and the minimum outer diameter (Dmin) is determined based on load uniformity requirements. Typically, Dmax – Dmin = (0.2 to 0.5) Dmax. For example, for a spring with Dmax = 100 mm, Dmin = 60 to 80 mm. The number of effective turns n ranges from 3 to 10, and the pitch p gradually increases from the ends to the center, with the largest pitch in the middle. This ensures uniform force across all turns when the spring is compressed. The direction of rotation must match the assembly part. Automotive suspension springs are mostly right-handed, while construction machinery springs are mostly left-handed. After parameter design is complete, a strength check is required to ensure that the spring’s stress at maximum operating load is less than the material’s allowable stress (65Mn allowable stress ≈ 600MPa).
Winding equipment for convex coil springs is categorized by degree of automation into manual, semi-automatic, and CNC machines. Manual machines are suitable for small-batch, large-diameter springs. Diameter variation is achieved by manually adjusting the mandrel position, resulting in lower efficiency but lower equipment costs. Semi-automatic machines, equipped with a mechanical cam mechanism that allows for a preset diameter variation curve, are suitable for medium-volume production, achieving a winding accuracy of ±1mm (diameter deviation). CNC machines utilize servo motors to control the mandrel, feed, and winding speed, enabling precise control of diameter variation and pitch , achieving a winding accuracy of ±0.5mm. They are suitable for large-volume, high-precision spring production. The winding mold consists of a mandrel, a reducing slide, and a guide wheel. The mandrel diameter dcore = Dmin – 2d (material diameter). The reducing slide’s motion aligns with the spring’s diameter curve. The guide wheel ensures smooth material feed, maintaining a controlled contact pressure of 0.1-0.2MPa.
The winding process for convex coil springs consists of three main steps: winding, quenching, and tempering. During winding, the spring steel wire is pre-treated: rust removal and lubrication (a mixture of graphite powder and engine oil) are applied. The wire is then fed into the winding die via a feeding mechanism. The rotating mandrel drives the wire, while the reducing slide moves along a preset trajectory, gradually increasing the spring diameter from the ends toward the center. The winding speed is adjusted based on the material diameter: 30-50 rpm for d = 5-10 mm, and 10-30 rpm for d = 10-30 mm. Excessive speeds can result in uneven spring diameters. Pitch control is achieved by adjusting the ratio of feed speed to mandrel speed. Feeding speeds are faster in the center where the pitch is larger, and slower in the ends where the pitch is smaller. Pitch deviation must be ≤ ±0.5 mm. After winding, the spring undergoes stress relief annealing (250-300°C for 1-2 hours) to eliminate internal stresses generated during winding and prevent deformation during subsequent processing.
Quenching and tempering are key heat treatment processes for ensuring the mechanical properties of convex coil springs. During quenching, the spring is heated to 830-870°C (65Mn steel) for a holding time calculated based on the material diameter (1-1.5 minutes/mm). The spring is then cooled in oil for 10 minutes or longer to ensure complete hardening of the core and a surface hardness of 55-60 HRC. The tempering temperature is determined by the load: 400-450°C for springs subjected to static loads and 350-400°C for springs subjected to variable loads. The holding time is 2-3 hours, followed by air cooling to room temperature. After tempering, the hardness is controlled at 40-45 HRC to ensure a balance between elasticity and toughness. After heat treatment, the spring needs to be straightened. The spring is placed on a dedicated straightening machine and its verticality is checked with a dial indicator. If the error exceeds 0.5 mm/m, a press is used to apply external force for correction. After straightening, a stress relief tempering (200°C for 30 minutes) is performed to eliminate the straightening stress.
Quality inspection for convex coil springs includes dimensional accuracy, mechanical properties, and appearance quality. Dimensional accuracy testing: The maximum and minimum outer diameters and free length are measured with a vernier caliper, with deviations controlled within ±1mm , ± 1mm , and ±2mm , respectively . The number of effective turns is measured with a tape measure, with an error of ≤± 0.5 turns. An angle ruler is used to check verticality, with an error of ≤0.5mm/m . Mechanical property testing: A load – deflection test is conducted on a spring testing machine , measuring the deformation under a specified load. The deviation must be ≤5 % of the design value. Fatigue testing is conducted, cycling between the maximum and minimum working loads 10⁶ times. No breakage or permanent deformation should occur. Appearance quality inspection: The surface must be free of defects such as cracks, folds, and rust. Magnetic particle inspection can be used to detect surface cracks, and sandpaper can be used to remove minor scratches. By strictly controlling the winding process and heat treatment parameters, convex coil springs can achieve a service life of more than 10⁵ cycles, meeting both shock absorption and load-bearing requirements.
