Winding of truncated cone coil springs
A truncated conical coil spring (also known as a tapered coil spring) is a coil spring with a straight axis and a gradually varying coil diameter from one end to the other. It features high load-bearing capacity, excellent cushioning performance, and a gradually varying stiffness (nonlinear stiffness). It is widely used in automotive suspension, machine tool shock absorption, and construction machinery. When subjected to a load, the large coils first contact and deform. As the load increases, the small coils gradually join in the deformation, achieving a cushioning and vibration-absorbing effect. Compared to cylindrical coil springs, the winding process for truncated conical coil springs is more complex, requiring precise control of coil diameter variation, pitch, free height, and other parameters to ensure that the spring’s performance meets design requirements. The winding process requires selecting appropriate winding equipment, mandrel, and process parameters based on the spring’s material properties, geometric parameters, and performance requirements.
The material selection and pretreatment of truncated conical coil springs significantly impact their winding process and performance. Common spring materials include carbon spring steel (such as 65Mn and 70 steel), alloy spring steel (such as 50CrVA and 60Si2Mn), and stainless steel (such as 1Cr18Ni9Ti). 65Mn steel offers high strength and elastic limit (σb = 980 MPa, σs = 785 MPa) and is inexpensive, making it suitable for general-purpose truncated conical coil springs. 50CrVA steel offers high fatigue strength, impact toughness, and excellent heat resistance, making it suitable for springs subjected to shock loads and high temperatures (such as automotive engine valve springs). 1Cr18Ni9Ti stainless steel offers excellent corrosion and oxidation resistance, making it suitable for springs used in humid or corrosive environments. The cross-section of the material is generally circular, with a diameter determined by the spring’s load-bearing capacity, typically ranging from 0.5 to 20 mm. Before winding, the materials require pretreatment. For carbon spring steel and alloy spring steel, annealing (e.g., heating 65Mn steel to 700-720°C, holding, and then slowly cooling) is necessary to reduce hardness (generally controlled at 180-220 HB) and improve plasticity for easier winding. For stainless steel with higher hardness, solution treatment (e.g., heating 1Cr18Ni9Ti to 1050-1100°C and water cooling) can be performed to improve plasticity. Surface treatment, such as rust removal and oiling, is also necessary to prevent scratches during winding.
The geometric parameter design of the truncated conical coil spring is the basis of the winding process. The main parameters include the maximum coil diameter Dmax, the minimum coil diameter Dmin, the free height H0, the pitch t, the number of coils n, the cone angle α, etc. The maximum and minimum coil diameters are determined by the installation space and load requirements, typically Dmax/Dmin = 1.5-3. The free height H0 is the height of the spring when unloaded and is determined based on the required deformation, typically H0 = (2-5) Dmax. The pitch t is the axial distance between two adjacent coils. The pitch of a truncated conical coil spring can be either constant or variable. Constant-pitch springs are simple to manufacture, while variable-pitch springs can achieve a more ideal stiffness change. The number of coils n includes the number of active coils and the number of supporting coils. The number of active coils is determined based on the load capacity, while the number of supporting coils is typically 1.5-2.5 to ensure smooth spring ends and facilitate installation. The taper angle α is the angle between the spring axis and the generatrix, α = arctan [(Dmax-Dmin)/(2H0)], typically α = 5°-15°. If the taper angle is too large, the spring is prone to lateral bending. If the taper angle is too small, the stiffness change is not significant. For example, a car suspension uses a truncated conical coil spring with a material diameter of d=10mm, Dmax=150mm, Dmin=80mm, H0=300mm, effective number of turns n=5, number of support turns 1.5, and a cone angle α=arctan [(150-80)/(2×300)]≈6.6°, which meets the design requirements.
The winding equipment and method of truncated conical coil springs are selected according to the size and batch size of the springs. For small truncated conical coil springs (material diameter ≤ 5mm), a common spring winding machine (such as CNC20) can be used to control the movement and rotation of the core shaft through programming to achieve a gradual change in the coil diameter. During winding, the spring material is fixed on the feeding mechanism of the spring winding machine. According to the change law of the coil diameter (linear change or nonlinear change), the core shaft is driven by the servo motor to move axially, and the main shaft drives the core shaft to rotate. The material is gradually wound into a cone under the guidance of the core shaft. For large truncated conical coil springs (material diameter ≤ 5mm), a common spring winding machine (such as CNC20) can be used to control the movement and rotation of the core shaft through programming to achieve a gradual change in the coil diameter. During winding, the spring material is fixed on the feeding mechanism of the spring winding machine. According to the change law of the coil diameter (linear change or nonlinear change), the core shaft is driven by the servo motor to move axially, and the main shaft drives the core shaft to rotate. The material is gradually wound into a cone under the guidance of the core shaft.
