Cylindrical coil springs are the most widely used elastic elements in mechanical engineering. They achieve functions such as shock absorption, energy storage, and reset through their elastic deformation. They are widely used in automotive suspensions, machine tool fixtures, instrumentation, and other fields. The winding process must ensure spring diameter accuracy (tolerance ±0.5mm), pitch uniformity (deviation ≤0.3mm), free length (tolerance ±1mm), and elastic properties after heat treatment (hardness 40-45HRC). The core of the winding process is to wind the spring wire around a mandrel according to predetermined spiral parameters (diameter, pitch, and number of turns), forming a helical structure with a specific stiffness. For example, an automotive clutch spring is required to generate a force of 500N at a compression of 20mm. Precise control of wire diameter, number of turns, and pitch is crucial during winding; otherwise, the spring force will vary by more than 10%. Therefore, the winding of cylindrical coil springs requires systematic control in terms of equipment selection, process parameters, and operating techniques.
The choice of winding equipment should be determined based on the spring specifications and production batch. Manual winding is suitable for small batches of large-diameter springs (wire diameter > 8mm). A bench vise is used to secure the mandrel, and a wrench is used to rotate the mandrel to wind the spring wire. This method is suitable for repair or prototyping, but the precision is relatively low (pitch deviation ±0.5mm). Mechanical winding equipment is divided into conventional spring winding machines and CNC spring winding machines. Conventional spring winding machines are suitable for medium-volume production (500-1000 pieces). The pitch can be adjusted by adjusting the gear ratio, with a diameter control accuracy of ±0.3mm. CNC spring winding machines are suitable for large batches of high-precision springs (wire diameter 0.5-20mm). Servo motors control the mandrel speed and wire feed speed, achieving a pitch accuracy of ±0.1mm and a turn error of ≤0.1 turn. They are suitable for high-end applications such as automotive and aviation. The mandrel diameter of a spring winding machine must be determined based on the inner diameter of the spring (mandrel diameter = spring inner diameter – 0.1-0.2mm; for example, a 10mm inner diameter spring uses a 9.8mm mandrel). Mandrel material should be 45 steel hardened to 50-55 HRC to prevent deformation. An automotive parts manufacturer using a CNC spring winding machine has reduced spring dimensional consistency from ±0.5mm to ±0.2mm, increasing the pass rate to 99%.
The preparation and clamping of spring wire have a significant impact on the winding quality. The material of the spring wire needs to be selected according to the working environment: low carbon steel wire (65Mn) is suitable for low stress springs (such as toy springs), music wire (SWP) is suitable for high stress springs (such as valve springs), and stainless steel wire (304) is suitable for corrosion-resistant environments. Before winding, the spring wire needs to be pre-treated: straightening (straightness error ≤ 0.5mm/m) and stress relief annealing (600-650℃ for 1 hour) to avoid distortion after winding. During clamping, the spring wire needs to be introduced into the spring winding machine through the guide wheel and tension device. The tension is adjusted according to the wire diameter (1mm wire diameter tension 50N, 5mm wire diameter tension 500N) to ensure smooth wire feeding and avoid pitch fluctuation. For spring wires with a diameter > 5mm, it needs to be preheated to 100-150℃ to improve its plasticity and reduce rebound during winding (the rebound amount can be reduced from 0.3mm to 0.1mm). A spring factory has reduced the free length error of the spring from ±1.5mm to ±0.8mm through strict wire pretreatment.
Setting winding process parameters is crucial for ensuring spring performance. The mandrel speed is determined by the wire diameter: 1000-1500 r/min for wire diameters <1mm, 500-1000 r/min for wire diameters 1-5mm, and 100-500 r/min for wire diameters >5mm. Avoid excessive speed, which can cause wire vibration. Pitch control is achieved by adjusting the ratio of wire feed speed to mandrel speed (pitch = wire feed speed / speed). For example, a mandrel speed of 500 r/min and a wire feed speed of 1000 mm/min results in a 2 mm pitch. During winding, reserve 1-2 wraps at the ends (with a pitch of 0.5-0.8 times the wire diameter) to enhance support stability at the spring ends. For compression springs, the number of effective turns, n, is calculated using the stiffness formula ( k = Gd⁴/(8nD³) , where G is the shear modulus, d is the wire diameter, and D is the pitch diameter). For example, a 65Mn spring with a wire diameter of 3mm, a pitch diameter of 20mm, and 5 effective turns has a stiffness k ≈ 50N / mm . During the winding process , the pitch and diameter are measured every 10 coils . If deviations exceed these limits, the wire feed speed or mandrel diameter is adjusted promptly. By precisely setting these parameters, a medical device manufacturer has achieved spring stiffness tolerances within ±5% , meeting the requirements of precision instruments.
Post-winding processing and quality inspection must be strictly adhered to. Completed springs undergo stress relief annealing (300-350°C for 1-2 hours) to eliminate winding stresses and minimize dimensional change during use (free length change ≤ 1%). Springs requiring high elasticity require quenching (oil quenching at 830-860°C) followed by medium-temperature tempering (420-460°C), with a hardness of 42-45 HRC to ensure the elastic limit. Surface treatment options include electrogalvanizing (corrosion protection, coating thickness 5-10μm), phosphating (for enhanced wear resistance), or painting (for decorative purposes). Quality inspections include measuring the outside diameter and free length with calipers (accuracy ±0.1mm); checking pitch uniformity with a pitch gauge; testing the spring force with a tensile testing machine (deviation ≤ 5%); and checking the helix straightness with a projector (≤ 0.3mm/m). An aerospace company conducts 100% spring force testing on springs to eliminate unqualified products and ensure the reliability of spacecraft mechanisms.
The winding of special cylindrical coil springs requires specialized processes. Variable-pitch springs (such as automotive suspension springs) require real-time wire feed speed adjustment on a CNC spring winding machine to achieve a gradual change in pitch from dense to coarse (pitch change rate ≤ 5%/turn). Support is added to the variable-pitch section during winding to prevent wire deviation. For winding left-handed springs, the mandrel must be rotated in the opposite direction and the wire feed direction adjusted to ensure the helical direction is consistent with the drawing (check with a left-handed template). Multi-start coil springs (such as shock absorber springs) require multiple starting points on the mandrel, allowing for simultaneous winding of multiple spring wire strands with a gap of ≤0.1mm between strands to avoid interference. Specialized tooling is used to secure the multiple wire strands during winding to ensure uniform tension (tension difference between strands ≤ 5%). Using specialized processes, a construction machinery manufacturer successfully wound a three-start variable-pitch spring, meeting the shock absorption requirements of large equipment.
