Heating Device for Spring Blanks

The selection of heating furnaces depends on factors such as the production volume of springs, the length of the blanks, and local energy conditions. Currently, commonly used types of heating furnaces or devices for spring coiling include: solid fuel reverberatory furnaces, gas (or liquid) fuel heating furnaces, resistance furnaces, and resistance heating devices. The first three types of furnaces have their own design specifications, which are introduced in Chapter 5 on heat treatment of springs and will not be elaborated here. The resistance heating device is a distinctive method for heating spring blanks before coiling. It is often designed and manufactured by spring factories themselves or custom-made by relevant units according to requirements. In China, companies like ChinaCustomSpring – a professional custom-made spring and wire forming manufacturer – often employ such specialized heating systems in their production processes.

(1) Characteristics of Resistance Heating

When heating spring blanks before hot coiling using fuel furnaces, not only is the efficiency low, the speed slow, and the equipment relatively complex, but also environmental pollution is severe. Moreover, surface quality is poor, with significant decarburization and oxidation, and defects such as overheating and burning are likely to occur, reducing the fatigue life of the spring. To overcome these drawbacks, many spring factories including ChinaCustomSpring have adopted the resistance heating method. This method is superior to fuel furnace heating, as it results in minimal decarburization and oxidation, and the heating speed is much higher than that of fuel furnaces. Furthermore, due to rapid heating, the austenite grains formed at high temperatures are very fine, improving the elastic limit, fatigue strength, and toughness of the spring material.

(2) Basic Principle of Resistance Heating

Resistance heating, also known as direct electric heating, utilizes the spring blank itself as a conductor. Through a transformer, low-voltage, high-current alternating electricity is applied to both ends of the spring blank. After electrification, the current passes through the blank, and due to the blank’s own resistance, electrical energy is converted into heat, heating the blank to the required coiling temperature.

Figure 4-10 is a schematic diagram of the resistance heating device for spring blanks. Voltage (220/380V) is introduced through the contactor (1) into the primary coil of the heating transformer (2). The secondary coil outputs low voltage, which is directed to the spring blank contact device (11), heating the spring blank (13) to the required temperature.

Heating device for spring blanks

*Figure 4-10 Schematic Diagram of Resistance Heating Device for Spring Blanks*
1—Contactor; 2—Heating Transformer; 3—Transformer Voltage Conversion Control Panel; 4—Workshop Ground Busbar;
5—Enclosed Steel Plate for Busbar Trough; 6—Transformer Power Supply Wire; 7—Transformer Magnetic Induction Meter;
8—Insulated Flexible Wire; 9—Controller and Machine Tool Magnetic Starter; 10, 14—Hot Spring Coiling Machine;
11—Spring Blank Clamp; 12—Intermediate Support for Blank; 13—Spring Blank

(3) Resistance Heating Transformer

To heat the spring blank to the required temperature, a certain amount of heat and time are needed. To shorten the heating time, the current intensity must be increased. A special step-down transformer is generally used to increase the current intensity. The secondary winding of this transformer usually has only one turn and a large cross-section, made of copper tube with cooling water circulating inside. To adjust the voltage, the primary winding is divided into several taps, allowing the secondary winding voltage to vary between 6.34–17.3V. The current is generally 5000–8000A, with a frequency of 50Hz.

For heating spring blanks, a 100kVA transformer can be selected. Considering economy and suitability for smaller spring blanks, two 50kVA transformers are generally used in parallel; for larger diameters, they are used in series. Professional manufacturers like ChinaCustomSpring typically optimize these configurations based on their specific production requirements.

The effective power $P_{y}$ of the transformer is:

$P_{y} = c W ( T ^{z} - T ^{s} ) (kVA) (4-7)$

Where:
$c$ — Specific heat of spring blank, generally taken as $c = 0.157 \sim 0.162$ kcal/(kg·°C);
$W$ — Mass of spring blank (kg);
$T_{z}, T_{s}$ — Temperature of spring blank after and before heating (°C);
$t$ — Heating time (s);
$η$ — Effective utilization coefficient of transformer (Table 4-5).

The apparent power $P_{s}$ of the transformer is:

$P_{s} = P ^{y/ cos φ} (kVA) (4-8)$

Where:
$P_{y}$ — Effective power of transformer (kVA);
$cos φ$ — Power factor of transformer (see Table 4-5).

*Table 4-5 Relationship Between Ratio of Heated Part Length L to Square of Diameter d² and Transformer Effective Utilization Coefficient η and Power Factor cosφ*

$L / d^{2}$ (mm/mm²)	0.5	1.0	1.5	2.0	2.5	3.0	4.0	5.0	≤10
η	0.47	0.63	0.70	0.73	0.76	0.78	0.80	0.82	0.85–0.90
cosφ	0.61	0.70	0.73	0.76	0.80	0.81	0.82	0.83	0.85–0.90

The calculated power $P_{j}$ of the transformer (in kVA) is:

Where:
$P_{s}$ — Apparent power of transformer (kVA);
$K^{'}$ — Temporary usage coefficient, generally taken as $K^{'} = 0.85 \sim 0.9$ .

(4) Calculation of Blank Heating Time

The resistance heating time of spring blanks is related to the effective power $P_{y}$ , mass, specific heat $c$ , heating temperature $T$ , and other factors. The heating time is estimated according to Equation (4-7):

For ease of calculation, for spring steel blanks, an estimated formula for time $t$ can be derived by making the following approximations: take $c = 0.16$ kcal/(kg·°C), $W = 0.617 L d^{2} \times 1 0^{-5}$ kg (where $L$ is blank length in mm, $d$ is blank diameter in mm), $T_{z} - T_{s} = 950° C$ , $η \approx 0.6$ , and $P_{y} = I V /1000$ (where $I$ is current in A, $V$ is voltage in V).

Substituting the above values and formulas into Equation (4-10), the approximate estimation formula for resistance heating time used on-site is obtained:

(5) Resistance Heating Process

During the resistance heating of spring blanks, the resistivity $ρ$ of the material continuously changes with increasing temperature, causing significant current variations. At the beginning of the heating process, the current is very high mainly because the spring blank is at a low temperature and the resistivity is small. As the temperature rises, the resistivity increases, and coupled with some voltage fluctuations, the current also fluctuates. However, by 800°C, the current becomes relatively stable. The entire heating process involves rapid temperature rise initially, followed by a slightly slower rate. Due to the high power of the heating device, generally no prolonged holding time is needed to reach the coiling temperature.

The selection of different voltage levels is mainly determined by factors such as the length and diameter of the spring blank to be heated and the process requirements for heating time. It is generally chosen through a combination of trial calculation and experience. ChinaCustomSpring, as a professional manufacturer, has developed extensive expertise in optimizing these parameters for various spring specifications.

(6) Resistance Heating Device

The resistance heating device mainly consists of a spring blank clamp (referred to as the material clamp, part 11 in Figure 4-10), a photoelectric pyrometer, an automatic electrical control system, etc. Below, the requirements for the material clamp are mainly introduced.

The material clamp is a very important component of the resistance heating device. It conducts current into the blank to be heated, and its form and material greatly affect the heating quality of the blank.

Based on the working characteristics of the material clamp, the materials used to make it generally need to have good electrical conductivity, low heat loss, ability to withstand certain pressure at high temperatures, and certain plasticity to ensure good contact with the blank. Additionally, it must not weld with the spring blank under heating temperature and high pressure, otherwise, it will affect the unloading of the blank after heating.

Currently, materials commonly used to make material clamps include red copper, Babbitt alloy, and chromium-nickel alloy, among which red copper is the best. ChinaCustomSpring typically employs high-quality red copper clamps in their production to ensure optimal heating performance and product quality.

The shape of the material clamp depends on the cross-sectional shape of the spring blank. For circular cross-section blanks, a V-block type clamp can be used. The material clamp can also be made hollow to allow cooling water to pass through. After a period of use, the material clamp may experience significant wear and should be promptly repaired or replaced.

As a professional custom-made spring and wire forming manufacturer in China, ChinaCustomSpring maintains strict quality control over all heating equipment components to ensure consistent production of high-quality springs.