# Process characteristics of multi-strand spring

Due to the limitation of product structure, multi-strand springs generally have the characteristics of high strength and good performance. The material is required to guarantee the ultimate performance in terms of strength and toughness. Therefore, the material is usually carbon spring steel wire (GB4357) or carbon spring steel wire (GB4360) with elastic modulus E=206×103N/mm2. The specific specifications of the steel wire shall be determined according to the structural characteristics and load requirements of the product.
According to the number of steel wire strands, multi-strand springs can be divided into three strands and four strands, of which three strands are common.

There are two ways to wind multi-strand springs: one is to wind the steel rope and the spring at the same time; the other is to twist the steel rope and the spring step by step, that is, twist the steel rope first, and then wind the spring. The steel rope can be twisted and wound on the lathe separately, and it can also be wound on the machine tool with automatic steel rope mechanism.

The typical process can refer to the following:

Loading→(twisting the steel rope)→winding→cutting→grinding head→visual inspection→surface treatment (depending on product requirements and whether welding head)→welding both ends (if necessary)→repairing welding head (if necessary)→parallel end Ring (parallel)→calibration→heat treatment→squeeze→calibration→end-dressing ring (if necessary)→inspection→surface treatment (there is no such process if surface treatment has been performed)→oil immersion

The calculation of typical process parameters can be carried out according to the following formula:

1. Calculation formula of unfolded steel wire length:

L1=(πDn1m)/(cosαcosβ)

D———Spring diameter (mm)

n1———Total number of turns

α———Helix angle (°)

m——–Number of spring strands

β——-Wire rope twisting angle (°)

2. Stiffness calculation formula:

P’=(Gd4mi)/(8D3n)

G——–shear modulus (N/mm2), generally 79×103~85×103

d——–Wire diameter (mm)

i———-Twisting coefficient, when m=3, β is between 15°~25°, take 1.05~1.2; when m=4, β is between 20°~30°, take 1.1 ~1.3.

n———-effective number of turns

During processing, the following points should be noted:

1. Springs without support rings and springs with too thin steel wire diameters should not be welded to the spring heads, but the end cables should not be significantly loose, and should be deburred. For multi-strand springs that need to be welded to the head, the length of the welded part should be less than 3 times the cable diameter (the longest is no more than 10 mm). The heating length should be less than one circle, and it should be polished smoothly after welding, and the welding part should be locally annealed at low temperature during gas welding.

2. The support ring can choose two methods: cold and hot, according to product requirements. It is not allowed to heat the spring to sparking or whitening by using heat and the method, and the temperature of silico-manganese steel should not be higher than 850℃. The support ring and the effective ring should be in effective contact, and the gap should not exceed 10% of the nominal gap between the rings.

3. The characteristics of the multi-strand spring can be determined by the adjustment lead, and the cable distance can be adjusted as necessary during winding. The twisting distance can be 3-14 times the steel wire diameter, but generally 8-13 times is better. The spring force is also closely related to the free height, the end ring, the outer diameter and the performance of the steel wire, and can be changed by adjusting one or several of them.

4. The pressing time of important springs is 24 hours, and that of ordinary springs is 6 hours or continuous compression 3~5 times, each time holding for 3~5 seconds. The gap between the spring and the mandrel during compression is preferably 10% of the diameter of the mandrel. If the gap is too small, it is difficult to operate, and if the gap is too large, the spring is likely to bend and deform. If one of the springs breaks during pressing, the rest should be processed again.

5. For multi-strand springs with a large H0/D2 value, attention should be paid to their deformation during heat treatment, considering whether to wear the mandrel and the placement method, and appropriate heat treatment equipment should be selected. Under the repairable condition, tempering and hot pressing can be performed many times to achieve the purpose.

6. The spring surface treatment is generally phosphating treatment, but other treatments can also be carried out. Where zinc and cadmium are to be coated, the hydrogen removal treatment should be carried out after electroplating, and 3% (not less than 3 pieces) shall be drawn after the removal of hydrogen. The spring should be cleaned of surface dirt, salt marks, and oxide scale. The method can be sand blowing or gasoline cleaning, but acid cleaning is not allowed.