Wire selection design
1. Selection of wire type
The selection of wire type in harness design focuses on the environment and function of the wire harness.
For example, the ambient temperature around the engine is high, and there are many corrosive gases and liquids. Therefore, the high temperature, oil-, vibration- and friction- resistant wires must be used.
The wires on the lid of the luggage compartment must maintain their elasticity at low temperature, so the cold elastic wires should be selected to ensure their normal operation; the wires on the automatic transmission must be resistant to high temperature and hydraulic oil, and their temperature stability should be good; shielded wires should be used for weak signal sensors, such as knock sensor, crankshaft position sensor, ABS wheel speed sensor, etc.; the inner wire of the door requires high bending resistance.
The wires commonly used in automobile wire harness usually are multi-stranded copper wires, and the insulation is PVC insulating material.
The wire for harness should have the characteristics of temperature resistance, oil resistance, wear resistance, waterproof, anti-corrosion, anti-oxidation, flame retardant and so on.
The types of wires commonly used in automobile wire harness are Japanese standard (AVSS, etc.), Chinese standard (QVR), German standard (FLRY), American standard and so on.
AVSS (AVS) wire is characterized by thin insulation and good flexibility; QVR is characterized by thick insulation, soft and good ductility; German standard wire insulation is thinner and flexible; American standard wire insulation is generally thermoplastic or thermoset elastomer, and processed by irradiation process. The appropriate type of wire can be selected according to the needs of users and different working environments.
2. Calculate the cross-sectional area of selected wires
The current of the circulating wire is calculated according to the power of the electrical device; the electric equipment that works for a long time can choose 60% of the actual carrying capacity; the electric equipment that works for a short time can choose the wire between 60% to 100% of the actual carrying capacity.
Change the cross-sectional area of the wire according to different working environment and temperature.
According to the direction of the wire, the number of connectors (that is, the scale of the voltage drop) to change the cross-sectional area of the wire.
With regard to the calculation of the cross-sectional area of the wire, some experts have summed up some empirical formulas: I=P/UsA=IρL/Ud.
In the formula: I-current; P-power; Us-system voltage; A-conductor cross-sectional area; Ud-maximum allowable voltage drop loss; ρ-copper resistivity; L-wire length.
Or according to the following empirical formula: the empirical theoretical value of the relationship between the current and the cross-sectional area of the wire (larger than the value calculated according to the above formula) is shown in Table 3.
Design of rubber parts
When the automobile wiring harness passes through the hole, the rubber part is generally used for transition to play the role of wear resistance, waterproof and sealing. It is mainly distributed in the following parts: the interface between the engine and the cab, the interface between the front cabin and the cab (left and right), the interface between the four doors (or the rear door) and the compartment, and the fuel tank inlet.
Commonly used materials are generally natural rubber, neoprene, silicone rubber, EPDM, etc.
Characteristics of natural rubber: good elasticity and mechanical strength, excellent flexural resistance, high tear strength and good cold resistance.
Disadvantages: The aging resistance is not very good, it is not resistant to oil and ozone, and it is flammable.
The characteristics of neoprene: ozone resistance, heat aging, oil resistance and other properties are good, with flame retardancy and self-extinguishing; but low temperature resistance is not good.
Characteristics of silicone rubber: good heat resistance, cold resistance and weather resistance;
The disadvantage is that it is not resistant to oil.
Characteristics of EPDM: weather resistance, ozone resistance, heat resistance, corrosion resistance, acid and alkali resistance are all good, and it has high strength and high expansion rate;
Disadvantages: poor adhesion, and not good as the elasticity of natural rubber, poor oil resistance.
In comparison, the comprehensive performance of EPDM is better, so now the rubber parts for automobile wiring harnesses generally use EPDM materials.
Full vehicle harness wrap and fixing design
1.Wire Harness wrapping design
The outer wrapping of the wire harness plays the role of wear resistance, flame retardant, anti-corrosion, preventing interference, reducing noise, and beautifying the appearance. Generally, the following wrapping design schemes are formulated according to the working environment and space size.
The working environment of the engine wiring harness is harsh, so it is all wrapped with corrugated pipes with high flame retardancy, waterproof and high mechanical strength; The working environment of the front cabin wiring harness is also relatively poor, and most branches are also wrapped with corrugated pipes with good flame retardancy. Some branches are wrapped with PVC pipe;
The working space of the instrument wiring harness is small, the environment is relatively good, and the tape can be fully wrapped or spiral wrapped;
The door wiring harness and the ceiling wiring harness have a small working space, which can be wrapped with tape, and some branches can be wrapped with industrial plastic cloth;
Thinner roof wiring harness can be directly attached to the body with sponge tape; Since there are many parts of the chassis wiring harness in contact with the car body, it is wrapped with corrugated tubes to prevent the wire harness from being worn.
2. Performance analysis of raw materials for wrapping
1).Corrugated tubes
Corrugated tubes generally account for about 60% of the wire harness, or even more. The main features are good wear resistance, high temperature resistance, flame retardancy and heat resistance in high temperature areas. The temperature resistance of the corrugated tubes is -40-150℃. Its material is generally divided into PP and PA. PA material is superior to PP material in terms of flame retardancy and wear resistance; but PP material is stronger than PA material in bending fatigue resistance.
2).PVC pipes
The function of PVC pipe is similar to that of corrugated tube. PVC pipe has good flexibility and resistance to bending deformation, and PVC pipe is generally closed, so PVC pipe is mainly used at the branch of the wire harness to make the wire transition smoothly. The heat-resistant temperature of PVC pipes is not high, generally below 80 °C.
3).Tape
The tape plays the role of bundling, wear resistance, insulation, flame retardant, noise reduction, marking, etc. in the wire harness, and generally accounts for about 30% of the wrapping material. Wire harness tapes are generally divided into three types: PVC tape, flannel tape and cloth tape.
PVC tape has good abrasion resistance and flame retardancy; the temperature resistance is around 80 °C, the noise reduction is not good, and the price is relatively cheap.
The material of flannel tape and cloth tape is PET. The flannel tape has the best wrapping and noise reduction properties, and the temperature resistance is around 105 °C; The cloth tape has the best wear resistance, and the temperature resistance is up to about 150 °C. The common disadvantages of flannel tape and cloth tape are poor flame retardancy and high price.
3. Fix design of Wiring Harness
The central electrical box is generally fixed with steel strips, bolts, etc., or directly installed on the car body with a fixed structure designed by the electrical box itself.
Each wire harness is generally fixed in the car body hole with plastic ties, buckles, etc. Most of the body holes are round holes or elliptical holes, generally with diameters ranging from 5mm, 6mm, and 7mm.
Each wiring harness are generally fixed collectively with sheath brackets and installed on the vehicle body. Larger models such as MPV, have long and thick body wiring harness, which are generally fixed on the body with guard plates to reduce vibration and noise.