Design Crimp Points for Connecting Wire Harnesses

Connection Harness crimping point, also called piercing point, contact point, splice point. The design of the crimping point has a great impact on the stability of the vehicle’s electrical functions and the cost of the wiring harness. This article discusses the design of crimp points.

Definition of crimp point:
The wire harness crimping point refers to the connection point where more than 2 wires in the wire harness are hinged.
As shown in the figure below, this crimping point (SP) consists of Wire 1, Wire 2, and Wire 3. The 3 wires are connected to each other inside the harness. The hinged connection point of the three wires inside the harness is the crimp point, also called the splice point, and the harness piercing point.
Type of crimp point
Automotive wiring harness splice points, commonly used methods include ultrasonic welding, U-shaped terminal crimping, etc.

Ultrasonic welding of wire harness crimp points

Wire harness contacts for U-shaped terminal crimping

Standard for multi-wire harness crimp points

Ultrasonic welding
The principle of ultrasonic metal welding is to use vibration energy at ultrasonic frequency (20-40kHz) to transmit vibration waves to the surfaces of two metal objects that need to be welded. Under static pressure, the two metal surfaces are rubbed against each other, thereby forming a fusion between the molecular layers.

As shown in the figure above, the 50/60 Hz current is converted into 15, 20, 30 or 40 KHz electrical energy by an ultrasonic generator. The converted high-frequency electrical energy is converted again into mechanical motion of the same frequency through the transducer, and then the mechanical motion is transmitted to the welding head through a set of horn devices that can change the amplitude. The welding head transmits the received vibration energy to the joint of the workpiece to be welded. In this area, the vibration energy is converted into heat energy through friction, melting the metal.
Ultrasonic welding can be used to join both identical materials and dissimilar materials, such as copper and aluminum. Because the metals are welded directly together, no additional materials such as ferrules, solder, or flux are required. In addition, ultrasonic welding produces low thermal stress on the parts (the welding temperature is much lower than the melting point of the parts), so the properties of the welded material itself and the properties of surrounding materials do not change. If the insulation sleeve remains intact.

Distance standards for wire harness crimping points

Wire harness buckle – tie distance

Set the position of the wire harness crimp point

3 wire harness crimp points

Ultrasonic welding standards
The most authoritative welding standard SAE/USCAR-38 jointly issued by the American Society of Automotive Engineers and the United States Automobile Survey Committee. A comprehensive definition of the ultrasonic welding process between automotive cables and terminals provides an important reference for industry specifications.

Advantages and disadvantages of ultrasonic welding:
The advantages of this method are fast speed, energy saving, high fusion strength, good conductivity, no sparks, close to cold processing, high efficiency, good conductivity, environmental protection and safety. It is the development trend of wire harness contacts.
The disadvantages are that the metal parts to be welded cannot be too thick (generally less than or equal to 5mm), the solder joints cannot be too large, and pressure is required. Welding equipment prices and maintenance costs are relatively high.

U-shaped terminal crimping
Crimping is a process that applies pressure to the contact area between wires and terminals to form a tight connection. During the crimping process, the conductor is squeezed by the sleeve and deforms in both longitude and latitude directions, forming a cold weld. The original oxide film on the surface of the conductor and the sleeve is deformed and ruptured, the conductor and the pure metal surface of the terminal sleeve are in full and close contact, and micro-movements occur during the extrusion and deformation process.
Physically and evenly crimping the copper wires of wires through U-shaped terminals is a commonly used contact method at present. The U-shaped terminal crimping diagram is shown in the figure below.

Figure U-shaped terminal crimping diagram

U-shaped terminal crimping standard
The USCAR standard is a specialized connector standard, and USCAR 21 is a terminal crimping performance specification. A comprehensive definition of the automotive wiring harness terminal crimping process is provided, providing an important reference for industry specifications.

U-shaped terminal crimping advantages and disadvantages:
The advantages of this method are easy operation, high efficiency, good conductivity, and cheaper crimping equipment and U-shaped terminals;
The disadvantage is that compared with ultrasonic welding, it requires the use of crimping materials and increases weight.

Selection of the location of the crimping point:
The location of the crimping point is not only related to the cost of the wire harness, but also to the quality of the wire harness. Unreasonable splice design will not only increase the cost of the wiring harness, but also increase the weight of the wiring harness, and may also cause assembly difficulties and other problems.

Choose a location where the wiring harness is relatively stable. Do not choose moving parts or locations with greater interference.
Arrange the crimping point near the intersection of each signal line and the trunk, which can effectively reduce wires and save costs.
3. Selection of contact locations to avoid loopback harnesses. Especially in the wet area, there are return harnesses, which is not conducive to splice point sealing.
Avoid common pressure of more than 10 wires at the same crimping point. Once more than 10 wires are pressed together, additional crimping points need to be set.
For the same crimping point, the diameter of the minimum wire diameter cannot be less than 10% of the sum of the wire diameters of all the wires at the crimping point.
6. The sum of the wire diameters on the left and right sides of the crimping point should be as equal as possible, and the smallest one should not be less than 50% of the other side.
7. The wire diameter of the minimum wire diameter at the crimping point should be as large as 16% of the maximum wire diameter at the crimping point.
8. When one wire connects two crimping points, the distance between the two crimping points shall not be less than 150mm. In the picture below, the distance between SP1 and SP2 is greater than 150mm.
The distance between the crimping point and the branch point, buckle/tie needs to be greater than 50mm, and the distance between contacts should also be greater than 50mm (except for the can line), so as to effectively avoid contact between contacts.

Crimp point protection
If the protection of the wire harness contacts is unreasonable, the insulation at the wire harness contacts may be punctured, or water may enter the contacts, causing a short circuit failure.
Generally speaking, the distribution of automobile contacts is divided into two categories: dry area and wet area. Contacts in the dry area need to be protected with special tape; contacts in the wet area need to be protected with glue-containing heat shrink tubing or butyl glue to achieve a waterproof sealing effect.
As shown in the figure above, the area below the wading line is called the wet area, and the rest outside the dehumidification area is the dry area.

Summarize:
This article systematically introduces the definition, classification, location selection and protection of crimping points. When doing layout design and principle design, you need to pay attention to the crimping points. If you want to quickly improve your automotive wiring harness design capabilities, please pay attention to the wiring harness courses of YAXUN wiring harness engineers.