Brief Discussion on Dimension Chain and Tolerance Design Method of Wire Harness

This paper briefly introduces the dimension chain and tolerance design method in wire harness development, the common dimension chain labeling forms and the influence of different labeling forms on dimension chain. This paper discusses the dimension chain and tolerance design of wiring harness from two aspects: forward development and design verification. Finally, combined with actual cases, it makes a comparative analysis of the influence of different dimension chain labeling methods on actual products under the same design scheme, and gives reasonable suggestions on the future development and promotion direction of wire harness.

1 Introduction of Wiring Loom Dimension Chain

The whole vehicle harness has more than a thousand harness dimensions. These dimensions are interrelated and form a number of closed size combinations in a certain order, which is called the harness dimension chain.

In the process of vehicle harness development and design, the dimension chain design of harness is an important link that affects harness assembly. The wiring harness runs through the whole vehicle, and there are wiring harnesses where there is electrical equipment. The dimension chain design of the wiring harness directly affects the assembly and reliability of the wiring harness. If the wire harness dimension chain is well designed, the assembled wire harness has good consistency and high reliability. If the size of the harness is not properly designed, it will be difficult to ensure the consistency of the harness after assembly, and the electrical reliability of the whole vehicle will also be affected, which may lead to the risk of wearing the harness, and even have adverse effects on driving safety.

2 Wire harness dimension chain and tolerance design

According to the V-shaped model of vehicle development proposed in ISO26262 and the actual development process of wire harness, the dimension chain design of wire harness can be summarized into four steps.

2.1 3D data design of wiring harness: 3D data design of wiring harness of the whole vehicle is completed by using the environmental data of the whole vehicle, including topology design of wiring harness, overall layout, outer diameter design, external protection, material selection of connectors and positioning pieces, envelope design of wiring harness between moving parts, confirmation of wiring harness assembly space, etc. A complete and accurate 3D data design scheme of wire harness is the premise to ensure the design accuracy of wire harness dimension chain.

2.2 Design of wire harness dimension chain The design of wire harness dimension chain is an important link in the process of comprehensive generation of wire harness drawings. Through the 3D data of CATIA, it is imported into the engineering drawings of CHS or CATIA to realize the conversion of harness size data. After the first edition of the wiring harness drawing is obtained, according to the design scheme and the loading verification, the wiring harness dimensions are optimized, the dimension closed loop is realized, the correlation among dimensions is established, and the dimension chain is formed.

2.3 Accounting and tolerance setting of wire harness dimension chain After obtaining the wire harness drawing, according to the limit tolerance and design requirements, check and confirm the dimension chain on the wire harness drawing one by one to ensure that the products within the tolerance range meet the design requirements. In particular, for the positions with small interior space, high dimensional accuracy or high assembly requirements, the dimension chain and tolerance requirements will generally be additionally set to ensure the loading state of wiring harness products. 1) For the dimension chain on a single branch, there is the following relationship: let the total length of the wire harness be L total, and divide it into N segments, each segment is L1, L2…LN, the tolerance corresponding to L total is δ, the tolerance of L1 is δ 1, and the tolerance of L2 is δ 2 … ln is δ n, then there is: the total size of the dimension chain is the sum of all segments, and the sum of each segment size and tolerance cannot exceed the total size and tolerance.

2) The outer diameter of the trunk of the harness is usually thick, and the relative position of the branch point on the trunk will be affected by the trunk size and tolerance. Therefore, in the dimension chain accounting, attention should be paid to the inspection of the position deviation of the branch point of the harness. Let the trunk size of the harness be L, with a tolerance of δ, and there is a branch point N on the trunk. The dimensions of both sides of the branch point N are L1 and L2, and the corresponding tolerances are δ 1 and δ 2. When the trunk of the harness is in extreme deviation, the offset of the branch point N perpendicular to the trunk is δ n, and the dimensions of both sides of the branch point N are X and Y.

In this case, when L1=L2, the maximum value of the offset ΔN will appear. At this time: In summary, the offset of point N is related to the length of the trunk of the harness, the trunk tolerance and the position of point N. If L=200mm, ΔNMAX=32.02mm, far exceeding the design tolerance requirement, and the branch of the harness at point N will also be offset by the same distance. That is, the design of the dimension chain of the trunk will affect the offset of the branch point, and further affect the overall position of the branch.

3）Through the foregoing discussion, the dimension chain and tolerance design of the wiring harness, the dimension chain and tolerance design of the trunk and branch points will all affect the assembly state of the wiring harness products, and the assembled wiring harness will be in a three-dimensional space in the whole vehicle, and will even deviate in the space. The wire harness is fixed by the positioning piece and bracket, so the setting of the dimension chain of the positioning piece and bracket is also very important. When designing the dimension chain between positioning points, it is necessary to fully consider the offset of the positioning points under the limit deviation of the wiring harness, and at the same time, it is necessary to avoid tolerance accumulation, so as to prevent small tolerance from accumulating into large tolerance, and finally the positioning points will be misplaced.

Theoretically, each dimension is within 5 tolerance, but in fact, 4 dimensions share 5 tolerance. The accumulated tolerance can be eliminated by setting dimensioning and measuring methods. As the wiring harness is a flexible part, it can't be 100% consistent after assembly. Therefore, it is recommended to select a relatively fixed position on the whole vehicle for the dimensioning and measuring points of the wiring harness, such as the sheet metal hole on the vehicle body, the grounding point, the interface of electrical components, the welding stud and other high-precision spatial positions. It is not recommended to use the branch point of the wiring harness and a certain point of the moving parts as the dimensioning points.

2.4 Design Verification and Optimization

After the design of the dimension chain and tolerance of the wire harness is completed, the design scheme of the wire harness is checked and optimized through theoretical accounting and real vehicle verification. Theory accounting, considering 3D data, dimension chain, tolerance design and other dimensions, fully identifies the 3D data envelope surface of wire harness under the extreme tolerance and the actual situation of products within the tolerance range of wire harness, and confirms the relationship between wire harness and vehicle environmental data. Real vehicle verification: by disassembling and assembling the wiring harness, checking the assembly relationship between the wiring harness and the whole vehicle and the actual vehicle state of the wiring harness, and further judging the fineness of the wiring harness design.

3 Factors affecting the dimension chain and tolerance design of wire harness

During the development of wire harness, there are many factors that affect the dimension chain and tolerance of wire harness. It is necessary to judge according to the actual situation, and fully consider the requirements of design, production technology and vehicle assembly, so as to achieve the optimal design of wire harness dimension and tolerance.

1）Key points of wire harness size setting in narrow space. With the increasing electrical equipment of the whole vehicle, theoretically, the space required for the wiring harness will be larger and larger. On the other hand, the gradually increasing use space of the whole vehicle will lead to the smaller and smaller space left for the wiring harness. Under this condition, the local narrow space puts forward higher requirements for the dimension chain and tolerance design of the wire harness. In the small space, the size of the wire harness should be strictly set, and the tolerance should also be strictly controlled, so as to ensure that the posture and space of the assembled wire harness meet the requirements, and avoid the assembly difficulty caused by the wide size of the wire harness.

2）Key points of wire harness size setting between moving parts. Usually, there are some wiring harnesses on the whole vehicle, which are connected between the moving parts. In addition to the design length of the wire harness between moving parts, the position envelope of moving parts should be fully considered to prevent the wire harness from being stressed or pulled when the parts move, resulting in unstable electrical connection. In design, the size of the wire harness between moving parts can be relaxed by increasing the slack of the wire harness data; Or by calculating the size of the harness at the farthest position of the moving parts, plus the allowance of the harness to buffer the moving parts, the set length is obtained.

3）Size setting of wire harness assembled on moving parts. The moving parts (engine, transmission, etc.) on the whole vehicle have small vibration with a certain frequency. The size setting of the wiring harness assembled on the moving parts should ensure a safe distance between the wiring harness and the moving parts, so as to avoid the relative movement between the moving parts and the wiring harness during vibration. Generally, the safe distance between the harness envelope and the moving parts in the engine room should be > ＞25mm. If it is limited by the layout environment, the safe distance between the harness envelope and the moving parts should also be greater than the maximum amplitude of the moving parts.

4）Influence of wire harness production technology on wire harness size setting. In the production process, it is necessary to ensure the size requirements of the finished wire harness from material preparation, semi-finished product processing and finished wire harness assembly. There will be material loss in the thick trunk, branches and bends of the harness, so attention should be paid to the requirement of setting the symbol of loss when preparing and producing the harness materials.

5）4 Common dimensions chain labeling methods

Appropriate marking method of wire harness dimension chain will directly affect the design accuracy of wire harness products and the tolerance deviation in mass production of wire harness. Therefore, it is an effective way to improve the consistency of finished wire harness products and eliminate the accumulated tolerance by choosing an appropriate method to mark the dimension chain of wire harness.

At present, there are several common dimensions chain labeling methods.

1）Dimension segmentation of harness. The total length of the harness shall be dimensioned, and the dimensions of each segment shall also be dimensioned. The total length tolerance and the tolerance of each segment shall meet the design requirements.

2）Dimension chain labeling based on the same measuring point. Use the same measuring point to dimension multiple dimensions.

3）Local dimension default labeling. In a closed dimension chain formed by multiple dimensions, one or more non-critical dimensions can be treated by default, and the overall dimension tolerance can be adjusted by using the default dimensions.

4）Special tolerance marking. For tolerance requirements other than the implementation standards, special marks can be made on the drawings.

5）Dimensions of different measuring positions. For the position where the outer diameter of the trunk is large, the trunk has a great influence on the branch size. At this time, the dimensions on the drawing can be measured to the inside of the trunk and the outside of the trunk. Measuring points in different positions have great influence on the accuracy of branch length.

5 Conclusion

Starting with the setting of wire harness dimension chain and tolerance, this paper shares the setting process of wire harness dimension chain and tolerance, introduces the factors that affect the design of wire harness dimension chain and tolerance, and illustrates several common dimension chain and tolerance marking forms with examples. With the increasing number of vehicle equipment, the layout of wiring harness on the vehicle is more and more extensive. The wiring harness design specifications with high precision, good assembly and good consistency are of great significance to the production and manufacture of wiring harness products, the assembly of the vehicle and the reliability of the vehicle. It is hoped that this experience sharing will enable you to have a deeper understanding and further improve the design of wire harness, and contribute to the development of wire harness.