USCAR-21 is the quality standard that can strike fear into companies who deal with automobile assembly companies. USCAR-21 is a universal standard which applies to suppliers of electrical assemblies to The Ford Motor Company, General Motors Corporation and Fiat Chrysler Automobiles.
Companies who are not involved with automotive wire harnessing can quickly dismiss this standard as not applicable to their business and industry. Although this is true, some of the components of USCAR-21 are valuable and can be directly applied to the validation of wire terminations for any industry. Companies not considering some of the key features of USCAR21 can miss out on the benefits to quality improvement.
We will drill down to reach those best practices which can be applied to your company.
Prior to the formation of USCAR-21, each automobile manufacturer had their own testing and validation standards. The adoption of a common standard meant more efficiency in the testing process, especially for suppliers who provided wire harness across a number of automobile OEM’s listed..
The USCAR-21 standard includes mechanical and accelerated environmental tests which are designed to duplicate potential operating conditions a wire to terminal connection can experience through the expected life cycle of an automobile. Demands on the electrical system are increasing with smaller wires carrying communication signals and larger wires used in current load in applications such as EV batteries. A comprehensive standard was required to address the performance requirements through the full range of harness size and complexity. At the same time, to reduce some of the bureaucratic burden from the supplier end.
Lets bring USCAR-21 down to it’s most basic objective: Electrical connections must maintain low electrical resistance through the expected life span of an automobile. This is achieved by adhering to critical crimp design and validation criteria.
Ultimately, monitoring the crimp process during production is important to ensure specifications from the validation stage are maintained.
Here are a few considerations to the actual electrical crimp.
- The terminal supplier’s recommended crimp height and width is the starting point of all initial and pre-production crimp validation. Wire terminations should perform acceptably though the tolerance range of the recommended terminal crimp height.
- Crimp compression, the process of encapsulating the wire into a crimp barrel with sufficient force to deform the strands from their round shape is a major factor in acceptable electrical resistance. Compaction of the strands by 15 to 20% provides the best opportunity to pass electrical resistance testing. There is a direct correlation between the amount of wire strand compression and electrical resistance. Especially leading up to and through the above compression range.
- Crimp barrel and wire size must be carefully matched to ensure the optimal compression is achieved. In combination with the proper crimp tool design. Factors indicating a poor wire to terminal match:
- Wire strands under-compressed.
- Strands not evenly distributed in the crimp barrel
- Crimp wings curl and touch the bottom (floor) or sides of the crimp barrel.
- Crimp wings with a gap at the top, with strands not fully encapsulated in the crimp or pressed on top during the crimp process.
Cross Section showing under compressed strands and voids.
- Serrations in the crimp are designed to break oxides present on the surface of wire strands, providing better electrical connection. Also to assist in the mechanical secure-ness of the crimp.
Consideration of measurement methods.
- Crimp Height is the primary test factor in a validation and in-process measurement.
- Tensile (Pull) testing is a secondary testing method to ensure the wire termination has “sufficient mechanical strength”. It is not a determining factor of electrical performance as low or high compression can affect not only mechanical but electrical performance.
- In all cases, insulation support on terminal crimps must be peeled back or not crimped at all for pull testing. Pull test readings should be made on the wire to terminal crimp only.
- Crimp Cross Section un-covers the actual crimp condition from wire/terminal match and crimp tool profile. As well, crimp applicator setup can be checked (i.e.: terminal feed position).
- Visual inspection criteria ensures the proper machine setup provides the best crimp. Visual factors of an acceptable crimp are:
- End of wire protruding on the contact side of the wire crimp. Known as brush.
- Insulation and wire (roughly 50:50) present in the “window” between the wire and insulation crimp.
- Bell mouth present on insulation side of the wire crimp to protect wire stands. Bell mouth can also be present on the contact side of the wire crimp.
- Insulation crimp wings should not penetrate the insulation and make contact with the internal stranding and should fully encapsulate the insulation.
Electrical and Electronic devices that are not assembled into an automobile will have their own performance parameters. In most cases they are not subject to temperature and humidity fluctuations that are present in an automobile. But totally dismissing good crimping practices and not applying some of these best practices can expose your products to premature failure which carries a high cost (and liability). That’s the important factor when Acting on the Cost of Quality, understanding the Benefits and avoiding the risks of Inaction.
WireProcess is equipped and ready to deploy the tools you need to take your crimp process to the next level. Connect Your Way to WPS.
