As I write this, a name brand manufacturer of appliances is adding to the list of recalled Dishwashers from defective power cords. The power cord can overheat, causing a serious fire hazard. The recall extends several years and affects a number of models under different brand names. I am not going to name names, a simple internet search can give you all the information on this recall. What is significant about this recall is that there were five reports of property damage which has affected over 600,000 models. A relatively small cost part over five failures has created a massive recall involving two countries (USA and Canada). Risking hundreds of thousands of lives on top of monetary cost and cost to reputation.
We are going to drill down to the cause(s) of the component failure. Also the greater implications of all manufacturers of electrical components. And how preventative measures by applying tools and quality practices can greatly reduce the risk of component failure.
What Causes a Power Cord to Overheat?
High electrical resistance at any point along the length of the cord is a typical culprit for excessive heat. High Electrical Resistance is caused by a number of factors. In the case of a wire assembly some factors are outlined below:
- Wire size too small for the rated current that flows through it.
- Connector Crimp and Wire are not properly matched.
- Poorly crimped terminals
- Crimp Equipment.
- Not set up correctly.
- Worn Tooling
- Inconsistent Crimp Press Force.
- Crimp missing strands.
Crimped terminals are a core connection in a wire assembly and can be a source of high electrical resistance. High resistance in crimped terminals is directly attributed to too low and in some cases too high compression of the wire strands within the crimp. There is a direct correlation between crimp compression and electrical resistance. Crimp compression cannot be determined without cross section analysis. Pull test does not provide an accurate measurement of crimp compression (and by extension: electrical resistance). In some cases, pull test can actually decline slightly before resistance peaks.
There is a direct correlation between crimp compression and electrical resistance.
Terminals that have a crimp barrel that is too large for the terminal can cause low compression. Improper crimp tooling can also contribute to inconsistent crimp quality. Worn crimp tools, improper crimp tool setup and press force that can vary from piece to piece can also lead to inconsistent crimp quality.
Wire stripping equipment can also cause inconsistencies. Worn blades or improper machine setup can cause cut strands. Cut strands can reduce the total compressed area in the crimp which can increase resistance.
Prevention is the Cure.
The difference between this recall taking on the dimension it has and not even happening at all could be simply a few missed checks in the assembly process. We are going to get really specific and focus on the terminal crimp process. Consider the following:
- Knowledge of the assembly process is critical. But it is not enough. Assuming wire assembly is a simple process that does not require constant monitoring can be a critical mistake.
- Understanding each factor in assembly and their inherent variability is important. In the case of crimping a wire to a terminal, there are five basic elements to consider. Any of these factor which are not in control can affect quality.
Where do you start?
Manufacturing Machines are well maintained and capable of repeatable piece to piece consistency. Static calibration and single piece measurement does not guarantee statistical capability. Dynamic measurement provides the way to measure process capability and repeat-ability.
Tooling is replaced before quality is affected.
Manufacturer’s Specifications are a critical start to a process validation. Suppliers spend a significant amount of resources in developing components and the associated assembly instructions and quality standards. For example, quality standards for terminals include crimp height measurements and minimum pull test standards. Crimp Height measurements are created which are based on optimum crimp compression. Validate using Crimp Cross Section Analysis.
Use the Right Measurement tools. For terminal crimp height, use crimp height micrometers. Other measurements can be made by blade micrometers or dial calipers. Pull test as a secondary measurement. Cross Section Analysis for more in depth analysis of the wire under compression.
Correct Materials are used. Ensure the wire is matched to the terminal based on the supplier’s specifications.
Measure when material lots change. Materials within or between lots can cause variation in the crimp process. Re-Measure to assure there no changes in materials.
When a failure occurs of this magnitude, it can be a wake up call for the companies involved. But it also can be a valuable lesson to others. Don’t take a chance with the future of your company. Ensure your crimp process is validated and monitored prior to and during the production process. Crimp Quality Solutions provides valuable tools and knowledge for the terminal crimp process.