Jacques Hoffmann 0000-00-00 00:00:00
The right leak testing method will help avoid the unnecessary costs of adding material and labor to fi x defective production components. In many automotive manufacturing scenarios, there are considerable advantages to focusing on in-process testing as opposed to end-of-line testing. As a part or assembly is built, material and labor costs are constantly added. The total value of a production component is usually a function of its percentage-completion and includes both labor and material costs. If one integrates testing throughout an assembly, one can avoid the costs of adding material and labor to defective production components. These are a few examples of how costs for rework and scrap can be avoided by in-process leak testing: • Leak testing following ultrasonic welding of two plastic housings. • Leak testing following spin-riveting of a cover to a pump housing. • Leak testing after the insertion of an “O” ring or other seal. Defective Parts or Defective Tests? The inability to differentiate good parts from defective parts may be due to faulty testing methods, which in turn are usually the result of inadequate understanding of real testing requirements. In leak testing, there are often problems with seal creep or fixtures that are not rigid enough. Start by Selecting the Right Test Method There are various methods that one might choose from—simple pressure decay testing, differential pressure decay testing; helium mass spectrometer testing; and mass f low leak testing. Let’s look at each option. Pressure Decay Testing The lowest cost method, pressure decay testing, actually has lower initial costs for fixtures, but may in fact be more costly due to longer test cycle times (pressure measurements required at the beginning and end of a known time interval). This method is especially vulnerable to outside interference, such as changes in ambient temperature, drafts, test-part deformity and seal creep. Differential Pressure Decay Testing Differential pressure decay testing methods are superior to simple pressure decay testing, but are similarly inadequate for testing parts where temperature compensation is an issue. Both pressure decay and differential pressure decay test methods involve increased cycle times compared to other dry air test methods. Helium Testing High cost helium testing, especially costly in the current context of rapidly rising helium prices worldwide, is usually justified in air conditioning and similar applications where potentially hazardous gas leaks less than 10-3 sccs need to be detected. Additionally, these instruments typically require more maintenance than other leak testing systems. Mass Flow Leak Testing In most applications requiring detection of low leak rates, the lower cost mass flow testing method using customized, leak test capable, hi-resolution sensors is actually preferable, since it has the shortest possible testing times, can readily compensate for temperature and, overall, delivers the best accuracy, even in applications where testing is required at 0.05 sccm or less. The larger the part volume, the better mass flow test methods are when compared to pressure leak testing methods. The Importance of Fixture Design The jigs and fixtures used in assembling an automotive product often have different and frequently contradictory requirements than those needed for accurate leak testing. That’s why upfront consideration of fixture design is usually required and tends to pay dividends in terms of accuracy and cost savings in the long term. And why off-the-shelf fixtures are almost always inadequate. For example, the physical dislocation of seals between the testing instrument and the component being tested, called seal creep, will compromise all methods of leak testing. Excessive clamping force on system joints can often mask leaks. Thus, expert fixture designs for dry air leak testing have several factors that they are managing: the part cannot have any creep or motion, it must be stationery; the part cannot be crushed or damaged; and the fixture design cannot mask leaks. Often a part being tested needs to be in the true car position, such as automotive valves. In some instances, assembly and test fixture designs must simulate the parts to which parts are to be joined. While leak testing a transmission casting you would try to mimic the paper gaskets that will be in use in the field, as opposed to actually using paper gaskets since these wouldn’t hold up during the testing process. Fixture design is often mediated by how completely the overall assembly and test operation is automated, including specific testing stations. In manual systems, the sealing and fixturing may be more difficult because of ergonomic issues. Clearance for loading and unloading may be an issue. Poka-yoke considerations (a Japanese term meaning mistakeproofing) are such that often a fixture design will anticipate human errors of placing wrong parts in the testing stations. A poka-yoke informed fixture design will make sure that only parts with the correct dimensions will be tested. Since fixture design can make or break your assembly and test solution, it is generally advisable to enlist only testing engineers with experience in building many turnkey testing solutions. One needs a nuanced understanding of leak and functional test requirements to bear on fixture designs that allow superior test instruments to deliver their promised performance. Specify Customized Software for your Data Highway Testing experts know that there are no one-size-fits-all generic testing solutions. Taking short cuts by selecting the lowest cost instrument from the shelf and placing it into assemblies rarely achieves gage R&R in acceptable ranges because the test sensors and software programming are not well-matched to application requirements. In most cases, custom software for test-intensive assemblies is actually less expensive than a generic off-the-shelf application because it is designed to meet real business needs and does not encumber users with the costs for program features and functionality that is not actually required. Moreover, many general applications are not especially easy to use, or do not reflect the real data handling requirements of the application. Most test applications require software that works in real-time, which is usually not the case with off-the-shelf packages. It is important to have the means to view real time traces of a test instrumentation transducers’ performance, as opposed to simple test cycle times. Slower and less detailed applications do not process data quickly enough to provide for meaningful test control and/or test method calibrations. If the required data analysis is more complicated than generating a simple line graph, the off-the-shelf software packages that are commonly used similarly fall short. The best-inclass custom software for test-centric assemblies will automatically calculate R&R percentages based on the number of trials performed. How networks are built is also important in fully integrating testing into assembly operations. When there are steep requirements for testing, there is inherently a lot of data that typically has to be processed and also sent to a plantwide network. BUS compatibilities need to be considered. State-of-the-art, test-centric assemblies have Ethernet capabilities that allow the huge datasets from intensive testing to be distributed to plant-wide networks for analysis. Internet-based remote diagnostics are another feature of the bestin- class, test-centric assemblies. They also interface with barcode scanners. The leak detector controls the test cycle and interfaces with both the scanner and the customer data highway seamlessly to provide total information flow. Custom software development for test-centric solutions starts with identifying business goals as to how the data and information garnered during testing will be used, not just in initial production, but also during the entire lifecycle of the product being produced in the assembly and test operation. This is of growing importance to automotive companies that now want processes to attach data to products from cradle-to-grave to assist with recalls or reducing time-to-market in new product development cycles. For instance, a leak detector can be interfaced with a bar code scanner and the customer data highway to provide a seamless f low of data. Insist on Guaranteed Gauge R&R A tell-tale sign of a less than optimal assembly and test operation is one where the machine builder defines testing gauge R&R in terms of those proffered by the manufacturers of the test instruments. Gauge R&R of the entire assembly and test solution is what counts, and this is what must be guaranteed to users of the test-centric assembly and test solution. This explains why the most savvy machine builders around the world are now incorporating applications engineers with a sole focus on testing into their project development teams. The improvements to be expected from a test-centric assembly approach are not trivial. Leak and functional test cycle times can be decreased 25–70% in the best-in-class examples and yields increased proportionally. NDT Jacques Hoff mann is President of InterTech Development Company, a world leader in test-centric assembly specializing in automated leak and functional testing with 7 patented mass fl ow and hydraulic technologies, as well as, helium mass spectrometry (ISO-9001-2000 International Standards for Quality Management). For more information, visit www.intertechdevelopment. com or call (847) 679-3377. TECH TIPS The inability to differentiate good parts from defective parts may be due to faulty testing methods, which in turn are usually the result of inadequate understanding of real testing requirements. The lowest cost method, pressure decay testing, actually has lower initial costs for fixtures, but may in fact be more costly due to longer test cycle times. Leak and functional test cycle times can be decreased 25–70% in the best-in-class examples and yields increased proportionally..
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