They’ve been through the quality department, and all of the parts are in spec. You’ve double-checked assembly processes, and the installation is perfect. Then you drive the test vehicle off the line, and you hear a squeak. Two pieces of plastic trim are rubbing against each other in a way designers hadn’t predicted, and drivers are going to notice.
It’s the kind of scenario that gives quality engineers nightmares. It’s not easy to fix minor cabin-noise issues after designs have been finalized, while vehicles are in final manufacturing proveout. Sending parts back for redesign can be costly and delay vehicle launches. Typically, companies have to use a variety of noise-cancelling technologies that add cost and build complexity to vehicles.
“We tend to come into the process late in the game, when there aren’t a lot of other options,” says Mike Norton, group manager of automotive and bearing industries for Klüber Lubrication, a supplier of specialty lubricants. “The OEMs do a lot of testing in the plant, and if a noise occurs, the engineers are told to fix it right away. That either means redesigning the part, applying Mylar tape, or using one of our lubricants. If you’re already in production proveout, you don’t want to start talking about part redesigns unless the problem can’t be solved any other way.”
Minor squeaks tend to be caused by a condition called stick slip, a major contributor to noise, vibration, and harshness (NVH) issues in motor vehicles. The condition exists when parts stick to each other until enough force causes them to slip against each other, creating a vibration that generates noise. The more force needed to move a connection from sticking to slipping, the more kinetic energy gets released when the parts slip – so greater stick leads to greater noise. Rubbing the inside of a plastic food container with your thumb gives a good simulation of the problem. A dry thumb against a dry surface can generate a loud noise. Lubricate the plastic with dish soap, and the noise levels drop.
That’s the strategy that NVH engineers tend to use – lubricate the mated surfaces so they don’t stick, eliminating the noise generator. Norton says the two leading methods to eliminate sticking are applying Mylar or felt tapes to mating surfaces or lubricating one of the surfaces.
Mylar tape works well, but it can be labor-intensive to install, difficult to automate, costly, add mass, and it can be visible. Norton says many automakers use tape on hidden plastic parts, such as points where air-conditioning vent pipes meet the instrument panel, but keeping such systems hidden can be a challenge.
Klüber Lubrication’s solution is the MR 3 line of spray-on oils – Klüberalfa MR 3, MR 3-500, and MR 3-800. The perfluoropolyether (PFPE) lubricants are not coatings that bond with the surface of the plastics. They are oils suspended in a carrier. Workers spray the material onto mating plastic surfaces, and the carrier evaporates, leaving behind a thin layer of the PFPE oil. The different formulations have different viscosities, and the higher the viscosity, the greater the visibility. The lower-viscosity materials are virtually invisible once applied, while the more viscous version provides a sheen.
“Chrome-plated plastics have gotten really popular in vehicle interiors over the past 10 years or so, and they’re often a major contributor to NVH. That shiny, glossy surface can create really tough stick-slip scenarios,” Norton says. “The other major trend that’s problematic is piano-black finishes. They’re so smooth that when they come into contact with other items, they can squeak quite a bit.”
The most common problem, he adds, are when different materials come into contact with each other – plastics rubbing against metal, rubber connecting to plastic, or hard plastics rubbing against softer composites.
Because NVH issues tend to arise so late in the vehicle-launch process, Norton says Klüber Lubrication tends to work with manufacturers as they build early test vehicles – cars intended to test manufacturing systems, not ones sold to the public. Engineers have created an anti-noise kit with several Klüber lubricants that can be pump-sprayed onto squeaking parts to eliminate noise.
When lubricating the part eliminates the unwanted noise, that step typically becomes part of the build process, and suppliers will add a lubrication step before shipping parts to the OEM for assembly. Norton adds that Klüber engineers have worked with companies that sell spraying equipment to make sure the company’s lubricants can be applied with standard, automated tools.
“With cars becoming quieter and quieter, noise is one of the biggest indicators of build quality,” Norton says. “It’s an especially big problem with electric cars. Without the engine noise, you can really notice every little squeak or rattle. So we’re getting a lot of calls from the OEMs when they identify a noise problem.”
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or firstname.lastname@example.org.