Poor Plastic Selection Caused Gas Pedal Failures

Click for larger image.

[Via Design News]

Toyota outlined the technical problems reated to the accelerator pedal in a letter to the NHTSA. Looks like it’s a matter of poorly-considered material choice. The accelerator pedal assemblies contained a friction lever made of the polyamide (nylon) 4/6 material. Toyota’s investigation found that the material was susceptible to humidity (when the A/C isn’t running), which could cause the friction lever to absorb moisture and swell. It is well known that nylon is a hygroscopic (water-absorbing) polymer.

What, you didn’t know that? Well now you do…

According to Toyota (see the illustration), friction builds up on contact points (in left image) when condensation forms on smooth plastic surfaces and the nylon material swells. In February 2008, the material of the friction arm was changed to polyphenylene sulfide (PPS), which is non-hygroscopic, while investigations continued. The latest fix is the insertion of a precision-cut steel reinforcement bar (right) to eliminate the excess friction.

We’ll see if this actually fixes the problem. So what’s the take-away? If you’re sitting in a design review for a product that has a chance of seeing any humidity or moisture, make sure the materials you’re using can deal with that moisture. This issue of nylon swelling due to moisture is a more common mistake than you might think. So the next time somebody mentions using nylon for an application, consider moisture and humidity and then blow the engineers minds by saying, “Yeah, but isn’t nylon hygroscopic?”


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