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Motion System Design

The problem with plastic

Fixing things is one of my favorite pastimes. Bikes, toasters, fans, fishing reels – it doesn't matter. I'll fix anything. In fact, I've been known to fix things that don't need fixing.

Lately, I've had plenty to keep me occupied, including a broken snow blower, a breadmaker, an electric sander, a garage door opener, and a sidewalk edger. And I'm happy to report, everything is working – for now.

If I sound a little hesitant it's because I know, that in a matter of time, almost everything I fixed is certain to fail again. And the reason: As the popular TV commercial goes, "It's plastic."

You know what I mean. Plastic gears. Plastic sheaves. Plastic cams. Plastic gives. Bending, breaking, flexing, flaking plastic.

I've got nothing against plastic. When it's used intelligently, it's a great material. But the way it's being used in consumer goods can't be helping its image. The problem is that millions of consumers don't enjoy fixing things the way you or I do. As a result, tons of perfectly good merchandise go into our landfills each week – because of plastic.

This puts a whole new perspective on my favorite pastime. With each gnarled plastic component I replace, I can feel like I'm doing my part to keep America beautiful. I’m so happy, I can hardly contain myself.

So let me tell you about some of the things I've rescued from the trash heap of broken plastic parts. I'll start with the snow blower, since it was the most recent.

I'd say it's about two years old. My father, who owns it, couldn't get it to start. He and I suspected the same thing – that the gears on the electric starter had worn down – and when I opened it up, that's what it was.

The starter mechanism on this particular model consists of an ac motor with a small plastic pinion that meshes, momentarily, with a toothed flywheel attached to the engine. The pinion gear, which rides on some sort of centrifugally actuated plunger, was pretty much destroyed. Knowing my father, who generally waits for the month of April to clear the snow from his driveway, I couldn’t imagine the gear failing from overuse.

That the pinion is made of plastic is not the only reason it failed. Poor design also played a role. The starter motor, for example, is held on by three bolts and a cable tie. There's no point in speculating whether the tie down was an afterthought, because even with the strap, the motor is so unstable you can see it pivot about its plastic mounting flange when the pinion and flywheel engage. If the new pinion lasts two more years, I'll be surprised.

The garage door opener also succumbed to the plastic plague. Here, the workhorse is a worm drive with a plastic gear. When I looked inside, I found that the gear was chewed to smithereens.

In the breadmaker, it was a plastic pulley. The pulley warped, causing the drive belt to continually run off track. Whenever the belt rode up onto the lip of the sheave, the resulting tension caused the shaft to seize. I was lucky the motor didn't burn up while I was trying to figure out what was wrong.

The sidewalk edger is a slightly different story. The gears are actually made of metal. The housing, however, is plastic. And despite the reinforcing ribs, it's just not strong enough to resist the forces produced by frequent high-speed collisions between the cutting blade and immovable concrete.

Plastic also took my electric sander out of commission. Twice. A plastic cam in the drive mechanism converts rotary motion to reciprocating or orbital motion. I suspect the cam overheats as the load increases, making it vulnerable to deformation.

Plastic may be great stuff, making our lives better just like the commercial says. But the way the consumer goods industry is using it in power-transmission components may be a bit of a stretch.

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