These injection-molded, high-precision plastic shafts and gears were made for a two-stage reduction transmission used in automotive power lift gates. The first-stage gear and shaft (far left and left) and second-stage output plastic gear (right) are injection molded from Celcon acetal copolymer (POM) M90 and Celcon GC25T, respectively. The second-stage output shaft (far right) is injection molded from Celstran PA 66-GF50-02.
As always, you provide a comprehensive look at some of the cutting edge in materials, Ann. Lightweight engineering plastics are particularly interesting because as you show, they have such a broad range of application. I'm especially interested in their use as lightweight materials for solar-powered vehicles and medical devices.
Thanks, Elizabeth. Since plastics are, in effect, always a custom mix, the resins can be made to fit a wide variety of spec combinations. But I was a bit surprised at the mention of solar-powered vehicle applications.
Being a car guy, I've had bad experiences over the years with "plastic/composite" parts failing in automobiles. One of the worst for me were the timing chain gears that GM used in the small block chevrolet V8 engines in the 1970s. But I've experienced plenty of smaller failures in plastics that just don't hold up over the years in the rough circumstances of the vehicular world. Silly little things like clips for hoses and wire assemblies are frequent failures, but there are bigger problems too.
Just last month I finally upgraded the plastic gears in my Trans-Am's headlight motors to a brass gear. The design of the headlight motor features an electric motor with a metal worm gear, that meshes with a plastic ring gear that's connected to the shaft which turns the headlight motor. Instead of incororating discrete limit switches in the design, they made a headlight controller which senses the high current spike when the headlight has reached the stops and can't spin any more. When it detects that high current, it turns off the motor. Unfortunately, that metal worm gear is placing a lot of pressure against that plastic rings gear and it eventually breaks teeth off of the plastic gear. There's a cheap fix to flip the assembly 180 degrees and use the other side of the gear (since the rotation only uses half of the gear), but eventually teeth on both sides break, causing the headlight to make a grinding sound when it reaches the limit. (The controller doesn't sense the current spike but has a failsafe to shut the motor off after a few seconds.)
Luckily, there are companies who machine nice metal gears for this application, but they are a bit expensive. Since my Trans-Am was my first new car which I'm keeping to pass down to my son some day, I finally invested in the new brass gears and they work well.
Thanks for sharing your actual experience with what we write about, Jim_E. I'm sorry to hear that about the plastic components in your cars. Much of the problem here, or elsewhere, is due to incorrect spec-ing of materials, sometimes because of engineers but often, as we hear a lot, because engineers spec the right material but management doesn't like the fact that quality costs more. That said, I'm surprised brass gears are OK, especially in a car. My bad experience with them is in a coffee-grinder: they wore down way too fast, changing the grind to very coarse by default.
Your Story of the headlight worm gear sounds familiar. Historically, plastic gears of any resin (often nylon) just didn't have the life-span that brass gears can offer. Under the hood environments are abusive, experiencing vibration, heat, dirt, and chemical spills. It's easy to understand why the Automotive OEMs would choose injection molded gears over brass, at a fraction of the cost to produce the parts. They don't have to last forever-- only 3 years or 36,000 miles!
Maybe Ann's new examples of the VALOX PBT and the XENOY blends will change all that, and low-cost injection molded gears can get a new reputation for longevity; starting today.
As a P.S., here are some new engineering plastics from DuPont specifically for car applications that need resistance to high temperatures and chemicals: http://www.designnews.com/document.asp?doc_id=269020
Ann, I think that you will find that a whole lot of automotive "engineering" winds up being done by purchasing people who get rewarded for cutting costs, and it seems that they are awarded by suppliers for delivering POs. The evidence in that area is more circumstantial, such as purchasing people sporting diamond encrusted gold Rolex watches.
Sometimes a plastic part that can easily handle the calculated average loads is just not up to handling those larger occasional loads, at which point the failure is permanent even if the part sort of works after the damage. Purchasing people are great ones for cutting safety margins in order to reduce costs. But the result is much lower quality. But that reflects on the engineers and so purchasing does not care about reducing quality.
William, I know what you mean. That's true for a lot of industries, not just automotive. A long time ago I wrote for an electronics purchasing publication, the old EBN, and I spent most of my time trying to educate them on what engineers (mostly) already knew, as well as on what they were learning about new technologies. But the whole concept of how they buy what they buy is different, and that becomes a crucial factor in quality.
Years ago at one employer I had a stamp for my released drawings that stated "This system will not function as required unless it is wired according to the circuit shown". I needed to add , " and built with the specified materials", but at that job the problem was panelmen who took shortcuts. BUT the principle is identical.
Oak Ridge National Laboratory has developed a surface preparation method to improve joining carbon composites with aluminum, with potentially far-reaching ramifications for high-volume industrial applications.
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