Plastics trim lawn tools

It wasn't all that long ago that yard work entailed lugging, pushing, and swinging heavy metal tools designed, it seemed, to test the stamina of Paul Bunyan. The equipment appeared more closely related to commercial landscaping and logging equipment than residential lawn and garden products. Thankfully, those days are gone, and much of the credit for their passing goes to the application of advanced polymers.

The transition was far from easy. Engineers fought for decades to overcome plastics' image as the poor-man's metal, and to make consumers see the advantages offered by what is arguably the most versatile family of materials.

"When we came out with our first chainsaw that used plastic back in the 1970s, customers labeled it as cheap," says Kevin Beaulieu, engineering manager for chainsaws at Poulan. "Today, it's a complete non-issue."

Why the change of heart? Because plastics help engineers grapple with tough problems, the kind that, if solved, make a more desirable product. Some of the design objectives that can be addressed with polymers include:

Reducing parts count

Often, the real world serves as the best teacher. The following examples illustrate how engineers at several big-name lawn and garden equipment companies leveraged plastics to keep their products in front of the competition. Each case study begins with the major objectives of the design. The ensuing explanation tells how plastics helped make these objectives a reality.

Goal: Lower cost, fewer parts. Poulan's CSI line of chainsaws (specifically models 2050, 2150, and 2175) demonstrate the gains realized by combining several components into one. Each is powered by a 2.0 to 2.1 cu-inch gasoline engine. Their 14- to 16-inch-long cutting bars make them perfect machines for trimming around the yard.

Predecessors of these saws used a separate oil tank, fuel tank (typically die cast from magnesium), and a handle. Assembly required gaskets at the mating interfaces--always a possible source of leaks. And the parts had to be painted, an increasingly unacceptable technique in an emissions-sensitive world.

The solution? "We took everything and incorporated it into a main housing that consists of two halves that are vibratory welded," explains Beaulieu. He chose a 30% glass-filled nylon 6, supplied by Allied Chemical and BASF, for its heat resistance and chemical resistance--and the fact that it could be vibratory welded. Subtle design touches include plastic return springs for the trigger mechanism molded into the housing. "The only metal parts are pivot pins," he says.

Advantages of the new design include an estimated 30% cost reduction and a savings of about 5% in weight. It eliminates two springs, at least six fasteners, and three big assemblies. The molded-in color doesn't chip or require painting. And the hermetically sealed tanks prevent leaks.

"When you looked for a material that would give the combination of heat resistance, chemical resistance, weldability, and molded-in color, the nylon 6 turned out to be the best material," Beaulieu says. "I think it's one of the neatest applications of plastics we've done."

The one-piece main housing, however, wasn't the only problem Poulan engineers solved with advanced polymers. They were also challenged by a carburetor adapter plate that suffered from reduced physical properties at high temperature.

"They were using a nylon 6/6, but the material would creep and lessen the preload on the attachment bolts," says Hans Ullmer, technical accounts manager for BASF. "The carb would come loose and generate air leaks that affected performance."

To increase the plate's resistance to heat, engineers turned to BASF's Ultrason(R) E 2010 G6. While perhaps 60% more expensive than the nylon, the polyethersulfone's heat deflection temperature at 264 psi is 414F, and it can be used continuously at 374F. "This material reduced the creep almost to zero," says Ullmer.

Goal: Toughness, improved ergonomics. Listening to the customer's needs and then developing a design that addresses them. That's what Toro did with its line of electric blower/vacs--the Air Rake, Rake & Vac, and Super BlowerVac.

During Quality Function Deployment studies, engineers found that consumers wanted a product that was durable, assembled easily, converted from blower to vacuum quickly, had a cord that wouldn't come loose, and was easy to use. Toro knew that, to sell well, the product also had to look good, be cost effective, and not weigh too much.

To meet these wildly disparate objectives, designers began with the main blower housing. It's made from Dow Plastic's Magnum 9030 ABS, a material selected for its high gloss, low weight, cold weather impact resistance, and low shrinkage. "There was a lot of value in the material for what it did," says Steve Svoboda, senior design engineer at Toro. To address concerns with the cord pulling free, engineers formed into the housing halves a cord-lock feature that is simple to thread, but prevents strain from transferring to the plug.

The blower tubes and bag collar are formed from AIM 4800, an advanced styrene resin from Dow that offered impact properties close to ABS, but at a lower cost. "It has better impact and much higher gloss than polystyrene," Svoboda says.

To make assembling the tubes easier, engineers developed the first snap fits for a blower/vac. Previous designs used screw-fits that were so subtle that customers often thought the threads were knurls and tried to push the tubes together. Explains Svoboda, "The snap fits are so intuitive they nearly eliminated assembly issues."

Converting from blower to vacuum takes just seconds. The vacuum tube is made from Dow's Dowlex^TM IP 10 HDPE, chosen for its flow characteristics and cold-weather impact resistance. A lockout switch prevents the motor from running during changeover, and Svoboda claims he can switch a machine from blower to vacuum in about 15 seconds--without tools.

Some of the smaller parts, like the inlet cover and on/off switch, are currently made from a high-impact polystyrene. But the company is phasing this resin out, and Toro will soon switch them to the AIM 4800 styrene.

A "competitive secret" material is used on the blower impeller. Unlike a vacuum cleaner, debris in a blower/vac actually passes through the impeller. Dealing with air speeds reaching 190 mph on high-end models, engineers had to ensure that sticks or rocks wouldn't destroy the fan blades. The final selection Svoboda only describes as a GE alloyed resin. "We considered maybe 10 to 15 materials before settling on this one," he says.

In all, plastics helped keep the family of blower/vacs under 5.2 lbs (6.1 lbs for the Super BlowerVac). "It's far lighter than metal for a hand-held product," says Svoboda, "but plastic still has the im-pact resistance we needed, and the high-gloss finish."

Goal: Weight reduction. The Weed Eater Featherlite^TM SST was already reportedly the world's lightest straight-shaft line trimmer. With a new plastic gearbox, it's even lighter.

Made from BASF's Ultradur(R) S 4090 G6, a 30% glass-fiber-reinforced nylon PBT/ASA alloy, the gearbox translates the angle of the line-trimmer's output shaft so that the trimmer head spins parallel to the ground. It replaces a precision aluminum-cast housing that weighed about 30% more.

Inside the housing, two spiral bevel gears and several bearings handle the work. The gears, made from powdered metal, replace the previous design's forged gears. Engineers cut the overall transmission unit's weight by about 50%.

Dimensional stability was the engineers' greatest concern about the new gearbox material. "We are trying to control the fit at the pitch line of the gears down to about four thousandths of an inch," says Jeff Sadler, director of new products at Poulan/Weed Eater. "So the gearbox needed dimensional stability of about plus or minus two thousandths in key areas."

At first, they tried a regular glass-reinforced alloy, but got unacceptable warpage. The addition of the ASA in the Ultradur S, however, cut the warpage by almost half, explains BASF's Hans Ullmer.

"We saw an opportunity to turn to plastics to get benefits in weight and cost," says Sadler. But even more than that, they did it to push the technology. "A plastic gearbox was something that no one in the industry had done before," he says.

Goal: Metal replacement. The machining and finishing required of many metal components make them prime candidates for replacement with plastics. Such is the case with an engine housing for an advanced light engine from Kohler, as well as a lawn mower deck for a push mower from Austrian-based Viking, a division of Stihl.

Hoechst Technical Polymers (HTP) supplies the Celstran PPG40-02-4, a 40% glass-filled, long-fiber polypropylene for the Kohler engine housing. It offers the impact strength required for the company's new OHC-16 engine and costs 30% less than the nylon used in the company's previous engine, the Command Twin. Earlier still, in the Command Single, engineers specified a costly metal-elastomer-metal sandwich composite housing that had to be welded and painted.

The opportunity to use Celstran emerged because the OHC-16 runs 50F cooler than the Command Twin. Cooler operation saves fuel and oil and extends the oil-change interval. "Kohler is the pioneer in taking small engines that previously used stamped metal and transitioning them to composites," says Eric Lee, HTP applications engineer.

At Viking, engineers faced a similar evolution of changes with the design of the deck for their latest mower. Poly-propylene had been used to replace metal in smaller decks. But for those larger than about 35 cm (13.77 inches) poly-propylene doesn't have the necessary physical properties, and ABS or PC/PBT had been used.

Engineers from DSM Engineering Plastics suggested Stapron(R) N NM-19, a UV stabilized, compatiblized alloy of ABS and nylon 6. Compared to ABS, Stapron N offers much improved fuel resistance, surface appearance, and acoustic damping. Best of all, it's 14% less costly than PC/PBT alloys, effectively illustrating the "more for less" philosophy that explains why designers of lawn and garden equipment keep looking to plastics to help solve their design dilemmas.

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Tractor wheel revolves around plastics

In engineering, as in the movies, some of the best ideas can end up on the cutting-room floor for reasons other than the quality of the work. Such was the case with Terry Hardesty's unique design for an all-plastic tractor drive wheel for John Deere.

The object was to address problems inherent in steel wheels and small pneumatic tires," Hardesty says. Intended to accept a semipneumatic tire, the molded wheels needed no cosmetic covers to dress them up. They also accepted a nicely integrated set of wheel weights that fit flush with the outer rim. "It all worked together," says Hardesty. "There was no one breakthrough."

The skeptical may point out that plenty of plastic wheels and semipneumatic tires exist on wheelchairs, carts, and the like. But none are drive wheels. To enable his design to accept the power needed to propel a 600+-lb tractor and driver load, he used a 20% talc-filled polypropylene/polyethylene copolymer that contained an insert-molded powdered-metal spinner at the hub. Three arms protruding from the spinner transmitted torque to the wheel, and the design proved quite robust. "We took it to 150% beyond what we normally test and were never able to fail even one of the wheels," he claims.

Hardesty developed the wheel as part of Deere's "Stealth" project intended to demonstrate an all-composite, monocoque-framed, rear-engine riding mower. Alas, the highly ambitious program was killed before production, but the wheel design lives on as patent 5116106. "There are no plans right now," he says, "but there's no reason it couldn't end up on a product some day."