You've surely noticed how magazines and newspapers fill their pages with stories on electronics, the Internet, bioengineering, artificial intelligence--take your pick. But it's mechanical systems and components for agriculture, manufacturing, automotive and other applications that enable industrial society to function. Electronic devices may provide a very valuable control layer that directs such mechanical components and systems, but the mechanical systems do the work.
|Sealing the joints of engineered chain significantly extends chain life in difficult applications such as cement plants and heavy construction equipment.|
As a prime example of such engineering creativity, take a look at a century-old product--power transmission chain. No glamour, nothing new, right? Wrong!
Few operations stress equipment more than the cement industry. Chain drives used on bucket elevators by cement manufacturers get eaten alive. "Cement is so abrasive that it gets into the chain's joints and erodes the pins, like wind and water and sand did the Grand Canyon. It gets that same pattern to it," says Product Specialist Tom Beranek of Rexnord Corp.'s Engineered Chain/Idler Division in Milwaukee, WI. To prevent destruction of chain joints, Rexnord's engineers developed a sealed-joint chain.
Each joint in the chain contains a four-lobed resilient seal. In early designs, engineers pressed the seal between the chain's side bars. Today a steel retaining ring made from powder metal holds the seal in place. Two of the seal's lobes press against the ring, two against the chain's outside side bar. It took time to find out how much press fit between seal and side bar was appropriate. An excessively tight fit hinders chain articulation; too loose a fit permits contaminants to invade the joint. Both the seal and retaining ring ride on a bushing, and the pin rotates within the bushing. The outer sidebar rotates as the chain moves, scrubbing against the seal. Lubricant can be sealed into the chain at the factory, or a grease fitting on the chain can permit maintenance personnel to replenish the lubricant.
In a cement elevator, bucket chains may last only one or two years. Sealed joints extend the life of such chains by 50% or more.
Other industries now use the sealed joint chain technology originally developed for the cement business. Nuclear power plants, and construction equipment like chain-driven scrapers, now make use of sealed-joint chain technology. "Externally, chains haven't changed much over the last 30 to 50 years," says Beranek. "But inside the plant, we've taken the technology and brought it up to the 21st Century. Fatigue strength is something we're really shooting for." Statistical process control, CNC machines and CAD each play a role in improving engineered chain.
Since the early 1970s, better surface finishes and more accurate parts have doubled and tripled chain fatigue strength, Beranek asserts. New materials, coatings, and heat treatments are important areas of study at Rexnord's engineered chain group as they strive to improve their product.
In another arena of chain technology, Philip Mott, chief engineer at Borg Warner Automotive in Ithaca, NY, sees two trends driving the entire power-transmission industry: Extreme cost pressures and extreme pressure to improve performance. In line with the idea that when things get tough, the tough get going, the industry must keep a process of continuous improvement underway. "We do computer simulations of chain loads, and use FEA extensively to improve link styles," says Mott.
In a major trend, rocker joint chain is displacing the venerable roller pin chain in automotive applications. "In the past, a round pin chain was stronger than a rocker joint chain. By using FEA, we've been able to make rocker joint chain as strong as round pin chain," he states. Given the much higher speed capability of rocker joint chains when compared to round pin chains, this improvement in strength represents an important product advantage.
And, keeping in mind the old axiom that a chain is no stronger than its weakest link, Borg Warner's engineers came up with ET guide links. Described in Design News, October 8, 1995, p. 145, these side links enhance load sharing across the width of a chain. Stiff side links carry a disproportionate amount of chain load, and upset load distribution to the internal links. "Ideally,"says Mott, "you'd like every link to share the load equally." The ET links look rather like a horseshoe with a plate at its toe. Because of this design, an ET link can open somewhat under load. So the chain's internal links carry load more uniformly.
Given the need to shrink chains, to make them take up smaller volumes in today's transmissions and engines, link load sharing becomes especially important. With fewer links in a chain, design engineers must find ways to see that each carries load. Thus the ET link.
Innovation in the supposedly static arena of chain design means employment and prosperity at Borg Warner. But innovation ends when engineers and management think of a product as a commodity. "A product only becomes a commodity item when you let it. If you continue to put higher technology into a product, it will not be a commodity," Mott insists.
Gearing up. In no area of technology can companies--or design engineers--stand still. Not in chains, and certainly not in gears. The Series 650 parallel shaft gearmotor line recently introduced by Bison Gear and Engineering Corp. of Downers Grove, IL, illustrates this point.
"We wanted to fill a gap in our parallel-shaft product line, and we wanted the ability to get down to very low speeds," says Chief Engineer George Thomas. With the five-stage gearing used in the Series 650, Bison can generate a reduction ratio of 2300:1. So Thomas and his colleagues can achieve speeds of approximately 0.7 rpm and torque to 720 lb-inches using a gearbox measuring 5 by 6-.25 by 5-.75 inches. Applications for the gearmotor exist in conveyor systems, food processing, pallet wrapping and other low-speed, high-torque equipment.
"It was an intricate design," says Thomas. "We decided to take a small number of gear meshes and use them at many places in the gearbox. We ended up with something like seven gear meshes used in different locations to give us 18 different ratios. Actually, a lot more potential gear ratios could be created just by substituting one mesh for another."
Since 1985, Bison has used CAD for gearbox design. On the other hand, Bison engineers continue to use non-CAD software for gear design. Thomas reports that CAD packages for gear design have not yet been accepted at Bison. "Gear design is, on the one hand, very simple, and on the other very specific. To present it as part of a general design package has meant that you had to simplify too many things--critical things."
Getting belted. Flying well below the radar of most folks who look for innovation, belt technology keeps moving along. Take a glance at drawings of early 19th Century textile mills and you'll see belt-driven equipment everywhere. Like chains and gears, these power-transmission components have been around for awhile. So surely the industry must be able to rest on its laurels?
Not really. Sawmills make extensive use of powerband belts: multiple-unit V-belts with a tie band (a layer of fabric) on top of the set of belts to hold them together. "They use these belts on chippers and saw drives, and they were typically getting three to six weeks of service before breaking them," says Gates Rubber Co. Application Engineer Gary Porter. To deal with this problem, Gates Rubber (http://www.gates.com/gates/ ) set about developing the Predator(TM) belt, which comes in standard outside circumferences ranging from 85 to 125 inches. Gates spokesperson Jerrold Donovan says the company can produce limited production runs of any size.
Designed for extreme shock load applications, this new V-belt consists of neoprene that forms the V-shape of the belt, tensile members that do the work of carrying load and resisting shock, cover material, and the tie band. Gates engineers use aramid fibers as tension members, rather than the polyester fibers typically employed. In this type of belt, the tensile members carry load, while all other components of the belt support, protect and position the tensile members.
The Predator is a covered belt. Fabric wraps around the entire V-section of the belt. In conventional powerband belts, manufacturers apply rubber to both sides of the fabric.
Unfortunately, the rubber coating has a high coefficient of friction. To deal with this situation, Porter and his colleagues used rubber only on the inside of the belt's cover fabric. Consequently, the belt can slip under peak shock loads, and generates less heat when slipping than a rubber-coated belt.
Belt technology will probably benefit significantly from new materials, Porter believes. "One of the problems you have is trying to get rubber to run in a dynamic state," he explains. "Some rubbers fracture or rupture if the belt's constantly flexed or bent. So you're going to see materials engineers blending materials to try to enhance the properties of rubber." And no matter what comes, he expects to see a good future for belts. "There will always be a place for belts. If you can't hook a motor to a pump directly, and you've got a space in between, a belt is a relatively inexpensive way to make that transition from the motor to the pump."
Shafting the future. What could be simpler than a shaft? Solid or hollow, flexible or stiff, shafts find applications throughout industry. Basic, reliable, and taken for granted, they may seem like the least likely candidates for enhancement in the power transmission industry. But when competent engineers apply their minds to a project, innovation happens.
Quite probably the biggest changes taking place in shaft de- sign involve composites. At Dana Corp.'s Spicer Division, for example, (you can reach Dana at http://www.dana.com ) glass/graphite driveshafts offer a range of new capabilities to users. These components consist of a filament-wound fiberglass tube core overlain with graphite and an E-glass hoop roving. This construction enables engineers to change the shaft's first bending resonance (its critical speed) and to tune the shaft's response to torsion. Further, the bending stiffness can vary independent of torsional stiffness. Composites possess excellent damping properties when compared to steel, which helps engineers control noise and vibration.
Aluminum-graphite shafts, now used on pickup trucks made by General Motors and Chrysler, are built up from layers of materials on the aluminum tubes. "Between the aluminum and graphite, we place an insulation layer--a surface veil--to break the electrical connection. There's a hoop roving of E-glass over the carbon fiber to give the structure hoop rigidity, and another layer of surface veil is placed on the shaft OD to improve the ease of processing, aesthetics and damage tolerance," says Jim Duggan of Spicer's Advanced Design Engineering Dept. Spicer typically uses about one pound of mid-modulus-grade (35 to 40M psi) graphite on its aluminum/graphite pickup truck driveshafts.
Opportunities may exist for new types of shafts in the heavy trucking industry as well. Spicer has produced all-aluminum driveshafts, and has prototyped and field-tested aluminum/graphite driveshafts. The composite construction offers some real advantages. "With the aluminum/graphite driveshafts, we've been able to span 120 inches in drive-line length with a single-piece shaft," Duggan states. No intermediate supports are needed. "We can eliminate not only the bearing and the yoke, and an extra universal joint kit, but also a non-structural cross-member. We've been able to take almost 75 lbs out of the weight of a heavy truck." Today, cost limits applications for the new driveshafts. But they work, and represent a new technology now available to designers of drive lines.
Steel rules in most areas where engineers need shafts. "It's readily repairable and everybody knows how to play with it," Duggan remarks. But niche markets use the new technology. Over the last five or ten years carbon-fiber shafts found a place in the cooling tower industry and for stationary-pump applications. "They're lightweight, and can be less corrosion-sensitive than steel," says Duggan. In such applications they make sense, because the driveshaft costs much less than the labor required to replace it. Also, when properly tuned, a composite shaft can easily handle the kind of torsional vibration produced by a five-bladed cooling tower fan. The composite shaft in that application replaces shafts that consist of tubes with rubber biscuits between them. This type of damped tube is usually costly and heavier than a tuned composite shaft.
Think about securing a hub to a rotating shaft. Simple. Just use a set screw or other clamping means to bite against the shaft. Of course you may cut up the shaft, and shaft damage can cause problems. But what else can you do?
|Integral hub fastening system, called Fairloc, holds hub in place without marring the shaft.|
Well, the Fairloc(R) integral hub fastening system is certainly one way to avoid shaft damage, according to Senior Design Engineer David Ladyzhensky of Designatronics Inc.'s Sterling Instrument Div. (http://www.sdp--si.com ) in New Hyde Park, NY. Used in the company's S50FP9 Series couplings, the hub fastening system consists of two slots machined into the aluminum hub. One slot is oriented radially, the other at an angle. This arrangement creates a transverse wedge that remains attached to the hub on one side.
A cap screw passes through a clearance hole in the solid part of the hub, and into a threaded hole in the transverse wedge. Tightening the screw causes the cantilevered clamping section to grip the shaft. "With the Fairloc-type hub we can achieve somewhat higher torque. If torque is exceeded the hub slips on the shaft without damaging it. Regular clamp-type hubs can damage a shaft," says Ladyzhensky.
Many observers might consider the technical arena where Sterling Instruments operates a static area. But according to Ladyzhensky, he and his colleagues are constantly trying to improve their company's products. "We're trying to use different materials and we're trying to improve our material capacities all the time," he asserts. "Just recently we changed our design for molded gears. We used to go with overmolded Delrin(TM) around an aluminum core gears. Now we've changed to buying the gears already overmolded around the stainless steel or aluminum cores and we just slice them, that's it."
These examples of engineering ingenuity illustrate an important point--one often forgotten by corporate management and accountants. Labeling a technology as mature does not mean that it remains unchanged. Engineering adds value to the most mundane operation. When first-class engineers become involved, all products can become more valuable to customers. The difference between a product that must be sold strictly on price and one for which users will pay a premium is engineering. And it's engineers like you who create that critical difference.