DN Staff

September 6, 1999

17 Min Read
Productivity Kit

Step-by-step guide to gear-compounding design

By John Kelley, Staff Research Engineer Shell Chemical Company Houston, TX

The number of injection-molded plastic gears produced each year runs into the billions. Engineers use them in vehicles, computers, electronics, industrial machinery, and medical equipment. Why? They provide a lightweight, low-cost means of transmitting motion and power.

But how does a design engineer know which type of plastic will work the best for the particular gear needed for the application under design? RTP Co. technicians found that many of its customers pondered this same question. As a result, they put together a step-by-step specification guide, "Specialty Compounds for Gears," to get them started in the right direction. For example, the guide states that neat (unfilled) gears are particularly ideal for small, low-power designs. However, applications that require precision motion control, load carrying ability, or temperature resistance demand the enhanced performance of special plastic gear compounds.

The guide adds that advances in computer simulation and design tools, resin technology, and processing controls allow designers and molders to hold tighter tolerances and to design in special features for higher-performance applications. Future developments, the guide continues, may include: ability to mold gears larger than three inches in diameter, increased load and power capabilities that can accommodate gearing above 5 hp, and provide holding tolerances to AGA-Class 6 or more.

The step-by-step guidance begins with a discussion of performance expectations and design requirements. This can cover anything from molding accuracy, dimensional stability, and chemical resistance, to impact resistance, mechanical strength and stiffness, electrical conductivity, flame retardance, and precoloring.

The next step looks at the best way to identify resins and additives that can optimize the design. It covers semicrystalline and amorphous resins, as well as such additives as PTFE, molybdenum disulfide, graphite, silicone, and glass, carbon, and aramid fibers.

The third step helps fine tune the gear compound by customizing materials that can meet demands for precision motion control, load-carrying, temperature resistance, color, and geometry (see table). It not only highlights specific industry applications, but rates dimensional accuracy, speed range, and load range of various materials used to design gears.

A final table lists some successful gear applications, the resin used, and the design considerations involved in arriving at the right material for the application. For example, Teledyne Water Pik turned to RTP for a precolored, glass-filled, lubricated nylon 6.6 for its tooth polisher's bevel gears. The addition of glass fibers improved the gear's stiffness, ensuring the teeth do not flex under initial torque. A 90-degree bend in the device made frictional forces a major concern, as significant slippage occurs during initial gear mesh. The addition of a lubricant minimized gear-face wear during startup.

You can obtain a copy of the guide by e-mailing [email protected].

Metal seals help keep space shuttle in orbit

West Palm Beach, FL -- Imagine temperatures reaching 1,347F. Now imagine the challenge of achieving hermetic sealing at such temperatures. Pratt & Whitney (P&W) Space Propulsion successfully addressed that problem on the hydrogen fuel pump of the Space Shuttle when the main engines fully engage.

"NASA contacted us 10 years ago to begin updating new turbo pumps for the liquid hydrogen and liquid oxygen delivery systems for the Space Shuttle Main Engine (SSME)," recalls Mike Paytas, a development engineer for P&W. "The high temperatures drove us toward metal seals. There is no way that rubber or plastic could hold up."

It may seem counter-intuitive to picture how a sturdy metal can be flexible enough to provide a hermetic seal and still withstand high pressure and heat loads. The metal compounds used in this application are quite hard. However, the metal's shape, along with its coating, provides the amount of elasticity and plasticity needed for the pumps.

Hydrodyne, F.P.I. Inc. has, over the years, demonstrated much success in producing metal seals and bellows for similar applications. "Hydrodyne is the only company we know of who makes the types of seals that can meet our demanding requirements," Paytas notes.

For the SSME project, P&W needed seals for the turbine housing on the fuel-pump side. The pressures generated here are in the thousands of pounds psi. Paytas had Hydrodyne design and produce some "W" seals and bellows for the project.

"A cross section of your basic metal seal looks like a sideways U," explains Yvon Carious, Hydrodyne's vice president. The load is borne upon the ends of the U, which allows some degree of elasticity."

The W-shaped seal is simply a doubling of a U-shaped seal. A metallic bellows is created by multiplying the U configuration over and over as required. "This unique shaping helps the seal to withstand a large load, without excessively deforming," Carious adds. "Under severe compression, regular O-ring, C-ring, or spring seals will become distorted. But U-shaped seals are often reusable because of their resiliency."

Each seal has a coating determined by the system's fluid, temperature, and plasticity demands. DuPont's Teflon(R) is commonly used because it is inert to most chemicals and is effective for cryogenic applications. Gold is occasionally used for its corrosion resistance and malleability, along with its ability to withstand temperatures of 1,500F or more.

Hydrodyne machines its bellows from solid stock. Even the flanges can be incorporated as a solid integral part of the unit. This helps eliminate areas of weakness that might be caused when pieces are welded.

Temperatures of 1,000F or more are routinely approached when the SSME are fired up. But the metal components might become frigid on the pad until such a time that the engines are engaged. These operating extremes presented the special challenges for the Space Shuttle seals.

"We define the environment and Hydrodyne's design engineers work with us," says Paytas. "We also have a lot of revisions and modifications, but they seem to work expeditiously on making adjustments to fit our operating needs."

As a result of the successful Space Shuttle applications, Paytas reveals that P&W is also looking at Hydrodyne's metal seals for its jet-engine business. "They are completely applicable for such uses," Paytas concludes.

Qs and As of acrylics

Engineers have long appreciated the versatility and beauty of acrylics, which can be used to produce many attractive and functional items. Increased knowledge of these adaptable materials can help bring about even better results for creating winning applications. Answers to some commonly asked questions about acrylics and their application can make the job even easier. Grant La Fontaine, sheet products technical service manager, CYRO Industries (Rockaway, NJ) provides them:

Q: What are some typical acrylic applications?

A: Acrylic sheet can be used as a safe alternative to glass in such applications as displays, picture frames, hockey rinks and arenas, signs, architectural and security glazing, and appliances. Applications for acrylic molding and extrusion compounds include, but are not limited to, automotive lighting lenses, instrument panel covers, toys, medical devices, lighting fixtures, transparent food and beverage containers, refrigerator drawers, and paper dispensers.

Q: What are acrylics' most outstanding properties?

A: Depending on the product, acrylics offer exceptional optical clarity, high impact strength resistance, rigidity, and outstanding weatherability. They resists many chemicals found in normal use, and the strength of acrylics is higher than that of most other thermoplastics. Acrylics do not discolor or shrink after fabrication. Acrylic sheet, like ACRYLITE(R) AR abrasion-resistant acrylic sheet, is among the most scratch resistant of all thermoplastic materials, possessing a "glass-like" surface coating that resists abrasion.

Q: How do you raise the impact strength of standard acrylics?

A: Impact modifiers (usually rubber) are used to raise the impact strength. Rubber-modified resins have many times the impact resistance of standard acrylic compounds, while retaining their high optical clarity. They can still be fabricated, machined, molded, and extruded with the same ease.

Q: Does acrylics release anything toxic when burned?

A: When burned, acrylics typically release carbon dioxide and water. They will release carbon monoxide, if insufficient oxygen is present. Since they burn very clean, acrylics generate much less smoke than most other plastic materials.

Q: Do acrylics resist gamma or Electron-beam sterilization for medical applications?

A: Many acrylic products resist gamma, e-beam, and ethylene oxide sterilization. CYROLITE(R) GS-90 and CYRO- LITE CG-97 acrylic-based multipolymer compounds are the most gamma- and e-beam-stable grades, experiencing no reduction in physical properties, and almost no yellowing at normal levels of irradiation. These compounds also resist plasticizers found in PVC tubing, and are alcohol- and lipid-resistant.

Q: Can you regrind acrylics?

A: Most acrylics can be reground and reprocessed, without adversely affecting physical properties. Reground material, however, can produce a slight shift in color. To avoid significant color change, mixing a maximum of 25% regrind with 75% virgin material is recommended.

Q: What steps should extruders follow to ensure a high quality acrylic product?

A: Extruders should maintain a clean screw, barrel, and die, as well as the proper melt temperature. They also need to use microfinished, chromed, and hardened polishing rolls. For injection molding, proper runner, gate, and mold design is essential. In all cases, process equipment should be properly sized to meet needed throughput requirements. Acrylics, being amorphous plastics, require low shear process and tool design. Molders and extruders should adequately dry the material before processing.

Q: What is the best way to bond acrylic?

A: Acrylic's bonding ease permits fabricating parts without visual interruptions that can distract from aesthetics. Solvent and adhesive bonding are both good ways to bond acrylics. Solvent bonding softens the bonding area to the point where molecular entanglement between the two surfaces occurs. When the solvent evaporates, the entanglement freezes in place. Bond strength will generally achieve about 25 to 50% of the strength of the parent materials.

Chart depicts the effect of gamma irradiation on the yellowness index of various CYRO acrylic-based multipolymers at 2.5 Mrads. Zero indicates no yellowness based on standard ASTM standards. Yellowness starts to occur at an index of five or higher.

Two-part acrylic cements can achieve bond strengths almost equal to the acrylic materials being bonded. Other adhesive systems, including two-part epoxies, 100% solids, UV-curable, and cyanoacrylate adhesives, will have bond strengths that depend on their adhesion to the parent material.

For more information about acrylics, contact: D Artz, CYRO Industries, Box 5055, Rockaway, NJ 07866; FAX: (9730 442-6117, or e-mail www.cyro.com/.

Urethane adds natural feel to electronic writing

Lincoln, RI"We wanted to simulate the experience of putting pen to paper," says David Arthur, vice president of engineering and operations for A.T. Cross Co. How did he accomplish this? By using a urethane foam material as the elastomeric surface of its electronic writing tablet.

In the application, Cross uses a 1/32-inch-thick piece of PORON(R) foam, supplied by Rogers Corp. (East Woodstock, CT) on the outside of its iPen(TM) and iPenPro(TM) electronic pen system. "Some pens make an irritating tap or squeak when moving across the tablet," Arthur notes. "Other have a plastic feel and draw heat away from the hand."

In an effort to overcome this, Cross measured the parameters of a typical high-quality writing experience and designed the iPen to duplicate this experience.

The iPen system consists of a wireless battery-powered pen and writing tablet hard-wired to the computer. The left mouse button is replaced with a tip switch on the pen, and the right button with a barrel switch. Users move the pen over the tablet to mark-up electronic files, including word documents and spreadsheets. The pen's synthetic ruby ball tip rotates as it moves across the tablet, giving the "feel" of a ballpoint pen.

An antenna grid beneath the foam layer intercepts RF signals sent out by the pen. The computer displays the user's handwritten comments on the monitor.

Cross also factored the resilience of the writing surface into the iPen's design. "We found people want to write on a full pad of paper. They don't like the feel of a single sheet on a hard surface," he continues.

Cross found that PORON had the needed properties to meet his design criteria. The foam provides enough cushioning to be pleasing to the touch, while having the necessary firmness to resist collapse, according to Arthur. Because of the material's neutral properties, PORON won't draw heat from the body or become warm with extended use. Other plus: the foam can be screen-printed. Cross incorporates icons on the surface of the writing tablet to assist in program navigation.

Products to watch

Sound-management foam

QUASH dB1(R) polyolefin foam has a sound-absorption coefficient of 0.77 at 500 Hz when tested according to ASTM E1050 at a thickness of 35 mm (1.4 inch). CFC-, HCFC-, and HFC-free blowing agent and accelerated curing systems reduce residually blowing agents in the foam to trace levels. Targeted applications include portable room dividers, acoustical panels, office doors, appliances, and automotive interiors

Dow Chemical,Box 1206, Midland, MI 48641; FAX: (517) 832-1465.

Lubricated cast nylon

Nylawear(R) lubricated cast nylon has a 0.10 dynamic coefficient of friction (at 20,000 PV, 50 fpm) and a limiting pressure velocity of 30,000 psi at 100 fpm. Material's lubrication qualities eliminate chatter on medium- to high-load, slow-speed applications. The nylon also can endure high-heat, pressure-velocity conditions of 300F. Typical applications include wear pads, guides, bushings, rollers, thrust washers, and slides for medium- to heavy-duty equipment.

Dielectric Corp., W141 N9250 Fountain Blvd. Menomonee Falls, WI 53051; FAX: (414) 255-2761.

'Blue Angel' composites

Line of ecologically friendly, internally lubricated, polycarbonate-based composites, Lubriloy(R) D FR ECO, meet Europe's "Blue Angel" Ecological Standard. The standard requires that the materials contain no halogen or bromides and have a maximum PTFE level of 0.5%. The composites feature a patented lubricant alloy instead of a traditional lubricant like PTFE, but are said to provide similar wear and friction performance. Other features of the material: satisfies UL requirements for electronic applications; has a low specific gravity that can translate into lower costs per cubic inch; and incorporates good dimensional stability and impact resistance.

LNP Engineering Plastics, 475 Creamery Way, Exton, PA; Fax: (610) 363-4749.

Dense fluoroelastomer

Extruded, dense fluoroelastomer compound has a hardness of 75 (5 Shore A). Resin resists chemicals, solvents, high heat, and flame and has good weathering resistance. Material is targeted for specialized intricate and hollow profiles that require firm sealing systems, whether static or dynamic. Applications include automotive, industrial, and aerospace components.

Lauren Manufacturing Co., 2228 Reiser Ave. S.E., New Philadelphia, OH 44663; FAX: (330) 339-7166.

High-speed steel

Micro-Melt(TM) Maxamet(R) alloy, a premium high-speed steel made by powder metallurgy, bridges the hardness gap between traditional high-speed steel and cemented carbide. The alloy has consistently attained a room temperature hardness of HRC 70 minimum. In field tests, the company reports, tooling using the alloy was run at speeds higher than those typically possible with traditional PM high-speed steels, and at speeds nearing those used with carbide tooling.

Carpenter Technology Corp., Box 14662, Reading, PA 19612; FAX: (610) 208-2858.

Iridescent color system

Iridescent, special-effect color system, Spectrachrome(R) Series, features as many as four distinct colors or shades in one product, depending upon the light source. System is said to work with PET, PVC, SAN, PC, PP, and PC resins. Available in 12 colors for such applications as sport/leisure, houseware, toys, lawn/garden, and bath/sanitary.

Clariant Masterbatches Div., 111 N. West St., Easton, MD 21601; FAX: (410) 770-3156.

'Designer' alloys

Company can produce as little as 250 lbs or as much as 7,000 lbs of Custom Vacuum Melted "Designer" alloys. The resultant melt could be further processed into almost any mill product (sheet, strip, plate, tube, forgings, or rolled rings). Current materials involved in the program include: stainless steel, PH grades, nickel alloys, super alloys, alloy steel, and carbon steel.

Specialty Steel & Forge, 26 Law Drive, Fairfield, NJ 07004; FAX: (973) 806-4488.

Perfluoroelastomer compound

Chemraz(R) "clean" perfluoroelastomer compound can sustain temperatures as high as 260C (500F). The material is said to support an infinite range of geometries and cross sections. Material is targeted for etch and CVD processes in semiconductor and sealing applications.

Greene, Tweed & Co., 2075 Detwiler Rd., Kulpsville, PA: FAX: (215) 256-0189.

Mildew-resistant sealant

Dow Corning(R)mildew-resistant silicone sealant bonds and seals under high-humidity, extreme-temperature conditions. The one-part, non-slumping white paste reacts with moisture in the air to form a durable, rubbery solid. The material will remain flexible over a long service life and resists shrinking, cracking, crumbling, and drying out. Targeted for non-porous stainless steel, porcelain, and ceramic surfaces. Dow Corning(R) 786

Dow Corning Corp., Box 0994, Midland, MI 48686; FAX: (800) 496-4586.






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