August 17, 1998 Design News

ENGINEERING PRODUCTIVITY KIT Power transmission

Gearbox-coupling package offers greater stiffness

Output coupling mounts directly on gearbox's planet carrier, eliminating conventional shaft

Charles J. Murray, Senior Regional Editor

Chicago, IL--In servo applications, a machine can never be too stiff. There, stiffness is one of the few universal "goods," enabling better dynamic performance and faster response.

Unfortunately, not every gearbox-coupling combination provides the stiffness needed to optimize servo performance. In many cases, engineers must dampen the machinery and "de-tune" the servo performance, just to accommodate the lack of stiffness in the gear reducer and coupling.

A new gearbox-coupling package from Gam Gear, however, solves that problem. Known as I.M.P.A.C.T. (Integrated Modular Planetary And Coupling Technology), it is shorter and stiffer than conventional planetary gearboxes and couplings. In essence, the I.M.P.A.C.T. is like a coupling with a planetary gearbox at its core. It consists of a corrugated metal bellows coupling on its input side, a planetary gearbox in the middle, and a corrugated metal bellows coupling on its output side.

The compactness and stiffness resulting from the design are said to be ideal for dynamic applications, such as packaging, stamping, and material handling machinery. "It creates a much stiffer system," notes Craig Van den Avont, general manager for Gam Gear. "With this coupling, you can increase the gains on your servo system and get much tighter system response."

The key to the I.M.P.A.C.T's compactness is its unusual mounting of the coupling directly onto the gearbox's planet carrier. Engineers from Jakob Antriebstechnik in Germany, designers of the new system, accomplished that by boring a hole in the center of the planet carrier and applying a unique connection concept.

The concept involves three main parts: a split-sleeve shaft that is integral to the coupling; a conical bushing; and a bolt that connects the planet carrier to the coupling. During assembly, the coupling shaft goes into the bored hole. The bolt fits within the shaft and is threaded through the bushing. As the bolt is tightened, it draws the wide end of the cone into the split-sleeve shaft, creating an interference fit between the shaft and the bushing. This interference fit serves as the connection between the planet carrier and the coupling, allowing the two to rotate as a single entity.

The conical bushing design reportedly generates twice as much clamping force as conventional radial clamping hubs. It clamps at about 4.5 times the torque capacity of the coupling, so in a worst case scenario the coupling will break before it slips.

By employing the conical bushing connection, instead of a side-mounted bolt, as most conventional couplings would, the new design eliminates any balancing problems caused by side-loading. It also simplifies maintenance. The reason: The conical bushing configuration enables users to easily remove the coupling. In contrast, conventional couplings require that users loosen a screw and open the gearbox housing.

For design engineers, the overall advantages of the I.M.P.A.C.T. are numerous. The most obvious of those is simplicity. Design engineers no longer need to select input and output couplings when they add a gearbox to a piece of machinery, since this design already includes it. Nor do they need to add a bell housing, since that, too, is included in the I.M.P.A.C.T. package.

On the input side, the I.M.P.A.C.T. coupling accommodates thermal expansion and compensates for misalignment. In contrast, many couplings don't allow for thermal growth of the motor, which can damage motor bearings or encoders. By using a metal bellows coupling on the input side, however, the I.M.P.A.C.T. design allows for thermal expansion of the motor shaft.

Most important, the new design reduces the length of the gearbox-coupling package, improves torsional stiffness, and reduces inertia. By eliminating the conventional output shaft, engineers say they've cut 20-45 mm of the typical length of the gearbox and coupling. The improved compactness and elimination of the shaft have, in turn, resulted in much greater stiffness.

Those advantages are expected to be critical for users, particularly in servo applications. "Whether it's printing, packaging or other applications, servo systems need to make very dynamic, quick moves," Van Den Avont says. "Any time you can improve the stiffness of a servo system it's an advantage, because it allows you to more effectively do those dynamic applications." 

Additional details in the U.S.?Contact Craig Van den Avont, Gam Gear, 7333 W. Wilson Ave., Chicago, IL 60656 (708) 887-5000

Additional details in Europe?Contact Achim Mayer, Jakob Antriebstechnik, Dieselstrabe 8, D-63839 Kleinwallstadt. Telephone: (0 60 22) 22 08-0.

Other Applications

Composite wear rings improve pump performance

Charles J. Murray, Senior Regional Editor

Kulpsville, PA--By employing rotating impeller parts made from a composite, a south Texas refinery has reduced maintenance costs and improved the mean time between failure for its pumps.

The refinery, which was experiencing frequent catastrophic failures of its boiler feed water pumps, was paying out $15,000 per pump for repairs at six week intervals. To solve the problem, the refinery replaced stainless steel wear rings on the pumps with the composite.

The composite, made by Greene, Tweed & Co. and known as WR525, consists of carbon fiber and polyetheretherketone (PEEK) resin. It serves more effectively in the wear rings because its lubricity makes it non-seizing and non-galling. Those characteristics were critical for the refinery application, where occasional cavitation caused galling of the wear rings.

One of the key reasons for the material's performance in high temperature, high pressure applications is its thermal expansion coefficient, engineers say. Because it has a different thermal expansion coefficient than steel, it expands at a slower rate. As a result, when the heat of the application increases, the WR525 wear ring grows tighter around the shaft. This makes it almost impossible for the pump to seize. It also reduces pump vibration and improves pump efficiencies.

Greene, Tweed & Co. engineers say that the material's performance in such applications is important to a rapidly growing user community. "A few years ago, our customers were happy to get one year intervals between maintenance," notes Jonathon Pledger of Greene, Tweed & Co. "Today, they want five year intervals."

In the Texas refinery application, WR525 has increased the mean time between failure and reduced the average part repair cost from $15,000 to $3,000, Pledger says.

Nickel-based alloy stops galling in food processing machinery

Charles J. Murray, Senior Regional Editor

Waukesha, WI--Using a nickel-based alloy, engineers may now have a more potent means to deal with the problem of galling in food processing machinery.

The alloy, known as Waukesha 88, is mostly nickel, but also includes tin, iron, bismuth and chromium. It solves the problem of galling of stainless steel materials, which are often used to combat corrosion-caused processing of aggressive foods and pharmaceuticals.

Although it may seem innocuous at first glance, galling of stainless steels can cause enormous problems for food processors. The worst problems occur when stainless steel surfaces rub against one another, potentially releasing tiny steel particles into the food or drug that's being processed. The problem is even more pronounced in food processing, where the use of lubricants is often not permitted, and galling is therefore increased.

Waukesha 88 solves that problem because it offers a combination of properties. Like stainless steel, it is corrosion resistant, making it ideal for use in the processing of aggressive foods and pharmaceuticals. Unlike stainless steels, however, Waukesha 88 also incorporates bismuth, which provides an anti-galling effect. That, in turn, enables its use in dynamic applications where galling can be a problem.

As a result, some engineers now employ Waukesha 88 in rotors for positive displacement pumps, in bushings for pump shafts, and as plungers and formplates for meat patty machines. The alloy was recently used as a piston material in multi-station filling machines, which fill cans and bottles with foods ranging from baby meals to ketchup.

Engineers say that the key is to use the material as one of two mating parts, because cost often prohibits its use in large shafts or housings. In the filling machines, for example, engineers used it as a piston but did not apply it to the mating part--a large cylindrical casting.

"It's more costly than stainless steels," notes Tom Kerwin of Waukesha Foundry. "But it lasts longer and solves the galling problems, so it makes sense in critical applications like food processing."

Spring design made easy

Kenneth C. Massett, Senior Product Design Engineer Smalley Steel Ring Co., Wheeling, IL

When designing a wave spring into an application, it can be time consuming and difficult to choose variables affecting the design. The engineer normally starts by defining the loads, work heights and free height. Next, he or she establishes the cavity that the spring must operate in.

Now that the major criteria have been estab