Machinedesign 2335 Frank Pedregon Drives 0 0

Design by Objective: Boosting performance

March 1, 2001
In a matter of seconds, Funny Car dragsters can scream past 300 mph. The components that go into these National Hot Rod Assn. (NHRA) vehicles must be of the highest calibe

In a matter of seconds, Funny Car dragsters can scream past 300 mph. The components that go into these National Hot Rod Assn. (NHRA) vehicles must be of the highest caliber. While these race cars may be an icon of high performance, the same components that provide them power and speed often turn up in less glamorous, but very powerful, industrial-grade machinery. The same 3.25-in.-wide belts handling 1,000 to 1,500 hp in a Funny Car, for example, also drive food-processing machines and lumber saws.

Today motion system engineers are faced with the challenge of drawing more speed and power from a smaller envelope. This inherently magnifies problems of heat dissipation. Other obstacles to achieving high performance include friction and heavier loads as well as heightened demand for greater torque and quickness. In response, component manufacturers are offering more niche products and customized solutions to conquer these performance whammies.

Speeding up

When performance is mentioned, one of the first things that comes to mind is speed. Faster components mean more widgets per hour, which equates to more money in the bank. So, today’s motion systems need to know how to move.

For instance, servomotors are the main source of power these days, and typically run at 3,000 rpm, changing direction at a moment’s notice. Imagine the strain on belts, bearings, gears, and other connecting components. The Denver-based Gates Rubber Co. did and, as a result, redesigned its PowerGrip GT2 synchronous belt, giving it twice the high-speed power rating (60 hp) of the previous generation.

Bearings, perhaps more than any other component, absorb the burdens speed presents. Today, the burden is around 650,000 DN (ID of the bearing multiplied by the rpm) – that’s the break-off point between standard and high performance, according to Russell T. Gilman Inc., an SKF company, Grafton, Wis.

Taking advantage of parent company SKF’s bearing technology, Russell T. Gilman is using hybrid ceramic roller bearings that can run at high speeds and provide high radial stiffness. One of the weakest points of a bearing is the cage containing the balls and rollers. Cages are typically made with a phenolic resin material, but SKF is starting to use PEEK, formally known as polyetheretherketone, which is strong, light, and heat resistant to help it withstand high centrifugal forces. Stronger race materials also help, such as closedgrain, high-strength steel for the outer and inner races. SKF has also developed bearing seals that contain the grease without slowing the bearing.

Taking the team approach, Russell T. Gilman has designed a machining spindle with a built-in motor. Their motor supplier, Ruland Manufacturing Co. Inc., Watertown, Mass., and drive supplier, Sincor Electronics Div., York, Pa., worked together to optimize the drive and motor so they function together at maximum performance under specified parameters. The spindles consequently gained speed with less vibration in a compact package.

Take the heat

In order to beat the heat characteristic of compact designs, designers are focusing on prevention of heat buildup.

“We think there are going to be more people using synthetics in the future,” says Jeff Lay, gear industry director, Nye Lubricants Inc., Fairhaven, Mass. “The synthetic lubricant industry is just really beginning to take off as far as commercial markets go.” Petroleum-based greases start to break down between 80 and 100°C, but synthetics like PFPE can withstand temperatures as high as 300°C and as low as 90°C.

Lubricant application methods are also being rethought. For example, some high-speed spindles are fit with an oil-air lubrication system, which periodically gives the bearings a squirt of oil. Most of these systems allot a healthy dose every few minutes. As you get to the end of a cycle, however, the lubrication starts to starve and the bearing temperature rises slightly. When the lubricant is refreshed, the temperature drops back down. This thermal cycling can cause a lot of stress on the bearing. SKF recently introduced a system that shoots out smaller amounts of oil more frequently to maintain a consistent temperature.

Another approach is to cut out heatcausing design elements. Some brake manufacturers use electrical coils to urge a cylinder against a brake, but that instantly introduces heat to a brake design. It heats up “like a light bulb” as soon as you turn it on, says Edd Brooks, senior technical representative, Nexen Group Inc., Vadnais Heights, Minn. Nexen instead uses static air pressure to do the job. Their metal interfaces have the heat sink capability to absorb the energy of braking without cracking, which can happen if too much heat is introduced at once.

It’s important to properly spec a brake to the load, and not try to stop the load too quickly. If a large brake in a sawmill is under speced, it could literally fly apart, says Brooks, a safety hazard to say the least.

Rubbing the wrong way

V-belts rely on friction to transmit power. Most components, however, aren’t so friction friendly. Because parts are constantly bumping and rubbing against one another, lubrication is often essential to preventing heat build-up and wear in motion system designs. There are many approaches to lubrication, though.

Most synthetic greases have been around for decades, but innovations today lie in new applications and additives.

Lay says the best way to lubricate a gearbox is with an oil, but this introduces problems such as leakage and gearbox venting to prevent over pressurization. To avoid these issues, soft greases that slump back into the gear mesh and provide adequate lubrication, typically NLGI grade 2, are being used. A switch to grease in small fractional hp motors is a fairly new approach, says Lay.

Semi-fluid greases help reduce wear and scuffing, which generate heat in a gearbox. Film thickness plays a significant role here.

“Getting the right chemistry is important, but certainly getting the right consistency and film viscosity which provides the right film thickness is also important,” says Lay.

Like gears, bearings would run beautifully with oil lubrication, says Dr. Michael Dube, R&D Manager, Nye. But, the oil would run out. He describes grease as “the sponge that keeps the oil in place and keeps the bearing running.”

When a bearing runs at high speed, load vibration is critical. It’s important for bearings to run quietly and smoothly to achieve high performance. If an agglomeration of thickener particles exists in the grease, the bearing can vibrate, compromising life span. Nye recently developed a manufacturing process for its Nytor Series that achieves the proper grease consistency with less thickener reducing solids in the grease for smoother and quieter operation.

Dube attributes new developments in grease lubrication to correct employment of additive, thickening, and base oil chemistries for application-specific lubrication.

As far as friction-susceptible surfaces, new options in materials are standing up to wear. It’s common to use ceramic balls for ball bearings, but, as mentioned earlier, ceramic rollers for roller bearings are making the scene.

With brakes, like V-belts, you want some friction. The friction material Nexen uses on its industrial brakes is similar to what’s in the shoes or pads on a car, but the exact composition is a mystery even to Nexen. Competition is stiff, and proprietary “witches brew” concoctions are rumored to contain things as off-the-wall as pecan shells. Brooks does confirm that some materials contain rubber “tire crumbs” and pieces of brass for wearability. In web process control, the brake needs to slip continuously, so it may contain pieces of a slippery material such as Teflon.

Stop and go

Servomotors can stop and start in less than 100 msec. In recent years the price of these motors has dropped, and they’re now commonly used in place of clutches. But, there are not many servo applications over 100 Nm. “You’ve got to make sure your servo can make the move and achieve the same thing that the clutch could achieve, that is stop and start very quickly,” cautions Carroll Wontrop, senior systems engineer, Kollmorgen, Radford, Va.

Even though clutches are being eliminated from designs, you still need a brake with enough torque to slow down a servomotor in case of a power-off situation or to dynamically stop a load. Because response time is critical, Nexen uses directional control valves, which can be designed for different flow rates, to direct air to the clutch-brake. Traditionally, Nexen’s brakes have been air-engaged and spring-released, but their new line of Eclipse brakes is spring-engaged and airreleased to fit servomotors.

High-torque braking applications, such as automotive assembly plants are a challenge. There you may have six car bodies hanging up 20 or 30 feet, you need safety brakes on the conveyor system. When considering brakes for any high-torque job, it’s important to keep in mind that with a static air brake you get a higher torque unit in a similar or samesize package compared to something electrically engaged, says Brooks.

Loaded down

Miscalculating the effects of a load can lead to system failure. Two common performance- compromising mistakes that Brent Oman, Gates, manager in power transmission product application, reports seeing in belt drive applications are misalignment and too little static tension. Under-tensioning does not lead to reduced bearing loads, as bearing loads are directly proportional to the dynamic load.

Oman says he feels not enough designers are aware of the capacities of today’s belt drives, as they continue to become more capable drive units.

“I think the trend is just going to be towards more power in a smaller space. Eventually, the problem is going to be how do you get power to the shafts of the systems. That’s going to be the tricky part. But, everything is always being looked at as far as material improvements and capacity increases,” he says.

Tough belt drive applications include positioning on printing presses, where stiffness capacity lets a belt transmit its load very accurately. Stiffness depends on the tensile cord, but also on the belt materials. A fairly rigid tooth is needed to prevent deflection.

One of the biggest changes Gates has made to its high performance belts is the hardware. The compact and narrow matched system with taper-lock bushings reduces bearing loads while boosting horsepower.

When a servomotor is coupled to a load, several issues affect performance, says Wontrop, such as the servo loop quality, the amplifier’s power capability, and motor feedback type. Other factors affecting performance are how much inertia you have, compliance in the load, and the motor design itself, Wontrop says. To reduce compliance issues, which entail a motor not being tightly coupled to the load, you can add software filters to a motor drive, add inertia to the motor, or lower the inertia from the load, he suggests. Feedback is a critical issue because the more resolution you have, the more feedback information you acquire and the better you may control the motor. Inertia mismatch, compliance, and bandwidth are three areas where engineers often run into problems, says Wontrop.

Another area where compliance can be addressed is at the coupling. Zero-Max Inc., Plymouth, Minn., recently developed a nonmetallic composite material disc pack for its CD coupling. It is more flexible, but yet torsionally stiff. Because the laminates can be put together at various angles, the disc pack can be modified to give different characteristics in terms of stiffness.

Doug Moore, vice president of sales and marketing, Zero-Max, says a common mistake in selecting couplings for high-performance is going with less than what’s needed to save on costs. But coupling failure can send designers back to square one. Following selection software guidelines, a fairly new tool, helps designers make better decisions the first time, he says.

Other developments in load carrying capabilities include a hydraulic bearing preload system from Russell T. Gilman to adjust spindle system stiffness to change the preload going into a bearing, and application of synthetic lubricants with better film thickness to help highspeed ball screws and lead screws carry heavier loads.

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