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Braking breakthroughs

Braking breakthroughs

Whether you use a diesel, an electric motor, or rubber bands to power your product, you must find ways to dissipate and transmit that power. Down through the years, engineers like you have turned to brakes and clutches to do the job.

Today's brakes and clutches greatly exceed the performance of older designs. In this fiercely competitive arena, companies strive to establish a position by developing products that offer users special capabilities. In this article we present a few examples of innovation by makers of brakes and clutches. The following examples illustrate some of the benefits these efforts bring to design engineers:

Totally enclosed brakes are naturals for the food processing industry, or where wear products from friction materials must be contained. But totally enclosed, non-ventilated (TENV) brakes do encounter some problems with heat dissipation. In fact, when compared to open designs, TENV units usually must be derated to prevent thermal problems.

In its new generation of electrically actuated TENV Super-Mod(R) clutch-brakes, Stearns Division of Rexnord Corp., Milwaukee, WI, claims a thermal capacity 96% greater than that of competing units. And they achieve this improvement without external fan modification.

Gene Poker, Stearns' product manager for dc products and electronics, says an investment-cast aluminum housing is the key to the new brake's improved thermal capacity. The housing minimizes air spaces around the brake's electromagnetic components and friction components. "This precision casting permits direct conduction of heat generated during braking to the housing," says Poker. Heat then leaves the aluminum housing by convection to the ambient atmosphere.

On the clutch side of the assembly, "the end bell is made in one piece to hold the magnetic components," says Poker. Just as on the brake side, heat moves out of the clutch by conduction through the housing. A bi-directional fan cast into the steel rotor moves air around inside the housing to eliminate hot spots.

Stearns makes the new TENV clutch brake in SM-50, SM-100 and SM-180 sizes. They generate static torques ranging from 16 lb-ft through 35 lb-ft.

Stopping earthmovers. Brakes for earth-moving machinery face fierce operating conditions. The International Standards Organization publishes regulations on the performance requirements (and test procedures) for braking systems used on all sorts of vehicles, including earthmovers. A design project carried out by engineers at Tol-O-Matic, Inc., Hamel, MN, resulted in a new family of brakes that conforms to ISO standard 3450-1985. Furthermore, Tol-O-Matic is now in the process of becoming a certified ISO 9000 supplier of such brakes.

Manufacturers of industrial construction machinery such as small backhoes and graders wanted the advantages of a caliper brake for their designs. But the brake had to produce a large clamping force in a small package. To break into this market (please forgive the pun), engineers Tim King and John Wodnick of Tol-O-Matic set out to design a high-torque disc brake able to fit into any standard 200 mm (eight-inch) wheel.

Called the 210 Series, it's offered in mechanical, hydraulic, and dual-function versions. The heaviest brake in the series-the dual-function version-weighs 4.5 lbs. All three of the versions employ a common integral bracket. Disc thickness on the 210 is 6 mm. Pressures within the brake hit 2,000 psi.

The fully booted two-piston brakes generate torque maximums ranging to 1026 Nm dynamic torque. Tol-O-Matic is now looking into the possibility of designing phenolic pistons for the new brake, to enhance its corrosion resistance. But for the present the pistons are made from conventional materials.

Wrapping it up. In some clutch applications, a clutch's output shaft can idle ahead or coast too far. For example, in paper transfer applications, power usually comes from a gear or pulley mounted on a shaft or clutch. Drive-through torque from the input bearings and the clutch's internal bearings can rotate the output shaft when the clutch is turned off.

George Larson of Reell Precision Manufacturing Corp., St. Paul, MN explains that the most common and least expensive solution to this problem is to put a continuous brake on the shaft. If the brake doesn't draw much power, and the shaft motor has the punch to drive through it, this approach works well.

Using an electromagnetic brake on the shaft allows the user to brake the shaft during deceleration and hold. Doing so eliminates heat generated by a continuous brake at full shaft speed. But, says Larson, it requires "a device that costs about as much as a clutch, a second set of input and output torque connections, and a separate electrical connection and control."

To handle this type of control problem, Larson and his associates designed Reell's EC-20 clutch-brake. The patented design uses the magnetic path of an electric wrap spring clutch to control a reverse logic brake. This brake provides stopping and holding torque in a paper transfer application. As magnetic flux rises to control spring engagement, it pulls the brake plate against the brake's shoulder assembly to release the brake. The two-pass magnetic path then holds the brake disengaged until the voltage turns off. Next, the clutch disengages and the brake spring pushes the control plate against the brake plate and friction hub. A forked bracket holds the coil connection anti-rotation and brake anti-rotation. Brake torque ranges from 8 to 35 oz-inches. Applications that require high acceleration torque with consistent time to speed fit this clutch-brake's performance profile. It can help decelerate the shaft and hold the output during the rest of the duty cycle. Overall the clutch-brake measures 1-3/8 by 1-5/16 inches.

Textile tension. A manufacturer of specialty textile manufacturing equipment came to Inertia Dynamics Div. of Reliance Electric, Collinsville, CT, seeking a low-cost clutch-brake for a machine that inserts buttons and snaps into clothing. The customer wanted a totally enclosed unit capable of 200 cycles/minute, with no adjusting of either the clutch or brake for a minimum of three million cycles. Just to make the job more fun, Product Engineer Kip Mentzer and his colleagues were given six weeks to design, develop, and ship the product.

They organized a work team that included engineering, manufacturing, production, purchasing and personnel from the assembly line. Bi-weekly update meetings helped ensure good communication between team members. Given specs and overall requirements, Mentzer decided to use a single armature (disc) to eliminate clutch-brake overlap. This approach, he says, enables the clutch-brake to more than meet the cycling requirement. A dual-armature system (one armature for the brake, one for the clutch) would fight itself and extend the time required for cycling, he explains.

Because the SMC-50 device is totally enclosed to permit washdown, there are no fins on the clutch-brake's housing. The field cups that contain the brake and clutch coils contact the housing. Heat rejection occurs into the housing, which functions as a sink.

With an OD of 6 .50 inches and 4 .25 inches long, the brake is rated at 22 lb-ft. Users adjust the unit manually. Mentzer says doing the project was fun, though hectic. "It was really a crash course-things happened very quickly."

The new clutch-brake can operate at a rate as fast as 400 cycles per minute. In addition to use in textile machinery, it may find other applications in conveyor indexing and packaging machinery.

Clutching for safety. Riding lawnmowers-both commercial and consumer units-commonly employ a blade clutch-brake to prevent the blade from rotating when the mower is not cutting. Typically, engineers used a clutch-brake originally designed for some other industrial or mobile application to control lawnmower blades.

Warner Electric saw an opportunity to produce a custom-designed clutch-brake that would provide superior performance in riding lawnmowers. Two products resulted from this perception, the MagStop(R) for consumer riding mowers and the MagStop(R) Commercial clutch-brake. The first is intended for riding mowers rated to 16 hp, the latter for riding lawnmowers rated at 17 through 28 hp.

"This is really the first clutch-brake of this type designed for the application," says Scott Fuller, principal engineer at Warner Electric Div. of Dana Corp., So. Beloit, IL. The unit contains a steel armature (disc) drawn to a steel clutch disc by an electromagnet. Energizing the electromagnet's coil pulls the armature into the rotating rotor driven by the mower's engine. This action engages the clutch and rotates the mower's blade.

When current flow to the coil ceases, leaf springs attached to the armature pull it back into contact with a permanent magnet braking surface. Braking torque generated by the ceramic magnets stops the blade within three to five seconds.

This clutch-brake requires no adjustment. Magnetic forces will draw the armature to the braking surfaces despite wear. Because magnetic forces exerted upon the armature remain constant, stop time remains consistent for the life of the mower, says Fuller.

Several versions of the MagStop clutch-brake are on the market. Consumer versions generate nominal static torques ranging from 40 to 80 lb-ft, while the commercial units produce from 105 to 175 lb-ft static torque. In either case, the clutch-brake is available in versions that draw about 1.5A, permitting manufacturers to use a low-cost power source. Clutch-brakes previously used for these applications drew 4 to 5A.

This lower current draw was a consequence of the decision to design a clutch-brake specifically for the application. "We were able to make a large enough cavity to install the ampere-turns we needed to keep current draw down," says Fuller. The coil, insert-molded into a PET shell, is riveted to the clutch-brake assembly. Insert molding enhanced vibration resistance and, somewhat surprisingly, says Fuller, heat dissipation. Riveting the assembly together makes it difficult for end-users to remove the blade brake, which would defeat the product's reason for being-user safety.

Warners Electric's consumer MagStop clutch-brake is in the market now; the MagStop Commercial will be available this autumn.

Squeeze it. Many applications use a rotating shaft to drive some sort of load. Often, it's important that the motion of that shaft be controlled; it must start and stop as desired, and must not coast.

In such applications, engineers often use a spring-set, electrically released brake to halt the shaft. Different applications require brakes of different torque capacity. Frequently the only major difference between two such brakes is their springs. "If the customer or the distributor can change the torque, they don't have to come back to the factory," says Haim Loran, president of Electroid Co., Division of Valcor Engineering Corp., Springfield, NJ.

Five models of the new GFSB spring-set brake are made by Electroid. All employ a simple, user-friendly adjustment mechanism that makes it possible for one brake to do many jobs.

Braking is performed by a sandwich consisting of two round steel plates on either side of a disc with friction material on both of its faces. At the top of the brake is a threaded plastic insert. This insert bears against a plate that, in turn, compresses the brake's set springs. The set springs force the components of the sandwich mentioned above into contact.

Energizing the brake's potted coil draws the spring-driven steel plate away from the dual-surface friction plate, and compresses the set springs. This movement frees the friction plate, which floats it freely on the splined shaft, and moves away from the second steel plate in the sandwich. The shaft can then rotate freely. De-energize the coil and the brake sets, halting the shaft.

To change the set spring force exerted upon the rotating dual-face brake disc, the user screws the threaded insert up or down, releasing or compressing the springs. One model's rated torque can be adjusted, for example, from 9 to 40 lb-inches. The torque rating of Electroid's biggest GFSB, the 18.5-lb GFSB-16MR, can be adjusted from 350 lb-inches to 885 lb-inches. A cam lever permits bi-directional manual brake release of all models.

"My experience has been that good things are simple," says Loran, "and this design is a good, simple approach."

No squealers allowed. Web systems use tension control brakes to help maintain proper loads on material passing through the systems. Basically, these brakes are always lightly engaged-that is, they constantly slip. Naturally the brakes tend to squeal as the web moves. That high-pitched sound makes communication difficult for people working near webs.

Many companies sell anti-squeal kits for noise control, but Horton Manufacturing, Minneapolis, MN, sought a different way to take the squeal out of the firm's XTB(TM) tension control brake. The brake features pneumatically controlled calipers that bear against an armature. Typically, the user chooses the number of calipers on the brake to match the maximum torque needed in the application. Users can vary line pressure in a number of different ways to control brake torque and tension a web.

Chris Greene, manager business development, web products at Horton, says the XTB is a quiet brake. But in some applications, especially low-speed, low-tension web applications, it did sing a bit. Much of the vibration that generates noise in a slipping web tension brake is associated with the pad surface.

LeClaire and his colleagues discovered that they could stop squealing with an isolator. "Basically, it's an o-ring placed between the back of the brake pad or friction plate and the caliper piston. That's sufficient to change the dynamics of the system so that it doesn't squeal," says Greene. The o-ring serves as a spring-damper, and inserts enough damping into the system to suppress squealing while still transmitting the necessary force to the pad. Horton has applied for a patent on the squeal-suppression system. Says Greene: "We're not claiming that we'll never again have a squeal. But we've not found any cases in which it does squeal."

Keep on trucking. In the very competitive trucking industry, it's critical to increase vehicle productivity. Engineers at Eaton Corp., Cleveland, OH, attack this problem with the ES(TM) Reduced Envelope Brake family. These brakes fit into standard 19.5-inch wheels, enabling truck builders to avoid using larger, heavier wheels and tires. Doing so increases the cubic capacity of vehicles-without reducing brake performance.

Jim Clark, product line manager, foundation brakes at Eaton Truck Components Operations, says designing the new brake raised serious packaging issues. Brake components must not hit the vehicle's axles. Simply scaling down a larger brake to fit the wheel can cause brake actuation components to collide with other under-carriage parts.

"Instead of a 7-inch-wide brake, we went to an 85/8-inch-wide design," says Clark. This change gets the brake out into the airflow, out of the tire's envelope. It runs cooler, and there's more swept brake area. Thus the brake area through which brake energy can flow increases. Thermal loading per unit of lining area decreases when compared to a 7-inch-wide brake.

"We worked with wheel manufacturers to help them come up with wheel designs that gave as much clearance to the drums as a standard wheel would. And we did considerable testing with the tire people in running brake and tire temperature tests," says Clark.

The 15- by 8-5/8-inch brake has a slightly larger swept area (about 12% larger) than a standard 16.5- by 7-inch brake. Packaging proved the main consideration in the design, says Clark. "We were trapped by several physical and thermal considerations, and making it wider was the secret."

Eaton's ES line employs a fabricated shoe and either a forged steel spider or a cast steel spider. The foundation brakes are designed for Class 6 (to 26,000 lb GVW), Class 7 (to 33,000 lb GVW), and Class 8 (more than 33,000 lb GVW) trucks. They employ a proprietary phenolic-resin-based lining. Using these brakes cuts truck weight by 230 lbs/axle, according to Clark. Although the brake is about as heavy as larger diameter brakes, it can be used with 19.5-inch wheels and tires, which are significantly lighter than the next-largest wheels and tires used on heavy trucks.

As these examples illustrate, applications for clutches and brakes are everywhere. And design engineers continue to find new ways to make them operate more efficiently. So although brakes and clutches are mature products, there's still lots of new life in this old technology.

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