Ingenuity combined with technical knowledge yields invention. Key elements
in many different types of power transmission systems, the clutches and brakes
described in this article, make it possible to bring applications out of the
workstation and into the real world. They illustrate the capabilities achieved
by manufacturers of these taken-for-granted components.
Servo performance. Combining an electric clutch or brake with an encoder and a control package enables engineers at Warner Electric, South Beloit, IL, to provide some of the major aspects of servo system performance. The encoder monitors shaft position and provides information on shaft position to the Model CBC1000 controller.
In a typical application, the CBC1000 counts a preset number of pulses, then energizes the brake or clutch and stops or starts the load. Should the actual stopping point differ from the preset point, the controller attempts to correct the error by energizing the clutch/brake a bit sooner or later. Warner's Unimodule brakes work with the CBC1000 to provide C-face mounting and both clutch and brake in one package.
Engineers at Warner Electric developed the CBC1000 to deal with a problem commonly encountered by builders of clutch-brakes. "We've always had what's called morning sickness. The brakes are an inductive product, and the inductance changes because of the change in coil resistance as the coil gets hot," says Senior Project Engineer Scott McCoy. Early in the morning, when an application applies, say, 90V to the brake, it exhibits very good performance. As the coil warms up, its resistance increases and the 90V applied to it will not provide the same current as earlier in the day. After a time, performance stabilizes and becomes consistent.
Attempts to deal with this problem using current-limiting controls proved rather expensive, and not always satisfactory with regard to performance. "Current-limiting systems have their applications, but weren't the universal control we were looking for," he explains. The solution to morning sickness actually lay in monitoring where the clutch-brake stops its load, then using that information to control the device.
Cycle to cycle, this clutch-brake control system achieves an accuracy of đ encoder counts. For a 1,000 pulse/revolution encoder, coupled to a four-inch-diameter conveyor-head pulley, one encoder pulse yields 0.0125 linear inch resolution. Such accuracy, according to Warner, could previously not be attained except by using Geneva mechanisms or servo systems.
No more master cylinder. Required system fluid volume and pressure limit the performance of conventional master cylinder brake systems. Providing more volume requires a larger cylinder bore, which, in turn, requires greater input force.
To avoid these constraints, engineers at Oildyne, a unit of Commercial Intertech Corp. located in Minneapolis, MN, designed an electrically powered sub-compact hydraulic power and control system for brake actuation. Invented by Fred Cords, Vehicle Systems Division engineering manager, this system can provide pressures to 3,500 psi and flows of 0.1 to 0.865 cu-cm/revolution. Its fixed-displacement pumps run on many different fluids, including brake fluid, kerosene, and hydraulic oil. "We can have manual push-throughs in a failure mode, and a dead-man switch," says Oildyne's Jeff Swanson.
During operation, an initial force input to the pump controller via the brake pedal starts the pump and motor. Initial fluid flow occurs at a low pressure, because fluid can flow freely back to the system's reservoir. Input force controls a poppet relief valve, and as input force increases, closing force on the poppet increases. This change boosts pressure supplied to the brake system. Pressure to the brakes increases in direct proportion to input force. As for flow volume, proper pump selection enables volume to increase as the system requires it.
In medium-duty applications, such as on lift trucks, this type of system remains independent of any on-board hydraulic system powered by the vehicle's engine. It's entirely dedicated to brake operation. "If a vehicle's main hydraulic system fails," says Swanson, "we will still be able to stop the vehicle."
Wrap it up. Engineers at Reell Precision Manufacturing Corp., St. Paul, MN, give their EC20 wrap-spring on/off clutch indexing capability by combining sensor technology with a proven mechanical design. The new clutch includes a totally enclosed Hall effect sensor that looks at a nylon wheel with 12 teeth loaded with ferromagnetic particles. Teeth can be magnetized so that either a north or a south pole addresses the sensor.
Any of the 12 teeth can be magnetized, and, of course, teeth can be left in a demagnetized state. "Let's say you magnetize all 12 positions, and you magnetize 11 so that south poles face the sensor, and one north," says Reell Precision's Joe Arnold. This difference, can be detected by the sensor, so "you would then know when you got back to your starting position." He refers to this performance as the clutch's "home-flag" capability. The new clutch, called the ED20P, requires a four-wire connection, consisting of 24 Vdc power, ground, sensor lead, and switch lead. Engineers designed the wheel to be splined onto the hub of the clutch, which, in turn, attaches to a driven shaft.
"We're working on an application where the user has five different positions they want to move to very quickly," says Arnold. The customer employs a 500 rpm input and five magnetized poles on the ED20P's sensor wheel to step from one position to the next. Employing the clutch's home-flagging capability will enable the user to find the process starting point, in the event of a power failure.
Normally disengaged, the clutch is typically used in systems that generate drag on the driven shaft. That drag stops the shaft when the clutch disengages. As in other wrap-spring clutches, the ED20P's spring has an input hub and an output hub. On one toe of the spring is a small friction disc clutch. It exerts about eight oz-inches of torque and causes the spring to wrap down on the output hub when the clutch is energized. That action causes the spring to wrap down on the shaft.
Reell Precision's ED20P will make its first appearance on a product in 1997. It's rated for a life of one million cycles at 20 lb-inches. Lighter torque loading extends the unit's life.
Low inertia, long life. Cycling drives used in material handling and packaging machinery constantly stop and start loads. In such applications, the inertia of the clutch-brake and load can combine to consume much of a motor's peak energy capacity. Also, the system must dissipate the kinetic energy developed by the rotating components. High-inertia components result in more kinetic energy, in the form of heat, that the clutch-brake must handle on each engagement.
An oil-shear clutch-brake employs stacks of discs. Thus, disc diameter, and clutch-brake inertia, can be kept minimal. Inertia increases by the fourth power of disc diameter, while torque increases in proportion to either disc diameter or the number of working surfaces. As a result, a multi-disc design can produce high torque with lower inertia than conventional clutch-brakes. During operation, an oil-shear clutch-brake must squeeze a thin film of oil from between a series of alternating friction discs and steel drive discs to achieve full surface contacts. Circulated by the pumping action generated as the clutch-brake operates, the oil absorbs the heat of engagement, cushions the engagement, and minimizes wear.
A typical multi-disc oil-shear clutch-brake, the new Model 01 Posidynefrom Force Control Industries, Fairfield, OH, transmits approximately 100 lb-inches at speeds to 3,600 rpm. Designed to serve in fractional to 1-hp applications, the unit is 75/8 inches long (C-face to C-face), has a maximum OD of 8 inches, and weighs 20.15 lbs. "There are still many applications that just don't require the complexity of a servo-type drive," explains Force Control Vice President Reg Kelly. "And because the Model 01 is an oil-shear type of drive, MTBF is expressed in years rather than months."
Heat generated during normal operation is conducted through the oil in the clutch-brake to its cast-aluminum housing, and dissipated by radiation and convection. Totally sealed and pneumatically actuated, the Model 01 provides a maximum rating of 1.0 thermal horsepower. It generates a cyclic inertia of 0.005 lb-ft2 and handles cycling applications to 600 cycles/minute. Ensuring low cyclic inertia for the Model 01 required new tooling for the small friction discs used in the clutch-brake. Also, "the clutch's non-cyclic thrust plate spins with the input shaft and does not start and stop," Kelly explains, "which helps reduce inertia."
Where the big guys live. Big machinery has a special charm. And the 66VC1600 clutch from Eaton Corp.'s Airflex Division is big. Designed for use in such equipment as grinding mills, marine propulsion systems, and oil-field machinery, the clutch can handle 5,600,000 lb-inches of torque at an actuation pressure of 75 psi.
Inside each spring-released, pneumatically actuated clutch, a loose, flexible actuating tube is contained within a housing formed by a rim and two side plates. "It actually looks roughly like a bicycle tire innertube, and it's built using an older tire-type technology, from cord plys and rubber," says Paul Showalter, Eaton Airflex product manager.
Elements called torque bars pass through cavities in the backing plates of friction shoes, located radially inboard of the actuating tube. The side plates hold them in position. To engage the clutch, users pressurize the actuating tube, which forces the friction shoes radially inward about 1/4 inch and clamps them down on a drum. Leaf springs in the torque bar cavities retract the friction shoes upon release of tube pressure.
In use, the input side of the clutch's outer element (side plate) is mounted to a rotating shaft via a spider assembly. When engaged, the clutch drives a 66-inch-diameter drum coaxial to the clutch, and the drum drives another shaft on the clutch's output side. The torque bars, which support the friction elements, transmit force from the input to the output side of the clutch.
Large air passages extend through the entire lengths of the friction shoe backing plates. When combined with the scalloped shapes of the side plates, this design allows cooling air to flow through the elements. Further, the design of the input-side spider helps pull air through the system.
The 66VC1600 clutch is "the mother of all VC elements," says Showalter. Smaller ones accommodate drum diameters as small as 11.5 inches. Clutch torque capacity depends upon applied pneumatic pressure and rotating speed. Maximum recommended pressure is 125 psi, but the company establishes its ratings at 75 psi.
Serving servos. A new servo motor brake line from American Precision Industries' Deltran Division, called the BRP Series, aims at producing reliable performance in worst-case conditions. Designed as static holding brakes, these spring-set, and electrically released brakes consist of an electromagnet assembly (which also houses the set-springs), armature, friction plate assembly, and flange, all splined to a shaft. Physically, the six brakes in the series range in size from 11/2 to 71/4 inches in diameter.
"It's easy to get torque out of a brake," says William Fierle, Deltran Division regional sales and marketing manager, "but if the electromagnet can't release it, it's not useful to the customer. So the key here is to make the brake release in a minimum ampere-turn condition." When a high-performance, brushless servomotor operates, it heats up. And the motor's brake absorbs that heat. Further, when the motor operates, the brake is electrically released--its coil is energized--adding heat to the environment. Also, it's not uncommon for voltage supplied to the motor and brake to dip some 10% below the nominal rating of 24, 90 or 100V. Despite these conditions, the brake must release.
Adverse conditions can produce a motor-caused ambient temperature rise of 100C and a brake-coil-induced temperature rise of an additional 40C. To achieve maximum magnetic circuit efficiency, and ensure that the brake can function reliably under difficult conditions, Deltran employs magnetic modeling. "We think doing so is unusual in brake design," says Fierle. "Magnetic modeling is very helpful to motor designers, but it isn't necessarily common to brake designers."
Servo motors can stop a load very efficiently and accurately, Fierle explains. But during system setup, the servo may not be used as the primary means of starting and stopping. Typically, it's a manufacturing engineer who puts the servo into a line. It's easy to hit the stop button when setting up the system. Each time the engineer does so, the brake dynamically stops and wear occurs. So Deltran designs for the manufacturing engineer who uses the brake dynamically during setup. In normal operation, the brake functions as a holding element, not a dynamic brake.
Different mounting configurations for the brake can be accommodated by the new line. Some motor manufacturers prefer to place the brake in the front bell, some inside or outside the rear bell. The BRP Series can accommodate all these mounting variations. Epoxy-coated coils and 180C-rated wire enhance the ability of these brakes to resist moisture and heat.
Bulking up a brake. When The Toro Company of Bloomington, MN, needed a transaxle for their new 3000 Series Groundsmaster mower, they turned to Sauer-Sundstrand. That company then went to Tol-O-Matic Inc., Hamel, MN, to obtain a parking brake for their IHT-M15 transaxle, then under design for Toro.
John Wodnick, Tol-O-Matic's OEM account manager, says engineers at Tol-O-Matic were developing the firm's 210-Series calipers, intended as a low-profile, high-torque hydraulic and hydrau-lic/mechanical combination brake. "Because the IHT's envelope requirements matched up with the 210," says Wodnick, "we designed the mechanical version of the 210 specifically for the IHT and Toro."
As development of the 3000 Series Groundsmaster continued, engineers at Toro asked that the brakes also provide traction control for vehicle operation on slopes and steering assist. (Steering assist requires braking one wheel, and rotating the vehicle around the wheel.) Meeting these new requirements made a redesign of various components necessary.
Finite element analysis (FEA) and early testing indicated a need to increase the yield strength of materials used in the brake. Heat treatment met this need by improving housing yield characteristics 225% under load. Redesign of the pivot pin and housing interface accommodated higher input force loading, and distributed the loading over a greater area to reduce stress. Because torque capacity had to grow, a redesign of the brake's cam lever was necessary. The cam profile was redesigned to enhance its mechanical advantage, while maintaining constant clamping force throughout lining life.
Higher forces attained by cam profile changes made it necessary to increase the lever's yield strength and the contact area of the cam and backing-plate interface. Increasing material thickness and through-hardening the component increased yield strength 275% when compared to the thinner standard case-hardened cam. Also, FEA allowed optimization of mounting bracket holes, and through-hardening increased the bracket's yield strength by 225%.
Carried out in collaboration with Toro, all of these changes increased the performance of the caliper disc brake from 6,000 lb-inches static torque to more than 12,000 lb-inches.
As these designs illustrate, whatever the future holds in store, engineers can be certain that they will have reliable power-transmission components to meet their needs.