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September 23, 1996
16 Min Read
Without the ability to transmit power reliably in large and small systems, no mechanical or electromechanical device can function properly. But how can you explain the importance of power transmission to non-engineers? Over the years, too many managers have come to assume that power transmission is easy. After all, everyone knows how to handle that sort of thing. Just go buy some gears, right?
Well, not really. Every project presents engineers with new problems in power transmission, and new problems call for innovation. In this article we present a few examples of engineering innovation in power transmission. They demonstrate once again that the design engineer's mind and experience make the difference between failure and success in industry.
Magneto-rheological bearing. In light trucks, engineers commonly use two-piece driveshafts to transmit torque to the rear axle drive. When traditional truck owners like contractors, carpenters, and other tradesmen use these vehicles, stiff suspensions provide good driveline orientation. Either the system does not generate transient driveline disturbances like center bearing vibrations, or the user doesn't regard the disturbances as objectionable.
On the other hand, no one riding in that light truck would mistake it for a passenger car. The general public is using more and more of these vehicles as transportation. And consequently ride quality has become important to light truck design.
Producing a passenger-car ride in a light truck requires reducing the suspension's stiffness. One result of this change is that the deflected shape of the suspension under high torque, especially in a heavily loaded startup situation, allows the axle pinion to pitch up. That pitch causes a mismatch in the driveline angles in a high-torque, low-speed condition. Because of the kinematics of a U-joint, the secondary force couple effect at the center bearing position causes a center bearing excursion in a plane normal to the plane of the true joint angle. As the bearing moves, the passengers feel a shudder until the vehicle's speed versus torque situation changes, and the axle pitch returns to the nominal condition.
One-piece composite or large-diameter shafts can deal with this situation by eliminating the need for a center bearing. But not all vehicle layouts can accommodate one-piece shaft designs. So engineers must use the center bearing support to eliminate the high-amplitude, low-frequency vibrations, while still isolating low-amplitude, high-frequency vibrations. "Rubber bumpers don't suppress shuddering very well," says James A. Duggan, chief engineer, advanced design at Dana Corp.'s Spicer Driveshaft Div. in Holland, OH. "And pillow block bearings stop shudder, but they transmit more objectionable vibrations than they can eliminate."
To finally solve the problem, Duggan and his colleagues decided to use magneto-rheological fluids in the center bearing's bracket. The materials contain suspended particulates that polarize and line up in proportion to the strength of the applied magnetic field. This orientation changes the shear characteristics of the fluid, and thus the apparent stiffness of the bracket structure. Depending upon the setup, sensors can detect driveline excursions at the center bearing position, or support stiffness can vary with shaft speed.
"The smart support can provide stiffness at low speed to eliminate startup shudder. But it can also isolate the high-frequency, low-amplitude vibrations associated with higher shaft-speed operation," says Duggan. Dana's Spicer Driveshaft Division has patented this driveline suspension concept and put the prototype through bench testing. Vehicle testing later this year will further demonstrate its effectiveness.
Coal mover. Trail Mountain Mine near Orangeville, UT, is operated by Portland, OR-based PacifiCorp. When running full-out, the mine produces approximately 3.8 million tons of sub-bituminous coal each year using longwall and continuous miner operations. All the coal moves to processing facilities over more than 7.5 miles of conveyors that run 24 hours/day, seven days a week. Downtime isn't permissible.
At Trail Mountain, management wanted to achieve 99% uptime for its six mainline conveyors. To reach that goal, mine equipment and engineering firm Long-Airdox of Oak Hill, WV turned to Alignment-Free DriveTM gear drives made by Falk Corp., Milwaukee, WI. Designed specifically for use on high-horsepower underground and surface-belt conveyors, Falk's alignment-free technology allows mounting of the drive directly to the conveyor. The bolt-on design also permits fast installation and maintenancewithout the need for time-consuming alignment of input and output shafts. More conventional drives require the user to build a concrete foundation for the drive, and to use a low-speed shaft coupling.
Falk's drive is alignment-free in two respects: first, it doesn't require parallel or angular alignment in relation to the conveyor. Richard Klug, product manager for heavy-duty gear drives at Falk, explains that an articulated torque arm on top of a concrete block partially supports the drive and motor, and allows the assembly to follow any conveyor headshaft movement. Traditional foot-mounted drive alignment can require 16 hours of work by a two- or three-man crew to complete drive-to-conveyor and motor-to-gearbox alignment.
The drive must mate with the motor at its input side and the conveyor at its output. On the drive's input side, Falk engineers employ a bell housing with a register that matches up to the motor's flange register. When installers bolt up motor and drive, the fitup of the registers automatically aligns the input side. On the drive's output, users can select either a hollow shaft or a solid shaft. If the hollow shaft option is preferred, the drive slips over the conveyor driveshaft. A shrink disk closes down on the shaft to complete alignment. From 673 mm to 1,000 mm of the conveyor driveshaft winds up in the drive, depending upon the model of drive employed.
When a solid shaft is used, Falk places a moment flange coupling between the output shaft and conveyor shaft. To complete installation and alignment, installers align register marks on the coupling's flanges (on the conveyor and drive sides) and use bolts to draw the halves of the coupling together. Alignment-free performance is especially attractive to users because shaft misalignment is a major cause of equipment failure and downtime.
Seven of the Trail Mountain Mine's eleven Falk alignment-free drives are connected through fluid couplings. The other three operate with dc motors. One conveyor, called the third west conveyor, is currently 1,000 ft long. Eventually the mine will extend it to 4,000 ft. Each of the two right-angle alignment-free drives on the conveyor transmits power from a 350 hp ac electric motor operating at 1,780 rpm. Both motors deliver power through Falk's new 1000 Series enhanced start fluid coupling to the Alignment Free gear drive, which has a 34.25:1 ratio. Total output torque for the drive system is 863,525 lb-inches.
Braking away. Open-loop drives using clutch-brakes work quite well in lots of indexing applications. But in many applications, demands for quick changeover and enhanced repeatability lead engineers to look at other types of drives--typically servos.
Oil-shear clutch-brakes, such as those made by Force Control Industries, offer inherent accuracy and consistency. A Posidyne(R) clutch-brake can cycle between clutch and brake in 20 to 40 mseconds, and stop consistently within 30 to 45 degrees, even in an open-loop system. By using a controller called the CLPCTM (Closed Loop Position Control), engineers at Force Control give new indexing accuracy to clutch brakes with ratings from 100 to 24,000 lb-inches of torque.
Specifically designed for use in indexing operations with oil-shear clutch-brakes, the CLPC takes data from an incremental encoder (typically on the clutch-brake's output). After calculating a running average of any error, the unit advances or retards the brake's trigger point to hit a position. Accuracy can easily hit plus or minus 3 degrees at 1,800 rpm, according to the company. "Even in the most backlash-free drive, small constant changes occur in the dynamic load because of variations in product," says Reg Kelley, vice president of Force Control Industries. "Friction in a drive can also change during warmup, causing a variation in drive loading. The CLPC constantly corrects for these changes."
Controller scan time is approximately 20 mseconds, and the control can function with almost any quadrature encoder now on the market. A watchdog timer with 16 selectable values senses jams and shuts down the drive if the timer doesn't see a specified number of encoder counts during a set time. When the drive operates, the controller tells the master machine control that the driven device is busy, ensuring synchronization with other equipment. In addition, it allows manual advance or retard of the brake trigger point via touchpad and an LED readout.
Kelly points out that using this combination of control and clutch-brake offers several advantages. The prime mover can be a stock electric motor. Also, the cyclic inertia of an oil-shear clutch-brake amounts to only 0.012 lb-ft2 for a 400 lb-inch unit. It rises to 3.18 lb-ft2 for a 24,000 lb-inch unit. So most of the motor's torque can be used to drive the load.
This type of control/drive setup can provide a wide torque control range by changing actuation pressure. Standard oil-shear clutch-brake units, according to Kelley, can generate acceleration/deceleration times ranging from 20 to 30 mseconds to 1.5 seconds. Also, given that heat generated by engagement mostly gets absorbed by transmission fluid, oil-shear clutch-brakes exhibit little in the way of torque change over their lifetimes, or during a cold-to-hot phase shift.
The initial application of a drive using an oil-shear clutch-brake and a CLPC involved a unit called an auto-catcher. Used in the manufacture of roofing shingles, an autocatcher collects 36-inch-long shingles that enter the catcher at speeds of 500 to 900 ft/minute, separated by 18-inch intervals. After catching seven or eight shingles on two counter-rotating blades called star wheels, the catcher indexes the wheels 90 degrees plus or minus 2 degrees in 60 to 90 mseconds, dropping them into a bundler-forming chamber.
Force Controls provides a drive for this application that's based on an oil-shear clutch-brake controlled by a CLPC. It indexes the blades accurately at rates from 30 cycles/minute to 180 cycles/minute, depending on the shingles handled. Clutch output runs at 933 rpm into a common shaft for 4:1 counter-rotating gearboxes. Acceleration/deceleration times range from 0.02 to 0.04 second. The kinetic energy per engagement amounts to about 50 ft-lb, and the clutch brake dissipates about 0.50 average thermal horsepower.
Small stuff. When engineers at Black & Decker sat down to design the firm's VersaPakTM Cordless Detail Sander (Model VP510T), they expected to use a small gearset to transmit power from motor to sanding head. But, for a variety of reasons, they also decided to make the orbital sander's housing from ABS.
It turned out, however, that the ABS housing would flex when the sander operated. Gearsets can't tolerate much flexing-induced misalignment. Black & Decker's engineers considered building a rigid metal frame to hold the motor and keep the gears in alignment, and/or using a glass-reinforced housing. Either step imposed costs that the engineering team didn't want to accept.
As an alternative to gears, engineers turned to Gates Rubber Company's 2-mm pitch, 4-mm-wide PowerGrip(R) GTTM belt. Like all PowerGrip belts, this unit employs a deep curvilinear belt tooth profile. Unlike the situation with a trapezoidal tooth, the PowerGrip profile produces full flank contact, eliminating the tooth stress line area seen on trapezoidal belts. This reduction in stress extends belt life and prevents tooth distortion. Furthermore, the deep tooth design improves resistance to ratcheting. In the orbital sander application, if pressure on the sander head stalls the drive, the belt withstands the torque load and stops the motor rather than slipping.
Misalignment of a few mils doesn't bother the belt. Driven by a dc motor running at a constant 12,000 rpm and a 15-groove pulley made from sintered iron, the 68-tooth belt delivers power to a 26-groove diecast zinc pulley. This diecast pulley drives the eccentric that generates the sander's orbital action. Gates supplies the pulleys as well as the belts, and the small drive system generates a 3:1 speed reduction. Finally, the belt drive is quieter than a gearset of comparable capacity.
Fiberglass tensile cords wrapped in a neoprene body give all of the belts in the PowerGrip GT line--made in 2 mm, 3 mm, and 5 mm sizes--flexibility and strength. "These belts are used in Europe and very heavily used in Japan," says Daniel W. Parsons, Gates project engineer. "They're gaining wider acceptance in the U.S." Gates sees good prospects for these small belts, particularly in various types of office automation equipment and consumer products. The company offers the user assistance in working with the PowerGrip GT. "We have a line of sprockets available off the shelf, primarily for prototyping or lower-volume OEMs. For higher-volume OEMs like Black & Decker, we'll work with them directly," says Parsons.
Talkin' big. Copperweld Corp.'s Copperweld Shelby Div., of Shelby, OH, makes Drawn Over Mandrel (DOM) seamless tubing from 2 to 8 5/8 inches in OD. One of the plants includes two large drawbenches used to make the DOM tubing. During this operation, workers coat lengths of tubing (shells) with drawing lubricant and neck-down (point) one end of the tubing so it will fit through a cold-draw die. Then the drawbench pulls the tube through a ring die and over a mandrel, exerting as much as 1,000,000 lbf on the tube in the process. Hundreds of tube sizes can be made from one shell size. Cold working of the steel tube during this process improves its surface quality and increases the strength of the finished part.
An extremely heavy multi-link chain applies the drawing force. Once the pointed end of the tube is placed through the ring die, a gripper holds it tightly. The drawbench chain engages a massive hook on the gripper carriage, and then pulls the carriage along the bench at 90 to 120 ft/minute. Speed depends upon the tube's cross-sectional area and the degree of tube diameter reduction desired. After the tube passes through the die, the spring-loaded hook disengages and the tubing rolls onto a transfer table for further processing.
The chain on one of the drawbenches at the Shelby plant, originally installed in 1989, had worn to the point where replacement became necessary. On a drawbench, explains Robert M. Phillips, manager-engineering & maintenance at Copperweld Shelby, the chain wears in two areas: "The surfaces between the pins and individual links are wear points, and there's a wearing action where the hook engages the chain to pull the carriage." Some wear also occurs on the drive sprocket where it engages the chain.
To replace the chain and sprocket, Copperweld turned to Rexnord Corp.'s Engineered Chain Div., Milwaukee, WI. After Copperweld specified the replacement chain's dimensions and characteristics, Rexnord recommended the materials and heat treating necessary to maximize wear resistance and durability. The chain consists of 184 lengths, each made up of alternating sets of three or four links joined by a round pin. Overall, the completed chain is 175 ft long. The Milwaukee company machined the links from alloy steel using a CNC vertical milling machine to control chain pitch and link dimensions.
Pins for the chain were made from carbon steel using CNC lathes. Rexnord used SPC on pins and sidebars to ensure link-to-link accuracy. To extend the chain's life, Rexnord added independent grease fittings to each pin for lubrication, and machined or drilled channels to distribute lubricant to all links on the single-point fitting.
During operation, the chain mostly experiences tensile loads between the drive sprocket and the point at which the carriage hook engages the chain. Phillips says tubing materials used vary within the carbon steel family, typically from 1026 through 1030 and 1035. The original chain lasted about six years. Given lubrication and improved metallurgy, Phillips expects the new chain to serve Copperweld Shelby at least twice as long.
Moving racks. Master Finish Co. of Grand Rapids, MI, does various types of metal plating, mostly for the automotive industry. In the process used by Master Finish, workers dip racks of parts into a series of tanks filled with chemicals. Until recently, a rack-push system that incorporated a rodless air cylinder moved racks horizontally from tank to tank. The system ran in an open-loop mode.
In this demanding application, the air cylinder caused trouble. Basically, says Maintenance Manager Robert Peterson, unavoidable changes in load size made the cylinder's operation inconsistent. "The equipment runs varying load sizes. Some weigh more, some less. You would set the pressures to run the system at one weight, and it would have trouble with the next load." A timer on the basket would dump the load into a tank after a set interval of travel.
Given that the cylinder would not always position the load correctly, the load could wind up going into the wrong tank, thus becoming scrap. Also, says Peterson, over time the piston began honing down the interior of the cylinder: "The piston would cock under load and cut into the bore as it moved." Air would then leak around the piston and system efficiency suffered.
To deal with the situation, Master Finish eliminated the pneumatic cylinder and replaced it with a belt-driven linear actuator controlled by a PLC. Manufactured by Warner Electric, the Model TP05 linear actuator moves the 90-lb racks of parts from tank to tank at a maximum speed of 45 inches/second.
Providing an 18-ft stroke length, the TP05 accelerates to its operating speed in one second. It also decelerates to a halt in one second. When you factor in the time required for acceleration/deceleration, the new system can move each rack through its path at an average speed of 2 ft/second.
A 3/4 hp dc motor provides power for the actuator via a steel-reinforced positive-drive belt. Operating at an input speed of 506 rpm, the actuator generates 33.45 lbf on the saddle that engages the racks. The system also operates with a position repeatability of plus or minus 0.012 inch. A SECOP Q7006 regenerative drive with full-wave, four-quadrant operation allows the company's production staff to control the new actuator. According to company officials, the redesigned rack push system eliminates the maintenance problems that plagued the old system.
Mechanical power transmission involves a world of constantly changing problems and adaptations to customer demands. Solving those power transmission problems can be intellectually stimulating--and fun.
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