March 2012 / Volume 64 / Issue 3|
Software simplifies linear drive selection
By Bob Eisele, Amacoil Inc.
CNC systems, metrology equipment and other types of machinery often require a linear motion component. This may be because of a need to move a cutting tool back and forth, position measuring probes, convey material or clear debris from the work area.
Selecting the right size linear drive to provide this linear motion can involve a series of complex and time-consuming calculations. Linear-drive selection involves analyzing and integrating many variables, including axial thrust requirements, weight of the tooling head being moved, desired linear speed, frequency of reversals, system component life expectancy and travel distance.
Joachim Uhing KG GmbH & Co., Kiel, Germany, manufactures “rolling ring” linear drives, which are friction drives for reciprocating or positioning linear motion. The company has equipped its distributors with proprietary software that simplifies drive selection. The Uhing UH5 software program rapidly analyzes linear motion application requirements and selects the correct size rolling ring drive.
In a rolling ring drive, the linear motion results from friction produced at the point of contact between the inner race of the drive bearings and the shaft. The amount of thrust depends on the shaft diameter and the number of bearings in the drive unit. But the thrust produced by even a large friction drive can be overcome by a payload that is too heavy or by trying to move the drive too fast. Selecting the right drive means calculating how the coefficient of friction is impacted by the weight of the load and the linear speed.
The temptation is to oversize the drive to allow for a comfortable margin of error, which increases the price. Also, it’s not always possible to oversize a rolling ring friction drive because of design limitations.
All images Courtesy of Amacoil
The manner in which the payload, such as a cutting tool or pneumatic hose, is attached to the drive may cause overturning moments of force, twisting or pulling the drive unit as it travels. This works against the thrust generated at the shaft/bearing interface and increases wear on the bearings, dramatically reducing the system’s lifespan. A machine designer needs to calculate bearing life to know if the drive system is large enough and whether or not the load is properly supported.
In the time-sensitive design of CNC machinery, delving into these types of analyses and complicated calculations is not an attractive proposition. The UH5 software processes and calculates complex drive selection criteria in seconds while minimizing the possibility of human error. Engineering and design personnel can focus on other aspects of the project without getting bogged down with the myriad details surrounding drive ––selection.
Among the program’s output data is shaft rotational speed, drive torque, drive bearing lifespan, power requirements and shaft sag. Additional output data includes Fourier transformation analysis graphs, which show drive displacement vs. distance traveled at a constant speed, and dynamic simulation charts for visualizing shaft deflection off the centerline in relation to shaft rotation speed.
The software was developed with the Technical University of Braunschweig, Germany, to determine linear drive application and selection criteria for specific linear motion requirements. The program provides linear-drive recommendations that fall within a 99.9 percent probability of accuracy.
In addition to saving time, the program helps prevent oversizing the system, which aids in cost control. Likewise, the software prevents customers from specifying an undersized drive, which could then require additional expense to enhance or replace. Because the software calculates bearing life, it enables end users to evaluate whether or not a particular size drive unit will withstand specific performance demands.
The software uses customer-supplied application input data, which is submitted by phone or online. Based on application criteria, such as drive-stroke length, payload weight to be moved and desired ramp-up time, technical support specialists can quickly calculate required side thrust, linear-travel speed, linear- drive size (by model number), shaft rotational speed, drive power/torque requirements, system life expectancy, shaft sag and other technical and performance data. A summary printout is sent to customers. The entire process takes about 15 minutes.
Uhing, however, did not try to make the UH5 software do everything in the drive specification process. “The software mainly looks at axial-thrust requirements and bearing life,” said Bob Jeter, an applications engineer at a Uhing distributor. “Rolling ring drives are sensitive to both application requirements and to the physical setup of the system. The software is great at quickly telling us what size drive is needed. But then we use our experience with the drives to determine if the customer’s setup and work environment will support the selected drive unit.”
Jeter added that when users provide accurate application input data, the software virtually guarantees selection of the correct size drive.
End users and machine designers have requested the UH5 software, but Uhing decided it would be more prudent to keep the software proprietary to its distributors because they are familiar with the nuances of rolling ring drive behavior. “Even though an engineer can pretty quickly determine what size drive to use,” Jeter said, “we have no way of assuring a customer the drive will work unless we understand the intended use and see the setup.” CTEAbout the Author: Bob Eisele is application specialist for Amacoil Inc., Aston, Pa., the exclusive North American distributor of Uhing rolling ring linear drives. For more information, call (610) 485-8300 or visit www.amacoil.com.
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