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Building a successful linear motion system starts with choosing the appropriate actuator. Between the different sizes, technologies, and qualities, hundreds of options exist. The trick is to winnow down to the actuator that will deliver the best results. Fortunately, that is not as difficult as it sounds. The requirements of the application will reduce the set of possible actuator solutions and the constraints of the project will determine the best fit.
(Courtesy of Peter Hoffman)
The process starts with considering the series of key factors listed here.
Speed
Speed is an important factor to consider when selecting an actuator. Although screw-type actuators are effective, economical components, at very high speeds, they suffer from a phenomenon known as screw whip, in which the screw bows out as it turns. Screw whip causes vibration and premature wear.
The threshold for screw whip, called the critical speed, depends upon the dimensions and material of the screw. Critical speed can be calculated analytically using well-known equations. If the speed is too high for the use of a screw-type actuator, consider a linear motor or a belt drive actuator.
Load
It is essential that the actuator be appropriately sized for the load. There are several factors to consider when sizing for load capacity: the radial load capacity of the guide bearings, the moment capacity of the support carriage, and the axial load capacity of the support bearings and the ballscrew. It is important to choose an actuator designed to address the loads presented by the application.
A common misconception is that only load capacity matters, and load capacity does make it possible to calculate the service life of an actuator under a given load. However, other factors need to be considered, such as the stiffness of the actuator in various load directions. The design team may run load-deflection calculations to determine whether the actuator will perform successfully in the application.
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