MOTION SYSTEMS HANDBOOK
Common linear-actuator drive mechanisms
2018 has seen the introduction of new linear actuators. This cylinder from Rise Robotics includes an electric-motor-driven windlass with steel belts. Recall that a windlass includes a solid bar or cylinder that rotates; cord wound around the drum attaches to load and fixpoint for power transmission. Rise Cylinders come in sizes to 650 mm in diameter; deliver thrust loads to 450 kN; and speeds to 1.0 m/sec.
BELT AND SCREW DRIVES are the most common types of electromechanical actuators. Most manufacturers of linear actuators offer both belt and screw-driven options. Belt-driven actuators pair with a variety of guide mechanisms, with plain bearings, cam-roller guides, and recirculating bearings (riding a profiled rail or round shaft) the most common. Because their strengths are high speed and long stroke, belt-driven systems are often housed in an aluminum extrusion or in open configurations without protective housing. Within the screw-driven category, there are ball-screwdriven and lead-screw-driven actuators. Ball-screw actuators have higher repeatability and thrust forces than lead-screw actuators, but both provide inherent gearing through the screw’s lead (pitch). The most common guide system for screw-driven actuators is profiled rail, although plain bearings sometimes guide lead-screw-driven actuators. Because screw-driven actuators need rigidly mounted end bearings, aluminum extrusions often enclose them. However, applications needing high travel accuracy benefit from ball-screw types with a machined steel housing. One design that doesn’t exist is a ball screw actuator with cam rollers as the guide mechanism. This is because the forte of cam-roller guides is high speed, whereas ball-screw actuators are primarily for high repeatability and high thrust force — with limited speed capabilities. Pneumatically driven actuators aren’t electromechanical devices like the other actuator types, but their prevalence in automated equipment makes them important. For more information on the two types — including slider and rodtype offerings — see the section in this handbook covering pneumatic actuators. For extremely long lengths and robustness against contamination, rack-and-pinion-
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driven actuators are often the most suitable choice. However, finding a comparably suitable guide can sometimes be difficult. Joined profiled rails sometimes work for extremely long lengths. However, where contamination is a significant concern, metal wheels are usually preferred. A unique feature of rack-andpinion-based actuators is their ability to drive multiple carriages independently. A common application for rack-and-pinion actuators is in overhead gantries in automotive production. Linear-motor actuators are also capable of long travel lengths with multiple carriages, but they’re primarily used for high-precision strokes and very dynamic motion. To complement the strengths of linear motors, these actuators use high-precision profiled rails, crossed roller guides, or even air bearings as their guidance. Linear-motor-based actuators can mount in an extruded housing or on a machined aluminum plate, but to meet the highest travel accuracy specifications, they can also mount on a machined steel plate or granite base.
The PBC Linear SIMO Series Linear Motion Platform is flexible and affordable.
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8/21/18 2:43 PM