Linear Guide Wheels, Linear Slides, Linear Actuators — Oh!
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Linear motion components, guides, slides and actuators find use in a variety of precision linear motion applications, from machine tools to medical equipment. Linear motion guides offer a wide variety of performance characteristics, with one of the most significant factors being the type of motion control technology employed. The two most common types of linear motion technology are plain bearing and rolling element. Rolling element-based linear guide can be further categorized as either re-circulating element or guide wheel.
Plain bearing linear guides have no moving parts and rely on low-friction sliding surfaces to move on smoothly their rails. In rolling element-based linear motion guides, sets of balls or cylindrical rollers are contained within the payload mounting components and support them on their rails. Relative linear motion between the guide components and their rails causes rotation in the intermediate rolling elements, enabling lower friction motion than plain bearings.
In a re-circulating element guide system, rolling elements are contained in circuits in the payload mounting guide block and continuously circulate within the circuit when there is relative motion between the guide block and rail. Each circuit has a specific section where the elements move into contact with the rail to support the guide block. Once a rolling element passes this section, the remainder of the circuit serves to feed it back to the beginning of the loaded section. The circuits are generally oval shaped in order to minimize their profile and maximize the number of rolling elements in contact with the track at any given moment.
Guide wheels are rolling element bearings with special circumferential profiles that enable them to roll on rails with complementary profile surfaces. The most common profile sets are vee wheel/vee rail and crowned wheel/flat rail. Guide wheels have sets of rolling elements that are contained within fully circular circuits and support loads between the guide wheel inner and outer races. Like re-circulating element guides, the rolling elements only support loads when they pass through one specific section of the circuit, with the remainder of the circuit existing to return the elements to the loading section after they pass through. A guide wheel-based guide system consists of a carriage with a set of three or more guide wheels running on a pair of rails. The inner race of each guide wheel is fixed to the carriage while the outer races contact and are free to roll on the rail surface. As the carriage moves relative to its track, the guide wheel outer races roll on the rails, and the guide wheel rolling elements enable the guide wheel inner and outer races to rotate smoothly relative to each other.
Although the rolling elements within both re-circulating element guide blocks and guide wheels perform similar functions, each technology lends significantly different performance characteristics to a linear guide. Guide wheel technology allows linear guides to operate more quietly and in dirtier environments than recirculating element technology.
In all rolling element designs, the rolling elements vibrate and generate noise as they travel in their circuits. The rolling elements in re-circulating element guide systems are subjected to sudden and intense accelerations as they transition between the straight to curved sections of oval circuits. In contrast, guide wheels operate more quietly since their rolling elements travel in fully circular paths which subject them to more consistent accelerations.
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Guide wheels are well suited for dirty environments since their rolling elements are subjected to significantly less environmental exposure than those of re-circulating element systems. The elements in re-circulating element systems can lose lubrication quickly and have significant exposure to debris since they directly contact exposed rail surfaces. In contrast, guide wheel rolling elements only contact enclosed surfaces, allowing guide wheels to have more robust sealing features for lubrication retention and protection from debris.
Guide wheels are more forgiving of mounting-surface irregularities than re-circulating element systems. Parallel-track guide wheel systems work fine when mounted on surfaces flat to ±0.004 in. For comparison, mounting surfaces for square rails are typically held to ±0.001 in., which can substantially raise installation costs. Guide wheels also enable linear guides to have user-adjustable preload, which provides additional customizability of fit and performance characteristics.
Guide wheel-based linear guides are more versatile than plain bearing and re-circulating element linear guides in terms of the rail compatibility. Guide wheels can be used with specially-designed profiles or rails made from common materials like angle irons (vee wheels) or flat surfaces (crowned wheels). In contrast, plain bearing and re-circulating element linear guides typically have to be used with either specially-shaped profile rails or simple round rail. Profile rails for re-circulating element linear guides are typically proprietary to each manufacturer and not interchangeable. Many round rail-mounted guides offer the advantage of being able utilize commonly available round shaft material as rails, but generally offer lower load capacity, rigidity, than similar size profile rail-mounted guides. Round rails are also less space efficient than profile rails and can be harder to mount due to their curved surface.
|Characteristic||Guide Wheels||Round Rail||Square Rail||Teflon Bushing|
|Ease of Assembly||Excellent||Good||Good||Good|