This guide provides in-depth insights into linear slides. Read further to learn more about:
Linear slides, also referred to as linear guides or linear-motion bearings, are types of bearings that allow smooth and frictionless motion on a single axis. Machine tools, robots, actuators, sensors, and other mechanical equipment often require moving components in a straight line in any of the three-dimensional axes. Whie in contact with another object, free translational motion is always countered by friction, the force exerted by two bodies moving against each other's direction of motion. The amount of frictional force exerted is governed by the load acting on the surface in contact and the surface property known as the coefficient of friction.
Low friction and high precision are the two key characteristics desired for movement with low power draw, less tool wear, and reduced heat generation. A linear slide is only one component of linear motion machines, along with power screws, actuated cylinders, linear motors, and rack and pinion assemblies. Linear slides are particularly used only for guiding motion. Other machine elements are utilized for power transmission.
The main component of linear slides is the bearing, which can be either a rolling-element, plain surface bearing, or magnetic bearing. A rolling-element uses balls or rollers to minimize the area of contact into against the rubbing surfaces. The surfaces where these elements roll are called races. Rolling element bearings that utilize rolling balls are called ball bearings, while the ones using rollers are roller bearings. Ball bearings reduce contact to a small point. In theory, the area of contact can be reduced to an infinitely small point provided that the surfaces can resist the infinitely high contact stress produced. In practice, the ball and race surfaces deform slightly, creating a finite area of contact. A small area of contact limits ball bearings from carrying heavy loads. To counter the high contact stresses produced, the number of balls in contact is increased. This is done by adding more rows of balls and races.
The rolling element that uses rollers is roller bearings, which are designed to carry a load and minimize friction. Roller bearings move loads using rolling elements that separate moving parts from the bearings. The area of contact between the roller and race surfaces, in theory, is a line. Like spherical ball elements, rollers tend to deform, creating a rectangular contact area. This rectangular area is larger than that of equivalent-sized ball bearings. Thus, the resulting contact stresses are lower, allowing them to carry larger loads.
Plain surface bearings use sliding surfaces instead of rolling elements to support a load and allow motion. Plain surface bearings utilize materials and finishes with a low coefficient of friction and lubrication. Some plain surface bearings are made from composite materials such as PTFE and graphite with a metal backing. This type is called dry lubrication. This allows no metal-to-metal contact between the sliding surfaces and provides heat dissipation characteristics from the metal backing. As PTFE and graphite bearings are abraded, the worn-off particles become a lubricating layer. A similar property is seen in plastic plain bearings, however, they lack heat dissipating properties and act as thermal insulators.
One type of plain surface bearings uses pressurized air, lubricant, or other types of fluid to counter the loads exerted by the surfaces against each other. Hydrostatic lubrication is the term used for pressurized liquid lubricants, while aerostatic is for compressed air. Dried air is generally used in linear motion applications due to its cleanliness. Hydrostatic linear motion bearings provide near frictionless motion, making them useful for very high precision machines. A downside of this type is the expensive auxiliary equipment required to produce the pressurized fluid.
Magnetic bearings, like hydrostatic lubrication, provide near frictionless motion by repelling or attracting the moving object. Instead of using pressurized fluid to levitate the object, magnetic bearings use magnetic forces created by electromagnets. Magnetic bearings can exert large forces, making them applicable for high-load applications. However, this type has limited applications due to possible interference from mounted instruments and relatively high energy consumption.
This chapter tackles the general components of every linear slide system. This includes the slide mechanism and components needed for actuation and position control. Depending on the intended application and manufacturer, other proprietary parts are available, such as air bearings, positioners, and so forth.
The most common among the three main classifications are rolling element bearings. In comparison with lubricated and magnetic bearings, rolling elements are more robust and highly versatile. They work better in both dynamic and static conditions. Also, their performance and service life can be easily predicted by industry best practices and standards. Hydrostatic and magnetic bearings are only used in a narrow range of applications, mostly in laboratories and specialized instruments.
Rolling element bearings used in linear slides can be classified according to rolling element recirculation. For configurations without recirculation, the distance covered by the linear slide is limited to the length of the rolling element row. The rolling elements rotate but do not travel completely with the carriage. Non-recirculating rolling elements move only at half the carriage's speed and thus cover only half the distance. For recirculating rolling elements, the race has a return path designed into the carriage. The rolling elements recirculate by traveling the looped path within the carriage. The enables both the rollers and carriage to travel together along the guide rail. Rolling elements for recirculating linear slides are ball bearings.
Circular and gothic arches both have two races that contain the ball bearings. The races only contact the ball in circular arches at two points; for gothic arches, contact is at four points. Initial intuition dictates that gothic arches are better since it can support heavier loads. However, an effect called differential slip causes negative consequences for gothic arches. Differential slip is caused by the different rolling diameters occurring in curved races. This results in different rolling speeds creating sliding friction. Differential slip is more evident on gothic arches since there is a larger difference between the effective rolling diameters. Therefore, a circular arch is preferred over a gothic arch. Gothic arch is generally used for small systems that need higher load ratings than similarly sized circular arc races.
The construction of roller guides includes linear rails with rollers, a moving component, and a mounting bracket. The raw materials for roller guides are aluminum or cast iron, which is why they can support heavy loads. In conditions where there is the potential for rust or corrosion, stainless steel is used to make the roller guides. The distance between their rails determines their capacity and needs to be increased as the weight of the application increases.
The rolling motion of roller guides removes the need for lubrication that is necessary with sliding motion linear slides.
With the different types of bearings, recirculating or non-recirculating constructions, bearing contacts, race profiles, drive units, and precision controls, there is a myriad of possible combinations that can serve a specific application. However, some combinations stand out potentially due to their simplicity, load-bearing capacity, rigidity, or versatility. These are continuously being engineered to fit their intended use. Enumerated below are some of the widely used linear slides available.
The bushings can also be classified according to their form, which can be closed or open. Closed bushings have a shaft that is supported only at the ends, while open bushings allow shaft supports directly underneath. Having support underneath the shaft eliminates deflection from carrying high loads.
A linear slide only guides the movement of the machine tool or instrument, but it does not generate the necessary force to accomplish this. A drive unit is used to generate that force, either mechanical or electromagnetic. A simple linear slide usually requires manual actuation. This can be by pushing and pulling, or by machines such as a hand crank leadscrew. Below are the types of powered drive units used for linear slides.
The previous chapter tackled the machines that create carriage actuation. On the other hand, devices that monitor such movements deserve equal attention. Actuation can be operator controlled or automatically controlled. Sensors are needed to generate feedback signals that are used by the controller uses to either energize, de-energize, or modulate the force produced by the drive units. Below are the typical sensors used in linear slide systems.
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