This guide gives you in-depth insights into linear slides. Read further to know more about:
- Overview and working principle of linear slides
- Basic components of linear slides
- Common types of linear slides
- Types of drive units
- And much more…
Chapter One – What are Linear Slides?
Linear slides, also referred to as linear guides or linear-motion bearings, are types of bearings that allow smooth and near-frictionless motion in 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. Free translational motion while being in contact with another object is countered by friction. Friction is the force exerted by two bodies against each other which are always against the 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 generated. 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.
Chapter Two – Working Principles of Linear Slides
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 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 into a small point. In theory, the area of contact can be reduced into 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 to carry 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. The area of contact between the roller and race surfaces, in theory, is a line. Similar to spherical ball elements, rollers tend to deform creating a rectangular area of contact. 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 while also providing 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 for 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 with mounted instruments and relatively high energy consumption.
Chapter Three – Components of Linear Slides
This chapter tackles the general components of every linear slide system. This includes not only the slide mechanism but also 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.
Bearings: As discussed in the previous chapter, bearings are the main component that provides free motion between the surfaces. Bearings used for linear slides are enumerated as follows:
Rolling Element Bearings
- Ball Bearings
- Roller Bearings
Plain Surface Bearings
- Dry Lubrication
- Hydrostatic and Aerostatic Lubrication
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 speed of the carriage and thus covering 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.
- Rolling Element Bearings
Carriage: The carriage is the moving part guided by the bearings. It is the part that supports the tool, instrument, or sub-assembly that requires linear movement. Their linear movement is mostly limited within the X-Y plane. Carriages moving in the Z direction employ power screws or screw drives. A drive unit is usually coupled to the carriage. This provides the necessary force or torque to set the carriage in motion.
Guide Rail: These are stationary surfaces where the gliding surfaces of plain bearings or the rolling elements slide against. Guide rails for plain surface bearings are basically flat surfaces with or without lubrication. They can also be cylindrical in shape, often called a shaft or journal. For rolling element bearings, the races are designed to balance the covered area of contact with the magnitude of contact stress. Since this is more evident in ball bearings, its race profile is purposely designed which is classified into two: circular arch and gothic arch.
Circular and gothic arches both have two races that contain the ball bearings. In circular arches, the races only contact the ball 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 to 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.
End Cap: These are installed at the front and rear sides of the carriage seen in recirculating rolling elements. The end caps guide the rolling elements from the load-bearing zone to the return path.
- Lubrication Port: These are integrated into the end caps used to introduce lubricating oil to the recirculating bearings inside the carriage races.
- Seals: These are integrated into the end caps used to prevent external contaminants such as dirt and metal debris from getting inside the bearing races. Dirt is highly abrasive and can scratch the surface of the guide rails and bearings.
Bellows and Covers: These are used to protect the surface of the guide rail. Protective covers are necessary for machines dealing with metal chips, abrasive materials, and coolants. This debris is mostly seen on lathe and milling machines.
- Impact Dampers: Dampers are located at the ends of the carriage which are used as a safeguard in case of excess travel.
- Control System: A control system is integrated for linear slides with drive units. These are used to control the movement of the carriage by energizing the drive unit or actuator from operator controls or feedback signals generated by sensors and switches.
- Drive Unit: The drive unit or actuator is the component that provides or transmits forces that move the carriage. There are various types of drive units available such as ball screw, toothed belt, rack and pinion, linear motor, and pneumatic systems.
- Position sensors: Sensors provide feedback signals to the controller and drive unit. There are two basic functions performed by linear slide position sensors. The first is to prevent the drive unit from moving the carriage beyond its intended travel. The second is to determine the position of the carriage.
Chapter Four – Common Types of Linear Slides
With the different types of bearings, recirculating or non-recirculating constructions, bearing contacts, race profiles, drive units, and precision controls, there are a myriad of possible combinations that can serve a specific application. However, some combinations stand out which can be due to simplicity, load-bearing capacity, rigidity, and versatility. These are continuously being engineered to fit for their intended use. Enumerated below are some of the widely used linear slides available.
Dovetail Slides: These are linear slides that employ plain surface bearings that rely on a low coefficient of friction and lubrication. They are called as such due to the dovetail-shaped protrusion that fits into an identical negative geometry. The protrusion is usually on the stationary rail or base while its negative is constructed into the carriage. This configuration is sometimes referred to as a dovetail table. Dovetail slides are robust and can withstand both radial and lateral loads. These are typically used for large machine tools such as lathes, shapers, and milling machines.
Boxway Slides: Like the dovetail slides, boxway slides are plain surface bearings. But instead of a dovetail-shaped protrusion, these have a square gib with flanges at the top forming a T shape. They can handle heavier loads than dovetail slides due to the larger projected surface area in contact between the carriage and the rail.
Sleeve Bearing Slides: Instead of a mating tongue and groove geometry, this type uses cylindrical surfaces. These surfaces are called bushings and journals. The bushing is like a hollow cylinder constructed into the carriage while the journal is a long shaft that acts as the guide rail mounted on the base. Advantages of using sleeve bearing slides are its simple construction and its ability to handle loads applied in any direction. However, they are not as strong as dovetail and boxway slides and can only be used for light to medium load applications.
Linear Ball Bushings: This type is similar to sleeve bearing slides, but instead of using plain bushings, it uses ball bearings. The bushings are designed to contain recirculating ball bearings. The recirculation can be either tangential or radial. In tangential recirculation, the balls the return path is directed from the side or tangent to the shaft. This permits a more compact construction. Radial recirculation, on the other hand, has the return path perpendicular to the axis. This allows more load-bearing rows to be installed and thus, larger load capacities.
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.
Linear Ball Slides: This is one of the most common types of rolling element slides. Linear ball slides are similar to linear ball bushings, but instead of using bushings, a runner block is used. The runner block can also be constructed with a return path for recirculation. Linear ball slides are better than linear bushings since they offer better versatility and load capacity. Since the races sit directly on the base, there is guide rail deflection. Also, there are several design variations available for the race profiles that can favor either load capacity or compactness.
Crossed Roller Slides: As the name suggests, this type utilizes rollers that are oriented at 45° and 135° relative to the horizontal. The rollers can be arranged into a single row with 90° alternating orientations, or into multiple rows where each row is oriented perpendicular relative to the other rows. This type has better load capacity than similarly sized ball slides due to the larger contact area inherent to roller bearings.
Ball Screw: This is a special type of linear slide that combines ball bearings with power screws. A typical power screw drive has an Acme profile that engages the nut integrated into the carriage through sliding contact. A ball screw further lowers friction by introducing balls as rolling element bearings. The nut is constructed to have a return path for recirculation.
Chapter Five – Different Types of Drive Units
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.
- Ball Screw Drive: This type of drive system converts rotational motion into precise linear movements. As discussed in the previous chapter, it is composed of two main parts: screw and nut. The action of ball screw actuators is the same as that of the power screws. Rotating either the screw or nut moves the other component linearly. Ball screws are preferred because of their robust construction, high driving force, and zero backlash.
Belt Drive: In this type, the carriage is attached to both ends of a toothed belt. A toothed or timing belt is used instead of a flat or V-belt to prevent slippage. This toothed belt is wrapped around two pulleys located at the ends of the guide rails. One pulley is connected to a motor known as the drive end, while the other pulley is only for providing tension known as the tail end.
- Rack and Pinion Drive: This drive is composed of a pinion engaged to a linear gear that operates by converting rotation into linear motion. The pinion is driven by a motor and is mounted on the carriage. Rotating the pinion causes the carriage to move along the rack. Unlike the ball screw and belt drives, multiple carriages can be installed along with the rack since the rotation of one pinion is independent of the others.
- Linear Motor Drive: A linear motor is a type of motor wherein the stator and rotor are not arranged in a continuous loop, in contrast with conventional motors. Its working principle is the same as that of rotary induction motors. An electromagnetic force is generated from the electromagnet mounted on the carriage which exerts attractive and repulsive forces on the permanent magnets mounted on the guide rail. Introducing electricity into the electromagnet directly produces the thrust force. The previous drive units are basically mechanical devices that convert shaft power which ultimately rely on rotating motors. A linear motor has no intermediate mechanical parts, thus no backlash and elastic deformation resulting in better positioning accuracy. Also, less moving parts mean less wear and maintenance.
Pneumatic Systems: Pneumatic systems are piston and cylinder assemblies where compressed air is supplied on one or both ends. Introducing compressed air causes increased pressure inside the cylinder moving the piston. A rod is attached to one side of the piston and is extended or retracted according to the piston’s action. A pneumatic cylinder can be classified as single-acting or double-acting. A single-acting cylinder has only one inlet port. One stroke is pneumatically powered while the return stroke is caused by other countering forces such as spring force. In a double-acting cylinder, there is one inlet port at both ends of the cylinder. This makes the return stroke pneumatically powered also. The simplest pneumatic actuators have the tool or part of the carriage attached to the end of the rod. However, this requires an overall length twice that of the stroke. The rod is then replaced with other modes of coupling such as cables, bands, and magnets. Pneumatic systems have high operating speeds. Also, since there are no mounted electrical components, they are suited for explosion-proof devices. However, unlike the other types, the carriage cannot stop at an intermediate position. The travel is only from end to end positions.
Chapter Six – Linear Slide Sensors
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 to either energize, de-energize, or modulate the force produced by the drive units. Below are the typical sensors used in linear slide systems.
- Limit Switches: These are the simplest types of switches used to cut power on the motor of the drive unit or to send a signal to the controller regarding the position of the carriage. Limit switches are mechanically activated by the direct contact of the carriage or one of its parts. A cam or lever is linked to the switch’s contacts which opens or closes the electrical connection.
- Reed Switch: A reed switch is a non-contact proximity switch that is activated by an electromagnet or a permanent magnet. This type of switch is composed of a pair of ferromagnetic metals called reeds that are hermetically sealed in a plastic or glass envelope. Like the limit proximity switches, they can be configured as normally open or closed.
- Optical Sensors: Optical sensors or photo eyes are non-contact proximity sensors that detect an object by beaming light (usually infrared) into the object’s path and detects the same transmitted light reflected by the object.
- Inductive Sensors: This is a type of non-contact position sensor that utilizes the principle of electromagnetic induction for detecting the presence of metallic materials. It consists of an induction coil and an electronic signal oscillator. Applying an oscillating electric current through the coil creates a changing magnetic field that induces eddy currents in nearby conductors. The closer the metal is, the greater the magnitude of the eddy currents induced. These eddy currents create a new magnetic field that opposes the magnetic field generated by the coil. Opposing the magnetic field in the coil causes a dampening effect in the amplitude of the oscillations. This is then detected by the sensor.
- Hall-effect Sensors: These are non-contact position sensors that are activated by a magnetic field. It works by applying a current through a thin conductive metal strip. In the presence of a magnetic field, charges flowing across the metal strip tend to localize on one side, depending on the polarity of the magnetic field. Measuring the voltage difference between the top and bottom sides of the metal strip gives an analog signal which can then be amplified and converted into a digital signal.
- Linear slides also referred to as linear guides or linear-motion bearings, are types of bearings that allow smooth and near-frictionless motion in a single axis.
- The working principles of linear slides are based on rolling element bearings, plain surface bearings, and magnetic bearings.
- The bearings, carriage, and guide rails are the major components of a linear slide. Additional components such as drive units, sensors, controllers, lubrication systems, and others make up the whole linear motion guide system.
- The most common types of linear slides are dovetail, boxway, sleeve bearing, linear bushing, linear slide, crossed roller, and ball screw slides.