September 2007
Linear motion systems consist of a mechanical drive, a linear bearing, and a structural support. When a new linear motion axis is required, a customer has two choices. They can create the design, purchase the components, and assemble the components into a linear motion system or they can purchase a device that already includes this functionality. This type of device is called an actuator. The advantages of purchasing an actuator include reducing the time to market, consolidating suppliers, and reducing risk. For many applications this also leads to a lower overall cost.
Actuators can be driven pneumatically, hydraulically, or electrically. This discussion will focus on those driven electrically. Electrically driven actuators come in a variety of designs. The primary differences are found in the support structure, the drive, and the linear bearing. These three items will ultimately define the performance of the actuator including the velocity, acceleration, load capacity, length, precision, accuracy, and reliability.
of aluminum, steel, or cast iron. Machined base plates are used when flatness and straightness are critical in the application or when higher rigidity is needed. Applications include measurement systems and grinding or machining systems.
Extruded aluminum profiles are the most common and are more economical than machined bases. Many of the features such as mounting slots and sealing can be incorporated into the shape of the profile. Applications include material handling, pick and place, and general automation.
The three most common types of drive systems are direct drive, belt drive, and screw drive. Direct drive systems or linear motor systems drive the load without a mechanical drive system. The motor coils are embedded in the linear carriage and magnets are arranged in a linear pattern so that
when the current is alternated through the different motor coils it induces linear motion. The advantage of this design is better dynamic response and higher speeds. Through the use of a linear encoder they also exhibit good accuracy capabilities. Applications include machine tool systems and high speed positioning systems.
Belt drives use a timing belt and pulley system to position the load. Advantages include long travel lengths, high speeds, and low cost. Applications include material handing, palletizing, and pick and place systems. Screw drives (See Fig 2) use a threaded screw and a nut to position the load. There are leadscrews which rely on a solid nut and ballscrews which use a recirculating ballnut.
Leadscrews are often used as a low cost alternative for applications that have a low duty cycle and load. Ballscrews are used for higher loads and continuous operation. Ballscrews are also much more efficient than leadscrews. Advantages of using screw drives are high thrust capacity, high accuracy, and high mechanical advantage. Applications include vertical lifts, part assembly, Cartesian robots, and general automation.
The five most common types of linear bearing systems are plain bearings, track roller bearings, profiled rail bearings, ball bushings and non-recirculating ball or roller bearings
| Bearing type | Advantages | Applications |
| Plain bearings | High resistance to contamination, low cost | Automation systems with low duty cycles and low loads. |
| Track rollers | High speeds, low noise, medium cost, resistance to contamination, smooth motion | Gantry robots, packaging machines, Pick & place systems, Pick & place systems |
| Profile rail linear | High accuracy, stiffness & load capacity, compact design | Machine tool systems, Cartesian robots, Transfer systems, General automation |
| Ball bushing | Low to medium cost, low to medium cost | Pick & place systems (Fig. 4) Part assembly, General automation. |
| Non-recirculating ball Very high load capacity Precise positioning (Fig. 3) or roller bearings | Very high load capacity, Very high load capacity, low noise | Precise positioning (Fig. 3) Part assembly,Alignment & measuring system. |
With demand for complete system solutions continuing to grow due to increased automation needs coupled with internal cost pressures and shorter lead-times, so will the demand for actuators continue to grow. Whether for simple material handling requirements or complex multi-axis Pick & Place applications (See Fig. 3 & 4), linear actuators continue to evolve into an ever broadening array of products to support the wide range of needs and devices.
BSA wishes to especially thank David Zoesch, Schaeffler Group USA Inc., and Jason Wong, NSK Precision America, for their help in preparing this Bearing Brief.