A: A basic linear motion system starts with a structural support, which can either be integrated into the frame of the machine, or it can consist of a separate structure such as an extrusion or a machined aluminum plate. A linear bearing system will then be mounted to the support structure, along with a drive system that will move the bearing back and forth. Seals and various accessories also may be required to complete the system.
Q: What are the different types of linear bearings used in a linear motion system?
A: Bearing choices for linear motion systems include a plain bearing; a shaft and ball bushing; a track roller; a profiled rail system; and a non-recirculating bearing. In addition to being the least expensive option, plain bearings (basically two sliding surfaces) handle shock loads very well because there are no small balls running on a raceway with a high stress concentration. A shaft and ball bushing potentially offers the advantage of being able to also serve as the system’s structure, which saves money. Track rollers have the unique benefit of allowing the designer to create an extrusion into which the shafting can be rolled. This results in a combination structure/bearing system that is very economical and adds many other features, such as channels to hide cabling and t-slots for fasteners. Finally, profiled rail product - whether ball or roller rail - offers very high capacity with minimal space requirements. However, this option also requires more machining and preparation of the mounting surface, which adds to cost.
Q: What are the different types of linear drives used in a linear motion system?
A:The basic choices for mechanical linear drive systems are screw; rack and pinion; belt; or linear motor. The screw category includes lead screws, ball screws, and roller screws. Lead screws are a very inexpensive option, and work well for intermittent applications with low duty cycles. However, they are not as efficient as ball screws, which provide very good accuracy, repeatability, and mechanical advantage. The roller screw’s primary benefit is its ability to generate two to five times the force of an equivalent ball screw, making it an excellent replacement for a hydraulic system. Due to the precise grinding tolerances needed to achieve the load-sharing properties, however, roller screws can be expensive. To address this issue, bearing manufacturing companies recently developed the differential roller screw, which offers the high forces of a traditional roller screw at a price point closer to that of a ball screw. Belt drives are very cost-effective, especially for long distances, but their disadvantage is poor mechanical advantage (i.e., more torque is needed to drive the system). Linear motors are typically best suited for applications requiring high accuracy and high dynamics. However, their complexity also makes them an expensive choice.
Q: What other accessories are required?
A: The most important accessory in a linear motion system is the motor. Typically, this will be a DC brush motor, a stepper motor, or a brushless motor. A brush motor’s main advantage is low cost, while drawbacks include poor efficiency and brushes that wear out. Both of these aspects preclude the brush motor’s use in high-end motion systems. Brushless motors offer higher performance and greater reliability, but they require complex and costly electronic controls. DC brush motors, as well as brushless motors, need a feedback device to provide position control. Since a stepper motor does not require a feedback device, it is a good option when position control is desired but a feedback device is cost prohibitive. In addition to feedback sensors, linear motion systems sometimes require a gearbox located between the motor and the belt drive.
Q: When does it make sense to custom-design a linear motion system components instead of purchasing off-the-shelf components?
A: Unless an off-the-shelf product can be used with minimal, if any, modifications, a custom system will have to be designed and manufactured. Hiring a design firm to develop a solution and then engaging a contract manufacturer to build it often leads to poor results, because the people who designed it did not build it. The best results are typically achieved by partnering with a linear systems manufacturer who can precisely meet your needs, either by taking an existing catalog part that can be modified, or by starting with a clean sheet and asking, “What are you trying to accomplish?” This approach delivers the best of both worlds: a technical expert to help you design your system as well as a manufacturing partner with the experience and capacity to produce it on a large scale for you.
Q: When using an online sizing tool for linear motion systems, what should one know beforehand?
A: If buying an off-the-shelf system or investing in a third-party-designed custom system are not viable options, but you still want to be able to have the tools to make some of the design selections yourself, then an online sizing tool from a major linear systems manufacturer may be a good alternative. Before getting started, you need to know system requirements such as the load that is being moved and its position relative to the bearings; whether the load is static weight or due to accelerations; the environmental conditions in which the system will operate; the move profile; the spacing of the carriages; the orientation; the duty cycle; and the system’s expected life.