The ability to provide accurate rotary motion is critical in a wide range of applications in the automation equipment, medical device, machine tool, energy, welding, robotics, automotive, aerospace, semiconductor, and heavy equipment industries, as well as many others.
Some of the key rotary motion technologies available to address these applications include belt drives, cam indexers, planetary gearheads, direct drives, and precision ring drives. It’s important to look carefully at the pluses and minuses of each of these technologies in order to ensure that you select the approach that provides the right mix of accuracy, economy, durability, speed, noise, etc., for each specific application.
A systematic selection and application process can help ensure that the rotary motion technology that is selected meets of all of the requirements of the application while maximizing the performance and minimizing the cost of this critical component. (See Figures 1 and 2)
Belt driven rotary tables generally offer the advantages of high speed and low cost in rotary positioning applications. Belts are typically made of fiber-reinforced elastomer and contain teeth that interface with rotor pulleys to efficiently transfer torque and prevent slipping. Typical belt-driven tables offer speeds up to 1,000 rpm, continuous torque to 6.6 N-m, and resolution down to 0.16 arc-second using ring encoders.
Additional advantages of belt-driven systems include the fact that they generate relatively little noise and that they require relatively little maintenance. Due to the potential for elongation of the belt, positioning accuracy of belt drives is often inferior to other alternatives, such as planetary gearheads or precision ring drives.
In summary, belt drives are a good choice for applications that require high speed and low cost, however, have several limitations including limited load capacity, limited accuracy, limited rigidity, and relatively poor life.
Cam indexers have been used in rotary positioning applications for many years and are frequently used in dial machines, conveyors, and linkages. There are two types of cam indexers. The most common is the fixed index cam indexer, which does not use a servo motor. With fixed index cam indexing, a mathematical motion curve is machined into the cam to provide accurate positioning to a discrete number of defined positions. During rotation of the cam indexer, maximum displacement velocity usually occurs around the midpoint of the index cycle.
Any fluctuation in cam speed tends to generate increased output torque at the high displacement portion of the cycle. These torque fluctuations sometimes generate irregular rotary motion during indexing, as well as audible noises when the indexer approaches a station. These problems can be avoided by maintaining shaft speed within a very narrow range. Fixed index cam indexers provide high-precision positioning at a reasonable cost for applications that will always index to the same angle and do not require high acceleration.
Fully programmable cam indexers combine a servo motor with a cam-driven index drive. This type of cam indexer is advantageous when a flexible motion pattern is required, such as when two different products that require different indexing patterns are run on the same machine. A fully programmable cam indexer is also beneficial for applications where extremely fast positioning is required followed by a long dwell period.
Planetary gearheads are frequently used on motion control applications that require a high torque to volume ratio. Planetary gearheads utilize an arrangement in which one or several planet gears rotate around a pinion or sun gear. The planetary gears rotate within an internal gear that is most often cut into the internal diameter of the gearhead. The planetary gear decreases the reflected load inertia at the motor shaft by the inverse of the square of the gearhead ratio, which increases the control system responsiveness and generally provides more consistent and accurate motion response.
The planetary gearhead offers the advantage of a wide range of gear ratios which, in many applications, will make it possible to operate both the motor and the application at their ideal speed. Single-stage planetary gearheads typically provide ratios from 3:1 to 10:1. Helical gearing improves the performance of a planetary gearhead over spur gears by increasing the contact load line. The potential drawbacks of planetary gearheads include their relatively high cost and the fact that they contain backlash and can be damaged by shock loads.
A direct drive rotary motor is typically a large diameter permanent magnet servo motor. The unique characteristic of direct drive rotary positioning systems is that the motor is connected directly to the load, eliminating all mechanical transmission components. Rotary positioning systems built around direct drive rotary motors are widely used in the factory automation, medical equipment, and energy industries. (See Figure 3)
Direct drive systems generate energy savings by operating at high levels of efficiency because the elimination of the power transmission system provides a substantial reduction in friction. Direct drive systems also have fewer components, which often reduces maintenance requirements and provides quieter operation because there are fewer parts that can vibrate. The elimination of the gear train also reduces backlash and compliance.
Sometimes, a direct drive system is combined with an encoder mounted on the rotary table to provide precision positional feedback and a high stiffness bearing to improve positional accuracy and repeatability. However, this approach is quite expensive relative to other technologies discussed here. While direct drive motors provide high levels of performance and efficiency, they are limited by low load capacity, high cost, and relatively low accuracy without costly ring encoders.