Efficient Automation with Rotary Indexers

For the assembly of a wide range of motors, Nexen offers the CRD MR series of precision rotary indexers, equipped with a large central opening, so that cable bushings, robotic systems, inspection cameras and other devices can be arranged in the centre. They also feature the RPS system, which converts rotary motion into linear motion, ensuring zero backlash.

Lead times are a big deal for systems integrators, since designing and building an automated assembly system can take a while. A simple project might take 12 to 14 weeks. A complex one could take 48 to 52 weeks. With a large project, approximately 25 % of the total project time will be spent on mechanical and electrical design and acquiring long-lead-time items. Another 25% will be spent on final run off, tear down, shipping, installation, and training. Anything that can shorten this timeline – and thus help a customer get its product to market faster – is welcome. It is not surprising, then, that delivery time is a major consideration when choosing suppliers of automation components. To meet this needing, Nexen’s new CRD MR precision rotary indexers series, based on the CRD family, is ready for shipment in just a few weeks. Moreover, it offers all the benefits of the company’s patented precision roller pinion drive design – zero backlash, high precision, high torque, high acceleration, smooth motion, and low maintenance.

Higher transmission ratio for higher inertial loads

One of those benefits is flexibility. Until now, the CRD indexer was available in three configurations, giving engineers options for high speed, high torque or both, depending on the application. The CRD MR line gives engineers a fourth option: to drive the indexer with their own motor or gearbox. This provides two big advantages: it lowers the cost, and it enables Nexen to deliver the indexer in just three to four weeks – half the time of a traditional cam-driven indexer. With our customisable input, the CRD MR product can accommodate a wide range of applicable motors and shaft-output gearboxes.
A high roller-pinion-to-gear ratio allows engineers to drive the system directly with a servomotor or even a stepper motor, eliminating the cost of a gearbox. For high-load applications, the customer can drive the system with a gearbox and motor. Additionally, the gear-to-pinion reaction loads are fully supported, so the motor or gearbox shaft is not subjected to radial loading. This eliminates the need for costly high output capacity reducers in high-load applications. All these features reduce engineering and installation time.

Increasing the pinion-to-gear ratio allows us to drive higher inertial loads on the output with a smaller motor on the input, giving the customer a great deal of flexibility. For example, the CRD PL 250 has a pinion-to-gear ratio of 3.8-to-1, while the CRD MR 250 has a pinion-to-gear ratio of 12.5-to-1. Since there is an inverse square relationship between the gear ratio and output inertia, increasing the ratio from 3.8 to 12.5 is a big deal. It is much better to divide a 50 kg load by 156.25 (12.52) than 14.44 (3.82). When the ratio from the input to the output is increased, the input speed must also increase to maintain the same output speed; design efforts with the new product’s development maintained the peak speed of the indexer. CRD MR indexers have the same capacity and speed as the original models while increasing the ratio.

The CRD MR indexers have a large, open centre, so engineers can mount cabling, robots, inspection cameras and other devices in the centre of the rotating plate. This saves space and makes for a generally efficient, “clean” machine design. Alternatively, engineers do not have to put anything in the open space. An ongoing project foresees multiple stations around the indexer. When the part is finished, it will be ejected to the inside of the indexer instead of the outside, and a conveyor will pass underneath the indexer to take the finished parts to a packaging line.
The CRD MR line is available in three sizes: 150, 250 and 350 mm. When the Precision Ring Drive product line is included, Nexen’s indexer line now spans a range from 150 to 1,500 mm in diameter. To offer engineers even more flexibility, the CRD MR line comprises three models: the MRS (motor ready sealed); the MRG (motor ready guarded); and the MRO (motor ready open). The CRD MRS is fully sealed and carries an IP65 rating. The CRD MRG is guarded but not sealed. This means less protection for the inner workings, but also less overall drag than a fully sealed system, so it is more efficient and the motor doesn’t have to provide as much torque. The CRD MRO is for applications that do not require sealing or guarding.
Depending on their size, the maximum speed for all three models ranges from 161 to 304 rpm. One-way positional accuracy ranges from 59 to 31 arc-seconds, and one-way positional repeatability ranges from 9.8 to 5.2 arc-seconds. Unlike other drive systems, there is zero backlash from the motor through the driven load.

Patented linear drive

Like Nexen’s other ring drive products, CRD MR systems are typically mounted horizontally to carry an applied load, like any rotary indexing table. They are ideal for precision rotary indexing applications, such as machine tools, semiconductors, robotics, automated welding, medical packaging, assembly and cutting systems. In one recent industrial application, a pharmaceutical company uses a CRD MR 150 indexer to quickly and smoothly position vials below a dispensing system that deposits a small amount of material in each vial. The indexer’s compact footprint makes it ideal for lab work, and its large open centre enables the dispensing system to be located in the centre of the dial, which saves space. In another application, a machine shop is using the CRD MR to accurately position parts for external processing. In this application, smooth, precise motion is more important than high speed. In yet another application, the indexer is used to position parts for a hunting stand so they can be welded by a robot. Unlike other indexer types, Nexen’s ring drive products can also be mounted upside down to carry a suspended load, or they can be mounted vertically. The CRD MR units are no exception. For example, one CRD MR system is being used as a trunnion, positioning parts for robotic welding. In this application too, the system’s open centre plays an important part. The welding gun passes through the centre of the indexer so that it can weld a seam inside the assembly being rotated.
The key to the performance of Nexen’s indexers, including the CRD MR line, is the company’s innovative Roller Pinion System (RPS), which was originally designed for linear motion. The RPS is a patented linear drive concept that combines the best attributes of existing technologies while eliminating most of their shortcomings. The RPS converts rotary motion into linear motion. The system advances the traditional rack-and-pinion concept by replacing the spur gear teeth with bearing-supported rollers that engage a unique tooth profile in the rack. Instead of the sliding friction of traditional rack-and-pinion systems, the rollers provide smooth rolling friction that converts rotary motion to linear motion with 99% efficiency.

The tooth profile of the rack causes the rollers to be loaded in opposition, which eliminates backlash. With this meshing geometry, each roller glides smoothly into the tooth face following a tangent path. The roller does not slap into the teeth, so there is less noise, vibration, and tooth fatigue. In addition, the RPS provides high positional accuracy, no cumulative error, low-velocity ripple, high speeds, high rigidity, low maintenance, corrosion resistance and long life. Unlike some cam-driven rotary indexing systems, the ring drive can start and stop at any incremental position. Engineers can change the motion profile by simply loading a new servo drive program. The ring drive also allows maximum acceleration or deceleration at any point without risk of damage.