Pharmaceutical and Medical Packaging News
Magazine
PMPN Article Index
Originally Published December 1999
PHARMACEUTICAL
Prepackaged Linear Guides Improve Packaging Machine Performance
When incorporated into packaging equipment, linear guides can reduce downtime, increase speed, and reduce maintenance.
by Scott Schuler, director, linear guides, Thomson Industries Inc., Port Washington, NY
Pharmaceutical and medical packaging engineers can benefit from improved machinery performance by asking their equipment manufacturers to incorporate prepackaged, ready-to-install linear guides into their packaging machinery. Off-the-shelf linear guides optimize functionality and throughput and minimize downtime, cost, and contamination. They provide guidance for critical movement in applications from product insertion into containers, bottles, boxes, and cartons to folding, assembly, and sealing. Linear guides enable:
- Higher speeds.
- Increased accuracy and reliability.
- Quick size adjustments and product changeovers.
- Environmental compliance.
- Linear guide selection should be considered as early as possible in the machine design process in order to achieve an optimum performance/cost ratio. For example, specifying a linear guide that is more rigid than the application requires will either increase installation costs or reduce product life.
LINEAR GUIDE TECHNOLOGY
Typically, a linear guide consists of a carriage or pillow block that rides along a guide rail. The linear guide supports the mass of the object being actuated, while ensuring a smooth, low-friction, straight-line motion. Linear guides are usually available in either round or profile configurations, depending upon the basic geometry of the guide rail. The shape of the inner race defines the performance characteristics of each guide type.
Round rail linear guides feature increased installation tolerances yielding lower cost and easy installation, a higher profile for easy actuator connection, smooth operation, and lower replacement costs. Profile rail linear guides offer greater lateral rigidity, yielding less deflection, resistance to roll movement, better damping characteristics, and greater table rigidity for critical positioning applications.
Figure 1: A robotic pick-and-place mechanism handles box packing and palletizing.
Both configurations offer various technologies to consider, including antifriction rolling element guides (recirculating balls or rollers) as well as friction-type, sliding contact guides. Fully supported and end-supported mounting types and above-, below-, and side-mounting styles are available. When combined with the technology options, these types and styles create a broad range of offerings for equipment designers.
In recirculating ball linear guides, rolling elements recirculate between an inner and an outer race. This antifriction technology is widely used in the medical packaging industry because it offers a low coefficient of friction, resulting in better resolution and reduced drive costs, better accuracy because of minimal difference between the static and dynamic coefficients of friction, higher speeds, predictable life, and less wear performance. Antifriction linear guides can be found in various materials, including chrome-plated high-carbon steel and 440 C stainless steel. Accessories such as way covers, scrapers, special machining, platings, and lubricants are available options to satisfy specific design needs. Self-lubricating linear guides eliminate the need for costly lubrication systems or for scheduled maintenance.
HIGHER SPEED AND ACCURACY
Packaging designers constantly strive for higher speeds. But speed must be combined with accuracy and reliability to ensure defect-free product. Linear guides can help designers attain such speed. They can achieve speeds up to 10 ft/sec, are accurate to better than 0.001 in./ft, and offer predictable travel life that can reduce or eliminate warranty issues.
In a robotic pick-and-place machine for box packing and palletizing, two linear motion components can be used to satisfy high speed and reliability requirements (See Figure 1). These components include a metric linear motion stage and a profile linear guide.
Located on the vertical axis, the metric linear motion stage is actuated by a steel-reinforced timing belt yielding carriage speeds up to 3 m/sec (10 ft/sec). In addition, the stage features a new bearing that can last up to eight times longer than linear bearings of equivalent size. The stage provides positioning accuracy better than 0.2/300 mm (0.008 in./ft) and repeatability within 0.10 mm (0.004 in.). Load capacity is up to 5446 N at 250 km (1200 lb at 10 million in.) of travel life.
A horizontal axis, which locates the pick-and-place gripper head, consists of the profile linear guide and is actuated by a twin-shaft web ball screw—driven system that is itself driven by a motion-control package and a planetary gearhead. The rotary gripper movement is also driven and controlled by a motion-control package and by a planetary gearhead.
FLEXIBLE SIZE ADJUSTMENTS
Linear guides enable quick and easy manual adjustment for product changes, and they can be automated to even further simplify a packaging machine's flexibility. This translates into significant time and cost savings. Because infinite adjustments can be fully automated, packaging equipment no longer needs different tools for different products. And, because this can be done quickly, throughput is increased and downtime is reduced.
Significant new designs in actuated linear guides and related bearing technologies address adjustment requirements in bottling lines. Figure 2 shows an actuated width adjuster for a bottle conveyor. Bottling conveyor widths are commonly adjusted manually, which can take up to an hour. One actuated linear guide permits automatic adjustment within seconds. It features a smooth, high-response ball screw and gearmotor and uses a proprietary guidance bushing for precision. Compact and waterproof, the linear guide provides closed-loop feedback, speeds up to 100 mm/sec, thrust capacity up to 450 N, and stroke length up to 40 mm.
Figure 2: The MaxJac linear actuator permits automatic adjustment within seconds.
In a second application, a new family of small, precise linear guides and stages actuates the gates on a bottle conveyor. Line gates stop the bottles on the conveyor for filling and capping. Typically, these gates have to be adjusted each time the line is changed. Air cylinders traditionally suffer from offset loading, leading to early failure. However, the small-sized linear guide can be used for automatic gate adjustment. The bearings in the carriage handle the offset load, and the leadscrew/stepper motor is easily controlled for positioning from the PLC. The linear guide reduces adjustments from one hour to seconds.
MINIMIZING CONTAMINATION
Accessories are available for linear motion products to maximize uptime. Bellows, or way covers, protect critical bearing components from contamination such as cardboard dust, which is found in many packaging applications. Another way to maximize uptime is to ensure that bearings are properly lubricated to avoid failure resulting from oxidation or wear. Maintenance-free, self-lubricating linear guides ensure proper lubrication and minimize cost by reducing or eliminating the need for expensive lube systems or manual maintenance.
A recent design includes an oil-saturated polymer, designed to lubricate the guide as the bearing moves. This ensures a constant film of lubricant between the guide's balls and races.
Noise is another issue that linear guides address. Low-friction linear guides driven by smaller motors eliminate the need for big system drives and motors, thereby consuming less energy and generating less noise. Furthermore, they are safer to use, more reliable, and precise in applications such as filling where exact volume is critical.
DETERMINING NEEDS
Because motion-control components are buried under guide rails and conveyors or behind glass, they are easy to ignore. However, to make sensible decisions when buying new or retrofitting existing equipment, engineers should talk to their equipment producers to determine how the components that actuate handling, filling, and assembly processes all work together. That way, engineers will know what to expect when specifying machinery to meet a certain line speed or accuracy. As the examples in this article demonstrate, modifications to motion-control systems can influence machinery downtime, maintenance, speed, and length of system life.
Back to the PMP News Dec. 1999 table of contents
