When it comes to the economics of assembly machines, faster is always better. Every minute that it runs, a fast machine produces more goods—and more profits—than an otherwise similar slow machine. So why don’t you simply crank up the speed on your existing assembly lines or design faster, new machines?
The answer is that speed, for all its economic advantages, has some steep costs from an engineering perspective. Faster assembly lines typically suffer from resonance and vibration problems. These fast machines tend to operate close to the critical speed of gears and other moving components, and they may have more violent reciprocating movements as well. As a result, they experience more wear and need more aggressive maintenance and lubrication schedules.
What’s more, the technologies that can boost line speed do not come cheap. Faster lines typically require higher-bandwidth controls, as well as larger motors, actuators and gearboxes. The machine frame will likely have to be beefed up as well. While these infrastructure upgrades may well pay for themselves if line speeds improve profitability, they also require a significant amount of engineering effort.
There is, however, a simpler way to improve line speeds. By upgrading motion components—such as cam followers, track rollers, gears, cams or linear actuators—engineers can typically improve the speed of a machine by 15 to 20 percent or more. In one case, we were even able to double the output of a caplet making machine.
While it’s easy to overlook the contribution of individual machine components to line speeds, upgrading these components doesn’t require the huge capital expenditures associated with infrastructure upgrades.
Not every cam follower, track roller or gear lends itself to speed. Engineers will need to pick components that reduce mass without sacrificing strength. Engineers will also want components that can resist abrasive wear without requiring excessive lubrication schedules.
Polymers Are Faster
One of the defining characteristics of fast, advanced machines is that they tend to incorporate components made partially or entirely from high-tech polymers. When properly designed and made from the right material, polymer machine components weigh far less than comparable metal components. For example, our cam followers, which combine a polymer-bearing surface with metal shafts and precision ball bearings, weigh about 40 percent less than all-metal cam followers of the same size. The weight savings reduces inertia, which, combined with low rolling resistance, allows for more efficient high-speed operation. The same logic and weight savings applies to gears and track rollers too.
High-speed machines with all-metal components require continuous lubrication that’s most often provided by central lubrication systems, oil-filled gearboxes or oil spray. An interruption of lubrication, like when a nozzle in a central lubrication system clogs, usually leads to catastrophic failure and downtime. Polymer machine components can solve the lubrication and wear problems and reduce the amount maintenance associated with all-metal systems. Wear—and the resulting need to constantly provide lubrication—slows down high-speed machines. Grease or oil can also contaminate the machine’s final product.
The best polymer motion components are made from self-lubricating materials, so they eliminate the need for lubrication. They also resist various wear mechanisms—such as corrosive wear and galling—that affect metal machine components. The result is that polymer components maintain smooth bearing surfaces over a long lifecycle.
Keep in mind, though, that not all engineering polymers will perform equally in machine components. Some polymers lack the required tensile properties. Others can’t meet the requirements for thermal, chemical or moisture resistance. Moisture, in particular, can be a problem, since it’s ubiquitous in many manufacturing locations. It will cause most engineering polymers to lose tensile properties or change dimensions.
Making sure that polymer machine components have the right properties for the job is an essential ingredient for success. At Intech, we’ve found that a variety of polymers can make good motion components as long as the properties are carefully matched to the application requirements. Acetal, a go-to engineering polymer for gears and other motion components, can work well in many applications. We use it ourselves all the time. But, to really push the performance envelope, engineers will need to consider other polymers.
Our choice for the most demanding motion applications is Power-Core, a material based on butadiene. It has a combination of physical and mechanical properties that makes it uniquely well-suited for use in motion components, particularly at high speeds. These properties include:
Stability and strength. Power-Core maintains its tensile properties and dimensional stability in persistent humidity or even total immersion. By contrast, most engineering polymers absorb moisture. These hygroscopic polymers can weaken by up to 50 percent and no longer carry their design loads. Or, they can swell by 3 percent or more, making them incompatible with any mating components.
No lubrication required. Power-Core parts have self-lubricating properties and smooth machined surfaces, so they eliminate the need for lubrication—and associated viscous drag.
Vibration damping. Like many plastics, Power-Core offers inherent damping capabilities that metals won’t provide. Unlike many plastics, Power-Core’s damping capabilities reach their maximum point close to typical operating temperatures of high-speed assembly lines. The damping capabilities of plastics let polymer motion components reduce noise and vibration as line speeds increase.
Plastics Have Limits
There’s a good reason metals still make up the majority of highly loaded gears, cam followers, track rollers and similar components. In these applications, even a well-designed plastic part of equal size may not offer enough structural strength to meet the application requirements.
While polymers and other composite materials don’t have the tensile or compressive strength of metals, there are many applications where metal components are not used to their full load capacity. These applications are where polymers shine. They’re lightweight, which means lower inertia, kinetic energy and forces. They also have self-lubricating characteristics, which reduces or eliminates lubrication and wear. Lightweight, low-friction components also help to reduce drive power requirements and energy cost.
To get the best of both worlds, we often take a hybrid approach that uses plastics and metals together—casting plastic bearing surfaces over structural steel or aluminum elements. For example, we can apply polymer-bearing surfaces over high-speed roller ball bearings, which is a natural fit for applications slowed down by the greased needle bearings used in most metal cam followers. We also employ metal shafts and gear hubs.
These hybrid systems require some extra design attention. Using advanced stress analysis, we can determine the right ratio of plastic to metal. A well-designed component will take the best of what each material has to offer—structural strength from the metal, such as using metal core for safe gear attachment to a metal shaft, and beneficial surface properties from the polymer.
Analyze and Redesign
To calculate forces and simulate the stresses in lightweight polymer components, it pays to understand the load characteristics of the entire system, bearing in mind that a plastic component the same size as the metal component it replaces may not carry the load.
When designing for higher speeds, you’ll often need to meet the requirements of larger component sizes. As long as there’s enough room, you can design a component to carry the load by increasing the width or outside diameter, for example.
Of course, gear tooth mesh and cam follower tire designs also play a crucial role. The design process requires close cooperation with the engineers in charge of designing the equipment.
One recent example involved the arm of a painting robot. To carry the payload and evenly distribute the stresses throughout its 1,000-millimeter length, we changed the original 100 by 100 millimeter square aluminum tube cross-section to a tapered shape with a 260 by 150 millimeter cross section at the wide end. The purpose of the change to plastic material was to provide electrical insulation to the robotic arm in an electrostatic paint process.
Another example is a 19-inch OD anti-backlash Power-Core gear in a diaper-making machine. This gear must withstand the forces associated with an e-stop, which is equivalent to the full torque of a 10-hp motor for 30 milliseconds, plus the shock load of the machine’s gear train mass. Our calculations showed that if we increased the gear width from 2 to 2.5 inches, the gear would carry this high emergency load. The increase in width was possible because the application had enough room on the drive end of the machine to accommodate the thicker gear.
Whether you have a new or existing production line, it turns out that metal motion components can impose speed limitations in several ways. First, metal components can limit motion systems by virtue of their sheer mass—heavier components have more inertia, lower critical speed, and require more force to accelerate. In addition, metal components require lubrication, and worn components must be replaced, which often causes prolonged downtimes. Finally, and most importantly, metal motion components have no damping capabilities. As a result, the production line may be unable to provide faster operation due to problems with shock, vibration and noise.
With these limitations of metals in mind, here are two examples that illustrate how plastic motion components helped increase the speed of assembly lines.
Metal cam followers, as well as the cams or rails they’re running on, are subject to metal-on-metal wear. Over-greased needle bearings in metal cam followers can lead to skidding and extra wear. Cam followers on high-speed machines are often pressed against the cam with heavy spring loads to prevent skidding. High spring loads only accelerate wear on both the cam and cam follower. Fitted with lubricated-for-life ball bearings, polymer cam followers eliminate the need for lubrication of both the bearing and the cam surface. And, when properly designed, they maintain full contact with the cam without any springs, simplifying design. Most importantly, they don’t wear the cam surface, preventing costly downtime.
Making gears out of maintenance-free polymers—which can be up to seven times lighter than metal—eliminates vibration common to comparably sized all-metal gear systems. This increases both speed and machine output. In the case of a caplet-making machine, we replaced a heavy 3.5 inch-wide cast iron gear with an outside diameter of 17 inches and weighing about 170 pounds, with a Power-Core gear containing a metal hub that weighs less than 40 pounds. This increased the machine output from 400 to 800 caplets per minute.
The customer came to us with the request to eliminate grease in its caplet production line and ended up with a machine that produced double the output of prevailing industry standards. Polymer components often produce unexpected benefits.
For more information, call 877-218-2650 or visit www.intechpower.com.