The design of an automated piece of machinery involves an array of technologies. In most cases, the machine design aspects that tend to get the most attention are the various motion control and robotic components or the software and machine interface. One of the most overlooked aspects of industrial machinery also happens to be a critical component to achieving long-term impact on profitability for both the OEMs and their customers: bearings.
Though not as sexy as robotics or software, bearings are key aspects of the connective tissue in any motion control system. And it is their ubiquity that often leads to their oversight when specifying machine components. Through years of practice, many engineers tend to rely on the well-known bearing options with which they have successfully worked for many years.
The trust engineers have placed in traditional, lubricated bearings, for example, is not without merit. Even with all the advances made in bearing technology over the past decade, when it comes to high-load, high-speed, high-temperature or precision applications, lubricated bearings remain a top choice.
Outside of such applications, what is often overlooked is the option for many machines to use non-lubricated bearings. By eliminating the need for lubrication where possible, OEMs can reduce production costs while making their equipment more marketable and less expensive to operate for end users.
When it is an engineer's option to choose a lubricated or non-lubricated bearing for use in automated machinery, the issue of bearing lubrication is no small matter. A recent study by SKF USA Inc. (a manufacturer of bearings, seals, ball and roller screws and other industrial equipment) found that 54 percent of bearing failures are lubrication-related (see chart). In another study by the Massachusetts Institute of Technology, it was estimated that approximately $240 billion is lost annually (across US industries) due to downtime and repairs to manufacturing equipment damaged by poor lubrication.
So for those machines in which non-lubricated bearings are an option, what do those choices look like and how do they work?
Self-Lubricating Options
Self-lubricating plastic bearings are made of high-performance polymers and, unlike rolling-element bearings, they slide instead of roll. These bearings consist of a base polymer optimized with fiber reinforcement and solid lubricants. The fiber reinforcements increase load-carrying capabilities and wear resistance. Solid lubricants in the bearing are transferred from the bearing to the micro finish of the shaft to reduce friction.
No external oil or grease is needed for operation; self-lubricating bearings operate completely dry. As a result, they are more commonly used in labs and food-processing machinery that require clean, oil-free operation. Plastic bearings have also been proven to perform well in dirty environments since they contain no oil to attract dust and dirt.
Cost Factors
Though they require no lubrication, high-performance, self-lubricating plastic bearings do sometimes need to be replaced. When this occurs, the replacement part (a small, inexpensive plastic sleeve) can be purchased for a fraction of the cost of an entire re-circulating ball bearing.
In addition, plastic bearings do not require the machining and other processes necessary to install ball bearings. They are less expensive and do not require grease fittings, lines or pumps. Plastic bearings also can be used on less expensive shafting, such as aluminum or cold-rolled steel.
Some companies offer online calculators to predict bearing lifetime to ensure the selected bearing is ideal for the application; this eliminates the need for testing and saves time and errors in material choice. You can find examples of these calculators at http://www.pacamor.com/technical/lifeandload.php and http://www.ahrinternational.com/bearing_life_calculator.htm.
Application Fitness
Few machine system components are equally usable in all environments, and plastic bearings are no exception. Some of the applications for which plastic bearings are not a viable system design component include:
• High loads with high speeds, as these lead to excessive frictional heat buildup and wear.
•Highly cantilevered loads. Since self-lubricating plastic bearings slide (unlike ball bearings that roll), linear applications with higher coefficients of friction may result in uneven movements for highly cantilevered loads or drive forces.
•Extremely precise applications. Plastic bearings have a higher running clearance than ball bearings, sometimes .001-in. to .002-in., and therefore are not ideal for applications needing extreme precision.
•Extreme temperatures. Plastic bearings are not recommended for applications with long-term temperatures exceeding 484 degrees Fahrenheit.
Aside from these applications listed above, end uses for which self-lubricating bearings are viable include:
• Harsh, extreme environments, such as agricultural and outdoor equipment.
• Sensitive, clean environments, such as biotech, lab machines and medical equipment.
• Wash-down applications in packaging and food processing.
• Weight-sensitive applications, where the goal is to reduce fuel consumption and/or lower the inertia of moving parts.
• Plastic Bearing Field Applications
As noted above, there are a number of applications for which plastic bearings have proven to be viable engineering options. Below are brief overviews of how specific plastic bearings are being used in a variety of motion applications.
Agricultural
The manufacturer of "The Pick Planter" equipment for the farming industry initially used oil-impregnated bronze bearings with graphite plugs in its Pick Planter, which is used to create individual planting rows using walking gauge wheels to deliver a consistent planting depth. Two separate issues with the bronze bearings arose for the manufacturer -- one on the West Coast and one on the East Coast -- which required replacement of the bearings two to three times a season. On the West coast, the bronze bearings were experiencing high wear and premature failure due to the abrasive conditions caused by high levels of volcanic ash in the soil. On the East coast, high salt content in the air caused corrosion and seizure.
To address the issue, the manufacturer replaced all 144 bronze bearings on the Pick Planter with iglide self-lubricating plastic bearings from igus. According to the manufacturer, the pick arms' lifespan was increased by 500 to 600 percent. As an added benefit to the OEM, the iglide bearings cost 70 to 80 percent less than bronze bearings.
Packaging
A manufacturer specializing in vertical, form, fill and seal packaging equipment builds machines capable of reaching up to 160 cycles per minute and withstanding loads up to 15 pounds, while operating at speeds of 750 feet per minute. This manufacturer shifted from the use of metal linear ball bearings after experiencing scored shafts and leaking grease on some of the machines using those bearings. Since replacing those linear ball bearings with igus' self-lubricating DryLin R linear plain bearings, the linear bushings have surpassed the 10-million cycle mark on some of the company's packaging machines with little to no noticeable wear.
Medical
A team of researchers from the Worcester Polytechnic Institute in Massachusetts has developed a specialized magnetic resonance imaging compatible piezoelectric actuated robot.
To facilitate different types of motion, the robot uses a DryLin linear guide system and iglide plastic self-lubricating plain bearings. The linear guides facilitate translational motion of the positioning module, which provides gross positioning for the robot's needle driver. The needle driver is a vital part of the system, as it enables the rotation and translational movement of the needle cannula: a flexible tube inserted into the patient’s body cavity for MRI-guided diagnosis and therapy.
The needle driver has a needle guide sleeve, a collet locking mechanism and passive optical tracking fiducial frame. Two plastic plain bearings are used in the front and rear of the driver to constrain the needle guide. The bearings enable the robot's motor to rotate the needle using the collet mechanism by way of a timing belt. This rotating needle is designed to reduce tissue damage while enhance targeting accuracy. Another 10 plain bearings were used to create a revolute joint, also known as a pin joint or hinge joint, to provide single-axis rotation.
The linear guides chosen are comprised of hard-anodized aluminum rails and carriages and high-performance plastic sliding elements, which do not interfere with the MRI procedure. The linear slides operate without messy lubrication, which is important in a sterile medical environment. They also feature a lower-profile for applications where installation space is an issue.
The specific plastic plain bearings used are comprised of FDA-compliant polymers specifically designed for applications with contact to food or drugs.
