Plastic plain and linear bearings have emerged as an off-the-shelf, dimensionally interchangeable alternative to metal and ball bearings in many medical applications. Lubrication-free designs are clean and hygienic, letting them eliminate timely, expensive maintenance regimes. Moreover, plastic bearings are typically less expensive than traditional metal bearings and have a lower profile. They also offer higher operational performance, such as chemical and corrosion resistance and minimal noise. Correctly specified plastic bearings can be used in adjustable medical beds, C-Scan and MRI machines, laboratory equipment, diagnostic devices, physical therapy equipment, prosthetics, and wheelchairs.

An important pre-requisite for the medical industry is for dry-running bearings. Messy lubricants are out of the question because they attract dirt and dust particles which are not conducive to a sanitary, sterile workspace. Lubricants also involve costly, often impractical maintenance plans. One common bearing solution is to have a metal backing with a Teflon liner. However, this Teflon layer does not withstand high edge loads or oscillating movements so it is gradually stripped off. The solid lubes in some plastic bearings are transferred onto the shaft during movement. A fiber-reinforced material inside the bearing withstands high forces and edge loads. Some linear bearings and guides also have polymer liners using this same technology.

Consider several design details when sizing plastic bearings. For example, must the bearing function quietly? Ideally, it should be near silent when running. Does the bearing offer a reliable, smooth operation? The bearing should have a constant coefficient of friction so it can operate consistently. Is the bearing corrosion-resistant? Harsh cleaning agents typically used in the medical industry can corrode metal bearings. The advantage of many polymer bearings is that they are impervious to such corrosive cleaners.

Component predictability is also an important issue. Design firms should work with bearing companies that can accurately calculate the wear rate of its bearings given specific design parameters. Easy-to-use software such as Igus' Expert System lets users enter maximum loads, speeds, temperatures, and permissible wear allowed for their design. The program then calculates which standard, off-the-shelf bearing is suitable, runs a lifetime calculation, and provides a supportive test report.

Bearings in practice

Lube-free bearings are used in a variety of medical devices such as prosthetics, hospital gurneys, and lab equipment. For example, transport-incubator designs have changed little since the 1950s. This means they are heavy, difficult to use, and offer a newborn little protection from noise and vibration during transportation. One company redesigned one such device with an enclosure offering lower noise and vibration. The unit is entirely double glazed for maximum heat retention and provides better access and visibility for caregivers than traditional incubators. Igus' DryLin W linear guides are now used to slide the bed out as far as possible for reintubation, a frequent procedure absolutely critical for a newborn's survival. The lubrication-free linear guides adhere to infection control protocol and fit in with the clean look of the unit. They also work quietly and need little maintenance. The guides' low profile gives maximum space to the infant.

Another patient group that benefits from plastic bearings are amputees. For people with prostheses, daily activities involve frequent impacts and rotational movement. A rigid prosthesis transfers these forces directly to the body where the two meet. One company's lower-limb prosthesis absorbs and reduces the forces in a resilient rod and spring action. The energy and torque-managing device originally used a Teflon-lined metal bearing. However, the bearing surface wore significantly when subjected to high loads, resulting in unacceptable play to wearers. Unlike many others, an iglide Z bearing proved able to withstand the high forces produced by patients weighing up to 275 lbs. The prosthesis works well for walking, running, or even when taking part in extreme sports.

In another case, replacing a squeak-prone metal bearing with a plastic one solved a noise problem and cut production costs for one centrifuge manufacturer. The company reduced bearing-associated costs by 93%, and cut 10 min. off assembly time. The iglide G300 bearing requires only standard machining tolerances, while the metal version needed precise machining and reaming. Maintenance, which was grudgingly performed by customers, is completely eliminated because the bearing is self-lubricating.

In a different application, students from a high school in Pennsylvania developed an omni-directional wheelchair for paraplegics after winning sponsorship from the Lemelson-MIT IvenTeams Program. The program aims to inspire creative thinking and innovation among high-school students by providing the resources to do some real-world inventing. The $10,000 grant went towards inventing a multi-directional wheel system that lets users drive in 360°. Fifty-four bearings and multiple pieces of shafting were used to create each wheel. Iglide Z is not compromised by the heavy loads and high surface speeds encountered.

Another medical application demonstrating plastic bearing use is an ambulance cot designed to reduce strenuous lifting and the associated risk of back injury. The battery-powered emergency cot raises and lowers patients at the touch of a button and fully retracts in 2.4 seconds, reducing load and unload times. A polymer flange bearing handles heavy loads and are used in the integral hydraulic cylinder. The plastic bearings resist blood, harsh cleaners, and bleaching agents.