Medical Device OEMs Turn to Plastic Bearings

Bearings have evolved over the last century just as medical device technology has. Sintered bronze bushings first appeared in 1930. These bearings are impregnated with lubrication, which capillary action prevents from dripping out. The lubricant only flows out as a shaft begins to rotate inside the bearing at high rotational speeds. These bearings are porous so they continue to provide the correct volume of oil necessary for continuous, self-lubricating operation.

However, oil migration and contamination issues have forced engineers to take advantage of more recent bearing designs, such as Teflon-backed metal plain bearings that first made an appearance in the 1950s. The shell of these bearings is made from polymer infused with layers of lubricant and offers a cleaner operation than sintered bronze bearings. It is more suitable for sanitary workplaces. The problem is that the Teflon layer can be gradually stripped off when exposed to high edge loads or oscillating movements, such as those encountered with prostheses, for example.

A more recent innovation in the last few decades is plastic plain bearings containing a solid lubricant. These provide dry-running operation and a long service life. They are available in different material blends for a wide range of different applications and offer corrosion resistance, dirt and dust resistance, low noise and low wear. These same self-lubricating plastics are also available as low-friction sliding elements inside linear bushings and linear guides. These are impervious to the harsh cleaning agents used by those in the medical profession. They replace ball bearings, which require messy lubricants and maintenance regimes impractical for medical device end users. Even if a ball bearing linear guide has seals to contain lubricant, these do not typically last the lifetime of a linear guide.

Plastic bearings are used in adjustable medical beds, C-Scan and MRI machines, laboratory equipment, diagnostic devices, physical-therapy equipment, prosthetics and wheelchairs. "The existing and potential applications for these components are far wide ranging", says Tom Miller, bearings unit manager at igus North America.

One reanimation device, for example, was designed to enable all helpers-from laymen to professionals-to conduct optimum heart-lung resuscitations with minimal effort using plastic bearings. The equipment is placed over a patient's chest and folds together until it touches. At the same time, it independently centers and automatically adjusts the correct indentation depth and breathing volume according to the patient's body size. Cardiac massage is undertaken by operating a lever. This was the first reanimation system designed for those other than professional emergency service workers and capable of both cardiac massage and simulated breathing. The equipment uses 30 plastic plain bearings, which afford it freedom from maintenance and a low weight. The bearings are also fast to install and low profile and so use minimal space.

Another medical industry application is an innovative ambulance cot designed to dramatically 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 loading and unloading times. Plastic plain flange-style bearings capable of handling heavy loads are integrated into a hydraulic cylinder. The cots are often exposed to blood, harsh cleaners and bleaching agents, to which these plastic bearings are also resistant.

Amputees attest that the comfort factor is the most important for a successful prosthesis, especially since daily activities involve frequent impacts and rotational movement. Whereas a rigid prosthesis transfers these forces directly to the body interface, causing discomfort, one company's unique lower limb prosthesis cushions shocks in its resilient rod-and-spring action. This energy-and-torque managing device uses a plastic plain bearing, which handles the high edge loads and shear forces produced by patients weighing up to 275 pounds. The finished prosthesis can be used when walking, running or even when taking part in extreme sports. Users have described the motion as particularly soft with high stability.

Similarly, a prosthetic hip compensates for missing muscles and reduces the amount of energy required by a patient when walking. With standard prostheses, patients have to lift their prosthetic leg high with their good side in order not to catch carpet edges or other obstacles. This improved design enables a cushioned and controlled gait; possible because the system, unlike standard prostheses, creates a three-dimensional movement. The forces occurring in the prosthesis under oscillating motion are reliably transferred without a noticeable increase in play thanks to four plastic plain bearings used on the two main axes. The prerequisite for using these plain bearings included their high wear resistance. 

An auxiliary device enables staff in the care sector to handle serious cases on their own by using lifting and moving technology in such a way that other hospital operations are not encumbered and patients can be moved easily. The mechanism's crossbar structure enables weight release on lifting, thereby relieving pressure on wounds. Plastic plain bearings are used in the crossbar because they can handle the loads involved, are self-lubricating, low wear and reliable.

Plastic bearings are found in many medical applications simply because they present a safe and reliable alternative to standard bearings. With hygiene and cleanliness a top priority, these components are suitable for diagnostics and laboratory research as well as revolving trends in the medical technology sector.