04-06-2010 | BARDEN CORPORATION | PLYMOUTH UK
The latest surgical powered hand tools require miniature, super precision ball bearings, which are not only capable of withstanding the high loads and operating speeds involved, but also to provide high reliability and therefore a longer life of the tool, says Barney Eley of The Barden Corporation (UK) Limited.
The latest handheld cutting and drilling tools used in surgical procedures are now more sophisticated, as well as more compact and ergonomic in their design. But in order to ensure that these tools meet their performance standards and expected life, super precision ball bearings are required, which support the drill or cutting tool during often very harsh operating conditions.
The type of power tool - whether electric, air or motor-driven - that is used by doctors today during surgical procedures, will normally require super precision ball bearings, often custom-engineered to suit the particular demands of the application.
Barney Eley, Product Engineer at Barden, describes a typical surgical application example: "In surgical procedures, the operator of the sawing tool will undoubtedly have to put a relatively large load on the ball bearings. Since the tool might be used to saw through bone or to cut through tendons, muscles, cartlilage and other bodily tissues, the operating environment for the bearings is very harsh. Typically, the bearings are located close to the end of the cutting tool, often working inside the patient's body and so resistance to acids and other corrosive media is critical."
"This means the manufacturer of the power tool is going to require a bearing that is both compact enough to fit into the slender design of the tool, but also able to cope with the high loads and relatively high operating speeds [up to 80,000rpm] involved. In addition, the bearings will have to withstand the corrosive, slightly acidic operating environment inside the patient's body," he adds.
"When you are designing bearings for surgical tools, there are many ways in which the bearing can be improved to suit the application. For example, we can alter the curvature of the raceways, as well as the height or size of the shoulders. Tighter curvatures within the raceways will lead to a reduction in contact pressure."
Another important design consideration for the bearings is resistance to temperature. As surgical tools require regular sterilisation in autoclaves, the bearings must be designed to withstand temperatures up to around 140 deg C. "Most of our bearing designs for surgical tools will be manufactured in corrosion resistant 440C stainless steel, which provides sufficient resistance to cleaning chemicals and other aggressive acidic media," confirms Eley.
"The bearings, which we supply as either deep groove or angular contact configurations, are normally provided with phenolic cages to provide further resistance to repeated sterilisation cycles."
The harsh operating environment for the bearings also means that protection is required against any fine particles of soft tissue and bone that may penetrate the contact regions between the balls and the bearing raceways, which would then lead to high stress concentrations and could lead to the eventual failure of the bearing.
In order to counter these problems, Barden can provide integral shields that are designed to help retain the lubricant (normally a food-grade grease in surgical tool applications) whilst preventing the ingress of contaminants. "This improved sealing design reduces the critical gap between the integral shield and the bearing inner ring to 60 per cent of that when compared to conventional shield and circlip designs. This means you get a reduction in operating noise, greater lubricant retention and improved protection from contaminants - all of which often leads to a longer life of the cutting tool." confirms Eley.
Relative scale of a surgical tool bearing
In surgical applications, the cleanliness of the bearings is essential. Barden's manufacturing plant in Plymouth has more than 1700 square metres of fully equipped cleanroom facilities, where all assembly, testing and packing operations take place. "On a recent surgical sawing tool project, the bearings we provided were custom engineered, angular contact ball bearings with an outside diameter of 4.76mm and contained balls just 0.79mm in diameter, which is the smallest diameter we have provided for this sort of application. Here, our cleanroom facilities were essential. Handling miniature ball bearings is not a simple task and we have the facilities and personnel here."
The bearings provided here were of a double row cartridge design with a single inner ring and two outer rings. "This design," says Eley, "enabled us to incorporate an integral, one-piece shield on both sides of the bearing in order to prevent ingress of contaminants and to retain the lubricant. This one-piece, pre-loaded and custom-designed bearing solution made assembly much easier for the customer, who simply had to 'drop' the bearing into position within the overall tool drive assembly."
Double row cartridge design with integral shields
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