Super precision bearings guarantee accuracy and flexibility of surgical and robot articulated arms
26-05-2009 | BARDEN CORPORATION | PLYMOUTH UK
Trevor Morris, Product Engineering Manager at The Barden Corporation (UK) Limited, discusses how super precision bearings are helping to guarantee the accuracy and flexibility of the latest articulated arms used in surgical and industrial robotics.
A wide range of measurement and inspection systems, industrial robots and medical equipment now use flexible arms to provide the degree of movement these systems require to carry out specific tasks such as handling, assembly, welding, measurement and surgical manoeuvres.
Flexible arms are used by industrial robot laser welding systems, portable measurement arms, laser inspection systems and the latest CO2 laser surgical arms used to perform intricate surgery on patients.
Many flexible arm systems have a number of articulated joints, enabling the end of the arm to be moved through 360 degrees, with three-to-six-axis of movement. Each of these articulated joints with require a bearing(s) of some kind, in many cases super precision bearings.
Trevor Morris is Product Engineering Manager at The Barden Corporation (UK) Limited, a Plymouth-based bearing manufacturer. Barden manufactures a range of super precision angular contact and deep groove ball bearings. The focus is on special configuration bearings that add value and provide engineered solutions for the customer. As Morris states: "Barden has recently engineered a number of bearing solutions for flexible arms. One particular project included a flexible CO2 laser arm for performing intricate medical surgery on patients. We have also provided bearing solutions for portable measuring and inspection arms, as well as for industrial robot systems."
The surgical laser arm is an excellent example of systems that require super precision bearings. All surgical laser units have a similar basic structure. The flexible arm is connected to a vertical pillar. This in turn is connected to the main body of the system, which contains the controls and dsiplay panel, as well as a firing tube system that contains the CO2 molecules to be "lased".
Barden Super Precision bearings gave added flexibility to the robotic arm of the "Phoenix" Mars lander. This arm was critical for analysing soil samples.
The flexible arm will normally require two or three articulated joints to give it the freedom it requires. A set of precision mirrors are positioned at each articulated joint and the pillar, which direct the laser beam in a unidirectional, straight line, from the firing tube at the base of the system, through to the end of the flexible arm, where the hand-piece is located, which in turn focuses the laser beam onto the patient.
The surgeon requires complete freedom of movement of the hand-piece, which is provided by the articulated joints in the flexible arm. However, this flexibility creates a potential problem for the laser beam, which must travel in a straight line at all times throughout the system, regardless of the movements made by the surgeon. Typical surgical arms are around two to three metres in length and so the laser has to maintain its position at all times. The slightest deviation of the laser beam from the centre of the arm will cause operational problems at the hand-piece. A small angular deviation of more than 1mm over a 2-3m length can cause large errors in accuracy.
Precision bearings are critical in ensuring that the laser beam maintains its position throughout the length of the system. As Morris puts it: "Normally, we would supply bearings for the base of the pillar for rotational movements, then sets of bearings for each articulated joint in teh flexible arm. As far as the customer is concerned, positional accuracy and controlling axial and radial runout of bearing sets are the key factors when it comes to selecting bearings."
A typical surgical arm will require bearings for articulated joints that guarantee no more than 5 microns of radial runout. These bearings therefore need to be manufactured to at least ABEC 7 standards, sometimes to ABEC 9 (ISO P2).
"Rotational accuracy of the bearings is also important," continues Morris. "This means providing a smooth running, low friction, low noise bearing that is lubricated for life. For flexible arm joints, the deep groove ball bearings are fitted with special oil-impregnated seals."
Barden's 'Flexeal' is an oil-impregnated, aluminium seal with a fibrous backing. Not only does this seal prevent loss of lubricant from the bearings, but it also prevents the ingress of contaminants, which could lead to increased wear of the bearings. These seals are fitted to a superfinished recess in the bearing.
Morris concludes: "Typically, bearings for articulated, flexible surgical arms are required to operate for at least five years, depending on the application of course. Therefore, if the bearings wear too fast due to poor running surfaces, cleanliness or lubrication, the life of the arm will be affected, which in turn affects the accuracy of the hand-piece. The same can be said for other flexible arm applications, such as robotics and portable measurement and inspection arms. Super precision bearings are essential components in these types of systems."
Barden's manufacturing plant in Plymouth boasts more than 1,700 square metres of cleanrooms for contaminant-free assembly and inspection of bearings. The company designs and manufactures super precision bearings to a minimum of P4/ABEC7 quality standards. Bearing sizes range from 5mm to 250mm outside diameter. Most bearings manufactured are either angular contact or deep groove types.
Barden is capable of manufacturing bearings to a geometric tolerance of P2 or better, and envelope dimensions to P4 or better. Raceway roundness is better than half a micron, with raceway surface finish better than 1 micro inch Ra.
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