Barden bearings make surgical robots more accurate

Trevor Morris, product engineering manager at Barden, describes how super-precision bearings help guarantee the accuracy and flexibility of articulated arms used in surgical and industrial robotics.

A large selection of measurement and inspection systems, industrial robots and medical equipment now uses flexible arms to provide the degree of movement needed 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 that perform intricate surgery on patients.

Many flexible-arm systems have articulated joints that enable the end of the arm to be moved through 360 degrees, with three-to-six axes of movement.

Each of these articulated joints requires one or more bearings of some sort, in many cases super-precision bearings.

Barden is a Plymouth-based manufacturer of super-precision angular contact and deep-groove ball bearings.

Morris said: '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 a system that requires 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 display panel, as well as a firing-tube system that contains the CO2 molecules to be lasered.

The flexible arm normally needs two or three articulated joints to give it the required freedom of motion.

Sets of precision mirrors positioned at each articulated joint and the pillar 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 2-3m in length, meaning 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.

Morris said: 'Normally we would supply bearings for the base of the pillar for rotational movements, then sets of bearings for each articulated joint in the flexible arm.

'Positional accuracy and controlling axial and radial run out 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 five microns of radial run out.

These bearings therefore must be manufactured to at least ABEC 7 standards, sometimes to ABEC 9 (ISO P2).

Morris added: 'Rotational accuracy of the bearings is also important.

'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.

It prevents loss of lubricant from the bearings and prevents the ingress of contaminants, which can lead to increased bearing wear.

These seals are fitted to a super-finished recess in the bearing.

Morris said: 'Typically, bearings for articulated, flexible surgical arms are required to operate for at least five years, depending on the application.

'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 designs and manufactures super precision bearings to a minimum of P4/ABEC 7 quality standards.

Bearing sizes range from 5-250mm outside diameter.

Most bearings manufactured are either angular contact or deep groove types.

Barden can make 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 one micro-inch Ra.

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