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Gearbox Bristles With Bright Ideas

01 November 2000

A most unusual six-output mandrel gearbox ensures pinpoint accuracy for
a rather special welding robot.

Machine builder, Bauromat (UK) set BSL a bit of a design challenge
recently when the latter was called upon to provide a bespoke gearbox for
an eight-axis, precision welding cell. The key design criterion required
the six output shafts to turn in unison without varying more than 0.5
minutes of arc from one to another. Moreover, the whole unit had to
operate in a clean room environment - hence the choice of a
sealed-for-life mechanical drive in preference to a pneumatic or
hydraulic alternative.

The welding cell consists of a central mandrel, mounted on a swinging
cradle arm manipulator inside a frame, and a laser-guided welding torch
attached to a robot arm. The gearbox forms the central drive unit for the
expanding mandrel, which holds a 3 tonne toroidal super-conducting
electromagnet in place while its stainless steel pressure jacket is
welded. The tight tolerance ensures that the weight of the magnet is
firmly supported and that there is no risk of the jacket becoming
distorted during welding.

The magnet is manoeuvred into place by a crane and clamped from inside by
six horizontal composite cushions mounted on bars. The bars are attached
to linear guides and driven outwards by six ball screw shafts extending
from the central gearbox. The magnet and mounting assembly are then
rotated as a complete unit during the welding process, which secures
stainless steel lugs to the outside of the jacket and produces a
continuous welded seam.

Keeping the vessel centred during the weld is vital on two counts. In
service, it holds liquid helium at -270oC, so dimensional stresses caused
by distortion could cause metal fatigue during repeated
temperature-induced expansion and contraction. It also ensures that the
welding guidance system remains within its field of vision to keep the
weld consistent.

Fitting the gearbox into a 600mm diameter space was also a challenge, In
order to ensure the strength and precision of the unit, the six output
shafts are mounted in a six inch thick annealed steel body by two opposed
tapered roller bearings. To prevent backlash, the output shafts have
internal pinion gears precision mounted on their ends and are driven by
two bevelled gears mounted at the top and bottom of a single drive shaft.
All the gears are matched sets for optimum drive accuracy.

The quality of the general advice and expertise available from BSL
ensured that the majority of the power transmission components for this
project was sourced form them, recalls Bauromat (UK)'s Gordon Biggs.
The gearbox itself fits our design specifications with a strong, simple
and practical solution ensuring that reliability and precision are
maintained throughout the life of the machine. The magnets are produced
by Oxford Magnet Technology and destined for use in medical body scanning
machines, so these attributes are paramount.

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