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Precision On A Large Scale: Worm Gear Sets For Astronomical Telescopes

01 September 2003

Holroyd is able to achieve a backlash level of just 11 arc seconds in a
405mm centre worm gear set, destined to position a new 2.6m diameter
radio telescope dish to an accuracy of 0.5 arc minutes

Probably the most conclusive piece of evidence supporting the Big Bang
theory of creation is the existence of an anisotropic radiation 'bath'
that permeates the entire Universe. This cosmic microwave background
(CMB) is thought to be the relic of thermal radiation, released about
300,000 years after the Big Bang. A new telescope is currently being
constructed by the Instruments Group at Cardiff University to investigate
this phenomenon and study the CMB's anisotropy in order to gain an
insight into the nature of the Universe as it may have been some 15
billion years ago. The 2.6m diameter radio telescope is due to be located
in the Atacama desert in Chile at an elevation of 5,000m by mid-2004.

One of its most important components is the worm gear assembly which sets
the radio dish inclination. Designed with a dual lead, 405mm centre
distance and a reduction ratio of 130:1, this assembly is being
manufactured by Holroyd at its Rochdale facility, which currently holds
the world record for worm gear efficiency at 98%. The Cardiff Instruments
Group telescope will require a pointing accuracy of 0.5 minutes of arc,
while the maximum permissible angular backlash must not exceed 11 arc
seconds, which places considerable demands on Holroyd's machining

Fortunately, the company is aided in this by a new worm gear contact
analysis program that reduces both the margin for error in the
manufacturing process and lead times from order to despatch. The program
also ensures greater consistency of manufacture and longer gear life by
providing benefits such as improved controllable contact spread, smoother
unscraped finish, enhanced lubrication and the facility to cut worm gears
off-load to offset distortion due to loading when in use. Holroyd's Steve
Whitehead takes up the story:

This program functions as an integral part of our worm gear design
process. it removes the reliance on traditional, declining skills-based
manufacturing, allowing us to eliminate trial and error from the design
and production process using contact prediction graphics and revised
calculations. The result is that we are able to manufacture accurate and
efficient gear sets right first time.

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