ZF’s Precision Planetary Gear Heads are providing the high levels of torque and accuracy required for Cambridge University’s new radio telescope installation.

The ArcMinute MicroKelvin Imager will soon be used to survey the sky, searching for signs of some of the earliest detectable galaxy clusters.  The installation comprises five pairs of 4m diameter parabolic dish antennae whose movement is controlled by a high reduction ratio servomotor system.

A small servo geared motor, drives 2 close coupled ZF Precision Planetary Gear Heads supplied by Drive Lines Technologies Ltd.  In conjunction with the final drive, the system achieves an overall reduction ratio of almost 100,000:1.

Using galaxy clusters in order to learn about the early Universe has long been recognised as an important area of astrophysics, but observing these distant objects using either light or X-rays has so far proved problematic. Since the discovery in the late sixties of a secondary effect* of cosmic microwave background radiation (CMB), astrophysicists have been building instruments to study the effects in order to measure the size of galaxies and calculate their mass. The most recent of which is Cambridge University’s series of ArcMinute MicroKelvin Imagers (AMI) installed at their Lord’s Bridge site.

Cambridge University’s new AMI installation is a series of ten antenna dishes that enables staff and students to survey the sky searching for signs of some of the earliest galaxy clusters, which are detected as ‘silhouettes’ against the backdrop of the CMB.  The AMI system will combine a large field of view with high sensitivity to faint objects, making it ideal for surveying the sky for the galaxy imprints and measuring the angular sizes of the objects in arcminutes rather than degree scales.

The parabolic dish antennae have 4m diameters, each weigh 1.5 tonnes and are designed to operate 365 days a year in winds up to 45 mph and survive gusts up to 115 mph.  In order to provide accurate results, the antennae need to remain fixed on a distant point in the sky during bad weather and compensate for the movement of the earth on its axis.  This means that the mounts need to rotate about their primary axis at nominally 15 degrees per hour, 360 degrees per day, requiring a high-ratio, slewing ring based, drive system.

From the servomotor speed range of 70 rpm to 3500r/min a slewing speed from 15 degrees per hour (for tracking) to a maximum of 12.5 degrees per minute (for positioning) was required.  As well as the slow speed, the level of accuracy required from the drive system is in the order of 20 seconds of arc with an encoder resolution of 10 seconds of arc.

‘To achieve the required reduction ratio of around 100,000:1’ explains Drive Lines MD Bob Clark, ‘the drive system incorporates 5 stages of planetary gearbox and a final drive pinion to slewing ring ratio of 5.5:1.  The selected servo motor was geared down via a single stage 3.7:1 box into two close-coupled dual-stage 70:1 ZF PG series planetary gearheads. The ZF PG units were the natural choice as they offered backlash levels of £2 arc minutes and a very high degree of torsional rigidity’.The use of grease as a lubricant, rather than oil, enables the PG gearheads to be mounted in any orientation on the equipment and, being sealed for life, allows the antennae to operate in all weathers without the need for maintenance.  Their compact size also makes  them the ideal choice for the application, which was, due to the need to constantly reposition the 10 antennae at the site, constrained in terms of both design envelope and weight.

ZF PG gearheads are available in single and two stage variants with a range of ratios between 3: 1 and 100:1.  The gearheads are rated for motor input speeds from 3000 to 10,000 rpm as standard, and are suitable for use with a wide range of motor frame sizes.

PG series gearheads are backed-up by the resources of one of the most respected manufacturers of driveline technology in the world.  ZF Industrial is part of the worldwide ZF Group of companies with production centres, partners and representatives on every continent.  In the industrial sector, ZF is at the forefront of developments, from individual units to complete systems solutions for all types of machinery and production units, including those used in robotic systems and the fast growing factory automation sector.

* The Sunyaev-Zel’dovich effect was first predicted in 1969 by Sunyaev and Zel’dovich and refers to the scattering of cosmic microwave background photons by hot, ionized gas in clusters of galaxies.

For more information on the range call Drive Lines on 01234 360689, log on to www.drivelines.co.uk or email [email protected] and request some literature.