Guarding hovercraft drivetrains

>Based in Southampton, Griffon Hoverwork has pioneered the development of the hovercraft and has been involved in the manufacture and operation of hovercraft since they were first conceived over 40 years ago. One of the company's most popular models with coast guards is the 8000TD, which can be configured to carry passengers, vehicles and equipment for disaster relief or medical evacuation.

>In 2001, the Indian Coast Guard acquired six 8000TDs, two of which were build at Griffon Hoverwork's UK premises and four in Calcutta. At 21.3m in length, with a payload of 8 tonnes, and with a maximum speed of 45 knots powered by two Iveco diesel engines, the performance of these hovercraft is impressive - so much so that Griffon Hoverwork recently secured a £34m contract to supply the Indian Coast Guard with 12 further 8000TDs.

>The craft proposed by Griffon Hoverwork were an upgraded version of the Indian Coast Guard's existing fleet, with several enhancements in design and technology. Among these enhancements is a condition monitoring system from Monitran. Designed and developed as a turnkey solution, the system employs 14 MTN/1100W general purpose constant-current analysis sensors with AC outputs, sealed to IP68 (submersible) and supplied with marine-approved cables. These feed into a bulkhead-mounted waterproof cabinet that contains 14 MTN/8066 g-mac signal conditioning units. Each g-mac unit provides analogue (AC) outputs proportional to velocity and peak g, which feed into a PCB-mounted microcontroller programmed by Monitran to drive a touch-screen display on the front of the cabinet. Several screen views are available but the most useful show:

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• The current numeric mm/s values of all sensors;
• The mm/s values of all sensors shown as a bar chart, with each bar showing current value, threshold and maximum value recorded;
• The mm/s values of all the sensors as lines on a 360 degree dial.
• All of the above also include a colour-coded status indicator; green to denote below-threshold and red to denote above-threshold. The cabinet also feeds a status indicator in the hovercraft's cockpit to give the pilot an indication of vibration levels.

>Tony Roxburgh, project manager at Griffon Hoverwork comments: "A hovercraft's drivetrain experiences a wide range of loads - in terms of torque and RPM - during operation as it tends to be worked hard when a crew is responding to an emergency. The Indian Coast Guard requested that their 8000TDs be fitted with an integrated engine and gearbox vibration monitoring system to warn the pilot if the drivetrain experiences any abnormal vibrations." Other requirements included that the system be lightweight, compact, compliant with marine regulations and itself tolerant to high levels of vibration. In addition, provision had to be made for maintenance engineers to view the vibration levels on any given sensor in order to identify which component(s) might be wearing or developing faults; a requirement that was easily met thanks to each g-mac having a buffered native accelerometer output presented on a BNC connector.

>Andy Anthony, Monitran's managing director, comments: "Monitran is increasingly undertaking turnkey projects, and developing engineered solutions from the sensor up. The project with Griffon Hoverwork also exemplifies what can be achieved with a relatively humble, general purpose accelerometer like the MTN/1100W."

>Recognising that monitoring variables such as vibration are key in a wide range of industries including aerospace, rail, marine as well as general plant and machinery, Monitran now offers a standard version of the microcontroller based condition monitoring system in the form of the MTN/5000-16. Containing up to 16 g-mac signal conditioning units and featuring a 2.8in TFT touch screen with an easy-to-navigate menu, it enables users to set sampling periods, ranges and accuracy levels, plus vibration threshold (alarm) levels on a channel by channel basis or across all channels. In addition, the system has 20 digital I/O channels, enabling the MTN/5000-16 to be integrated with other systems. The MTN/5000-16 is initially available in a 350x250x200mm cabinet, though other cabinet sizes may become available.