Compact actuators bring safe landings

Tom Shelley reports on a dramatic application of an actuator that is new to the UK

12V electrohydraulic actuators with a variety of uses have been chosen for a system that raises and lowers safety nets round warship decks, particularly helicopter landing pads. The devices come from the US civilian sector, where they are used on power boats and large trucks. They are at the same time, very rugged with higher power densities than are possible with purely electric actuators, yet are capable of being integrated into sophisticated control systems. The nets are made by Permali Gloucester, with glass reinforced plastic supports and polyester nets capable of catching and retaining a 150kg sandbag dropped or a 90kg steel weight dropped from 1.5m. They have to be turned down flat, outwards, so as not to obstruct a helicopter during landing, and then pulled upright afterwards and locked in position. Their real importance is in a rough sea, perhaps with waves breaking over the stern of the ship and the crew hampered not only by the wind and the sea but a build-up of ice on their clothing. The nets are there both to catch personnel who might otherwise be washed overboard, but have also been known to catch the nose wheels of helicopters, in danger or sliding off, as well as items of ordnance. The system has been sold to navies worldwide, including the Royal Navy, with cords to manually pull the net sections upright. However, an obvious improvement is to introduce power operation of the safety nets that could be accomplished remotely. For this reason, Permali turned to Parker Hannifin, which came up with a US developed and manufactured electrohydraulic actuator that is much more compact than an equivalent electric actuator, but unlike conventional hydraulic actuators, is completely self-contained. The 'Compact EHA' as it is designated, uses a hydraulic cylinder to deliver forces of up to 22kN (5,000lbf) at up to 130mm/s from a unit that weighs around 5kg, and is about the size and shape of two hydraulic cylinders beside each other. Made by Parker's Oildyne Division in Minneapolis, it uses a permanent magnet DC motor, either 12V or 24V, and is pre-flushed, filled and sealed. Parker's Martin Latimer told us that it can be used in simple on-off mode, analogue-controlled, or integrated into the company's IQAN, CANbus-based control system. Compared to an electric ballscrew, it is both smaller and less expensive. Light weight is ensured by use of an anodised aluminium housing. Maximum stroke is 203mm, and the unit is very quick and easy to install. It is normally just bolted in and connected up. It has internal relief valves so driving its load into end stops is not going to break it. Although new to the UK, the product is widely used in the US. Latimer told us that it is used to lift up the very large outboard engines that are popular on American power boats, proving its suitability for a marine environment, and for tilting the cabs of that country's big '18 wheeler' trucks to access engines and mechanics. Other applications include: opening hatches and doors, ambulance tilt beds, jack plates, mower deck lifts, hospital stretchers and ATV (All terrain vehicle) attachments. Latimer told us that because of interest on this side of the Atlantic, the company is looking at the possibility of manufacturing them in the UK, where wages are lower than in the US and they could avoid EU import duty.