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Amphibious RIB Optimisation of hydraulic latching mechanisms The design brief A hydraulic actuator had been built for a prototype marine vehicle. The requirement was difficult, because the mechanism used a single ram to move an arm through its travel and also to automatically lock it at either end of its extent. In the event of total hydraulic failure, the arm would remain locked but could be manually unlocked, moved and re-locked. Fine clearances were not acceptable, because the marine environment could introduce grit and mud into the mechanism. The prototype actuators worked, but were unreliable. They used springs which were prone to getting trapped and bent, particularly when dirt was introduced. The brief was to develop a new design which would still automatically lock at both ends of the travel, have a failsafe in the event of hydraulic failure, be reliable and robust. |
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Initial findings The existing design was modelled in Solidworks 3D CAD. The physical simulation mode recreated the movements of the prototype. By making small changes to the starting position of various components, it was possible to show how the prototype was able to function correctly and also why it was unreliable. The model demonstrated the sensitivity to starting position and showed how components could jam. The prototype actuators were located inside the hull of the marine vehicle, where they could be reached and moved manually if the hydraulics failed. The actuators penetrated the sides of the hull, which made them very difficult to assemble. Water-tight covers were needed on the outside of the hull, which were awkward and vulnerable to damage. New concept Coupland Bell suggested a different layout, which had a number of advantages. The actuators could be moved outside the hull, into moulded pockets. These were useful structurally and deleted the hull penetrations and vulnerable water seals. |
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The pockets would pick up location details from the hull mould, and give un-restricted access from outside, speeding up assembly of the actuators. However, this new position would prevent users moving the actuators if the hydraulics failed, so a new failsafe was needed. The hydraulic pump or the engine were the most likely causes of potential failure. On the other hand the hydraulic pipes were inaccessible and very unlikely to be damaged. A new failsafe was suggested, using isolation valves and a manual pump. Detail design The unreliable springs and catches in the prototype were deleted, by aligning the ram axis carefully with the latch positions. A symmetric design was used, to reduce unbalanced movements under load and improve reliability further. The cam was enlarged to make it less sensitive to grit. This was the only CNC part. Laser cut and folded steel sheet was incorporated as much as possible. This minimised the number of welds and reduced the production cost. Low-cost plastic bearings were employed, further improving reliability in the marine environment. |
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Solidworks 3D CAD was
used to model the new design. This time, the
physical simulation mode confirmed reliable movements under all
starting conditions. Solidworks was used to make a video of the
new
mechanism, showing it unlatch, move through its travel and latch, then
go through the cycle in reverse, using only a single hydraulic
ram. The software also showed that the latches would be effective in preventing unwanted movement when the ram is stationary. |
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Coupland Bell Ltd, The TechnoCentre, Puma Way, Coventry, CV1 2TT tel: (+44) 01926 863563 - email: web_enquiry@couplandbell.com