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A test rig for helicopter gearboxes

04 January 2012

An hydraulics specialist turns its hand to precision electronic control in the development of a special rig to test that most critical of components – the helicopter gearbox

Branch Hydraulic Systems has built and supplied a test unit for Compact Orbital Gears (COG), a specialist gearbox and test rig manufacturer based in Mid Wales. As part of a supply chain partnership, the Branch test unit is being integrated with a high-speed slave gearbox, designed and manufactured by COG, for the testing of main rotor gearboxes for a leading helicopter manufacturer.

The new test unit allows the helicopter gearboxes to be tested in hours rather than days - a significant reduction in the test cycle that helps to reduce the risk of component damage, as well as cutting unnecessary man-hours, minimising critical maintenance and downtime, and ultimately saving money.

Following an evaluation phase, which included parts supply, rig construction, installation, testing and commissioning on-site, Branch was satisfied that the new system and process not only met but exceeded its client’s specifications.

Automating the test
Branch’s concept for the test system combined hydraulic technology with high precision electronic control. With the hydraulic cylinders under closed loop PID control, Branch developed a control algorithm to balance the output torque conditions within the gearbox; the entire system being controlled by a PLC incorporated in the unit.
 
The team was able to provide a semi-automatic test environment facility to test the gearboxes under conditions of equal and unequal torque. By driving the proportional valve natively from the PLC, Branch was able to avoid the need for manual intervention in torque balancing. This not only minimises the risk of damage to the production gearboxes during testing but also means that personnel with specialist knowledge are not required to tackle any issues or make changes during the test cycle.

Torque distribution
A standard helicopter gearbox functions with a single input and equal 50:50 outputs, on the port and starboard sides, when operating normally. However a situation may arise, for example, where if a shaft were to become jammed this would cause an uneven load on the outputs and so one of the requirements of the unit was to test the gearbox under uneven torque conditions.

Together the COG and Branch engineers developed a control program to achieve this and designed a facility so that an operator could select either 50:50 torque balancing or biased torque balancing; for example, between 40:60 and 60:40 at the port and starboard output ends.

A helicopter gearbox has an input shaft speed as high as 19,000rpm and this requires precision control when testing. Branch installed high-precision Moog servo valves on each of the hydraulic cylinders. By changing the pressure applied to each cylinder and continuously monitoring the piston positions using LVDT sensors, torque is distributed from port to starboard side, depending on the selected ratio. In order to maintain precise control under test conditions (where a torque differential of less than 6.5Nm is common), Branch implemented proportional control to control flow and regulate pressure within the hydraulic circuits.

The working concept of the test system was proven within five days, a time scale that certainly impressed COG.

 


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