Test rig simplifies impeller inspection

Leading aero engine specialist Sigma Aerospace has developed a simple but exceptionally accurate test rig for inspecting aluminium alloy impellers for the Rolls Royce Dart engine.

Leading aero engine specialist Sigma Aerospace has developed a simple but exceptionally accurate test rig for inspecting aluminium alloy impellers for the Rolls Royce Dart engine.

The rig checks the impeller for service defects like cracks or fatigue, and produces a tamper-proof record of the inspection results which meets the requirements of international aviation authorities.

The rig uses the latest Eurotherm data acquisition management unit, the Type 5100V, to display and record the test results.

Sigma Aerospace has been overhauling and repairing the Rolls Royce Dart engine and its propellers since 1962, and since that time has processed around 3000 engines.

Although the Dart engine is now in its twilight years, it continues to successfully operate throughout the world and the company is committed to fully supporting the engine until it is eventually withdrawn from service.

A key element of the service support operation at Sigma are a number of rigorous inspection processes of both high and low pressure Dart engine impellers prior to re-assembly of the engine after repair or overhaul.

Part of this inspection process consists of an eddy current scan of the rear hub abutment face, to detect surface and subsurface defects such as fatigue cracks, machining defects or casting imperfections.

The results of such a critical inspection process do, of course, have to be fully recorded in a way that satisfies the demanding requirements of civil and military aviation authorities around the world.

To meet this challenge, Sigma engineers have developed a semiautomatic test rig which allows a detailed eddy current inspection to be carried out quickly, while at the same time displaying the probe signals and recording test data in a tamperproof, traceable record form for archiving purposes.

The test rig consists essentially of a motor driven turntable on which each impeller is securely placed and centred, over which is an adjustable gantry holding an eddy current probe within a finely adjustable head.

The probe is connected to a corresponding eddy current test instrument, in this case a Hocking Phasec 2200, the output signal from which is fed directly into a data acquisition management unit, in this case a Eurotherm Model 5100V.

Once the surface probe is in place on the face of the impeller, the turntable is rotated by a direct drive electric motor at about 5rev/min.

A 720-degree scan is carried out (ie over two revolutions) and then the probe is indexed horizontally across the face by 25thou for the next scan.

The entire inspection process consists of about 25 such scan increments, resulting in an inspected band of about 0.625in across the critical face of the impeller, carried out over 50 revolutions of the turntable.

The Eurotherm 5100V data acquisition unit displays the probe instrument output live on its quarter-VGA colour screen, annotated by unit output, angle of turntable rotation, time and date.

At the same time the instrument records the test results onto a removable 1Gbyte PC Card for further processing on an associated PC.

This card is used to produce three verifiable records: a printed test result docket, which stays with the tested impeller as part of the whole engine documentation; an electronic archive record, which is then stored within Sigma's central computer data base in perpetuity; and a test log book entry together with the assigned number given to this particular test procedure.

The 5100V is a new kind of data acquisition and recording unit which acts as both as a process viewer and as a high security data-recording source.

Based on leading-edge 32bit RISC technology it was chosen by Sigma engineers specifically because of its high data recording integrity.

During each recorded process, live data is logged within the instrument into the onboard nonvolatile Flash memory.

It is then saved to the PC Card in the form of encrypted data records.

The creation of a secure, verifiable record is achieved using a proprietary Eurotherm logging and archiving strategy that protects data from loss, corruption or tampering.

To meet the growing demand for such high integrity data acquisition systems, all Eurotherm 5000 Series instruments incorporate an Auditor feature specifically designed to encrypt and protect data.

This software includes a secure audit trail, electronic signing and authorising, password protection, signed operator notes, and several other protective features.

An added bonus is that 5000 Series instruments are inherently designed for networked environments and can therefore be accessed via a local area network, dial-up connection, Intranet or Internet.

They can thus provide a seamless interface between the process level and the corporate or departmental IT environment.

Mark Bown, Sigma's Business Process Improvement Analyst, was instrumental in getting the new rig developed for the purpose of moving to electronic test records.

He comments: " We saw the need some time ago to get away from paper records - however secure - and to archive highly secure, traceable test records on our central data base system.

And we needed an approach that could satisfy even the most demanding international aviation requirements.

We found this in the 5100V, and have been extremely impressed with performance to date.

I would also hope that we can soon implement the next step, that of getting the rig 'online', so that the use of an intermediate local PC will become unnecessary".

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