Return line filters clean up hydraulic systems

Specialist filtering equipment has reduced running costs on large five-axis milling machines used in the production of aircraft landing gear at Messier-Dowty in Gloucester.

Careful analysis and retrofit of specialist filtering equipment from Wyko has resulted in significantly reduced running costs for large Giddings and Lewis five-axis milling machines used in the production of aircraft landing gear at Messier-Dowty in Gloucester.

Problems with reliability due to frequent pump failures on the hydraulic systems were leading to downtime and high maintenance costs, after repairing the hydraulic pumps Wyko Fluid Power filtration specialist Derek Campion was called in to determine the root cause of the failures and offer a more permanent solution.

Production at the Gloucester facility of Messier-Dowty, a Safran Group company, is focused on complex major structural components including main fittings and bogies for large commercial aircraft supplied to Airbus, Boeing and others, plus main fittings and larger components for military aircraft such as the Eurofighter Typhoon.

As aircraft represent such complex engineered systems, build scheduling is crucial for the manufacturers, hence the supply of essential assemblies has to be assured, making any downtime on machine tools central to production at Messier-Dowty a major issue.

Problems had arisen over previous years with pump failures on the hydraulic systems of the seven individual Giddings and Lewis five-axis milling machines at Messier-Dowty.

During March this year Wyko was asked to overhaul the failed pumps and to determine the root cause of the failures.

Oil samples were taken and analysis was done via the patch test method.

The results indicated that the systems were contaminated with black and bright metal and were running above the industry standard ISO cleanliness level of - 15/14/12.

The first step after refitting the repaired pumps was to fit an offline filter unit to the hydraulic system to clean the fluid and improve the contamination levels.

Routine fluid checking procedures were then established to monitor contamination levels on an ongoing basis after the fluid had been cleaned.

Less than a week after the offline unit was removed the ISO contamination code ramped back up to the previous level and it became obvious that periodic filtering was not going to provide a solution.

It was evident that the pressure-line filters already fitted to the machines were working but significant contamination was being ingested into the system as the fluid travelled around the machine and was then being brought back into the tank.

This contamination would then pass through the pump causing abnormal wear before going into the pressure line filter.

Based on previous experience, and discussions with Pall representative Tim Wright, Campion opted to conduct a trial using specialist Pall SRT (12um) filtration assemblies as permanent additional return-line filters.

The trial consisted of two Pall SRT return line filter assemblies with filter blockage indicators.

A baseline ISO cleanliness level of 19/16/14 was established for the oil prior to the installation of the filter assemblies and a target set according to British Fluid Power Association (BFPA) guidelines of 15/14/12.

The trial commenced in March 2005 and it was agreed to continue for a three-month period.

Samples were taken periodically to make sure the filtration not only initially improved the ISO cleanliness level but also that it was being maintained.

In addition to the installation of return line filter assemblies, it was also decided to upgrade the tank breather filters and to filter any new oil entering the system.

The target level of cleanliness was then maintained throughout the duration of the trial.

Prior to the system being installed pump failures would occur on average every 18 months on each of seven machines.

Overhaul costs on pumps combined with the labour, downtime, an oil change and disposal of waste oil were in the region of GBP 16,000 pounds sterling per machine, per annum.

The new filtering system is intended to increase the mean time between failure from a calculated 1600 hours previously to over 3800 hours, this translates to a reduction is overhaul maintenance costs of over 50%.

The projected operating figures are based on a BHRA study of 117 hydraulics machines that established a clear correlation between fluid particulate levels and average time between failures based on ISO cleanliness codes.

The trial officially concluded in early July, and the remaining six machines are now being converted using the same system.

Taking into consideration the overhaul costs incurred in maintaining the Giddings and Lewis five axis milling machine, the potential savings in installing the filter assemblies mean the full ROI on a capital expenditure of GBP 1000 per machine (fully installed) can be achieved just 6 months.

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