To apply a cost effective control programme, it is important, obviously, to ensure all root causes of wear, such as alignment, balance, correct selection of lubricant etc are sorted first.
Moving onto the lubrication, relying on system filters alone will shorten the service life of these expensive elements, as well as allowing wear to occur during the clean-up periods. It is far simpler, and far more cost efficient, to ensure all entry points in the system are thoroughly sealed against contamination ingress.
Built-In Contamination
During manufacture, there is significant potential for contamination within the machine prior to commissioning. Twenty years ago, very little effort was applied to what is now termed ‘roll-off’ cleanliness qualification. Now, during the build phase each sub-assembly is flushed to ensure the residual build debris is removed, such as swarf and casting sand. On completion of the build, the system is usually flushed again prior to start-up. The way in which this is undertaken will depend on the nature of the machine and the ease of circulating a flushing fluid under no-load conditions. Recent advances in this field have lead to the formulation of several ISO standards. Manufacturers and users can now impose certification requirements for the achievement of a satisfactory cleanliness level prior to commissioning. The level to which this control should be taken will depend on the reliability and criticality needs of the user with respect to that machine.
During routine maintenance and service, the opportunity for built-in contamination continues, and again, sensible steps taken at this stage will minimise the risks to the machinery. Much of this is basic engineering housekeeping – for example, ensuring the exposed parts of the machine are protected as much as possible from atmospheric contamination such as dust and moisture. More importantly, any new sub-assemblies being fitted should be done so in a clean manner. These should also be bought with pre-qualified levels of cleanliness. Hoses are a typical example – the hose should be supplied pre-cleaned and capped, and remain sealed until the actual fitment to the machine.
The final, and perhaps most significant aspect is the storage, handling and dispensing of new oils. New oil can be full of solid particulates and therefore needs pre-filtering prior to use. In the case of complex hydraulics, barrels of new oil may be up to 128 times dirtier than the cleanliness level required, and the potential damage and reduced filter life on the machine is a greater cost than the cost of taking basic steps to ensure a clean oil supply.
In ensuring clean oil supply, one can request the lubricant supplier to pre-filter the oil to a desired level for an additional cost. This additional cost will be lost unless users carry out some basic on-site oil management techniques.
First, users can switch from barrels of grease to grease cartridges. This minimises waste and avoids the all too common scenario of the grease container being left open.
Oil must be stored in an enclosed shelter or building, not just for contamination control, but to comply with environmental legislation. Heating and/or cooling the building to maintain a constant temperature of 20°C will minimise potential oil damage. Pumped, filtered air pushed through this building using positive pressure displacement will help keep the building safe and contaminant free. In addition, barrels should have a breather fitted, not just for solids, but also for moisture removal. Where several barrels are racked, a manifold system with one main breather will reduce the cost of the breather replacements.
A filter cart will filter oil dispensed from barrels another time. These carts – increasingly common in industry – are especially useful if the barrel has been left open or has been stored outdoors for any length of time. If a barrel has stood for some time a barrel roller will re-suspend the additives and contaminants in the oil, which the filter cart can then remove.
Users are often likely to need oil in small quantities for refills. In the case of the small volume needs, oil should be dispensed into clean, sealed containers such as the Oil Safe variety. These Oil Safe units allow for either manual pouring from a resealable spout, or for pumping of the oil through a flexible hose and fitting. In this instance the drum pumps for filling these containers must have a filter and dust cap and should be properly fitted to the barrel, not just loosely inserted.
For dispensing of the oil directly from barrels, a filter cart is again advisable. The machine will need equipping with quick connectors for both filling and draining purposes, but these can be adapted through the use of stock Tee-piece fittings that fit to the breather/or filler and drain ports. In addition these can be upgraded to included oil analysis sample ports to minimise effort of sample collecting.
It is also best practice to have a set of tools dedicated to an oil type, which will minimise the risk of cross-mixing of oils.
Ingested Contaminants
Any point of contact between the inside of the machine and the atmosphere is a potential source of entry for contaminants. In the specification stage, this will mean ensuring fitting quality seals and breathers, as these are the two weak areas once the machine is operating.
Labyrinth or double-lipped seals are better at minimising dirt entry, though regular maintenance is required.
Where oil is leaking from a seal, dirt is entering. Minimising solid contamination will also minimise seal damage and leakage. Protecting the outside of the seal with shields or, on cylinder rams, with convoluted boots will avoid seal damage through dust and moisture adhering to the seal face.
Breathers are required to stop dust particles of less than 10µm and a simple solution is to use a spin-on oil filter canister of 5µm rating. Simple U-tubes and plugs with small 2mm diameter passages are inadequate.
On gearboxes, the purpose of the breather is to allow for air expansion to avoid seal damage. A better option is an expansion chamber that incorporates a bladder to allow for air expansion but also seals the gearbox from the atmosphere. This is very important in outdoor applications where both dust and moisture is an issue. On tanks where the oil level changes, a desiccant breather is not only an ideal improvement, but is also an improvement to minimise oil vapours outside of the tank for health reasons.
A final point is to ensure that all access points, such as hatches and cover plates, are properly closed and have adequate sealing to prevent dirt entry.
Contamination within the Machine
If the above steps have been taken, then the actual role of the filtration fitted to a machine should be one of protection, rather than control. First, one should assess what sort of filters have been installed. Not all machines are fitted with filters, either through design or for purchasing reasons. Where filters are fitted, again these may not be adequate to protect the machine.
Where no filtration is fitted, users may be able to install permanent off-line units with a pump and filter housing. Large gearboxes, in particular, will benefit from this treatment. Keep in mind that the suction pipe should be as far as possible from the return pipe to ensure full clean up. Where the use of permanent filtration is not an option, then the filter cart can be connected periodically, allowing the volume of oil to circulate 5-7 times through the unit before disconnecting. The filter cart can be used as additional off-line filtration on tanks where maintenance has just been undertaken to ensure faster cleanup.
On hydraulic systems where extremely fine contamination control is required, the use of electrostatic separation filters will prove beneficial in removing sub-micron sized particulates resulting from oxidation problems, thus increasing the potential service life.
Where water ingress has occurred, the easiest method is gravity separation from a settling tank. However, for in-service applications, a centrifuge will remove free and emulsified water. To further dry out the system, a vacuum dehydration unit or air stripping filtration system is required. For small volumes of water, desiccating filter elements will be more cost effective.
On hydraulics and engines, better quality, finer filters will ensure cleaner oil and improve the reliability and machine service life. Contrary to normal thinking, a finer filter will last longer in service than a coarser rated filter. Avoid locations that suffer from excessive vibration and pulsating flow, as these factors will reduce the efficiency of the filtration.
