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These days, best-practice contamination control is more like an accepted pre-condition for reliability. Given contemporary advances in technology for excluding and removing contaminants, it could be said that failure to control contamination is a failure of machine design rather than a failure of maintenance.
That said, effective contamination control is not something to be taken for granted. The results you get are only as good as those you demand, which is why it never hurts to be reminded of the reliability benefits of kicking fluid cleanliness up a notch. Consider the following case study:
A sugar mill was operating a fleet of more than 20 sugar cane harvesters. The typical fluid cleanliness of the hydrostatic transmission for the ground drive on these machines was ISO 22/20, and they were suffering regular pump failures - three pumps per machine, per season, on average.
The sugar mill contracted a local hydraulic engineering firm to investigate the recurring pump failures. They recommended a specification change to the ground-drive hydraulic motors and an upgrade of the filtration.
One machine was modified as a prototype, and after showing promising results, two more machines were modified in the first season. The ISO cleanliness code on the three modified machines was 18/15 or better.
|71%||of machinerylubrication.com visitors consider contamination control targets before purchasing new equipment|
By the fourth year, 15 machines had been modified. The mill was now changing out one variable piston pump per machine every three seasons - a nine-fold increase in pump life.
Armed with this data, the sugar mill convinced the cane-harvester manufacturer to incorporate the same transmission and hydraulic filtration design at the factory.
This is not a scientific study into the benefits of improving fluid cleanliness alone, because clearly, other changes were made to the hydraulic circuit in addition to upgrading the filtration. We’re also not told what influence (if any) these modifications had on other important operating parameters such as pressure and temperature.
Example of Hydraulic Fluid Cleanliness Targets
But what can’t be disputed is the drastic improvement in pump life. As a result, the equipment end user demanded that the machine manufacturer improve the specification (and initial cost) of the equipment they were purchasing. Of course, this was after the economic benefits of doing so had been clearly demonstrated to the end user.
For this hydraulic equipment owner, it was a case of “I once was blind, but now I see.” Prior to this education, they likely would have looked at two cane harvesters of similar capacity from competing manufacturers and bought the cheapest one - with little or no regard to machine reliability or life-of-machine operating costs.
There are two important factors for hydraulic systems that can help you set target cleanliness levels. One is how sensitive the components are to contaminants. This is called contaminant tolerance.
The second factor is pressure. There is a disproportionate relationship between pressure and contaminant sensitivity. Basically, the greater the pressure, the far greater the contaminant sensitivity the components have to contamination.
After you have considered the component type and the pressure, also consider the duty-cycle severity, the machine criticality, the fluid type and safety concerns. All of these factors collectively can be used to set target cleanliness levels in hydraulic systems.
Even though they got it the wrong way around, this machine owner got it in the end. If you’re a hydraulic equipment buyer/owner, the key takeaway of all of this is that the best time to consider these issues is before you purchase a piece of equipment.
By starting with the end in mind, you get the maintenance and reliability outcomes you desire - before the machine even gets delivered. Like in the cane harvester example, you specify the contamination control targets you want to achieve based on your reliability objectives for the piece of equipment and instruct the manufacturer to deliver the machine appropriately equipped to achieve these targets.
Based on the weight and viscosity index of the hydraulic oil you plan to use, you determine the minimum viscosity and therefore the maximum temperature at which you want the machine to run. You then instruct the manufacturer to deliver the machine equipped with the necessary cooling capacity based on the typical ambient temperatures at your location, rather than accepting hydraulic system operating temperatures dictated by the machine’s one-size-fits-all designed cooling capacity - as is the norm.
For example, say you are about to purchase a 25-ton hydraulic excavator that is fitted with brand “X” hydraulic pumps and motors. According to the pump manufacturer, optimum performance and service life will be achieved by maintaining oil viscosity in the range of 25 to 36 centistokes. You also know that in your particular location that you expect to use an ISO VG 68 weight hydraulic oil, and the brand of oil you are already buying has a viscosity index of 100.
This being the case, the pump manufacturer tells you, based on the viscosity and viscosity index of the oil you plan to use, that if your new excavator runs hotter than 70 degrees C, the performance and service life of the pumps and motors will be less than optimum. Not only that, with 70 degrees C as the maximum operating temperature, the oil, seals, hoses and almost every lubricated component in the hydraulic system will last longer.
So being the sophisticated hydraulic equipment user that you are, you say to the manufacturer before you order the machine: “I expect ambient temperatures at my location as high as 45 degrees C, and under normal conditions (i.e., no abnormal heat load in the system), I require this machine to run no hotter than 70 degrees C. If you deliver it to the site and it runs hotter than 70 degrees on a 45-degree day, then I’ll expect you to correct the problem - at your cost.”
You could continue by specifying other requirements that have an impact on hydraulic component reliability, such as that all hydraulic pumps have a flooded inlet, that no depth filters or screens be installed on pump intake lines and that no depth filters be installed on piston pump and motor case drain lines.
At the very least, as the cane harvester story demonstrates, the next time you or the company you work for are purchasing hydraulic equipment, be sure to define your fluid cleanliness and operating temperature/viscosity targets in advance and make them an integral part of your equipment selection process.