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It is widely recognized that contamination creates problems in hydraulic fluid power systems. The explanation is straight-forward - minimizing the gaps or clearances between moving parts improves efficiency and permits precise machine control. Particles compromise efficiency through abrasive wear and interfere with machine control when they become lodged in hydraulic valves.
Hydraulic equipment manufacturers have found that built-in contaminants introduced through the assembly of dirty components increase warranty costs. Three common strategies are employed by hydraulic equipment manufacturers to minimize the impact of built-in contamination:
Table 1. Volume-to-area ratio
of hydraulic components
Establishing the contamination limits for components, like most other engineering decisions, involves a cost/benefit analysis. While it is obvious that a clean part is preferable to a dirty part, there is a cost associated with achieving a given level of cleanliness. Therefore, it is prudent to consider the contamination sensitivity and working pressure of a hydraulic system when establishing contamination limits.
Generally, contamination limits for components are specified in terms of the milligrams (mg) of contamination and the length (longest chord) of the largest particle. As shown in Table 1, the volume-to-area ratio of hydraulic components covers several orders of magnitude. (Area, in this case, only applies to wetted surfaces that are in direct contact with the hydraulic fluid.)
In order to account for differences in volume and wetted surface area, different units of measure are used to define built-in contamination levels. Table 2 provides a list of contamination limits for new components expressed in common units of measure. For components that have a high volume-to-area ratio, mg/liter is a common unit of measure. For components that have a low volume-to-area ratio, mass per unit weight or mass per unit area is more appropriate. The main reason for this is that the lower limit of detection in gravimetric contamination analysis is between 1 and 2 mg. This limit is not a function of accuracy of the analytical balance but is the result of variation in the solvent and moisture absorption by the filter membrane used in gravimetric analysis.
Table 2. Upper control limits for built-in
contamination of hydraulic components
A common mistake in establishing component cleanliness specifications is to express limits in terms of an ISO contamination code. This is understandable since ISO codes are well-known shorthand for specifying hydraulic fluid cleanliness. However, contamination codes, as defined by ISO 4406, only apply to fluids, so to specify a maximum ISO 18/16/13 contamination code for a hose, tube, valve, cylinder, pump, reservoir, etc., is incorrect. This limit might be suitable as a roll-off cleanliness specification for an assembled vehicle, but to use it for components is problematic on several levels. Of particular concern is the fact that the ISO 4406 contamination code makes no distinction between 15, 150 or 1,500 micron particles. Large particles are of great concern in new hydraulic components because they can cause severe damage the first time the system is powered up.
Figure 1. Debris from a new hose Figure 2. Debris from a new valve
Figure 3. Debris from a new reservoir Figure 4. Debris from a new cylinder
Built-in contamination may be extracted from hydraulic components through pressure rinsing, ultrasonic cleaning, agitation and functional bench-test methods. A light petroleum distillate, such as filtered mineral spirits, is the preferred extraction fluid because it is an effective cleaner that does not promote rust or interact with the filter membrane used to quantify the contamination level.
Graph 1. New component contamination levels
The ISO 4405 gravimetric procedure is the standard method for quantifying the contamination level of hydraulic components. In this test method, particles are separated from the extraction fluid through vacuum filtration. The filter medium is dried in a non-ventilating oven, and the weight change (in milligrams) is recorded. Usually the length of the largest hard particle is also determined through microscopic inspection of the filter membrane.
The Fluid Power Institute recently evaluated the contamination level of more than 100 new hydraulic components. This data set included hoses, tubes, fittings, valves, cylinders, pumps and reservoirs. As can be seen in Graph 1, the contamination level in one-third of the new components exceeded 8 mg. Abrasive dirt and debris from these components will attack the rest of the hydraulic system as soon as the machine is powered up.
The process of manufacturing components produces contamination. Cutting a hose to length makes rubber and metal particles, as shown in Figure 1. Machining a valve manifold creates cutting chips, as shown in Figure 2. Fabricating a steel reservoir generates welding spatter, as shown in Figure 3. Welding the end caps on a hydraulic cylinder produces iron-oxide, as shown in Figure 4. In all cases, post-processing is required to prepare components for use in a fluid power system.
The assembly and filling process introduces particles into a hydraulic system. Therefore, it is a common practice to verify the contamination level of new systems as they are undergoing functional tests on the assembly line. Portable online particle counters are the preferred instruments for checking the cleanliness level. Online particle counters provide a rapid means of analysis and are not susceptible to the pitfalls of bottle sampling.
Table 3. Roll-off cleanliness targets for
new hydraulic systems
The roll-off cleanliness target for a hydraulic system should be based upon contamination sensitivity and working pressure, as shown in Table 3. In some equipment, the system can be cleaned by simply cycling the actuators. If the system incorporates components that do not completely discharge or return fluid to the reservoir, it may be necessary to use an auxiliary flushing cart to achieve the desired cleanliness level.
Once a system is assembled, purified and shipped to the customer, it should remain sealed to prevent contamination. Oil changes in the first 2,000 hours of machine operation frequently do more harm than good. This is because new oils are not necessarily clean, and end users often lack the equipment required to filter fluid into the system.
Starting a machine with a clean hydraulic system is the first step in achieving long and reliable equipment life.