It is desirable to assemble any component or system with clean parts in a controlled manufacturing environment. However, this may not always be possible. It is sometimes necessary for the entire hydraulic system to undergo a cleanup process after final assembly to reach the desired roll-off cleanliness level. This article provides a theoretical calculation of appropriate flushing requirements. Adherence to established roll-off cleanliness levels will provide the OEM with a better product and fewer warranty claims.
Experts estimate that 75 percent of hydraulic component and system failures are caused by contamination. Contamination causes premature wear and lost efficiency which can result in catastrophic failure. Typically, sources of contamination can be characterized as:
Inadvertent contamination left in the system or a component during initial assembly or a system rebuild. Examples include weld splatter and cleaning rag fibers.
Contaminants internally generated during system operation, or caused by wear, corrosion, agitation, oxidation or fluid degradation.
Externally introduced contamination that enters a system from various openings such as breathers, worn cylinder wipers, improperly sealed access covers, etc.
This article discusses built-in contamination, specifically particulate contaminants, and how to clean up the system following final assembly. Typically, particulate contaminants include weld splatter, dust, fibers, paint chips and other undesirable and potentially abrasive particles. Many of these particles are below the human visual threshold of 40 microns. Although they cannot be seen, they can be damaging to a system.
The main purpose of roll-off cleanliness is to minimize damage to the various system components in their infancy. To underscore the importance of establishing roll-off cleanliness standards, the International Organization for Standardization (ISO) is developing new standards outlining the cleaning of components and systems. One draft standard, ISO/WD 16431, describes “roll-off cleanliness of an assembled hydraulic system upon release from the production area.” This title may change as the document is finalized, but it is obvious that the target is to provide the cleanest possible equipment to the customer.
Figure 1. Hydraulic System Lines, Fittings
and Components That Come in Contact with Fluid
There are many ways to clean a system, and it is up to the manufacturing group of a company to decide which method(s) to use. The ultimate goal is to reach the desired cleanliness level at the most reasonable cost and minimum time interval. Some methods of achieving this are:
Figure 2. Off-line Filtration System
It is not economically feasible to remove all contaminants from a system. Most systems operate trouble-free with a small amount of contamination present. The amount of contamination that can be tolerated in a system depends upon the sensitivity of the most critical component. System reliability continues to improve, however, as ideal conditions are reached. Diminishing returns on increasing effort is the limiting consideration. This threshold for the contamination level is established by the component manufacturer and ultimately by the system builder.
The size and type of filter used are important in making calculations for cleaning a system. The analysis presented here makes use of the following assumptions:
Real applications will vary from this idealization to some degree, but the variation is not expected to significantly affect the results.
Figure 3. Off-line Filter and Power Source
to Cycle the System
The classic filtration equation that applies to roll-off cleanliness is:
Simplifying the filtration equation using the assumptions above:
No = initial particle count @ x micron per ml
Nd = downstream particle count @ x micron per ml
V = system volume in gallons
Q = flow rate through filter in gpm
b = Beta ratio
R = rate of ingression
t = time in minutes
These equations can be further simplified by assuming no ingression. By substituting R = 0:
Solving this equation for 5 m and 15 m particle sizes and using Beta ratios at these sizes, the downstream count in a system can be obtained. This can be related to the ISO 4406 contamination code. The same iteration can be used for 4 mm(c), 6 mm(c) and 14 mm(c) size particles to obtain the ISO Code per the ISO 4406-19991 standard. It is important to use the proper Beta ratios, which correspond to old or new particle size references. ISO 4572 (defunct) and 168892 provide multipass testing standards.
Calculations can be complex and time consuming. Some programs will return theoretical particle counts at 4 mm(c), 6 mm(c) and 14 mm(c) sizes and the ISO 4406-1999 contamination codes after certain key parameters are entered. Run the system and take periodic samples during roll-off. When specified cleanliness levels are reached, the equipment may then be certified for shipment.
This subject becomes complicated when additional dirt is introduced to the system during cleanup. The advanced mathematical models used for further analysis are beyond the scope of this article. Such models have been developed by other sources and may be used to solve complex systems.
Proper roll-off cleanliness procedures protect equipment in its infancy and provide for fewer warranty claims. The end-customer is provided with a high-quality system with clean components that meet his initial use needs. Roll-off cleaning however, is only the starting point for trouble-free system operation. The final responsibility in controlling contamination lies with the user. Users must maintain proper filtration and practice responsible contamination control in the system to keep the hydraulic fluid clean.
Note: Author recommends consulting reference nos. 4 and 5 for information on how to clean various hydraulic systems.