Click here to read the blog “What is Stiction?”
Stiction is a phenomenon that is beginning to become an accepted theory among engineers. It is no longer a question of is this happening, but how does it happen and when does it start to affect me? Different experts find this in varied ways, but are all honing in on the same time frame.
Mechanical engineers and technicians who routinely work with solenoid valves are familiar with the experience of trying to “stroke”, i.e., move through its range of motion, a valve that has been stationary for an extended time. They report that it is not uncommon for extra force to be required to move the valve after it has been stationary for a month or more. However, they are unable to say when within the month the increased stiction reaches a critical level that causes the valve to stick because it cannot generate sufficient force to overcome the increased stiction.
O-ring Handbooks note that the “coefficient of starting friction”, i.e., stiction, increases when the O-ring has been stationary for between 1 week and 1 month, after which time the stiction plateaus. One specific graph (Figure 5-7 in [1]) of “break out friction”, i.e., stiction, vs. “delay between cycles” shows the stiction force plateauing at approximately 300 hours. “The theory has been proposed and generally accepted that the increase in friction on standing is caused by the rubber O-ring.”
Experimental studies in similar fields have introduced the concept of equilibrium rest time, i.e., the time by which the maximum stiction is reached due to lack of motion. They found that a maximum stiction is reached by approximately 275 hours or about 11.6 days [2].
In a declaration [3], regarding the reliability indicators and information for use of a specific solenoid actuated pneumatic valve with respect to the safety standard EN ISO 13849-1 [4], it is specifically stated that the “valve must be operated at least once per week or once per shift to insure the intended function.” The data supporting this declaration was obtained by cycle testing during which the solenoid valve was never at rest for any significant time. This statement suggests recognition that the results of cycle testing are valid only if periodic valve movement is maintained. In the absence of such periodic movement, the failure rates derived from the results of cycle testing [5] cannot be considered valid when applied to components in applications where these components spend significant times at rest.
From this multi-source evidence, we can conclude that, as a rule of thumb, O-ring stiction increases with valve inactivity and reaches a maximum value at approximately 275 hours. Further, stiction levels can be returned to nominal or near nominal levels if the increased stiction can be overcome by valve motion. This suggests that increased stiction can be dealt with in a more cost effective way than proof testing and that solenoid valve reliability and safety performance can be significantly improved.
Stay tuned for more entries on stiction. I will also be presenting a white paper at GCPS/AIChE conference on 5/1/13 in San Antonio, TX.
[1] Parker Hannifin Corporation, Parker O-ring Handbook: ORD 5700, 2007.
[2] Z. Zhao and B. Bhushan, Effect of Lubrication Thickness and Viscosity and Rest Time on Long-Term Stiction in Magnetic Thin-Film Rigid Disks, IEEE Transactions on Magnetics, Vol 34. No 4, July 1998.
[3] Rexroth Bosch Group, Declaration: Reliability indicators and informations for use with respect to the utilization of EN ISO 13849-1:2008-12, 2009.
[4] ISO 13849-1 (2009), Safety-related parts of control systems—Part 1: General principles for design, Geneva: International Standards Organization.
[5] TÜV Rheinland Report No. V372 2010 S1, TÜV Rheinland Energie and Umwelt GmbH, 2011
Tagged as: stiction solenoid valve reliability Loren Stewart gcpsaiche en iso 13849-1