Fast screening test for tribological contacts in pneumatic valves
Primarily soft-seated valves are used for pneumatic switching valves, which are subject to a wear process during operation. This wear limits the service life of the valves and can currently only be determined by time-consuming and cost-intensive tests. The aim of the research project is to develop and test a test rig with which it is possible to estimate the tribological behavior of the valves over their entire service life within one month.
| Benefit | Procedure |
|---|---|
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Reduction of testing times |
Determination of reference values |
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Simpler and less expensive new development and optimization of pneumatic valves |
Development and construction of a test rig for accelerated wear testing |
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Characterization of the tribological system (friction, leakage & wear) |
Execution of tests and tribological analyses |
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Validation of the test rig |
Wear behavior of pneumatic switching valves
For the purpose of automation of large plants, primarily soft-sealed valves are used. They have elastomer molded seals (e.g. HNBR or polyurethane) either in the bushing or on the spool and thus ensure a better sealing effect.
The disadvantage is that the seals have to travel over the control edges with every switching operation, resulting in increased wear of the seal, which ultimately limits the service life of the valves. Since the relationship between pressure and friction forces is usually much less favorable in pneumatic valves than in hydraulics, the design of the tribocontact between the valve spool and the bushing is of particular importance. The quality of its design is largely responsible for dynamics, leakage and wear. Furthermore, development trends in pneumatics are also moving in the direction of cycle time reduction. With the high volumes to be handled, every millisecond of time saved per cycle has an impact on process costs.
This requires that all tribological contacts achieve as constant a low friction and leakage as possible over the service life, which also requires low wear. However, even with ideal design and operating conditions, seal life is limited. A change in the travel speed of the pneumatic slides or a change in the temperature of the system will result in a change in the behavior of the elastomer and grease. The modulus of elasticity of the slides increases with increasing travel speed or with decreasing temperature. Due to the large number of influencing parameters on the tribological seal system, the failure behavior cannot currently be calculated; rather, it can only be estimated or determined in tests. At the same time, however, a seal failure can lead to the shutdown of an entire production plant and thus to considerable costs.
The service life of pneumatic valves is defined by the B10 value. This value indicates how many switching cycles have resulted in a failure of 10% of the valves. For pneumatic switching valves, B10 values of 65 million cycles are common. Assuming a test duration of 0.5s per cycle, this corresponds to a test duration of approx. 1 year. These comparatively long test durations lead to delays in the development or optimization process of pneumatic valves and tie up test bench capacities.
Accelerated wear test for pneumatic switching valves
The currently long, necessary test times result in the need for a test methodology with which it is possible to quickly evaluate the potential of pneumatic valves and their tribology systems. It should be possible to estimate the wear behavior over the entire service life within one month, thus making the development of pneumatic valves simpler and more cost-effective. It must be ensured that the results obtained from the accelerated test execution are comparable with the reference test. This means that the conditions in the sliding contact must be comparable. On the one hand, there must be comparable kinematics and surface characteristics, but on the other hand, the factors influencing the deformation of the seal must also be comparable. This includes the preload force set, the friction that occurs and the hydrostatic pressure drop.
The test setup, which will be set up as part of the project, is intended to enable a service life estimate of pneumatic valves and their tribological behavior within one month to be made for the overall service life. During the wear tests, leakage, friction and the force required to move the spool are recorded.
The following figure shows a simplified circuit diagram of the test rig, which will be used to measure 5/2 directional control valves.
Acknowledgement
The IGF project 21028 N/1 of the Research Association Forschungskuratorium Maschinenbau e.V. - FKM, Lyoner Straße 18, 60528 Frankfurt am Main, Germany, was funded by the Federal Ministry for Economic Affairs and Climate Action via the AiF within the framework of the program for the promotion of joint industrial research and development (IGF) on the basis of a resolution of the German Bundestag.