5 months ago

MDA Technologies 2/2015

MDA Technologies 2/2015

CASE STUDY 03 Finite

CASE STUDY 03 Finite Element Analysis of the pressure relief function (check valve principle) on a BU rod seal. The graph shows the distribution of radial stress levels to improve the wiper function in mobile machines features the following characteristics (Figure 02): n The dirt shield and dirt sealing edge along the groove side prevent ingress of dirt and moisture, also in vertical cylinder applications; n Pressure relief function with an opening toward the cylinder rod space; n Leakage oil remains in the cylinder, and in the case of a corresponding pressure gradient between the wiper and the atmosphere flows outward and is evenly distributed on the piston rod; n The interference fit in the groove guarantees a stable position. The wiper cannot be pushed out of the housing; n Robust wiper lip with optimized geometry. The standard material, P5008 polyurethane, exhibits good wear resistance and low compression set. The sealing material can be changed to the hydrolysis-resistant P5000 polyurethane compound, depending on the field of application. Compact rod seal as a primary seal The BU profile Ultrathan rod seal is a compact seal with an integrated anti-extrusion ring. It is used as a buffer seal within this sealing system. Cylinders such as those utilized in mini excavators frequently operate under conditions in which pressure peaks up to 1,000 bar (100 MPa) are not uncommon. The BU rod seal is installed in front of it to isolate the load from the secondary seal. Due to its special shape the seal is able to return the pressure that can build up between the primary and the secondary seal under certain conditions to the hydraulic working system like a check valve (Figure 03). This prevents damage to the secondary seal due to excessive intermediate pressures and the resultant reduction of service life. The BU rod seal features the following characteristics: n Excellent sealing performance, also in non-pressurized conditions ; n Robust seal profile for harshest operating conditions; n Extreme wear resistance; n Insensitivity to extreme pressure peaks; n Maximum extrusion resistance; n Short axial installation length. The results of the Finite Element Analysis show that, due to its special design, the primary seal is able to return the intermediate pressure that has been generated between the primary and the secondary seal to the system like a check valve. As shown in this calculation, the dynamic sealing lip of the BU seal opens at a pressure of approx. 10 bar (1 MPa) or higher, thus preventing a pressure buildup between the primary and the secondary seal. However, there is no absolute requirement for the BU rod seal to be used as a primary or ‘buffer seal’ in a tandem sealing system. The BU seal is also frequently utilized as a single seal, for instance in the case of significantly varying pressure parameters. Parker recommends the P6000 polyurethane compound to satisfy the demanding operating conditions and to guarantee optimal dimensional stability and wear resistance. The hydrolysisresistant P5000 polyurethane compound can be utilized as well, depending on the field of application. The anti-extrusion ring made of glass fiber-reinforced W5059 polyamide additionally protects against extrusion in the case of large gap dimensions or pressure peaks. Low-friction secondary rod seal A friction-minimized seal profile such as the HL rod seal is suitable as a secondary seal for tandem configurations. The ‘secondary seal’ typically operates as a nonpressurized seal. The HL rod seal offers the major advantage of hardly generating any additional friction due to its exceptional design even though it essentially operates without pressure. Due to the cascading dynamic sealing lips the seal is automatically controlled by the system pressure. When pressure increases, the individual sealing lips consecutively latch on to the mating surface. As can be seen in Figure 04, the HL rod seal, in non-pressurized conditions, latches on to the mating surface only with the ‘tip’ of the first sealing lip, thus ensuring optimal ease of movement. In the event that the residual oil film should increase during the course 50 MDA Technologies 2/2015

CASE STUDY of seal life due to damage or wear the HL seal intervenes and adjusts to the new working conditions. In a nutshell: friction is reduced to a minimum and optimum ease of movement is ensured not only in non-pressurized conditions but in all pressure ranges as well. Whether it is utilized as a tandem seal or a single sealing element, the HL seal, due to its properties, supports minimized breakaway friction while delivering good sealing performance in all pressure ranges. Product features of the HL rod seal: n Extremely low friction; n No stick-slip, even in the case of low surface speeds; n Optimal sealing function; n High wear resistance; n High extrusion resistance; n Increased temperature resistance; n Easy assembly; n Short housing. The advantages of the geometry are systematically supported by the utilization of the P6030 seal compound. The material that has been specifically designed for fluid power applications delivers good media resistance and, among other things, exhibits increased temperature and wear resistance, as well as low compression set. Robust piston seal The OK piston sealing set has been primarily developed for heavy-duty hydraulics and is preferably utilized in double-acting cylinders. The design of the OK piston sealing set and profile properties superbly fit the requirements and operating conditions in mobile hydraulics. As a result, this sealing set, in addition to its extreme wear resistance, exhibits further positive properties: n Insensitivity against extreme pressure peaks; n Maximum extrusion resistance in the case of high pressures and large gaps; n Easy installation, without tools, on single-piece pistons due to split design of the sealing ring; n Dimensions according to ISO 7425-1; n Short axial installation length. The sealing ring of the OK piston sealing set consists of the filled modified W5019 thermoplastic compound that is characterized by high extrusion resistance, even in the case of high pressures and large gaps. Reliably centric guidance In mobile hydraulics, Parker-Prädifa prefers guide rings based on duroplastic synthetic resins with fabric reinforcements to keep the piston rod in a centric position even in heavy-duty operating conditions. This corresponds to the FR and FK guide rings using the Q5038 compound. The low guidance lash, due to minimal manufacturing tolerances, and the higher permissible surface compression of the guide materials ensure reliably centric guidance of the piston rod in the application. As a result, neither the surface of the piston rod nor the sealing elements are affected. The FK piston guide rings are preloaded on the outer diameter and the FR rod guide rings on the inner diameter for optimum functionality (Figure 05). Compared with thermoplastics and other guiding tape materials, duroplastic synthetic resins exhibit the following advantages: n Very high pressure resistance; n Significantly higher permissible surface compression; n Extremely high wear resistance; n Improved anti-frictional properties; n High resistance against aggressive media; n Easy snap assembly on singlepiece pistons and in closed and undercut grooves. Conclusion The Parker tandem sealing system has proven its viability in diverse applications and ambient conditions. Due to the continuous further development of the seal design and sealing materials the sealing systems are continually modified to meet the user’s requirements. In addition to the seal design, the selection of the materials satisfies the demands made by the relevant operating and application conditions, legal requirements for pressure fluids in certain regions, as well as special climatic conditions. Photographs: Parker-Prädifa About 05 Illustration of the preloading force for the FR (on inner diameter) and FK (on outer diameter) guide rings Company name: Parker Hannifin Division: Praedifa Sealing Established: 1918 (Group) Headquarters: Cleveland, OH, USA (Group) Turnover: $ 13 bn (Group) Employees: 57,500 worldwide (Group) Products: Seals and rings THIS ARTICLE IS A CONTRIBUTION BY 04 The Finite Element simulation shows the operating principle of the HL rod seal in various pressure ranges MDA Technologies 2/2015 51


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