Engineering Design Guidelines
Introduction
The fact that assembly criteria for sealing and guiding elements in fluid components tend to be seen as presenting no particular issues nowadays speaks for the advanced state which sealing technology development has achieved.
Nevertheless, to avoid engineering design errors, the necessity of adhering to certain basic principles must be pointed out.
The following examples and illustrations are based on typical application examples drawn from the design practice of the various user industries.
Even a seal optimally selected in terms of compound and geometry can only perform its intended function if the installation space and the entire assembly situation have been designed to meet its particular characteristics as well as enabling easy installation.
Basic Function of the Seal
Seals are not to perform guiding functions. For this purpose, special guiding elements are to be used,
which can absorb even major lateral forces.
Seal Assembly
Dynamic sealing lips and bodies must be capable of being installed without suffering any mechanical damage, i.e.:
--no sharp edges
--lead-in chamfers 15-20 °
--width of lead-in chamfers min. 0.5 x seal width
Sealing Surfaces
Under pressure-loaded dynamic operating conditions, seals must not be moved over gaps or borings.
Gaps
The gap dimension between moving assembly components must be kept at a minimum in order to
avoid gap extrusion of the sealing bodies (relief provided by anti-extrusion/back rings).
Protection by Wipers/Scrapers
To prevent contaminating substances from being dragged into the system by the return movement of the cylinder piston rod, resistant wipers/scrapers must be installed
Flow Turbulences
Strong flow restriction [throttling] of the operating medium in close proximity to seals must be avoided due to the danger of air release and its resulting consequences, like cavitation, diesel effect, etc.
Excess Pressure Loads
High pressure peaks and repeated excess pressure loads by drag pressure must be avoided, i.e. by inserting a spiral groove into the rod guidance
“Seal-friendly“ Surfaces (1)
Any surface coming into contact with a seal should be free from sharp, pointed machining traces. Grinding as the final finish for dynamically sealing surfaces is insufficient from the perspective of sealing technology. Grinding must always be followed by polishing.
“Seal-friendly“ Surfaces (2)
l Dynamic sealing
0.8 µm < Rt1 < 2.5 µm (Rt 2.5 µm: Ra ≈ 0.28 ... 0.6 µm, RMS ≈12.5 ... 28.3 µin)
80 % < tp1< 95 % (Rt 0.8 µm: Ra ≈ 0.28 ... 0.18 µm, RMS ≈ 3.3 ... 8.6µin)
l Static sealing
Rt2 < 6.3 µm (Ra ≈ 0.81 ...1.59 µm, RMS ≈ 35.6 ... 76.3 µin)
*tp2 > 60 %
l Non-sealing surfaces in installation spaces and lead-in chamfers
Rt3 < 15 µm (Ra ≈ 2.2 ... 4.0 µm, RMS ≈ 97 ... 194 µin)
Rt4 < 10 µm (Ra ≈ 1.4 ... 2.6 µm, RMS ≈ 62 ... 125 µin)
Concentricity
Attention must be paid to geometric concentricity of the assembly components/installation spaces.
