DESIGN NOTES
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The design notes as a PDF download available here
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CONSTRUCTION INFORMATION FOR CYLINDERS AND PISTONS |
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We recommend to make use of the existing international (ISO) and national
(DIN) standards for new constructions and their recommendations
about mounting spaces for seals. The mounting spaces for seals are also
indicated in the data sheet of the relevant type of profile even if special
mounting spaces (e.g. special seals, valve seals, rotary seals) are required.
The indicated surface qualities and lead-in chamfers have been proved
as far as possible. These guidelines can be found in the standards for
this reason. The indicated sizes, tolerances and surface qualities have to
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be necessarily observed. A simple installation and observance of the indicated
maximum operating conditions is only possible by following the guidelines.
Moreover damages of the seal respectively consequential mounting
damages can be avoided. The indicated proposals are based on our
experiences for many years and serve as an orientation for users. Due to
the interactively influencing factors, these proposals only have restricted
validity for the individual case.
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1. ONE-PIECE/MULTI-PIECE PISTON |
Image 1: One-piece piston within a cylinder |
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The assembly of seals is more difficult when using a one-piece piston. It
has to be preferred from the economic point of view because of the lower
manufacturing costs. Nevertheless it is useful and in some cases indispensable to manufacture a multi-piece piston. The idea behind this is the
easier assembly of the seal. For large piston diameters the expansion of a
seal has little effect on its original condition than on seals with small diameters. The thickness recovery to its original condition is no longer given
after assembling on small piston diameters. An expansion could probably
lead to a direct and complete uselessness of the seal. We recommend
multi-piece pistons when working with smaller diameters than 30 mm and
PTFE face ring seals.
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2. SURFACES |
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Special attention should be paid to the surface, because it is decisive for
the service life of a seal. It is not sufficient for dynamic sealing surfaces
when grinding is the last work process. The surface quality and bearing
ratio that can be achieved here are insufficient. A polishing process
should follow at all hazards.
The following surface qualities and accuracies have to be complied with:
Dynamic Sealing Surfaces:
0,8 μm ≤ Rt1 ≤ 2,5μm (Rt 2,5 μm ≜ Ra ≅ 0,28 ... 0,6 μm, RMS ≅ 12,5 ...28,3 μin)
For rubber and PTFE seals:
80 % ≤ tp1* ≤ 95 % (Rt 0,8 μm ≜ Ra ≅ 0,28 ... 0,18 μm, RMS ≅ 3,3 ... 8,6 μin)
For polyurethane seals:
60 % ≤ tp1* ≤ 80 % (Rt 0,8 μm ≜ Ra ≅ 0,28 ... 0,18 μm, RMS ≅ 3,3 ... 8,6 μin)
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Static Sealing Surfaces:
Rt2 ≤ 6,3 μm (Ra ≅ 0,81 ...1,59 μm, RMS ≅ 35,6 ...76,3 μin)
tp2* ≥ 60 %
Non-sealing Surfaces in Mounting 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)
* Measured at a cutting depth of 25% of the Rt-value based on an imaginary
zero line where the bearing ratio is 5%.
Rt = maximum roughness depth
tp = bearing ratio
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3. RADII
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You will find the required radii in and around the groove in the respective
profile data sheet or the current standards. Radii are characterised by the
big letter R and a digit (e.g. R 1). The digit always provides information
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about the circle radius with the unit millimeter. If the digit is subscript (e.g.
R1), this only provides information about the designation. It follows that a
declaration must be found where this radius is clearly defined.
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Image 2: Design of a groove including radii sizes without installed seals
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Image 3: Design of a groove including installed seals (wiper and rod seal) and radii declaration
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4. LEAD-IN CHAMFERS
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Lead-in chamfers are used to install machine components easily and
seals non-destructively. This applies primarily for the seal because they
are the most sensitive components within a cylinder. Lead-in chamfers
also avoid damages on the piston what leads to scoring on the cylindrical
wall and as a consequence to damages of the seals. For this reason all
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new construction has to be designed with lead-in chamfers. Up to a cylinder
diameter of 230 mm a lead-in chamfer should be equipped as presented
in image 4 and attached detailed close-up “A”. Please pay special
attention to the absence of burrs and roundness of edges. Specifications
for lead-in chamfers are indicated in the following table:
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Image 4: Detailed close-up: A |
Dimensional recommendations for lead-in chamfers |
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5. PERMITTED GAP SIZE
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The gap size or sealing gap „e“ describes the gap between the machinery
components cylindrical bore and piston or rather housing wall and rod
that can be different depending on the application. By pressurizing, a
migration or extrusion of this seal can occur on the pressure averted side.
The utilization of the maximum tolerances and eccentricity between the
machinery components are taken into account when making reference to
the gap size.
At high pressure the gap size has to be smaller, because not following the
recommendations can lead to “gap extrusion” and in consequence to a
shearing of the seal. This is a common damage symptom when using elastomer seals without back-up rings.
The image illustrates the locations of the gap size, pressure direction and the possible gap extrusion at the pressure averted side.
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Schematic illustration of a piston seal when pressurizing including gap size „e“ and the location of a possible gap extrusion as a result of high
pressure
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A nomogram which can be found on the following pages assists to determine the ideal gap size for polyurethane (PUR) respectively laminated fabric seals
[hardness range ≥ 85 Shore A] as well as elastomer seals made of NBR, HNBR and FKM [hardness range 70 up to 85 Shore A].
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EXPLANATION FOR THE USE OF THE NOMOGRAM
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Please find out the operating conditions such as maximum operating pressure, maximum operating temperature, dynamic sealing diameter, the material and the radial height respectively profile width of the seal first. The
dynamic sealing diameter is the size on which the seal does its work. In
contrast to this size there is the static diameter at the groove base. The
dynamic diameter when operating with a piston seal is the outer diameter.
It is the inner diameter when operating with a rod seal. The determination
of the gap sizes takes the most extreme conditions (shore up way of movement & softest material) into account. If a shore up way of movement prevails, the gap size can be enlarged up to 25 %. The gap size can be enlarged with additional 15 %, if a material is used that is 15 Shore A harder.
Please interconnect the d/D-axis and S-axis and extend this line until there
is a intersection point with the Y1-line. Also proceed like this with the P- and
T-axis and extend the line until there is an intersection point with the Y2-line.
Afterwards interconnect these two intersection points and read off the
value for the gap size on the e-line.
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[Requirement: The use of the determined gap size “e” requires finishing the
surfaces according to the construction recommendations (see “surfaces”)
and usage of a lubricating fluid. In the event of special operating conditions,
e.g. non-lubricating fluids such as water, acids or bases please get in contact
with our applications engineering]
Example for a seal made of PUR/laminated fabric (≥ 85 Shore A)
→ It results in a gap size of around 0,26 mm Beispiel für eine Dichtung aus NBR/HNBR/FKM (≥ 70 Shore A):
→ It results in a gap size of around 0,225 mm |
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MAXIMUM GAP ALLOWANCE „E“
For PUR- and laminated fabric seals (≥ 85 Shore A)
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MAXIMUM GAP ALLOWANCE „E“
For NBR, HNBR and FKM seals between 70 up to 85 Shore A
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