Parker O-Ring Handbuch Seite 119

6. Sealing Terminology
Surface finish often is quantified in terms of R
6.14, Roughness terminology). R
the highest and the lowest peaks in a roughness profile over a
test length l . R is increasing, being replaced by R
m t
mum depth of roughness. Rmax is the greatest single roughness
found in 5 consecutive single trace lengths l
6.14 by the roughest profile Z
include extreme roughness peaks as is the case of R
The middle roughness value R
ponents of the roughness trace within the trace length l
average roughness value of 5 consecutive trace lengths l
ten preferred to R .
a
If R is known, R can be taken from Fig 6.15 and vice versa. Fig
a z
6.15 is taken from DIN 4768, part 1, attachment 1. Should R
reach the upper portion of the graph it can be assumed that the
specified R values will not be exceeded. The lower limits would
a
be taken if a R value should be specified.
z
Finally, the depth of roughness R
vertical distance between the highest point on the roughness
trace and the center line of that trace.
R = vertical distance between highest
t
and lowest point
and
depht of roughness
middle roughness value
Fig. 6.14 Roughness terminology
and R (see Fig.
t a
is the vertical distance between
t
, the maxi-
max
. This is given in Fig
e
. In this case Z = R does not
4 4 max
.
t
is an arithmetic mean of all com-
a
. R
m
e
also is of interest and is the
p
, he
z
is of-
z
Table 6.6 Diagramatic representation of surface profiles
Fig. 6.15 Relationship between R
Values for R are of very little assistance in reaching a conclu-
t
sion regarding the suitability of a surface roughness from the
sealing point of view. Table 6.6 shows that for a similar R
levels of roughness can be produced. Ra values are unsuit-
able for comparison because profiles 6 and 7 have the same
R value. R values without reference to the load area tp also
a p
gives a false impression of roughness. A static sealing surface
R ≤6.3 µm (old:
t
DIN IS0 1302) is rougher than the dynamic surface requirements.
Seal manufacturers recommend a roughness R
dynamic sealing surface (R
ness DIN 3141; new: √R
load area is over 50%, or when the surface finish roughness R
is under 50%. These limitations often are overlooked, neverthe-
less the connection between surface finish and load area is very
important because an "open" profile can have sharp edges (e.g.
profiles 2 to 6 in Table 6.6). These open profiles are a product
of cutting processes such as turning or grinding. A much larger
load area is produced by cold forming processes such as rolling,
drawing or sinking.
119
R R R t (%)
t p a p
µm µm µm 0.25
1 0.5 0.5 50
1 0.5 0.25 25
1 0.5 0.25 25
1 0.75 0.28 12.5
1 0.25 0.28 62.5
1 0.785 0.188 3.5
1 0.215 0.188 65
1 0.5 0.39 43
Upper limit for
Devia-
R when
z
tion
transposing
from R to R
a z
Upper limit for
R when
a
transposing
from R to R
z a
R -value in µm
z
and R
a z
roughness DIN 3141; new: √R
= 0.25 to 0.5 µm ) (old:
a
2,5 roughness DIN ISO 1302) when the
t
O-Ring Handbook
Parker Hannifin GmbH
O-Ring Division Europe
0.50 0.75 R
t
50 50
50 75
50 75
25 37.5
75 87.5
14 35
86 96.5
50 57
all
t
6,3 roughness
t
≤ 2.5 µm for a
t
rough-
p