Parker O-Ring Handbook.pdf

Dynamic O-Ring Sealing
5-4
a) Dimensional Deviations
b) Surface Undulations (Waves)
c) Roughness
d) Superimposition (with a or b)
Figure 5-3: Surface Finish Structure
This is given in Figure 5-4 by the roughest profi le Z4. In this
case Z4 = Rmax does not include extreme roughness peaks as
is the case of Rt.
The medium roughness value Ra is an arithmetic mean of all
components of the roughness trace within the trace length
lm. The average roughness value Rz of fi ve consecutive trace
lengths often is preferred to Re.
If Ra is known, Rz can be taken from Figure 5-5 and vice
versa. Figure 5-5 is taken from DIN 4768, part 1, attachment
1. Should Rz reach the upper portion of the graph, it can be
assumed that the specifi ed Ra values will not be exceeded.
The lower limits would be taken if an Rz value should be
specifi ed.
Finally, the depth of roughness Rp also is of interest and is the
vertical distance between the highest point on the roughness
trace and the center line of that trace.
Values for Rt are of very little assistance in reaching a conclusion
regarding the suitability of a surface roughness from
the sealing point of view. Table 5-1 shows that for a similar
Rt all levels of roughness can be produced. Ra values are
unsuitable for comparison because profi les 6 and 7 have the
same Ra value. Rp values without reference to the load area
tp also gives a false impression of roughness.
A static sealing surface Rt ≤ 6.3 µm (VVV roughness DIN
3141) is rougher than the dynamic surface requirements. Seal
manufacturers recommend a roughness Rt ≤ 2.5 µm for a
dynamic sealing surface (Ra = 0.25 to 0.5 mm) (VVV roughness
DIN 3141) when the load area is over 50%, or when the
surface fi nish roughness Rp is under 50%. These limitations
often are overlooked, nevertheless the connection between
surface fi nish and load area is very important because an
“open” profi le can have sharp edges (e.g., profi les 2 through
6 in Table 5-2). These open profi les 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.
Parker O-Ring Handbook
a
b
c
d
R-Value in μ inch
a
2000
1000
500
250
125
63
32
16
8
4
2
1
R Value in μ m
a
R t = Vertical Distance Between
Highest and Lowest Point
R and R
max z
R p = Depth of Roughness
R = Middle Roughness Value
a
50.000
31.500
20.000
12.500
8.000
5.000
3.150
2.000
1.250
0.800
0.500
0.315
0.200
0.125
0.080
0.050
0.032
0.020
Z 1
e
R a
Figure 5-4: Roughness Terminology
R a
R a
Relationship Between R and R
a z
Rz
Upper limit
for Rz when
transposing
from R to R
a z
Deviation
Upper limit
for Ra when
transposing
from R to R
z a
0.16 0.40 1.0 2.5 6.3 16
0.25 0.63 1.6 4.0 10
RValue z inμm 40
25
100 250
63 160
Figure 5-5: Relationship Between Ra and Rz
Z 2
R t
l m
Z 3
l =5xl
m e
Z = R
4 max
R = 1
z (Z 1 +Z 2 +Z 3 +Z 4 +Z) 5
5
l m
R p
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
R z
Z 5