Parker O-Ring Handbook.pdf

When a cylinder rod extends out into a dirty environment
where it can pick up dirt, lint, metal chips, etc., this foreign
material can nullify the effect of the best lubricant and cause
rapid abrasive wear of both the O-ring and the rod. Equipment
exposed to such conditions should be fi tted with a
wiper/scraper ring to prevent the dirt from reaching the O-ring
seal. It is also good practice to install a felt ring between the
scraper and the seal to insure proper lubrication of the rod on
its return stroke. Figure 5-2 illustrates this concept.
A felt ring may cause corrosion in some installations, as felt
also tends to collect moisture. A second O-ring may be used
for the wiper, but it must not actually seal because a pressure
trap condition is likely to develop between two reciprocating
Cut Wiper O-ring
in Two to Prevent
Pressure Trap
Cylinder End Cap
Enlarged View
of Felt Wiper
Gland with Felt
Installed in Groove
in Squeeze Condition
3/18
Ref.
Alternate Design One
Hole in Top of Cylinder
Parker O-Ring Handbook
1/8 Ref.
Felt
Ring
Prefered Design
Oiling Hole
Direct to Felt
Snap Ring
Washer
Scraper
Felt Wiper
O-ring Packing
Piston Rod
Bearing
Figure 5-2: Lubrication Between Scraper and Seal Rings
Problem: To design a wiper gland for a 25.4 mm (1.000 in.) OD piston rod.
Procedural Steps: Example:
O-ring seals. This can be prevented by cutting the outer
O-ring so it cannot seal. Since this can easily be forgotten,
it is preferable to provide a vent hole between the two O-rings.
It should vent downward so it will not become clogged with
dirt. The sample problem provided in Table 5-1 explains how
to design the gland for such an O-ring wiper.
5.3 Surface Finishes
Finishes of contact surfaces have much to do with the life
of dynamic O-ring seals. Limits of maximum roughness for
glands are given on the drawings accompanying the design
charts in this section and represent accepted practice for
military and industrial use. Surface roughness values less
than 5 micro-inches are not recommended for dynamic seals,
however, as an extending rod will be wiped completely dry
and will not be lubricated when it retracts. The surface must
be rough enough to hold small amounts of oil. Ideally, a
microscopic “orange peel” type of surface is best, presenting
smooth rounded surfaces for the O-ring to slide on, with
small crevices between to act as oil reservoirs. This kind of
surface may be approximated by peening the rod with metal
shot or glass beads. An even better surface can be obtained
by electropolishing. The most desirable surface roughness
value is from 10 to 20 micro-inches.
The roughness of a surface as measured comprises several
elements which can be handled separately according to
DIN 4760:
Level 1 — dimensional deviations within
tolerance band
Level 2 — surface undulations (waves)
Levels 3 to 5 — range of roughness
All these deviations from the ideal fi nish are superimposed
as measurements are carried out and represent the surface
roughness (see Figure 5-3).
Surface fi nish is often quantifi ed in terms of Rt and Ra (see
Figure 5-4). Rt is the vertical distance between the highest and
the lowest peaks in a roughness profi le over a test length lm.
Rt is increasingly being replaced by the maximum depth of
roughness, Rmax. Rmax is the greatest single roughness found
in fi ve consecutive single trace lengths lm.
(A) Select O-ring with actual ID slightly smaller than Rod OD, B. (A) Parker No. 2-214 (ID = 0.984)
(B) Divide the actual minimum squeeze given in Design Chart A6-5 for
this O-ring size by two (the same squeeze is permissible in most cases).
(B) Squeeze 0.012/2 = 0.006
(C) Add this amount to both max. and min. gland depth, L, given in
Design Chart A6-5 to get proper gland depth for wiper, LW. (D) Calculate balance of gland dimensions same as for piston rod seal.
Table 5-1: Wiper Gland Design Example
(C) L W min = 0.121 + .006 = 0.127
L W max = 0.123 + .006 = 0.129
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
5-3
Dynamic O-Ring Sealing