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| Double Viper PT1 | Novathon U-Rings T 20 | T 20 Installation Tool |



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The MERKEL PT1 double
wiper consists of a PTFE
profile ring with a sealing
edge and a wiper edge and
two O-rings as the elastic
preloading elements. The
profile ring is manufactured
from the MERKEL special compound MT28 (PTFE/
bronze) which is characte-
rised by particularly good pressure and abrasion resistance. The double wiper can be combined with NBR
or FPM O-rings to suit the medium and temperature.
The sealing edge formed on
the PTFE profile ring in the
direction of the pressure
space reliably wipes off any
adhering residual oil film on
the out stroke of the piston
rod. The residual oil volume
is transported back into the
pressure space during the
return stroke when a seal
with good return capability
(e.g. OMS-MR) is used.
If the main seal is defective,
the PT1 double wiper can be
subjected to the full operating pressure for a short period. There is therefore no need for
a pressure hole.
(The dimension "n" should be designed to suit the maximum operating pressure and the material of the cylinder head.)
The wiping edge on the atmospheric pressure sides wipes off even firmly adhering dirt on the in stroke of the piston rod. The PT1 double wiper therefore offers reliable protection against dirt in the hydraulic system.
Both the static seal at the bottom of the groove and the even distribution of the pres-
sure forces on the sealing
and wiper edges are ensured
by the use of two O-rings. These two elastic preloading elements therefore provide specific support for the functions of the sealing edge and the wiper edge.
The double wipers of the PT1 series with internal diameters
> 30 mm can be fitted without auxiliary tools into housings which are not axially acces-
sible. For installation purpo-
ses, the O-rings arefirst introduced into the groove;
the profile ring is then defor-
med into a kidney shape and sprung in. It is necessary to ensure that no sharp kinks appear on the profile ring.
When fitting the profile ring,
it is also necessary to ensure the correct arrangement of the sealing edge relative to the pressure direction.
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Application Limits
Material : PTFE/bronze - NBR
Media : hydraulic oil
Temperature (C) -30...+120
Sliding speed (m/s) 5
Application Limits
Material : PTFE/bronze - FPM
Media : hydraulic oil, HFD fire-resistant fluids
Temperature (C) -10...+200
Sliding speed (m/s) 5
Installation chamfers
Nominal d a
           <   50 3
>   50..< 120 4
>120...< 200 6
>200...   1000 8
Surface quality
Roughness depths Ra Rt
Bottom of
the groove
< 1,8 m < 10  m
Sides of
the groove
< 3   m < 16  m
The surface roughness of the running surface (piston rod) has to be appropriate to the sealing element used.




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U-ring made of polyurethane material, particularly resistant to wear and extrusion.

Application Limits
Media : Hydraulic oil, water, emulsions
Pressure (bar) 400
Temperature (C)
Hydraulic oil
HFA, HFB fluids
HFC fluids
- 40...+110
+ 5...+ 50
+ 5...+ 40
Sliding speed (m/s) 0,5
Lead-in chamfers
Cross section B a
<    4 2,0
>    4...5 2,5
>    5...7,5 4,0
>   7,5...10 5,0
>   10...12,5 6,5
> 12,5...15 7,5
Surface finish
Ra a
< 0,3 m < 3 m
Surface of
housing o.D.
< 1,8 m < 10 m
Sides of
< 3    m < 16 m
Clearance fits
Diameter d Rt
<   80 H 9 / f 8
>   80...120 H 8 / f 8
> 120...200 H 8 / f 7
> 200 H 7 / f 7

A tolerance of H11 for the outside diameter D of the seal housing is recommended if the rod is not positively guided by the cylinder end cap or gland arrangement.

Max. extrusion gap
Pressure (bar) s max.
< 100 0,5 mm
< 250 0,35 mm
< 400 0,25 mm
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Series T 20 U-rings for sealing rods and rams are made of NOVATHAN material. This material is a polyurethane which is particularly resistant to abrasion and extrusion.
Further outstanding features are :
  • good thermal stability
  • good resistance to different media and
  • good elasticity
of this material as well as the low compression set of only 26% after 24 h at 70 C.
The U-rings are asymmetric in section with a shortened inner lip. To achieve an initial sealing force, the U-rings are produced with lips that are wider than the nomnal section. During operation the system pressure is superimposed on this initial pre-load, thereby increasing it. The lips are designed for optimum sealing in all pressure ranges as well as in a "zero pressure" condition.




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Series T20 U-rings can be snapped
into groove-type housings with the installation tool as illustrated.

We can supply design drawings for
this installation tool on request. Only two different tool sizes are required
for the dimension range 25-115 mm. Refer to the design documentation
for further details.

The photos illustrate the simple handling of the installation tool.
  1. The arms of the tool are
    opened out. The U-ring is
    then placed between the
    three pins.
  2. The U-ring is deformed into
    a kidney-shape by simply closing the installation tool.
  3. The U-ring deformed in this manner can then be easily installed; position the U-ring
    into the seal groove and
    remove the tool.
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