Service guide
PAHT / PAHT G pumps
PAHT 256-308 / PAHT G 256-308
Disassembling and assembling
hpp.danfoss.com
Service guide | Disassembling and assembling PAHT and PAHT G 256-308
Table of Contents 1. Introduction .............................................................................................................................................................. 2
2. Disassembling the pump ..................................................................................................................................... 3
3. Assembling the pump .......................................................................................................................................... 6
4. Disassembling and assembling of the swash plate ..................................................................................10
5. Disassembling and assembling of cylinder barrel and valve plate .....................................................11
6. Changing pistons ..................................................................................................................................................12
6.2. Assembling ..............................................................................................................................................................15
7. When should the pistons be replaced ...........................................................................................................17
8. Spare parts list for PAHT 256-308 .................................................................................................................... 18
8.1 Exploded view PAHT 256-308 ........................................................................................................................... 19
9. Spare parts list for PAHT G 256-308 ................................................................................................................20
9.1 Exploded view PAHT G 256-308 .......................................................................................................................21
1. Introduction
This document covers the instructions for disassembling and assembling the axial piston pumps
PAHT and PAHT G 256-308.
Important: It is essential that the pump is
Tools provided with toolset 180B4259
10 mm combination wrench
• 13 mm combination wrench
• 6 mm allen key
• Adjustable pin wrench
• Stop for retainer plate
• Press bush for valve plate
• Shaft seal extractor Ø45
serviced in conditions of absolute cleanliness.
Place the pump on a pallet or other stable
surface above the ground. Ensure that the pump
cannot roll. It must be possible to place the pump
vertically with the shaft pointing downwards. This
can be done between two pallets or between
two boards on a pallet provided that the distance
is minimum 50 mm.
• M8 allen screw (20 mm and 140 mm)
• Press bush Ø45 (Plastic)
• M8 mm eye bolt
• M8 mm nut
• M8 x 70 mm screw
For a better understanding of the pump, please
see the exploded view on page 19 and 21.
WARNING:
Do not reuse disassembled O-rings or shaft
seal as they might be damaged. Always use
new O-rings.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
2. Disassembling the
pump
1. Disconnect the pump from the rest of the
system.
2. Using a 10 mm combination wrench,
unscrew the four bolts from shaft seal
ange.
If shaft seal ange is stuck, screw in two
bolts in holes in the ange to remove it.
3. Remove ceramic ring from ange by
gently pushing it from the back of the
sealing ring.
6. Carefully remove the shaft seal assembly
using the shaft seal extractor supplied in
the toolset. The extractor must t
underneath the shaft seal. Press the arms
of the extractor together when turning the
bolt.
4. Mount the M8 allen screw from the tool
set in the top of the shaft.
5. Wet the shaft and shaft seal with clean
ltered water.
7. Turn pump into vertical position with shaft
pointing downwards.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
8. Using a 13 mm combination wrench,
remove all the bolts on the mounting
ange except the three shown in the next
picture.
WARNING:
Do not loosen the two screws keeping
swash plate in place.
9. Unscrew the remaining three bolts. Turn
each bolt one round at the time to make
sure that the ange is removed as straight
forward as possible.
10. Screw the eye bolt in the M8 hole in the
middle of the ange. Pull it straight
upwards.
12. Remove by hand the pistons one by one.
Be careful not to scratch the pistons. Tilt
the retainer plate to horizontal position for
easy removal of pistons, if required.
WARNING:
Do not use any tools.
11. Swash plate must be placed so that its
surface is not scratched. For further
disassembling of swash plate, see page 10.
13. Remove the retainer plate and the retainer
ball.
Note: As the retainer ball is not attached
to the retainer plate, it might fall out.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
14. Remove the retainer guide, the 4 springs
and the spring guide.
15. Mount a 8 mm eye bolt in the cylinder
barrel. Pull straight upwards. A continuous
lift will elevate the cylinder barrel out of
housing. This can only be done if shaft seal
is removed.
WARNING:
If the cylinder barrel is dropped or
lowered too fast into housing, the main
bearing/shaft bearing might be
damaged. It is not replaceable.
16. Place cylinder barrel upside down.
For further disassembling of cylinder
barrel and valve plate see page 11.
18. Remove, by hand, the two pins.
Note: The following operation is only
necessary if O-ring on port ange is to
be changed.
17. Remove the port plate by hand.
19. Place the pump horizontally.
Remove the remaining screws in port
ange by using a 13 mm combination
wrench. Carefully separate house and port
ange. Ensure that pin for the positioning
house is not lost.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
3. Assembling the pump
WARNING:
Do not use silicone when assembling the
pump. Do not reuse disassembled O-rings;
they might be damaged. Always use new
O-rings.
Note:
Place the pump on a pallet or other stable
surface above the ground. Ensure that
the pump cannot roll. It must be possible
to place the pump vertically with the
shaft pointing downwards. This can be
done between two pallets or between
two boards on a pallet provided that the
distance is minimum 50 mm.
1. Lubrication:
• To prevent seizing-up, lubricate all
threads with PTFE lubrication type.
• O-rings inside pump may be lubricated
only with clean ltered water.
• O-rings for port ange, mounting ange
and ushing valve must be lubricated.
• It is important to lubricate ALL parts to
be assembled with clean ltered water
(Especially all PEEK parts).
Important:
It is essential that the pump is serviced in
conditions of absolute cleanliness. All parts
must be absolute clean before mounting.
2. Place port ange with O-ring pointing
upwards.
3. Mount O-ring.
Insert pin for positioning housing
on port ange.
Ensure that two screw holes can be
reached from below.
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4. Position housing alingning pin hole
over pin. Carefully press the port ange
downwards. Do not squeeze the O-ring. If
the O-ring is damaged, the pump will leak.
Screw in at least two screws.
Service guide | Disassembling and assembling PAHT and PAHT G 256-308
5. Place housing horizontally. Screw in the
rest of the screws on port ange. Tighten
screws to a torque of 30 ± 2 Nm.
6. Place pump vertically. Place 10.5 mm pins
in port ange.
Position port plate by using the two pins.
Do not use force for this operation.
7. Ensure port plate is tted tightly against
the bottom.
IMPORTANT:
Lubricate port plate with clean ltered
fresh water.
If valve plate is disassembled from
cylinder barrel please see page 11
before continuing.
8. Screw eye bolt in cylinder barrel.
9. Make sure there is enough free space for
the shaft beneath the housing. Gently
lower cylinder barrel into housing.
WARNING!
If cylinder barrel is dropped or lowered
too fast into housing, main bearing and
shaft bearing might be damaged.
Replacement can only be done at
Danfoss, Nordborg.
10. Unscrew M8 eye bolt.
Place the four springs and spring guide in
cylinder barrel. Springs must be
positioned in the holes.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
11. Retainer guide, retainer ball and
retainer plate must be mounted as
indicated in below picture.
13. Place retainer plate in cylinder barrel.
Note: If pistons are replaced, place new
pistons in clean ltered water for a
couple of minutes. Exercise piston shoes
to make them “run” smoother.
12. Ensure that retainer plate is oriented
correctly, i.e. with its slant surfaces
pointing downwards.
14. Place pistons in retainer plate and
cylinder barrel.
When pistons are placed, tilt retainer
plate for easier placement of swash plate.
If swash plate has been disassembled from
mounting ange, see page 10 for assembly
of swash plate.
15. Lubricate piston shoes and swash plate
with clean ltered water. Remember to
place the pins in housing.
16. Place three bolts in mounting ange.
Turn each bolt one round at a time to
ensure mounting ange is mounted as
straight downwards as possible. Be careful
not to squeeze O-ring.
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17. Screw in the rest of the bolts and cross
tighten all screws to a torque of
30 +/- 2 Nm.
Service guide | Disassembling and assembling PAHT and PAHT G 256-308
Shaft seal
18. Place pump horizontally to get access to
shaft.
19. Place stop for shaft seal on shaft.
WARNING:
Ensure that carbon ring is pointing
outwards.
20. Use plastic assembly tool provided with
large diameter pointing downwards, to
press seal home against shoulder of stop
for shaft seal.
22. Remove old O-ring and t new one on
shaft seal ange.
21. Press new ceramic ring into retainer ring
by using plastic tool provided.
WARNING:
Ensure that the face with rubber seal is
positioned against shoulder in shaft
seal ange.
23. Place shaft seal ange on shaft. Tighten
the bolts with a torque of 10Nm +/-2 Nm.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
4. Disassembling and
assembling of the
swash plate
WARNING:
Make sure that the surface on the swash
plate does not get any marks.
1. Place the swash plate upside down.
Remove the 2 bolts holding the swash
plate. Remove the mounting ange from
the swash plate.
2. Check that the O-ring and pin are in good
condition (Provided with sealing set).
3. Change the O-ring on the mounting
ange.
4. Mount the swash plate on the 2 pins.
Carefully by hand tilt the unit to
horizontal position and mount the 2 bolts
which hold the swash plate. Tighten the
bolts with a torque of 30 Nm +/-2 Nm.
Finally check the surface on the swash
plate for any marks or foreign particles.
Mount the assembled part on the pump.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
5. Disassembling and
assembling of cylinder
barrel and valve plate
1. Push a screwdriver into the hole between
cylinder barrel and valve plate. Carefully
push downward the screwdriver so that it
makes a gap between cylinder barrel and
valve plate. Use this gap to put in another
screwdriver and loosen the valve plate
from the cylinder barrel.
Mount
backup
rings rst
Mount then
O-rings
2. Remove the O-rings and backup rings. If
they have been removed they can not be
reused.
Mount the new back-up rings on the valve
plate rst and then mount the O-rings.
3. Lubricate the new O-rings/back-up rings
and the liners in the cylinder barrel with
clean ltered water. Lower the valve plate
upside down on the cylinder barrel. Place
the press bush for valve plate (provided in
tool set) like on the picture. Screw the bolt
into the shaft of the cylinder barrel. Turn
the nut slowly clockwise. The valve plate
must slide carefully into the cylinder
barrel. Stop when the gap between
cylinder barrel and valve plate is 1-2 mm.
4. Remove the press bush again by screwing
the nut counter-clockwise.
Turn the nut slowly clockwise. The valve
plate must slide gently into the cylinder
barrel. Stop when the gap between
cylinder barrel and valve plate is 1-2 mm.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
6. Changing pistons
Note : It is essential that the pump is serviced
in conditions of absolute cleanliness.
Tools needed:
• 13 mm combination wrench
• 6 mm allen key
• Tool set 180B4347
Stop for
retainer ball
6.1 Disassembling
1. Disconnect the pump from the rest of the
system.
WARNING:
Never unscrew the 4 screws marked
with coloured sealer.
2. Loosen all screws on the pump except the
three screws as shown in the picture
above.
Note: There is still water inside the
pump.
4. Unscrew the remaining three screws by
using a combination wrench to turn each
screw a couple of rounds at a time so
the ange is removed as straight outward
as possible.3. Mount the guide bolt in the top hole.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
Drain plug
5. If the ange does not move forward loosen the drain plug to empty the pump
from water by releasing vacuum.
Swash plate
surface
7. Carefully turn the ange and push it
backwards to make it rest on the housing
surface. Ensure not to scratch the swash
plate surface.
6. Remove the ange when the remaining
three screws have been loosened. The
guide bolt must remain mounted.
8. Adjust the retainer plate to be parallel to
the end ange.
9. Screw the stop for the retainer ring into
the centre to keep the retainer ball in
place.
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10. Carefully remove the pistons one by one.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
11. WARNING:
Ensure that the piston shoes and the
piston surfaces are not damaged during
removal. It is recommended to place the
pistons upside down on an even and
clean base/surface.
12. Remove the retainer ring by sliding it
across the stop for the retainer ball. If
necessary, tilt the ange.
13. Inspect the piston liners. Replace any worn
parts.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
6.2. Assembling
2. Insert the pistons randomly.1. Carefully push the retainer ring in place.
5. Tilt the ange and replace the ange
O-ring.
4. Place the retainer ring in an odd angle
corresponding to the orientation of the
swash plate.3. Remove the stop for retainer ball.
6. Turn the ange and gently push it into the
housing.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
7. Mount the three screws as shown above.
Tighten them to a torque of 30 Nm ± 3 Nm.
9. Bleed the pump.
8. Remove the guide bolt. Connect the pump
to the rest of the system.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
7. When should the
pistons be replaced
This section provides guidance on, how to
determine whether the parts of the pump are
worn and should be replaced.
In case of doubt - the pistons must be replaced.
The pictures below are meant as a guideline for
evaluating the wear of the sliding surface.
Note: If the pistons break down, the pump
will suer a disastrous breakdown.
1. No wear or cavitation of the piston
shoe. New inspection is required in
4,000 hours.
2. Cavitation of the piston shoes.
New inspection is required in 2,000 hours.
3. Cavitation of the piston shoes.
All pistons must be replaced within the
next 1,000 hours.
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
8. Spare parts list for
PAHT 256-308
Pos. Qty. Designation Material
Pos Qnt Designation Material
1 1 Housing Duplex / PEEK
2 2 Pin ø6 x 10 AISI 316 x
3 4 Bleeding plug G 1/4" Super duplex x
4 4 O-ring ø11 x 2 NBR x
5 28 Screw M8 x 30 AISI 316 x
9 1 O-ring ø228 x 4 NBR x x x x
10 2 O-ring ø9,19 x 2,62 NBR x x x
11 1 End cover CW Duplex
13 3 Screw M8 x 16 AISI 316 x
31 1 Swash plate Super Duplex x x
34 2 Pin ø10,5 x 20 Duplex x x x
61 1 Cylinder barrel Super duplex / PEEK x
62 4 Spring Duplex x
63 1 Spring guide Polypropylen x
64 1 Retainer Ball Super duplex x
65 1 Retainer plate Super duplex x
66 9 Piston Super duplex / PEEK x
67 1 Key 12 x 8 x 70 AISI 316 x
71 1 Retainer Guide Super duplex / PEEK x
91 1 Port plate Super duplex / PEEK x
92 1 Valve plate Super duplex x
93 9 Back up ring PTFE x
94 9 O-ring ø30,2 x 3 NBR x
121 1 Port ange Duplex
122 1 O-ring ø68 x 2 NBR x
123 1 O-ring Ø228 x 4 NBR x
124 1 Shaft sealing Hasteloy C276 x
125 1 Cover Super duplex
126 2 Pin ø10,5 x 20 Super duplex x x
127 4 Screw M6 x 16 AISI 316 x
128 18 Screw M8 x 90 AISI 316 x
132 2 Bleeding plug G 1/4" Super duplex x
133 2 O-ring ø11 x 2 NBR x
142 1 Washer for shaft seal Polypropylen x
147 3 Screw M8 x 30 AISI 316 x
1 Service instruction x x x x x x x
180B4082 - Screw & seal set (PAHT 256-308)
180B4079 - Cylinder barrel set
(PAHT G 256-308)
180B4078 - Valve plate set (PAHT 256-308)
180B4084 - Retainer set (PAHT 256-308)
180B4083 - Piston set (PAHT 256-308)
180B4080 - Swash plate set (PAHT 256)
180B4081 - Swash plate set (PAHT 308)
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Service guide | Disassembling and assembling PAHT and PAHT G 256-308
8.1 Exploded view
PAHT 256-308
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Danfoss A/S
High Pressure Pumps
DK-6430 Nordborg
Denmark
© Danfoss | DCS (im | 2021.06
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