Technical Information · issue 36 of 1998
Friction Clutch HT 75
Model HT 75 Pole Pruners - Series 4138
IssueTI 36/1998
Full text
Page 1 of 4
Technical Information
36.98
Model HT 75 Pole Pruners - Series 4138
Contents
1. Friction clutch
2. Oil pump / strainer
1. Friction clutch
A new gearbox (1) with friction clutch (2) for
the chain sprocket (3) will be installed from 1
serial number 2 39 949 387 to further improve
dependability. fo
1.1 Purpose
The friction clutch is designed to help reduce
the load on the multi-part drive shaft in the
telescopic drive tube.
An overload situation can occur if the cutting
attachment has a dull chain and is suddenly
applied to the cut.
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1.2 Construction and function
Refer to description in 1.2 of Technical Information 3 2
bulletin 40.94 on introduction of the friction clutch
in model E 20 chainsaws.
1.3 User instructions
It is essential to point out to HT 75 users the
importance of keeping the chain sharp at all
times, i.e. it must be resharpened regularly.
1.4 Service notes
1.4.1 General
Repair procedures not described here are given
in Technical Information bulletin 03.97.
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The collar screw (4) that centers the cup springs
has a left-hand thread.
Cup springs (5) must always be replaced in
complete sets of three.
Always replace the washers (6) along with the 8
chain sprocket (3). The washers are supplied
with the replacement sprocket for this reason.
3
1.4.2 Design of friction clutch 5
at start of production 4
At the start of production, a washer (7; 0.4 mm
thick) was installed together with the ring (8; 3.6 76
mm thick). These two parts will be replaced later
6
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by the new ring 4138 642 8201 (4 mm thick).
Only ring 4138 642 8201 will be supplied as a
replacement and is part of the friction clutch kit
4138 007 1001 (see 1.4.3).
If the ring (8) has been replaced during a repair, it New gearbox with friction clutch
and the washer (7) must still be replaced by ring
4138 642 8201. fo
1.4.3 Backfitting
Every HT 75 can be converted to a friction clutch.
All the parts necessary for the conversion are
included in the friction clutch kit 4138 007 1001.
The instruction sheet supplied with the kit shows
the parts that have to be replaced.
1.4.4 Chain sprocket replacement
A special tool must be purchased or made up
in the workshop.
• Shorten a standard commercial T-handle,
5mm square section Allen key to the length
shown -
shorten T-handle, 5mm hexagon section
Allen key to the length shown (STIHL Allen key
(0.196")
5 mm
5910 890 2410).
Modifiy the profile as shown.
• Remove the gearbox from the shaft.
5910 890 2410
25 mm
(1")
100 mm
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(4")
)PSKVU=J=P
• Clamp the gearbox in a vise.
• Push special tool into shaft’s square socket
in the gearbox as far as stop.
• Unscrew and remove collar screw for cup
springs and chain sprocket clockwise.
• Pull the cup springs - make a note of installed
positions - and the worn chain sprocket off the
shaft.
Replace damaged cup springs.
• Fit new chain sprocket and cup springs on the
shaft.
• Fit the collar screw and torque down
counterclockwise to 6 Nm (4.4 lbf.ft).
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1.4.5 Checking clutch slip torque
It may be necessary to check the slip torque
after carrying out a repair.
• Purchase a standard commercial square
section 5mm bit for a torque wrench -
or 9
modify (do not shorten) STIHL 5mm bit 5910
890 0102 - 10
as shown under "1.4.4".
11
• Remove the gearbox from the shaft.
• Clamp the gearbox (by its hook) in the vise.
•
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Remove the complete chain tensioner.
• Position the guide bar against the gearbox.
• Push guide bar (9) toward chain sprocket and
engage its upper end between two teeth
(arrows).
•
Fit several A 8.4 mm washers (10; total
thickness 6 mm) or ring 0000 961 1409 on the
stud.
• Screw on the nut (11) and tighten it down -
the chain sprocket is now locked in position.
• Use the torque wrench to check the slip torque:
Reading must be between 5 and 14 Nm
(3.7 - 10.3 lbf.ft).
In case of deviations, replace the washers,
chain sprocket and cup springs.
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)PSKVU=J=Q
1.5 Summary
Item Part Name Original New Rem.
version version
1 Gearbox; 4138 640 0105 4138 640 0106 1)
including items 3, 6, 7, 10 thru 12:
2 Friction clutch kit; --- 4138 007 1001
consisting of items 3, 6, 7, 11, 12:
3 Pinion set; 4138 640 7300 4138 640 7301 1)
consisting of items 4, 5:
4 Shaft -*- -*- 2)
5 Pinion -*- -*- 2)
6 Ring 4138 642 8200 4138 642 8201 1)
7 Chain sprocket 3/8" Picco, 7 T; 1206 642 1301 4138 642 1350 1)
consisting of items 8, 9:
8 Washer (2x) 0000 958 0506 1208 162 8900 1)
9 Chain sprocket --- -*- 2)
10 E-clip 8 x 1.3 9460 624 0801
fo --- 1)
11 Cup spring 20x10.2x1.25 (3x) --- 9485 648 1694
12 Collar screw --- 1208 162 4200
All other parts as before
Modification to be introduced: in progress
Remarks
1) Original version of part remains available as spare
2) Part not available as separate item
2. Oil pump / strainer
In the case of trouble with the lubricating oil
supply, clean the oil channels. It is essential to
remove and clean the strainer (2) in front of the
oil pump (1). 1
2
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Last Technical Information on model HT 75: TI 30.98
ã 1998 Andreas Stihl Andreas Stihl - D1/MTK-na
Technical Documentation 6399 Page 2 of 4
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