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Chamberlain 3850 - Assembly Step 1: Rail to Motor Unit; Caution: Prevent Opener Damage; Assembly Step 2: Set Belt Tension

Chamberlain 3850
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6
ASSEMBLY STEP 1
Attach the Rail to the Motor Unit
To avoid installation difficulties, do not run the garage door
opener until instructed to do so.
Remove the two washered bolts mounted on top of motor unit.
Align rail and styrofoam over belt pulley. Cut tape from rail,
chain and styrofoam.
REMOVE STYROFOAM.
Insert both washered bolts through the rail into the motor unit.
Tighten bolts securely.
Use only these bolts! Use of any other bolts will cause
serious damage to door opener.
Position belt over the motor unit belt pulley.
To avoid SERIOUS damage to opener, ONLY use bolts
mounted in top of motor unit.
ASSEMBLY STEP 2
Set the Belt Tension
By hand, thread the spring trolley nut on the threaded shaft
until it is finger tight against the trolley (Figure 1). Do not use
any tools.
Insert a screwdriver tip into one of the nut ring slots and brace
it firmly against the trolley (Figure 2).
Place a 7/16" open end wrench on the square end. Rotate
about 1/4 turn until the spring releases and snaps the nut ring
against the trolley (Figure 3).
This extends the spring for optimum belt tension.
Washered Bolt
5/16"-18x1/2"
Rail Hole
Motor Unit
Belt Pulley
Rail Hole
USE ONLY THIS
TYPE AND SIZE
BOLT
Trolley
Threaded
Shaft
Spring/Trolley Nut
Nut
Ring
Trolley
Threaded
Shaft
Nut Ring
Slots
Square
End
Nut Ring
Trolley
Nut Ring
Trolley
1" (2.5 cm) 1-1/4" (3.18 cm)
BEFORE AFTER RELEASE
Figure 1
Figure 2
Figure 3

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