Chamberlain Merlin Whisper Drive MT3850 Installation And Operating Instructions Manual page 8

ASSEMBLY STEP 7
Set Belt Tension
• Thread spring nut onto the carriage bolt until finger
tight. Insert a screwdriver tip into one of the slots of the
nut ring and brace it firmly against the header sleeve.
• Place an open end wrench on the square end of the
spring nut, slightly rotate nut about 1/4 turn clockwise
until nut ring is released against header sleeve.
This sets spring for optimum belt tension. Belt may slip off
pulley if belt is too loose. If belt does slip, re-tighten spring
nut by turning nut clockwise a half turn.
Do NOT over-tighten belt.
Trolley Nut Ring
BEFORE
25mm
ASSEMBLY STEP 8
Attach the Belt Pulley Cover
• Position the belt pulley cover over the opener belt
pulley so the three holes in the cover align with the
three holes in the mounting plate. Attach with hex
screws provided.
HARDWARE
Trolley
Nut Ring
AFTER RELEASE
32mm
Hex Screw
To avoid possible SERIOUS INJURY to fingers from moving
garage door opener:
• ALWAYS keep hands clear of belt pulley while operating opener.
• Securely attach belt pulley cover BEFORE operating.
Hex Screws
7
Belt Pulley Cover
Opener Belt Pulley
Mounting Plate