HOW I DO………
CLOCK MAINSPRING
REPLACEMENT & SERVICE
By Lindsay Bramall, Jan. 2011
Open and enclosed mainsprings.
TOOLS.
Open mainspring
clamps
Let-down keys
English style mainspring winders.
Lock
Spring outer
end clamp
Clamp bar
Barrels 30mm
to 60 mm ID
Winding
arbor
Barrels below
30mm ID
Mainspring winders offer safe, controlled manipulation of mainsprings
and do not induce “coning” as does winding in/out of barrels by hand.
Unlocked and free to
rotate for winding
and de-winding
springs
Winder locked to hold
the wound up
mainspring whilst a
barrel is removed or
replaced.
Lathe as a mainspring
winder.
( Another method for open
springs )
Well used
heavy leather
glove.
Spring
cleaning
board
Gas
torch
Hole punch
Step 1.
“LETTING DOWN” THE
MAINSPRING
Remove the click spring
and fit the let-down key
Hold the movement, take the
spring force with the key and
let the power down.
Fit the let-down key
and wind up the
spring until….
….the clamp
drops on.
With the spring tension under the
control of the hand held let-down key,
release the click spring and click
and….
….let the spring down
into the clamp.
Springs secured
and removed from
the movement
Springs ready to remove for cleaning or
replacement.
Step 2.
REMOVING THE MAINSPRING
( DE-WINDING )
a. Enclosed
b. Open
Clamp for spring
outer end.
“Safe mode”
lock
Mainspring
inner hook
Firmly lock the
winder in a bench
vice.
a. Enclosed mainsprings ( de-winding )
Present the mainspring
and engage the inner hole
on the winder arbor hook
Drop the clamp bar into the
top of the winder and insert
the open clamp tongue into
the outer coil about 20mm
from the hook. Tighten the
clamp to grip the spring.
Grip the barrel tightly
and wind up the
mainspring until the
outer coil can be seen on
the barrel hook.
Clamp bar is locked securely
by the two forks.
The winding arbor is
locked safely here
All spring force is now being taken by the
winder and the barrel may be removed.
Barrel off and spring now
ready to be de-wound.
Note the “coning”
induced when this spring
was previously hand
wound into the barrel !
Controlled unlocking
of the winder and
de-winding of the
mainspring.
b. Open mainsprings ( de-winding )
Engage the winder
clamp adjacent to the
loop and insert the
winding arbor and
hook the centre hole.
Winding arbor locked
and the mainspring
clamp removed.
Spring now ready to
be wound up to
release the
mainspring clamp.
Spring de-wound for
servicing or discarding
All spring force now
taken by the winder
ready to de-wind.
Step 3.
SERVICING THE EXISTING,
OR NEW, MAINSPRING
BEFORE FITTING
Only use the old mainspring if it has not lost its strength or shape.
Thoroughly clean all mainsprings with solvent and for
reuse of existing springs clean with fine steel wool or
2000 grit “wet & dry” paper, then clean with solvent.
Repair the end hole if required.
Punch a new hole and shape to suit the barrel hook
New end
shaped to fit
the barrel hook
Punch & die
Step 4.
SIZING THE MAINSPRING
• You cannot assume that the old spring was correct for the
barrel.
• Measure the depth of the barrel from the cap lip to the
bottom of the barrel. Subtract 0.5 to 1.0 mm to give the
correct spring height.
• Use the “hundredth rule” to estimate the spring gauge.
ie. a barrel ID of 30 mm will use a 0.3 mm thick spring.
A 45mm ID barrel will need a 0.45mm thick spring.
• Measure the barrel ID. and the arbor OD. Using these
measurements and the spring gauge calculate the spring
length using :
(0.393 x ((bID x bID) - (aOD x aOD))) / sGauge
Mainspring calculatorI.xls
• Compare these measurements to the old spring.
• Order a new spring by:
Height x guage x length ( or barrel ID)
ie. 22 mm x 0.45mm x 1680mm ( 45mm ID )
Step 5.
REPLACING THE MAINSPRING
Smear the full length of the spring, the
cleaned barrel interior and the barrel arbor
pivots with oil, then…
simply reverse the removal processes.
WINDING OPEN SPRINGS
DIRECTLY ONTO
( OR OFF ) THEIR ARBOR IN A
LATHE.
Engage the spring centre
hole on the arbor hook
and the outer loop on a
long rod in the tool
holder.
Grip the arbor up
against the wheel.
Rotate the chuck by
hand to wind the
spring onto the
arbor.
Loop engaged
on the rod in
the tool post
Continue to rotate the
chuck until the clamp
will fit over the spring,
then let go of the chuck
and the spring will run
back into the clamp.
Withdraw the rod and
the spring is now ready
to refit to the
movement.
SIMPLE ISN’T IT ?
These are my methods, however, there are many others,
equally as good.
This is the most dangerous operation in clock repair so
have good tools, safe methods and keep the eyes and
mind focused on the job.
Things can go awfully wrong in a split second !!!