HOW I DO………

CLOCK MAINSPRING REPLACEMENT & SERVICE

By Lindsay Bramall, Jan. 2011

Open and enclosed mainsprings.

TOOLS.

Let-down keys

Open mainspring clamps

English style mainspring winders.

Barrels below 30mm ID

Barrels 30mm to 60 mm ID

Mainspring winders offer safe, controlled manipulation of mainsprings and do not induce “coning” as does winding in/out of barrels by hand.

Lock

Winding arbor

Spring outer end clamp

Clamp bar

Winder locked to hold the wound up

mainspring whilst a barrel is removed or

replaced.

Unlocked and free to rotate for winding and de-winding

springs

Lathe as a mainspring winder.

( Another method for open springs )

Well used heavy leather

glove.

Spring cleaning

board

Hole punchGas torch

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. Enclosedb. Open

Firmly lock the winder in a bench

vice.

Mainspring inner hook

Clamp for spring outer end.

“Safe mode” lock

a. Enclosed mainsprings ( de-winding )

Present the mainspring and engage the inner hole on the winder arbor hook

Grip the barrel tightly and wind up the

mainspring until the outer coil can be seen on

the barrel 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.

All spring force is now being taken by the winder and the barrel may be removed.

The winding arbor is locked safely here

Clamp bar is locked securely by the two forks.

Barrel off and spring now ready to be de-wound.

Controlled unlocking of the winder and de-winding of the

mainspring.

Note the “coning” induced when this spring

was previously hand wound into the barrel !

b. Open mainsprings ( de-winding )

Engage the winder clamp adjacent to the

loop and insert the winding arbor and

hook the centre hole.

Spring now ready to be wound up to

release the mainspring clamp.

Winding arbor locked and the mainspring clamp removed.

All spring force now taken by the winder ready to de-wind.

Spring de-wound for servicing or discarding

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

Punch & die

New end shaped to fit

the barrel hook

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.

Grip the arbor up against the wheel.

Engage the spring centre hole on the arbor hook and the outer loop on a

long rod in the tool holder.

Loop engaged on the rod in the tool post

Rotate the chuck by hand to wind the spring onto the

arbor.

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 !!!