Workshop Hints
Automatic Mainsprings
By Alan Burtoft FBHI
The strength of a mainspring is most important in maintaining the balance amplitude and resulting timekeeping
(see article on Positional error in hints and tips.)
In automatic watches this presents a problem when the spring is fully wound. The slipping spring or 'Bridle' must slip round the barrel wall before excess pressure is passed through to the train causing 'banking '(this happens when the balance amplitude us too high and the impulse jewel strikes the back of the horns of the pallet fork).
The bridle must also grip the barrel wall enough so as not to slip down ether rapidly, or slowly ('mainspring creep') resulting in shortened reserve power.
This condition is more critical in higher grade watches. Therefore the correct greases should be used.
Use Mobious 8200 or 8201 (with Molybdenum) for brass barrels and 8212 for aluminium barrels.
Checking for creep is much easier if you work on the same calibre's, by keeping dummy barrel covers with inspection holes.
Hold the barrel arbor in barrel tongs and holding the barrel wind the spring to the top and hold it still .
Observe the spring through the inspection holes for a few moments and check for creep.
Some barrels have scallops cut in the barrel wall to prevent the mainspring creeping down. 
If you are using the correct grease and the spring still slips then there is an anti slip grease mobious 8217.
Or shortening the bridle by 0.5 mm. and squaring the end of the spring should prevent slipping.
After making any alterations to the spring it is advisable to check the completed watch for amplitude, half wound, and fully wound this exceeds 320 Deg. and 310 Deg. respecavly then fit a weaker spring. (or new spring if it has already been modified)
If the amplitude is low half wound and too high full wound then the bridle may be griping the barrel wall too much.
Assuming the movement is clean and lubricated correctly and the escapement is adjusted correctly (see Checking a leaver escapement Hints) .
Alan Burtoft FBHI.
