Monday, 11 May 2015

Watch Faults - Slipping Cannon Pinion.

One in an occasional series on common watch faults.


This is a fairly common fault, particularly after the watch has been disturbed when serviced, usually it comes to light immediately or during the initial test but is can develop after a few months of use.

Symptoms:

The watch runs and the second hand moves but the hour and minute hand do not move at all  (a severe and obvious case!) or pinion slips, and the hands stop occasionally or move slowly giving the impression that the movement is loosing time badly, possibly in a random fashion.


What it is:
The Cannon pinion (left) Connecting to the movement
(under) and to the first part of the motion works which
in turn connects to the setting gear, top right.
Click on the image for a larger view.

The minute hand is mounted on the cannon pinion which also drives the under dial motion gear that moves the hour hand.

The cannon pinion is friction mounted on the central shaft of the watch, this has to be firm enough to turn the hands but has to be loose enough that it will slip when the hands are adjusted either by the key or, in a keyless design, when the setting mechanism turns the motion gear.

The pinion can start slipping if one or more of the following happens:
  • Oil migrates onto the shaft - this happens occasionally on unrestored watches when people splash oil all over the place or spray it with WD40 to try and get the watch working.
  • The shaft becomes too smooth for the cannon pinion to grip.
  • The pinion becomes worn.
  • Some pinions have a cut out and what is essentially an integral spring to maintain pressure on the shaft, this can get weak or break off altogether.
It is thought that setting the watch by turning the hands backwards can cause a cannon pinion to start slipping so only make small adjustments backwards and do so slowly and only if you know that the particular watch will not be damaged in other ways by turning backwards as many complicated watches such as repeaters can be.

The motion gear complete with the hours gear added, the hour
hand attaches to this and the minutes hand attaches to the
Cannon pinion.

How it is fixed:


Usually 2 or more actions are required in addition to degreasing the parts:
  • The shaft is roughened with a file to provide better grip.
  • The top of the pinion is bent inwards using a concave punch to make it grip.
  • The side of the pinion is nipped in (more) with a special tool (which does not work well on some metals used by English watch makers) or with a hammer and straight edged stake to provide some grip, this is somewhat dangerous as it is easier to over do it and break the pinion or tighten it too much which can result in quite a lot of labour to open it out again.
  • Replacing the pinion if one can be found of the correct size.



No comments:

Post a Comment