Overbanking question

Discussion in 'Watch Repair' started by Jeff Salmon, May 20, 2020.

  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  1. Jeff Salmon

    Jeff Salmon Registered User
    NAWCC Member

    Apr 11, 2002
    670
    6
    18
    Even though this repair is on an early Chelsea clock, before the platform escapements, I am wondering why the clock runs so well for hours and 2 or 3 days, then suddenly overbanks. The time is regulated to 18,000bph
    motion of the balance wheel looks great. I did find a loose banking pin, but I repaired that and now have much better motion to the balance, but it still overbanks after 2 days. Suggestions appreciated.
     
  2. Skutt50

    Skutt50 Registered User

    Mar 14, 2008
    3,541
    123
    63
    Male
    Gothenburg
    Country Flag:
    I am not familiar with Chelsea clocks but I assume it is a stationary clock. Then the first thing that comes to mind is that the clock after 2-3 days is about to come to a halt and the overbakning happens because the lever is not snapped fully against a banking pin.

    The cause however is likely because the guard pin is a tiny bit too short.....

    Adjust the guard pin a tiny bit and see what happens......
     
  3. Skutt50

    Skutt50 Registered User

    Mar 14, 2008
    3,541
    123
    63
    Male
    Gothenburg
    Country Flag:
    Additional thoughts:

    You mention that the motion of the balance wheel looks great. Just to make sure:

    - If jeweled, check the balance jewels for damage, if not jeweled, check that the hole is not worn so you have excessive side shake.
    - Also make sure the balance pivots are straight.....

    Any problem here may contribute to or even cause overbanking.
     
  4. DeweyC

    DeweyC Registered User
    NAWCC Member

    Feb 5, 2007
    2,074
    750
    113
    Watchmaker
    Baltimore
    Country Flag:
    #4 DeweyC, May 21, 2020
    Last edited: May 21, 2020
    Overbanking is caused by a failure of the safety function of the escapement.

    Since you made repairs to the banking pins, read up on how to adjust the escapement and adjust the banking. I doubt it will resolve the overbanking but it is an essential step.

    I believe your Chelsea uses a single roller escapement in which the dart is a vertical post near the notch. This dart (looks like a pin) keeps hits the roller if the lever is unlocked at the wrong time. It stops the lever from passing to the other side. This clearances between this dart and the safety edge of the impulse roller are adjusted to be at a minimum when the impulse jewel is out of the notch yet enough to allow the lever to pass when the jewel is in the notch.

    For complete instructions you should refer to any of the watch books that discuss setting up the lever escapement.

    As Skutt said, there are other causes that relate to the heights of the dart and roller table (most likely excessive endshake at the balance). Since the banking pin was excessively adjusted, you might check this as well.

    This Chelsae escapement is unjewelled at the EW and Pallet. It is also worth checking sideshakes as Skutt suggested. If you need to repair the holes, they do have to be more accurately placed than typical clock bushings.
     
  5. Karl Burghart

    Karl Burghart Registered User
    NAWCC Member

    Jan 30, 2012
    186
    18
    18
    Male
    Retired
    Ballston Spa, NY
    Country Flag:
    Region Flag:
    A good test on these is to rapidly move the minute hand backwards. If it's going to overbank at all it will when you do that. If you have side shake issues on the pallet or escape wheel take a look at installing jewels rather than bushings. I've had god luck using them for the pallet and escape wheel.
     
  6. DeweyC

    DeweyC Registered User
    NAWCC Member

    Feb 5, 2007
    2,074
    750
    113
    Watchmaker
    Baltimore
    Country Flag:
    I just stop the seconds hand when it is present. We are both using a test I was shown a long time ago.

    Caveat: Do not try this with a detent escapement!

    While we are talking about safety functions: A single roller is just as effective as the double roller.

    I have not seen an analysis (physicists out there?) but the explanation for the preference of the double roller is that the dart, when called into play operates on a radius that is 1/2 that of the single roller safety edge.

    But, the lever arm of the pallet has increased. The dart of the DR extends well into the horns while the dart for the SR is behind the impulse notch. That added length increase the lever arm of the pallet. While the force of the tendency toward lock gets deducted, we still have the residual force that forced it out of lock. Since the lever arm of the dart on the double roller is longer this results in greater force at the dart than would be in a single roller.

    As far as timekeeping, I have seen no obvious differences in 992 SR and 992 DR watches when it comes to consistency of rate. This is far from conclusive since I do not have a shake table.

    It would be interesting to see an analysis by someone skilled in the analysis of forces. On an intuitive level, I suspect the advantage of the DR is marginal. But thought experiments do have their limitations.
     
  7. Smudgy

    Smudgy Registered User

    May 20, 2003
    2,872
    23
    38
    Country Flag:
    Region Flag:
    You may want to check where the escape wheel teeth land on the pallets, making sure they land on the locking face. I wouldn't expect a stationary clock to overbank even if the roller and dart were removed. Without something to cause the lever to cross over, like the clock moving or being bumped, there is little to cause the overbanking issue.
     
  8. Skutt50

    Skutt50 Registered User

    Mar 14, 2008
    3,541
    123
    63
    Male
    Gothenburg
    Country Flag:
    Agree... I have however seen this happen when the gear train stops putting force on the pallet fork like when the mainspring is fully unwound and the fork does not snap to one side.... The watch should stop before this situation occurs but as I said I have seen it happen more than once.......
     
    darrahg likes this.
  9. Jeff Salmon

    Jeff Salmon Registered User
    NAWCC Member

    Apr 11, 2002
    670
    6
    18
    Thanks for all the suggestions. I worked with the banking pins some more. It seemed that the entrance pallet was a little too deep. I am still checking the running, but after 2 days it seems fine. Knock on wood.
     

Share This Page