Bulle Bulle clock fork, pawls and current

Discussion in 'Electric Horology' started by chrisuk, Sep 12, 2019.

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  1. chrisuk

    chrisuk Registered User
    NAWCC Member

    Sep 4, 2010
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    Clock repairer & organist
    Staffordshire , UK
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    Here are some notes relating to a Bulle clock repair I did a few years ago, the clock is still running fine.

    The first problem was a worn silver contact on the fork, someone had attempted to spread it out by hammering, not successfully. In this model the fork is insulated from its shaft so there is no fibre insulator on it. The remains of the silver contact could be straightened out, a small piece of heat resistant insulating material (cardboard) placed between it and the fork and a new piece of silver fitted and filed to shape.
    Fork-Worn-1.JPG
    Repaired-silver-conbtact.JPG

    The pawls were notched by the crown wheel, a similar action to that of brass wheel teeth on steel pinions.
    Pallet-Groove.JPG
    I filed out the notches however later checking showed that the pawls were now just too short to work correctly, the lower (driving) pawl hit the underside of the locking pawl as the driver was moved back to collect another tooth on the wheel. A new driving pawl was made from a piece of an old broken spring of the sort used in pocket watches. The mechanism could then operate correctly. When the driver is fully retracted there are about 2½ teeth between it and the locking pawl and clearance between the pawls. When the driver is fully extended ½ a tooth space remains between the pawls.

    Pawls-apart-fix.jpg Pawls-together.JPG

    The cutaway disc holding the pawl shafts into their bobbin was bent, this was flattened before the reason for it being bent in the first place was discovered. The shaft of the driving pawl was fractionally too long so flattening the disc caused it to bind. A very slim washer was fitted under the disc to resolve this matter whilst keeping the mechanism looking neat.

    When all was assembled on a testing stand the pendulum drove the mechanism correctly but with nearly no excess swing so occasionally the pawls would fail to drive the contrate wheel forward. The magnet which fits through the pendulum was removed and remagnetised, after this the driving of the mechanism was fine and the clock could be regulated.


    I made some measurements of the current drawn from the battery by the clock by putting a small resistance in series with the battery, connecting a oscilloscope across it and then taking photographs of the screen. The only way I could get a decent picture was by blacking out the room, keeping the camera shutter open a long time and doing single shot display on the 'scope.

    Scope-20mV-50mS-1.JPG
    The display resolution was 20mV/cm vertically and 50mS/cm horizontally and these were my results.

    Series resistance 26 Ω (clock coil resistance about 1,100Ω)

    Duration of pulse c 260 mS Mean voltage across the 26Ω resistor c 33 mV

    Current 0.033 / 26 = 1.27 mA

    Ignoring the 33 mV across the test resistor and taking 1.5 V as the supply across the coil gives a power rating of 1.5 x 0.00127 = 1.9 mW

    Charge used per oscillation of the pendulum 1.27 X 0.260 = 0.33 mAs.

    Pendulum rate c 10,060 beats per hour so charge per day 22 mAh.

    Energizer alkaline D cell c 18,000 mAh at low discharge rate so c 818 days running

    I hope that this is of some interest to you all,
    Chris
     
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  2. praezis

    praezis Registered User

    Feb 11, 2008
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    Hi Chris,
    yes, it is of interest, thank you!
    What attracted my attention:
    1. Closing time of the contact is rather long, most electric clocks show about 100...120 ms.
    2. Your current measured is nearly the same as as V_batt / R_coil. As current is
    (V_batt - EMF_coil) / R_coil
    the EMF of your coil seems to be rather low, aka magnet of your pendulum is rather weak.

    Frank
     
  3. chrisuk

    chrisuk Registered User
    NAWCC Member

    Sep 4, 2010
    39
    5
    8
    Male
    Clock repairer & organist
    Staffordshire , UK
    Country Flag:
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    Hi Frank,
    the closing time of the contact will vary with the shape and size of the contact and the depth of the pin within the slot on the fork. In this particular case it seems that the pin had to be set rather deep before the clock would work reliably.
    Repaired-silver-conbtact.JPG
    As a matter of general interest how would you measure the duration of contact closure?

    Best wishes, Chris
     
  4. praezis

    praezis Registered User

    Feb 11, 2008
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    Hi,
    exactly as you do, with an oscilloscope.
    Frank
     

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