Gent pulsynetic C7 pendulum bob weight

Discussion in 'Electric Horology' started by djrm, Mar 20, 2017.

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  1. djrm

    djrm Registered User

    Mar 18, 2017
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    Greetings everyone,my first post here.

    I have just acquired a Gent Pulsynetic C7 master clock made in 1970, and although mostly complete there is no pendulum bob weight, just an empty cover.
    I need to know the height and weight of the original bob weight. Does anybody have this information please? Could somebody measure and weigh their own for me?

    The clock is now working with some arbitrary weight added to the pendulum but it is consistently fast even at the maximum length, I think I need more weight nearer to the bottom of the pendulum.
    I have found out everything else I need to know about this clock on this site except for this detail.

    Kind regards, David.
     
  2. rogerj

    rogerj Registered User

    Dec 21, 2014
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    #2 rogerj, Mar 21, 2017
    Last edited: Mar 21, 2017
    Hi..As it was no doubt manufactured in imperial measurements, here's what I see:
    Main body: H = 10", Diameter = 2.25", Bore though middle 5/8" (to suit the flat rod)
    There is a recess at the top 3/16" deep dia. to suit the cover plate you have.
    In addition the to main body there is an extension at the bottom with reduced diameter (1.25" diameter), part turned ,and part a "washer" drilled to suit the rating thread.
    Total height of this section is 3/16". The " washer" is fixed with two 6ba CS screws (same as top plate)
    The material is ferrous; probably mild steel
    The recess that remains after fitting the top cover plate is probably for safer locating of the fine tuning weights.
    The "rating nut" at the bottom is a substantial affair and has a screw for locking it off..There is a thin metal pointer on the front face of the bob that doubles as a click against the "teeth" on the outside edge of the rating nut..Say if you haven't got that part.

    Hope this helps...Roger
     
  3. djrm

    djrm Registered User

    Mar 18, 2017
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    Thanks fro the reply Roger, that covers most of it. I would like to know the weight of the bob for confirmation if you know or have an opportunity to weigh it. My pendulum is a shiny chrome cover without the recess in the top, unlike any I've seen in the online photos.
     

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  4. rogerj

    rogerj Registered User

    Dec 21, 2014
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    ooops..I forgot that....about 4Kg..The weight won't be critical as long as the height is correct....but I have no idea what that shiny cover is about. the details I gave are of the black painted bob on mine...and one other I had previously..The "cover" I thought you were referring to is the 2" diameter "washer" screwed in the recess at the top which is to adapt the 5/8" bore to firmly match the flat rod. The slot for the rod also includes a central round hole to allow the rating thread to pass through..Someone else will have to explain the shiny cover...Could it just fit over the bob for aesthetic reasons ?
     
  5. djrm

    djrm Registered User

    Mar 18, 2017
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    Thanks Roger, your information has helped me very much, thank you.
    Attached are some pictures of the pendulum bob casing from my clock.
    Kind regards, David.
     

    Attached Files:

  6. rogerj

    rogerj Registered User

    Dec 21, 2014
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    Perhaps it was a version with the tube filled with lead..Your photos show that you have the top and bottom caps..
    A former would be required to preserve the bore.
     
  7. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    I always thought they used solid bobs with center
    attachment to the rod.
    The rod like like brass??
    Tinker Dwight
     
  8. djrm

    djrm Registered User

    Mar 18, 2017
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    Greetings Tinker,

    I have made a temporary bob out of two nine inch sections of old cast steel land rover half shafts, about 1 kg each, these just fit in the space available. This is working better than the twenty used AA cells I had previously put in the bob cover space!

    The clock now runs properly and I can make it too slow or fast. I shall have a go at getting it regulated.It gained two minutes overnight, the best its done so far. I need to find the correct bob somehow though or at least get some more mass into the case.

    One idea for a weight would be to wind some roof flashing lead around a former, like a massive toilet roll.

    I don't understand your question, The pendulum shaft looks like others I've seen described with hatching along its length. It looks original.

    I have some ideas to minimize the noise by use of a fancy driver for the electromagnet which will operate it in a constant current mode - more on this later.

    Kind regards, David.
     
  9. rogerj

    rogerj Registered User

    Dec 21, 2014
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    Couple of points David..The weight of the bob does not affect the going rate..It should however ideally fill the entire height of the chrome tube. It should also be secure ie no movement of the internal parts...this can give rise to erratic timekeeping - probably as you experienced with the bunch of AA cells. Coiled lead with that proviso should work..However we don't know if the bob was an original fitment..Mr Gent may have calculated that the downward expansion of the Invar rod could be cancelled out by the upward expansion of a bottom supported, 10" tall steel bob. Lead has a greater expansion coefficient than steel and so the temp comp may not be as good if my supposition is correct..
    The current rise time in the coil circuit is tamed a little by its inductance. If you attempt to change this with a constant current - which is only partly achievable anyway - you may increase the acceleration and the final speed resulting in even more clunk. You may also adversely affect the contact wear a bit.
    I would say just tweak the resistance in circuit for the minimum needed for reliable operation.
    From memory there is a felt pad in there somewhere..see that that is in good order.Basically, if all is correctly set up as best it can be, you'll probably have to live with the remaining noise..R
     
  10. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    To make it run slower, shorten the piece of half shaft.
    It isn't a matter of more weight!
    Remove about 1/4 inch and try again.
    Tinker Dwight
     
  11. djrm

    djrm Registered User

    Mar 18, 2017
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    Greetings, The reason I want to add some more weight is to bring it closer to the factory weight of 4 kg previousley mentioned. I'm sure there are reasons why it should be better like that, I know it struggled to run with much less weight. Now I have added my new weight with lower centre of gravity the clock adjustment is already in the usable range of the setting wheel. I'd like to keep with the original material too if possible to minimise any unwanted temperature effects.

    I have increased the resistance and the clock is quite happy on 200 mA maximum current. I see the current pulse duration is 140 mS. When the resistance was nearer to the minimum the duration was only around half this and hit around 300 mA maximum. The photo shows the voltage across a 1 ohm resistor in series with the supply when the contacts close. horizontal=20 mS/div, vertical = 100 mA/div

    The current is more uniform with the higher resistance, I may increase the voltage supply from around 15 V at present to 27 V to allow me to have an even higher resistance, this should make the drive current more linear. I'm hoping this will also reduce the noise more as the speed of movement should be less at the end of the stroke.

    I'll run it like it is for a few days and see if I can get the time keeping right.
    I'm looking for some slave clocks now to complete my installation.
    Thanks for the suggestions, David.
     

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  12. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    A higher resistance and voltage will give you faster rise time
    but not necessarily improve the operation of the clock.
    I believe the maximum current and duration are all that are needed.
    Tinker Dwight
     
  13. djrm

    djrm Registered User

    Mar 18, 2017
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    Hi Tinker,
    My thinking is that a faster rise time coupled with a lower overall current would lead to a slower final speed when the movement stops as it hits the limit, and that this would be quieter. I wont know for sure until I try it though. I think I read somewhere what the duration should be bet I don't recall what it said.
    Kind regards, David.
     
  14. rogerj

    rogerj Registered User

    Dec 21, 2014
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    #14 rogerj, Mar 23, 2017
    Last edited: Mar 23, 2017
    Hi David...Think of the the electro-magnet as a motor..If you could tweak it electrically to gently and steadily lift the gravity arm back on it's catch in 29 seconds the clock would work just as well - and most of the clunk would be gone. As I'm sure you know, with your electronics background, the higher the source impedance of the supply the more it approaches being a constant current source, which I think will increase the rise time of the current and hence the acceleration. Rather than resistance, additional inductance in the loop might help though. I think I read somewhere that in large master clock installations with large numbers of dials ( ie added inductance) and supply voltages of 50v or more the time taken for the gravity arm to be reset did increase appreciably.(though not to 29 seconds :)
    Extra inductance doesn't equate as stongly to the need for extra voltage though because the high voltages mentioned also accounted for extra resistance in the loop which could be significant.
     
  15. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
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    Actually, the higher the resistance the quicker the rise time. Just the
    opposite for coils than from capacitors.
    Faster rise time, more clunk.
    Tinker Dwight
     
  16. djrm

    djrm Registered User

    Mar 18, 2017
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    Greetings everyone,

    I'm glad to say that my wife noticed the clock was quieter after I increased the resistance, she had been complaining about it.

    I seem to have hit the sweet spot now as the clock is just about right after running all day.

    Today a Synchronone slave clock has just been added into the circuit, it seems to be compatible. I measured the resistance of it at less than three ohms.

    David.
     
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