American st electric motor question

Discussion in 'Electric Horology' started by mrpat2, Feb 27, 2019.

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  1. mrpat2

    mrpat2 Registered User

    Aug 17, 2018
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    Hi Guys I have a ST electric that has 2 self winding arbors, model b 1700 see pic. It seemed to run a tad slow, maybe a few min over couple days. So, took out the movement for a closer look, and was checking the motor when I gave a slight tug on the shaft and the flywheel came right off! that might explain it, I think it should be more of a tight fit so it doesnt slip. My theory is that when it is rewinding the chime springs, that puts more load on the gear train and that is when it would slip.
    Also there is some slop in the bushing that is closest to the hour/min centerline (see pic). Id like to rebush that, but its one of those that has a shoulder inside. Does the entire bushing get replaced or do I ream out this one and put a "bushing in a bushing"?
    Thanks in advance
    Patrick

    st electric1.jpg st electric2.jpg st electric3.jpg st electric4.jpg
     
  2. mrpat2

    mrpat2 Registered User

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    So heres an update. I pulled apart the front plate and found the pivot worn down, so I polished and smoothed it down and rebushed the hole for a better fit, It still seems to run slow, losing about a min a day. The rest of the gear train looks ok, nothing dragging, etc. I hooked up a labscope to see what was happening on the voltage and current to the motor. See the pic. Has anybody done an analysis of voltage/current on electric clock motors? Notice the current lagging the voltage, is this about right for this motor? The blue trace is current, yellow is voltage (60 cycle 120 V). One other point, is that if I stall the motor with my finger, it doesnt seem to change the current pattern. I would think that the current draw would go up

    stelectricvoltcurrent.jpg
     
  3. David S

    David S Registered User
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    When you stall the motor do you see a change in phase shift between current and voltage?

    David
     
  4. davefr

    davefr Registered User
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    Voltage will lead current in an inductive device and I doubt you'd see any significant current inrush as you approach stall speed. (not like you would on a typical split phase inductive motor).

    In my experience coils either work or they don't. I've never witnessed one ever becoming weak.

    IMHO your problem is mechanical. Either your motor isn't producing sufficient torque due to excess friction or bad alignment between the main shaft/bushing or the clock movement is putting excessive load on the motor due to wear or other factors. Have you tested the motor separated from the clock movement and does it maintain it's rated speed?

    I might have some of these motors. Can you show an image of the back side?
     
  5. mrpat2

    mrpat2 Registered User

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    Thanks for the comments and questions. I didnt see any phase shift when stalled. I also didnt disconnect the motor and test I think thats something I should do. I am familiar with inductive motors and how the current lags the voltage but never have tested one like this. Ill recheck the mechanicals to see if Ive missed something and see if I can get a pic of the motor too
     
  6. mrpat2

    mrpat2 Registered User

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    Another question.. what kind of rpm tester is appropriate for measuring speed on these electric clocks?
    It occurred to me I could check the speed of the motor installed and compare to removed and running free. I see some inexpensive ones on amazon but are they accurate enough to rick up the difference of only a min/day?
     
  7. davefr

    davefr Registered User
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    #7 davefr, Mar 8, 2019
    Last edited: Mar 8, 2019
    A tachometer isn't accurate enough. You need to test it against a known good precise clock.

    For example if it's a 1 RPM motor, just mark the motor's shaft or pinion gear and make sure it passes the same spot at the same time a known good clock's second hand passes a specific time. If it's 3.6 RPM then every 5 minutes it should be exactly in sync with the known good clock. You can do this for almost any speed motor. Perform this test over a long period of time to ensure it's always 100% in sync at specific times.

    This test is a good initial sanity check but often times a motor will run accurately without a load but then slow down under the clock movement's load. You can measure the torque with various methods/instruments and compare it to values from known good motors.

    You can also test the motor in a known good clock movement if you have one that you're 100% confident in.

    It's best to do all three methods. Your style of motor produces pretty low torque so they're unforgiving to excess friction.
     
  8. mrpat2

    mrpat2 Registered User

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    This brings up an interesting point of study. Through gear reduction, of x # of gears, what would the rpm of the motor need to be to achieve the final rpm of the min hand of 1/60 rpm. on this clock, there is the motor which spins without any reduction gears, the pinion is on the motor shaft. Then there are 5 more shafts/pinions then the final min hand shaft. Do I count the teeth of each gear? Or is there an easier way? Im thinking I could put a mark on one of the gears? There are variables, such as every 15 min there is a cog that lifts the trip lever for chiming, and there are 2 chime spring barrels that need to be kept wound. How would I know if it is running slow just during these events but not in between?
     
  9. flynwill

    flynwill Registered User
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    The easiest way is to count the teeth on all the wheels and pinions and then do the calculations. I have found that the easiest way to count teeth is to photograph the wheel and then in photoshop (or similar photo editing program) mark every 5'th tooth around the wheel (which is easy to do visually). Then counting marks multiplying by 5 and adding the remainder.
     
  10. davefr

    davefr Registered User
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    The best method would be to temporarily disable the chiming/cog lifting and spring winding and see if normal time only operation is accurate. However that really doesn't solve the problem of determining if the culprit is the clock or the motor or both.

    It might be time to disassemble the clock movement, clean it, and check for any parts that are worn or out of tolerance. A good motor won't compensate for a problematic clock movement.
     
  11. mrpat2

    mrpat2 Registered User

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    Thats a good idea, at least it might isolate the problem to the chime area or time/motor part. But youre right nothing beats a complete tear down and inspection, its just that this clock "looked" ok on initial lookover. But it did have the one pivot and bushing problem, maybe there is some more in there. I was originally hoping that this one pivot and rebush job would have fixed it but no such luck. Ill report back in a few days or so after some more work. Thanks for all the advice!
     
  12. mrpat2

    mrpat2 Registered User

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    Ok Ive been through this clock 100%. Tore it down complete, checked all the pivots bushings etc. The only pivot problem was the orig one and then I had a problem with that wheel snapping in 2 pieces, but got all that fixed ok. I started another thread in the repair forum, as I didnt think it was a question for this one. So, all mechanicals look good, no dragging anywhere that I can see. It is still losing time, now after running for almost a week, about 3 or 4 minutes. The only thing I can think of is the cam that starts the chiming mechanism every 15 minutes. Is there any way that this system could put undue stress on the time train so that it would slow down ever so slightly during those events? I can lift the lever to start the chime and it doesnt seem tight or binding. I cant believe that through gear reduction from motor to final minute hand shaft that this wouldnt be able to tolerate this kind of extra toque. But I am at a loss to figure why it loses time. Unless there is something wrong with the motor?
     
  13. mrpat2

    mrpat2 Registered User

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    So now I think Ive isolated the problem. I have disabled the chiming levers so that it cant trip every quarter hour. Still runs slow. Then I have read some posts here about the self winding spring barrels for the hour and quarter hour mechanisms. There is a slip mechanism built in to the barrel so that once it is fully wound it will slip and not put any undue friction on the minute arbor. Maybe mine are not slipping and releasing as designed.. so its back to pulling it apart, but how does one disassemble this barrel assy? it looks all sealed up and crimped shut. I propped up the levers to release the spring tension ok. just letting it run till it ran out. Any ideas?
     
  14. mrpat2

    mrpat2 Registered User

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    one other point I noticed that when i released the trips so the spring would wind down, both the quarter and hour springs ran quite a while before they stopped. I have read elsewhere that they are supposed to be good only for a few chime cycles but mine would have chimed for many more. This indicates that they were wound pretty tight
     
  15. mrpat2

    mrpat2 Registered User

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    got the spring barrels out. I can wind them in my hand but I cant get them wound enough to feel any slip, so they must be winding up over time and the creating power thieving which slows down the clock. I can remember the first reassembly not seeming to lose time for a few days then it seemed to get worse. How do I take this apart?

    st electric chime springs.jpg
     
  16. mrpat2

    mrpat2 Registered User

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    I used a barrel tool to tease off the cover and sure enough its bone dry and just beginning to rust. Ill service the spring just like a normal mainspring and it should work better. Interesting how they made the extra tail on the end of the spring

    st chime spring.jpg
     
  17. Karl Thies

    Karl Thies Registered User
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    #17 Karl Thies, Apr 15, 2019
    Last edited: Apr 15, 2019
    Do NOT use grease or oil on these mainsprings. The only lubrication to use is graphite. If you grease or lube them they will no grip the barrel properly and you will have little power for the chime springs. The bridle has to slip on the barrel so that it can continuously wind but it also needs to grip enough to provide the proper power. Hope you didn't lose the tension washer on the motor when the rotor came off.
     
  18. mrpat2

    mrpat2 Registered User

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    well its been over a week now since I reassembled the movement after servicing the chime springs. I didnt know any different, as no one had replied to the last couple posts. Anyway, I cleaned the springs with steel wool and thinner, dried them, then added mainspring lube on reassembly. It doesnt lose time anymore (one of the orig problems) and it chimes correctly. For a week now. These things get a complete turn of winding every hour, since the barrel gear is the same size as the min hand gear. When chiming, they dont use up a turns worth of winding, so there is more than enough winding going on to keep them up. I noticed that when I reassembled the barrel, I could wind it up a fair amount before I could feel it slip in the barrel. Im not inclined to pull it apart just to change the lube... As far as a tension washer.. the only one I saw was a small brass one under the rotor. It got put back, the motor is working properly also
     
  19. Karl Thies

    Karl Thies Registered User
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    I found that using regular oil or grease made the barrels too slippery and then made the chimes run very slow. If it works then that is fine. The small brass washer is very important on the motor, without it the motor can get noisy.
     
  20. mrpat2

    mrpat2 Registered User

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    does this info come from known sources or is this something that is learned by experience?
     
  21. Karl Thies

    Karl Thies Registered User
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    A little of both, articles on the springs appeared on the forum years ago, and I also learned it the hard way by trial and error. You learn a lot of tricks in 60 years of clock repair.
     
  22. mrpat2

    mrpat2 Registered User

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    then I bow to the clock repair gods as I have only been at this for about a year. Nothing like experience esp when you can learn from others!
     
  23. Karl Thies

    Karl Thies Registered User
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    For one year experience you are doing very well. Many clock repairers won't even attempt an electric clock repair, they say it's not worth their time. I love these ST electrics, especially the Medbury 5e's. I have 5 in my collection.
     
  24. Mike Heffernan

    Mike Heffernan Registered User

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    What happens to the ST is that one of the gears is 2 pieces with a friction slip to allow for the Time Set. Over time with the back tension of the 2 chime main springs this gear begins to slip. This slippage shows up in the form of a slight loss in time. You have to replace of repair this gear.
     
  25. Karl Thies

    Karl Thies Registered User
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    .
    While the pinion slippage could cause a loss in time, it is not because of tension from the winding springs. The springs are wound by the fixed part of the wheel. The only thing that causes drag on the slip pinion is the hands and strike levers which are actuated by this part of the wheel
     

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