How do you figure the length of the crutch?

Discussion in 'Clock Construction' started by Rob P., Jan 13, 2012.

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  1. Rob P.

    Rob P. Registered User

    Dec 19, 2011
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    Some day (when I'm rich and famous) I'd like to build a clock from scratch. One question that is bugging me is how the length of the crutch is determined.

    In a way, the crutch sets the swing distance of the pendulum and also limits the length of the pendulum rod. For example: Too short a crutch and a long rod means no impulse strength (unless you use a lot of power).

    So, how do you figure the length of the crutch?
  2. Allan Wolff

    Allan Wolff Moderator
    NAWCC Member

    Mar 17, 2005
    Tulsa, OK
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    The crutch can be any length you want it to be, so its length is usually determined by other factors such as finding an available attachment point to the pendulum. In fact, the grasshopper clock I built according to W.R. Smith's plans has no crutch at all; the pendulum attaches directly to the pallet arbor. The amount of swing of the pendulum is determined by the geometry of the escapement.

    Think of it this way. When the minute hand of a clock moves from 12 o'clock to 3 o'clock, the tip of the hand has moved through an arc of 90 degrees. and it is not just the tip, every point along the hand has moved through the same 90 degree arc.

    Here is a link to a previous discussion on this topic.
  3. Tinker Dwight

    Tinker Dwight Registered User

    Oct 11, 2010
    Calif. USA
    The crutch is just a lever arm, drivng another lever
    arm with the fulcrums located at vertually the same place.
    From that point of view, it make no difference what length
    it is.
    From a mechanical point of view, the shorter it is
    the more the play between to crutch and the pendulum
    or leader becomes a higher percentage of the swing.
    Not a good thing.
    Make it too long and it adds weight to the center of
    the pendulum, that effectively shortens the pendulum some.
    This isn't a big factor though.
    The last is resonance of the rod it self. You don't want to
    waste energy ringing the rod.
    For a uniform rod, it would be best to avoid distances
    along the rod, like 1/3, 1/4, 1/5 etc. Actually it is
    a little more complicated than that because it has to do
    with impedance matching.
    Basically you don't want to drive it at a point that
    makes the rod ring when it is hit. At least not
    close to its first and second resonance points.
    It is difficult to know these points until you actually
    put it together.
    You can tap the rod at several locations and note
    if it vibrates much. You'll most likely not hear it
    but feel it with a light touch of you finger.
    Good luck.
    Tinker Dwight
  4. jhe.1973

    jhe.1973 Registered User
    NAWCC Business

    Feb 12, 2011
    Arizona, USA
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    #4 jhe.1973, Jan 20, 2012
    Last edited: Jan 20, 2012
    Hello Rob,

    Some great advice already here so I'll just throw in a couple of photos of what I did and am now doing.

    The first regulator I built had a long crutch for no other reason than that I think it looks more elegant than a short one. My next one however has a rather short one and it still is being finished so I'll just contribute my thoughts.

    I made it shorter because I think the stiffness will impart a better impulse. I also incorporated the beat adjustment and verge location all into one assembly so that the escapement arbor doesn't transmit any torque.

    I couldn't come up with any reason for the time honored right angle connection of crutch to pendulum if a round pendulum rod is used.

    To keep the crutch contact as a point rather than a flat against a round, I have about a .040 inch HSS drill blank pressed into the side arms and the arms machined away just enough to allow the round drill blank to touch the round pendulum shaft.

    Can't say better or worse yet, it's just that I have found reducing flex as much as possible throughout the entire mechanism pays dividends in the form of less power required.

    I just brought a thread back from the dead that I had started and I will be discussing a lot of what I learned during the build process. You might find some help there too.(shameless plug) :D

    Crutch.jpg Crutch.jpg
  5. jhe.1973

    jhe.1973 Registered User
    NAWCC Business

    Feb 12, 2011
    Arizona, USA
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    I think that maybe Alan is being modest but I'm not :D so I'll mention his terrific thread in case you haven't already followed it:

    To help get you convinced to do a build w/o waiting until you are rich & famous his thread is hard to beat.

    Of course maybe Alan really is rich and famous .......... in that case never mind :D
  6. jhe.1973

    jhe.1973 Registered User
    NAWCC Business

    Feb 12, 2011
    Arizona, USA
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    I just noticed I misspelled your name Allan. I really am sorry about that.

    Maybe I'm being punished for being silly?

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