Starrett mercury plumb bobs
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    Default Starrett mercury plumb bobs

    I started my aprenticeship as a Millwright in 1969, the common thought about Starretg plumb bobs was the mercury dampened vibration, I called Starrett and was told the mercury added weight to increase the ratio of weight to diameter, I filled my plumb bobs with mercury and they settled faster, the problem as I saw this was the error really was transfering the vertical line to the part in question, Optical Jig transits solved this problem but with extra expense and work, but a great increase in accuracy.

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    I started my carpenter apprenticeship in 1962. I saw a mercury plumb bob used by the layout crew on their transit. The bob would quiver and come to a stop quickly. This plumb bob was to expensive for a carpenter.
    mike

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    I've made several large plumb bobs out of big chunks of brass and steel- construction friends got a kick out of bringing out a multi pound weight for different simple tasks. But lasers are so much more covenient!

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    Hmmmm...
    Mercury is remarkably free flowing , hence the name(s), I'd be surprised if it dampened vibration , but maybe something in the container helped?

    Clocks also use mercury as weight (and temp compensation) and their pendulums would not want to be damped at all.

    I'm intrigued to know how the starett damper works :-)

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    Quote Originally Posted by Billtodd View Post
    Hmmmm...
    Mercury is remarkably free flowing , hence the name(s), I'd be surprised if it dampened vibration , but maybe something in the container helped?

    Clocks also use mercury as weight (and temp compensation) and their pendulums would not want to be damped at all.

    I'm intrigued to know how the starett damper works :-)
    Summary: both are bobs, but have different suspensions and needs.

    Suppose the mercury in the clock pendulum completely fills the cavity in that pendulum. As the pendulum swings there is no movement of the mercury relative to the pendulum’s metal case. Therefore, no energy is lost stirring the mercury. And since the clock pendulums are always suspended on a solid shaft of some sort, the pendulum is not free to rotate about its junction with the shaft. It’s mercury does increase its density, though and does help it store more energy so that cutting through the air tends to affect its motion less than a less dense pendulum.

    Now, consider the mechanic’s plumb bob on a string. It can have three different motions. It can spin around the axis of the suspending string. It can rotate “side-to-side” around the point from its suspension point where the string joins the bob—-jiggle if you will. And it can swing “like a pendulum” in an arc with a radius defined by the string length. Filling partially (best) or even fully (good) will cause damping of the jiggle and the spin as both of those motions involve stirring of the mercury as the plumb bob shell will tend to move relative to the contained mercury and thus create some heat. The simple swing of the pendulum induced by touching of the pendulum will not be reduced significantly, though a partially filled bob would stir the mercury a tiny bit due to wind resistance. But, the final benefit of the mercury is increased density of the bob and therefore reduced sensitivity to air movement in its environment. I would much prefer the mercury-filled bob as rotation and jiggling are a nuisance when using a bob and air movement can be a problem, though not always.

    Denis

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    Any ideas why they aren't using depleted Uranium for plumb-bobs now???
    ...lewie...

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    Quote Originally Posted by Lewie View Post
    Any ideas why they aren't using depleted Uranium for plumb-bobs now???
    ...lewie...
    My guesses: Probably for the same reason new car models are not named Edsel, new planes are not named Hindenburg, and cruise ships are not named Titanic. There might be a bit of a perception issue with the public. Then there is the uncertain actual risk of general population long-term exposure to DU. (I hear even nano-second exposure to .50 cal DU rounds can be lethal. )

    Denis

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    Just learned about mercury plumb-bobs last year and have one on my wish-list. I think having 'mercury' in the name is a deterrent to the few who would still use them (such as old school framing carpenters and those doing old Millwright tasks) due to all of Mercury's negative press in the last 50 years. A number of eBay sellers note that the Mercury has been removed (probably for USPS concerns). IMO, anyone looking for one would do better to know the model they're after and leave 'Mercury' out of the search. Many sellers probably don't know what's inside.

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    The same principle is applied to tall buildings, mass tuned damper, the liquid provide a resistance to motion using inertia, it wants to stay still, building moves inertia provides an opposing force, same as a mercury damped Bob, I think it was in a book called critical damping ( there’s critical and sub critical from memory) water tanks on top of buildings stop sway, mercury in bobs stops swing.
    I’ve seen one but they were “banned” (though still sneaked out, they were heavy, support was piano wire, sometimes several hundred feet long) these guys came in to build a slip form chimney at one of the power plants, the casting stage had one in the centre with a reel of piano wire, form moved up, plumbob lowered, the guy said it wasn’t really a plumbob as there wasn’t lead in it!
    Plumbum is Latin for lead, modern parlance vertical so I’m not too sure.
    Mark

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    I can see how the spin and jiggle would be reduced , the mercury will stay still as the bob spins so angular momentum (AM) is reduced compared to a solid bob , similarly, the jiggling would benefit from reduced AM .

    Are you saying that the swinging time is typically similar to a solid bob? If so, then I've probably misunderstood the benefits described by the OP.

    Thinking: a gimbled mass would have lower AM , so might be an alternative to the heavy liquid.

    Quote Originally Posted by dgfoster View Post
    Summary: both are bobs, but have different suspensions and needs.

    Suppose the mercury in the clock pendulum completely fills the cavity in that pendulum. As the pendulum swings there is no movement of the mercury relative to the pendulum’s metal case. Therefore, no energy is lost stirring the mercury. And since the clock pendulums are always suspended on a solid shaft of some sort, the pendulum is not free to rotate about its junction with the shaft. It’s mercury does increase its density, though and does help it store more energy so that cutting through the air tends to affect its motion less than a less dense pendulum.

    Now, consider the mechanic’s plumb bob on a string. It can have three different motions. It can spin around the axis of the suspending string. It can rotate “side-to-side” around the point from its suspension point where the string joins the bob—-jiggle if you will. And it can swing “like a pendulum” in an arc with a radius defined by the string length. Filling partially (best) or even fully (good) will cause damping of the jiggle and the spin as both of those motions involve stirring of the mercury as the plumb bob shell will tend to move relative to the contained mercury and thus create some heat. The simple swing of the pendulum induced by touching of the pendulum will not be reduced significantly, though a partially filled bob would stir the mercury a tiny bit due to wind resistance. But, the final benefit of the mercury is increased density of the bob and therefore reduced sensitivity to air movement in its environment. I would much prefer the mercury-filled bob as rotation and jiggling are a nuisance when using a bob and air movement can be a problem, though not always.

    Denis

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    eBay won't allow anything containing mercury, but you'll always find old standard cells (voltage reference batteries) for sale that are full of mercury and cadmium!

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    I have a little dish of mercury that I gathered up after it leaked from an old electric level switch.

    I have no idea what I should do with it.

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    The swing time (period) of the pendulum is only slightly affected by its mass. In a vacuum two bobs with different densities but the same center of gravity have the same period. In air the denser of the two has a shorter period due to wind resistance affecting the lighter one more. Hanging from the same string in a vacuum the denser one has a CG further from its suspension point and a longer (slight) period. In air the wind resistance is overcome better by the denser material increasing its tendency to have a shorter period. Think of a bob swinging in dense syrup—-long period with quick damping. So, in some ways a completely mercury-filled bob would be a disadvantage if you look only at period damping. The 87B Starretts were partially filled and “sloshed,” BTW.

    Denis

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    Quote Originally Posted by Billtodd View Post
    Hmmmm...
    I'm intrigued to know how the starett damper works :-)
    L S Starrett patented it, in 1906. It's US patent number 833,699.

    Go to Google Advanced Patents and enter the number as US833699 for a copy.

    It's Starrett No 87, and lots of eBay listings do mention that it is Mercury-filled.
    Last edited by Joe Gwinn; 06-07-2021 at 10:13 AM. Reason: Add note.

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    Having read that patent in the past, it was of interest to me that the patent holder specifically mentioned completely filling the plumb bob with mercury and leaving no free space between the screw down lid and the mercury. Apparently the goal was originally *not* to have any sloshing going on. I would be curious to try it both ways (both with and without an airspace) and see what actually worked better. I should think that swing would be more vital to eliminate than local vibration, hence a completely filled plumb bob would be the way to go

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    From memory, the swing T = sq root (L/g), L obviously is the length and g is 9.81, though g varies where you are which is confusing but there we go, I remember going to the science museum in London as a kid they had a giant pendulum up to the top of the dome, it showed the earths rotation, facaults pendulum I think, it was fascinating.
    Mark

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    The mercury in a clock pendulum is for temperature compensation. The bob filled with mercury is suspended from the bottom of the rod so that the rod is in tension and the bob is in compression. The theory is that by using materials that have different temp coeffs, the lengthening of the rod with temp will be compensated by the change in the bob in the opposite direction, thus keeping the center of oscillation of the pendulum constant.
    The problem with many regulators of this sort is they used glass jars, thus creating a bad dT/dt response. This was solved by Riefler who figured out that you could get excellent compensation by using a tube for the rod and partially filling it with mercury.

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    Hard kicking rifles for dangerous game (like elephant, rino, etc) often have a mercury dampener installed in the stock to lessen recoil. I have several such rifles but am not sensitive to recoil so I have never installed one.

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    Quote Originally Posted by Superbowl View Post
    Hard kicking rifles for dangerous game (like elephant, rino, etc) often have a mercury dampener installed in the stock to lessen recoil. I have several such rifles but am not sensitive to recoil so I have never installed one.
    Interesting. Again i suspect this is adding mass without increasing angular momentum. While the gun would be heavier with the mercury 'damper', but it would still feel easy to maneuver as the mercury would roll around . When the gun recoils its whole mass is shifted thus acting to reduce the recoil shock.

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    Quote Originally Posted by Joe Gwinn View Post
    L S Starrett patented it, in 1906. It's US patent number 833,699.

    Go to Google Advanced Patents and enter the number as US833699 for a copy.

    It's Starrett No 87, and lots of eBay listings do mention that it is Mercury-filled.
    Thanks, I've just read it.

    It makes no claims to add damping , the claim for the filled mercury vial , is essentially added mass because of mercury's higher density.
    The other claim is the hook for using it with a shortened string.

    A bit of a disappointment really :-|


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