Subject:
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Escapement efficiency (Re: Gear train friction?)
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Newsgroups:
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lugnet.technic
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Date:
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Wed, 6 Dec 2000 17:50:39 GMT
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Viewed:
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2844 times
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A 1 rotation/ 36 second escapement is going to be really tough. Mine is 1
turn / 8 seconds, and is is close to the edge of the reliability envelope.
The big problem is that the escapement must come to a complete stop with
every pendulum swing, and you need to overcome static friction to restart
it, and it needs to provide sufficient power to your pendulum. Your pendulum
will need to be very long and heavy, so it is hard to get a lot of power to it.
I built a reduction with the large gear wheels, and it seems to have very
low friction, but it is so big and ugly I am tempted to stay with my current
setup. Instead, I worked on the pallets. I redesigned the anchor to take a
pair of tiny hinge elements as pallets, and these can be angled to maximize
energy transfer. I am using my double-paddle wheel escapement design shown
on my web site. Currently I have 1:2 8:24 8:40 8:40 8:40 as a reduction
system. The initial 1:2 is provided by a pulley on the weight. Last night my
clock ran for 8.5 hours and the 2 lb weight fell 18.5 inches, so my
efficiency is 2.75 hrs/ft-lb. The new escapement was running strongly enough
that I think I might be able to swap an 8:40 for the 8:24. By hanging the
weight on a shorter string from the pully, I could get another 2 inches of
drop, so this design could possibly run for 15 hours. With another 2 lbs,
and a set of pulleys, I should be able to get my goal of 24 hours.
One of the tricks I used to boost my efficiency was a new rewinding
mechanism. Before, I used a differential and a pawl. Under power, the pawl
would lock, and the differential would pass power to the escapement. When
the weight was rewound, the pawl would disengage the power. This works well,
but the power needs to pass through a set of bevel gears under high torque,
and this adds a lot of friction. My new method uses a little ratchet device.
It has an input shaft, and an output shaft. When you turn it in the power
direction, it acts as a solid shaft, so the whole device turns with the
shaft. When you turn it in the rewind direction, it acts as a broken shaft,
and does not power the escapement backwards. I'll post a picture now if
anyone cares to see it, or later when my clock is finished.
I also started some work on measuring friction. My previous calculations are
wrong, because I didn't calculate the pressure on the teeth and the shafts.
I've been looking for a M.E. text that has the standard methods for
calculating friction, but I haven't found anything yet. I made a little
friction measuring shaft. It has a gizmo in the middle of a shaft. The gismo
has a pointer that turns to indicate which end of the shaft has more torque.
You put a geartrain on each end of the shaft, spin the gizmo, and the
pointer points to the geartrain that has more resistance, so you can compare
two trains or two bearings for efficiency.
Regarding electric rewind, this shouldn't be a problem. Then power is no
problem, and you can concentrate on accuracy.
Please send me a photo of your escapement design, and I am interested in any
efficiency numbers you can measure.
Check out my clock page:
http://www.best.com/~amnon/Homepage/Games/LegoClocks/
-Amnon
In lugnet.technic, Chris Daniel writes:
> In lugnet.technic, Amnon Silverstein writes:
> > One of the tough problems in building a clock is to design a very low friction,
> Hello, I stumbled across your posting about the continuos loop chimer and it
> blew my mind. So I decided to join lugnut I'm also curently working on a
> weight driven pendulem clock and I'm going to try to build your device. I
> originaly tried to build a clock several years ago and I gave it up at the
> escapment.Then I stumbled onto Leo's site. Anyway to overcome lack of
> runtime, friction, and garbagy lego gear mesh, My friend and I concluded we
> needed a slow running clock. Our logic was less rotations less friction. I
> have a leo inspired escapent with 18 teeth conected to a second hand threw 5/3
> ratio or 24t:40t.So the escapent turns once every 36 seconds with one second
> between each tic and tock. Unfortunitly this set up requires a long pendulem
> and therefor more freggin legos.I don't currently have a way to post any
> pictures but I can e-mail some pics of the escapment to you if you would
> like.I also think one solution to increase run time might be a powered
> automatic winding system. It would utilize motors, polarity switches,a gear
> drive with a rocker arm that enables one way rotation only regardless of input
> direction,and a device to shut itself off when winding was complete(timer).
> These devices do exist in one form or another at
> http://public.surfree.com/werdna/lego.htm#legocad invented by Andrew Lipson
> I'm looking forward to your conclusions on the turn tables.Also I noticed
> the small gear doesn't mesh well. What do you think? well good luck to both
> of us on 1:300
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Message is in Reply To:
 | | Re: Gear train friction?
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| (...) Hello, I stumbled across your posting about the continuos loop chimer and it blew my mind. So I decided to join lugnut I'm also curently working on a weight driven pendulem clock and I'm going to try to build your device. I originaly tried to (...) (24 years ago, 6-Dec-00, to lugnet.technic)
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