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One of the tough problems in building a clock is to design a very low friction,
very tall gear ratio drive. I want the spool to turn around somewhat less
than once every half hour, and the escapement to turn once every six seconds,
so I need around 1:350 drive. I've been using:
8:24 -> 8:40 -> 8:40 -> 8:40 = 1:375
For every one turn of the spool, all of the axles in the drive train make a
total of:
1+3+15+75+375 = 469 axle turns worth of friction
Also, I can total up how many times a pair of teeth mesh each time the spool
turns:
24 + 120 + 600 + 3000 = 3744 geartooth meshings of friction
I would really like to reduce the friction. I am going to try to use the large
turntables fitted with plates to make axle holes. This could give a train like:
8:56 -> 8:56 -> 8:56 = 1:343
1+7+49+343 = 400 axle turns worth of friction
56 + 392 + 2744 = 3192 geartooth meshings of friction
So this drive train should have about 15% less friction.
If Lego provided a huge 2800 tooth gear, I could have a system that reduces the
friction another 13% or so:
8:2800 = 1:350
350 axle turns worth of friction
2800 gear tooth meshings worth of friction
Lubrication might help, but that is a sort of non-Legoy solution, and I am not
sure what to use. Maybe a dry lubricant like graphite or spray teflon would be
best.
Any suggestions for designing an efficient drive? Am I analyzing friction
correctly?
Reducing the friction and improving the energy transfer of the escapement would
lead to the most dramatic improvement, but this is really tricky.
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Amnon,
I've also found a not in-significant source of friction is the bushes
against the beams (or whatever your axle goes through). I've reduced this a
bit on occasion by not using bushes to hold axles in position, but having a
brick at each end, that the axle (almost) butts up against. It does allow a
bit of length-wise motion, but reduces the friction.
ROSCO
Amnon Silverstein <amnon@best.com> wrote in message
news:G526GG.IKB@lugnet.com...
> One of the tough problems in building a clock is to design a very low • friction,
> very tall gear ratio drive. I want the spool to turn around somewhat less
> than once every half hour, and the escapement to turn once every six • seconds,
> so I need around 1:350 drive. I've been using:
> 8:24 -> 8:40 -> 8:40 -> 8:40 = 1:375
> For every one turn of the spool, all of the axles in the drive train make • a
> total of:
> 1+3+15+75+375 = 469 axle turns worth of friction
> Also, I can total up how many times a pair of teeth mesh each time the • spool
> turns:
> 24 + 120 + 600 + 3000 = 3744 geartooth meshings of friction
>
> I would really like to reduce the friction. I am going to try to use the • large
> turntables fitted with plates to make axle holes. This could give a train • like:
> 8:56 -> 8:56 -> 8:56 = 1:343
> 1+7+49+343 = 400 axle turns worth of friction
> 56 + 392 + 2744 = 3192 geartooth meshings of friction
>
> So this drive train should have about 15% less friction.
>
> If Lego provided a huge 2800 tooth gear, I could have a system that • reduces the
> friction another 13% or so:
> 8:2800 = 1:350
> 350 axle turns worth of friction
> 2800 gear tooth meshings worth of friction
>
> Lubrication might help, but that is a sort of non-Legoy solution, and I am • not
> sure what to use. Maybe a dry lubricant like graphite or spray teflon • would be
> best.
> Any suggestions for designing an efficient drive? Am I analyzing friction
> correctly?
> Reducing the friction and improving the energy transfer of the escapement • would
> lead to the most dramatic improvement, but this is really tricky.
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"Ross Crawford" <rcrawford@csi.com> writes:
> I've also found a not in-significant source of friction is the bushes
> against the beams (or whatever your axle goes through). I've reduced
> this a bit on occasion by not using bushes to hold axles in position,
> but having a brick at each end, that the axle (almost) butts up
> against. It does allow a bit of length-wise motion, but reduces the
> friction.
The length of one axle is normally slightly shorter than a corresponding
beam, ie. an axle #4 is slightly shorter than a 4 stud beam. So this is
probably why you achieve some slack when putting bricks on either side
of the axle to support it.
But wouldn't your solution be analogous to using bushes to support the
axles, but letting there be some slack between the bushes and the
supporting structure? It appears to me that the effect of this would be
the same as the effect of removing the bushes completely.
Fredrik
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Fredrik Glöckner <fredrik.glockner@bio.uio.no> wrote in message
news:m3k89e4iva.fsf@crossblock.localdomain...
> "Ross Crawford" <rcrawford@csi.com> writes:
>
> > I've also found a not in-significant source of friction is the bushes
> > against the beams (or whatever your axle goes through). I've reduced
> > this a bit on occasion by not using bushes to hold axles in position,
> > but having a brick at each end, that the axle (almost) butts up
> > against. It does allow a bit of length-wise motion, but reduces the
> > friction.
>
> The length of one axle is normally slightly shorter than a corresponding
> beam, ie. an axle #4 is slightly shorter than a 4 stud beam. So this is
> probably why you achieve some slack when putting bricks on either side
> of the axle to support it.
>
> But wouldn't your solution be analogous to using bushes to support the
> axles, but letting there be some slack between the bushes and the
> supporting structure? It appears to me that the effect of this would be
> the same as the effect of removing the bushes completely.
The difference is, when the bush is rubbing against the beam, it's a much
greater surface area than when the axle-end rubs the end-stop.
ROSCO
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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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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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