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Worm gear

This topic has 9 replies, 7 voices, and was last updated 9 May 2018 at 23:22 by Graham Compton.

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19 March 2018 at 18:59 #346741
Steve Crow
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@stevecrow46066
Hello,

I'm quite new to things horological so I'm going to ask stupid questions. Here's one of them.

Is there any reason why worm gear can't be used in a clock mechanism?

I want a shaft to rotate once a year driving one or more cams and a calendar hand. This will be driven by the hour hand shaft making a ratio of 1:730.

I looked at conventional gearing methods when it dawned on me that a worm on the hour shaft driving a 10T wheel then a worm on that shaft driving a 73T wheel would be a very simple and compact way of doing it.

Also, the worm method gives a continous motion. With conventional gearing you'd have to have a ratchet somewhere to get the reduction so the motion will be intermittent, minimum of 12 hours per impulse.

I know worm gear is inefficient but we are talking large reductions here.

Is this idea viable or am I missing something completely?

Cheers

Steve
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19 March 2018 at 18:59 #3787
Steve Crow
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@stevecrow46066
19 March 2018 at 20:16 #346756
John Haine
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@johnhaine32865
Why would you need a ratchet? Is it because of the large ratio needed of 1:73? Philip Woodward used an elegant ratchet mechanism in one of his clocks where two pawls were driven in antiphase to give continuous movement of the ratchet wheel.

But given that the worms would be on the hour shaft, and you don't need much output torque presumably, you could have a pretty sloppy fit between worm and wheel to minimise the friction.
19 March 2018 at 20:23 #346760
Michael Gilligan
Participant
@michaelgilligan61133
Steve,

Worm gearing has been used in several clocks, but these are typically electrically powered.

… You have already identified the main limitation.

MichaelG.
19 March 2018 at 21:22 #346765
Bazyle
Participant
@bazyle
Two ideas
There is a type of high reduction gearing that works as follows. A large gear wheel is on a shaft and another gear with say one more tooth runs on the same shaft. One of the wheels is secured to the shaft and the other to be non rotating. A small gear, any number of teeth, is rotated around the big gears in mesh with both. The axle moves one tooth for each rotation of the small gear.

Worms have the problem that they are high friction as the flank rubs the tooth of the worm wheel. Some mechanisms may have one gear with the teeth made up of small rollers to reduce friction. You can use the same technique on a worm wheel using rollers on radial pins. It would probably help to use a large diameter for both
19 March 2018 at 21:42 #346767
Old School
Participant
@oldschool
I am no clockmaker but my father was he built orrery clocks they had worm wheel in them as did the Mudges moon clocks.
19 March 2018 at 22:04 #346769
roy entwistle
Participant
@royentwistle24699
Often used on moon phase dials and date mechanisms. Not too sure about 10tpinion driving worm to give 1:730

perhaps others can comment on that

Roy
19 March 2018 at 22:16 #346770
Michael Gilligan
Participant
@michaelgilligan61133
Posted by roy entwistle on 19/03/2018 22:04:05:

Not too sure about 10tpinion driving worm to give 1:730

perhaps others can comment on that

.

Roy,

I believe that Steve is proposing 2-stage worm gearing 1:10 then 1:73

[quote] a worm on the hour shaft driving a 10T wheel then a worm on that shaft driving a 73T wheel would be a very simple and compact way of doing it. [/quote]

This is mathematically sound.

MichaelG.
20 March 2018 at 09:56 #346794
roy entwistle
Participant
@royentwistle24699
Thank you Michael At 10 pm my mind's not really in gear

Roy
9 May 2018 at 23:22 #353428
Graham Compton
Participant
@grahamcompton76871
Having only recently joined the forum I'm coming late to this thread.

It's true that clock makers generally avoid worm gearing because of the friction, but elsewhere I've been describing bits of my Wilding Hipp clock and this successfully uses a 1:60 worm gear to divide down for the minute hand. Music boxes often use worm gears in the step up direction but they are too lossy for the step-up needed in a mechanical clock, particularly since they really do need lubrication for step-up and that is avoided between gears in clocks. My clock's step-down worm isn't lubricated and shows no sign of wear after some 18 years. In terms of tooth form. Wilding specified straight flank teeth, which for the worm is, I think, an involute form, the equivalent of a rack. The wheel is cut with something looking like a symmetrical ratchet cutter, probably not ideal in terms of the involute form but it has worked ok.

regards

Graham
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