GearHeads Corner

Hi Art,

I am using GM version 4.890 in imperial units to help me design a new dead beat escapement wheel for my 19th century turret clock which was manufactured here in the UK.

The escapement wheel has 30 teeth and has a CCW rotation. It is exactly 3 inches in diameter giving a diamentric

pitch of 10. The shaft centre to the escapement shaft which drives the pendalum is also exactly 3 inches.

The tooth span is just over 11.

My first problem is that I cannot get the mechanism to rotate in a CCW direction.

My second, and perhaps far more serious problem, is that GM is not calculating any where near the correct distance

for the escapement centres and is behaving quite strangely when moving up from a tooth span of 10.

Here are my observations

Tooth Span 9.998    Centre Distance 2.9989

Tooth Span 9.999    Centre Distance 2.9995

Tooth Span 10.000    Centre Distance 2.7541

Tooth Span 10.001    Centre Distance 3.0005

Tooth Span 10.002    Centre Distance 3.0011

Tooth Span 10.003    Centre Distance 3.0016

etc

Tooth Span 11.001    Centre Distance 3.6888

As you can see GM doesn't like the tooth span of 10 - all the others increment between .0005 and .0006 inches. But

worst of all is when the tooth span is just over 11 the distance is more than half an inch in error.

Incidentally the tooth form on my escapement wheel differs from that which GM is calculating. The root of the tooth

is wider and there is about a quarter circle undercut near the tooth tip - I presume this is to give a lower drop angle but I'm not sure.

Jim[/size]

Jim:

  Escapements arent an exact science in that the math varies from clockmaker to clockmaker, so
Im not too surprised you cant exactly match an older clocks center distance or tooth span. My version
is typically best on about 25% of the deadbeats teeth as a span, the centers are based on the current
math used by many on variosu websites. You cant switch directions, but from GM's perspective you
dont need to , a CCW is a CW when viewed from the rear. So rotating the deadbeat and the pawl on
their shaftschanges from CW to CCW with no other effect, so GM didnt bother having the selections necessary to
show them in reverse. The theory was to design as if viewing from the back if one wanted a CCW as opposed to CW.
  I suspect your center distance varies from GM's as a function of the difference of the deadbeat type and perhaps the
types of pallets on the one you have. Both will change the escapement in pretty drastic ways in terms of distances
between shafts and such.

Personally,  Id go with Tooth Span 10.001    Centre Distance 3.0005 , which is as close as you can get, but
you may find its hard to get a deadbeat to match an unknown design of pawl...

Art

 

Art,
Thanks for the swift response.
I'll go with your suggestions.
I'll probably get the wheel cut using a local water jet company.

I used them recently to cut some T10N pulley profiles generated by GM which I used in the gear train of a battery driven car built by a school for which I'm a CAD/CAM advisor. The car's first outing was last week-end and the car managed a creditable 4th doing the equivalent of 2,600 mpg! We couldn't has done it without your help.

Thanks

Jim

Jim:

  Excellent. Thx for the gear story, Its great to hear how GM's gears and pulleys are used..


Art