The Beall Clock Forum

General => Other escapements => Topic started by: dcunningham2 on January 28, 2007, 04:43:25 PM

Title: Arnfield gravity escapement - most accurate possible?
Post by: dcunningham2 on January 28, 2007, 04:43:25 PM
In analysing the single and double armed gravity escapements, it had occurred to me that there could be a slight variation in the friction required for the pendulum to release the escapement wheel.  I wondered if there might be some way to further isolate the pendulum.

Turns out, I'm too late -- the problem is already solved.  Apparently, as recently as 1986, a method was developed to completely isolate the pendulum from the releasing friction.  From what I understand, a single armed gravity escapement is configured so that the weighted arm itself (instead of the pendulum) contacts the other arm and releases the escapement wheel.  

The following links provide a full description and an animation of the escapement in action:

The pendulum is almost completely isolated from the triggering process.  Its only engagement is to pull the weighted arm 180 degrees away from a bearing surface.  A standard gravity escapement involves a slight shearing friction at 90 degrees during the triggering process.

If I'm thinking correctly, a wooden clock constructed using this escapement would be potentially more accurate than any other possible purely mechanical design, except for a slave/free pendulum system as designed by Philip Woodward and described in his book, My Own Right Time.

I can see only six possible sources of variation in time-keeping for this design:

1.  Variation of air resistance felt by the pendulum and escapement arm.
2.  Variation of pendulum length due to temperature and humidity,
3.  Variation of escapement arm length and other escapement dimensions due to temperature and humidity,
4.  Variation of pendulum pivot friction,
5.  Variation of escapement arm pivot friction
6.  Variation in the "stickiness" between two dry surfaces that are bearing directly upon one another.

I would think #2 & 4 would account for the bulk of the effects, and both of these can be minimized through commonly employed methods.  So unless there is some oscillator more regular than a simple pendulum, I'm thinking this should be the most accurate possible mechanism.

I'll build it and challenge all y'all to an accuracy contest.
Title: Arnfield gravity escapement - most accurate possible?
Post by: rabbit on January 29, 2007, 09:10:15 AM
my Gravity clock has a modified Arnfield gravity escapement. The weighted arm releases the escape wheel by pushing "through" the pendulum rod to the other arm. (the weighted arm releases the other arm, rather than the pendulum doing it.) this avoids the complicated trigger of the real Arnfield. it does introduce "error" in the fact that the weighted arm actually impulses past center. (it also makes this piece have the craziest "tick-tock" you've ever heard!) but, it works wonderfully.
this, and the fact that this clock is enclosed in a case, make this one of the most accurate wooden clocks i've made.
Title: I could have guessed ...
Post by: dcunningham2 on January 29, 2007, 07:58:46 PM
I could have guessed that you would already have built one of these escapements!

Here's my question -- it would seem that even though the weighted arm is acting to push the pendulum arm through the trigger point, the pendulum is still exposed to both the releasing friction as well as the load of lifting the unweighted leg and overcoming its pivoting friction.  It seems to me that the Arnfield is expressly designed to isolate the pendulum from just these effects.  

It also seems to me that these effects are almost insignificant and I wouldn't be surprised to find out that the variance in the length of a wooden pendulum arm due to temperature and humidity is a much larger contributor to inaccuracy.  I actually have another question about how to compensate for this, but I'll post it separately.

So anyway, I suspect that the reason your clock is so accurate is due to the inherent accuracy of gravity escapements in general.

You mention the crazy tick-tock -- is it a "tick-Tick .... Tock", or something else?