Re: Pallet design


James Nye
 

Dear John,

 

You wonder about the origins of the shape of the pallet. Apologies if this is already on your shelf, but the received wisdom is generally taken to be as set out in Miles’ book. Two relevant passages:

 

“In 1906, there was a serious train accident near Salisbury, and since this was attributed to excessive speed, it became clear that a more accurate measurement of the speed of trains was needed.  By 1907, [Shortt] had designed and patented timing equipment for measuring the speed of trains by recording the time taken to travel between two fixed points.  He also devised a graphical method for improving the curves in railway lines, the other contributory factor in the accident.”

From later in the book:

 

“Hope-Jones then went on to describe how the impulse given to the pendulum should ‘begin gently, increase in value until it reached a maximum at zero and then tail off with equal gentleness’, as shown in Figure 3/35 but the shape of impulse pallet, had been empirical with no theoretical basis.  In the audience was William Hamilton Shortt and as recorded in Chapter 1.  This seems to have been their first meeting.  Shortt lost no time and submitted notes in time to be published in the ‘discussion’ at the end of the account of the meeting a couple of months later.  He first postulated that by delaying the impulse, the escapement error could be made to compensate for the circular error (i.e. the error caused by increasing the amplitude of swing of a pendulum see Chapter 2).  As an engineer, Shortt recognised that, in a cam and roller system, what matters is the path of the centre of the roller, in relation to the cam.  He then submitted an accurate graphical analysis of a revised pallet design and his optimisation of the pallet shape is shown in Figure 3/35.  By removing the curve at the top of the pallet, he caused the impulse to rise more rapidly and by curving the slope, he made the end of the impulse fall off more gently.  He derived a mathematical expression for the optimum shape of the curve but in practice this was approximated to a part of a circle.  The pallet profile was filed with a curve of 7/16  in. (11 mm), later altered slightly to 3/8 in. (9.5 mm) radius.  There is a drawback in this optimised design of pallet since if the roller remains in contact with the pallet right to the end of the curve, closure of the contacts could be uncertain.  In actual practice, the contacts are arranged to close just before the roller arrives at the flat part of the pallet curve so that they do not make contact too slowly for reliability.  Final positioning of the pallet on the pendulum was dependent on the installer and the design had to allow for this.”

Cheers

James

 

 

From: synchronome1@groups.io <synchronome1@groups.io> On Behalf Of John Haine
Sent: 10 January 2019 20:37
To: synchronome1@groups.io
Subject: Re: [synchronomeelectricclock] Pallet design

 

Wow!  Thanks for all the quick replies.

Ian, noted that the ideal shape is probably not going to work any better.  Even the "ideal" shape is based on the notion that a raised-sine impulse waveform is optimum, with no real reason why.  As I have a little CNC mill I could form the shape almost as easily as the circular arc.

Peter, I think those pages are almost the same as what I have in "Electric Clocks and How to Make Them" - again the drawing lacks one dimension.

Ian, thanks for that article - very useful!  I think that is from HJ?  Could you say when it was published please?  Now that HJ is online from 1858 to 2000 I should be able to download the whole series.

Geoff, "HSN" is a newsletter published by the NAWCC, sort of the American equivalent of the BHI, it's for oddities like me who still find the physics of pendulums and things interesting.  Not a very wide circulation I fear.

Thanks again all - I'm sure I'll have many more questions.

John.

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