PERFECTING THE HARRISON TWIN PIVOT GRASSHOPPER ?· The first grasshopper escapement created by John…

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    PERFECTING THE

    HARRISON

    TWIN PIVOT

    GRASSHOPPER

    ESCAPEMENT

  • 2

    Perfecting the Harrison Twin Pivot Grasshopper Escapement

    David Heskin 2009

    First Edition

    All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or me-

    chanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the author and the publisher.

    You must not circulate this publication in any other binding or cover and you must impose the same condition on any acquirer.

    The author and publisher of this book accept no responsibility whatsoever, however caused, for any loss, damage, injury, death or any other consequence as a result of the use of this publication, its contents and/or any errors and/or any omissions contained

    within it.

    Correct and normal workshop practices and timepiece operation must be observed at all times and the author and publisher of

    this book accept no liability for any accident, injury or consequence, no matter how caused, by following the procedures presented herein, which are given in good faith.

    The term replica must not be taken to mean exact copy.

    This publication is not intended to be a thorough or complete constructional guide. Information has been supplied, in good faith, in order to assist construction by those with sufficient skills and experience.

    All figures and diagrams are illustrative only.

    This publication is an extract from

    Perfecting the Harrison Grasshopper Escapement,

    ISBN 978-0-9555875-4-2,

    published 2009.

    Guidance in basic concepts from an earlier chapter of the

    parent publication has also been included, in order to create

    a 'stand-alone' publication.

    As much possible, inapplicable references to excluded parts

    of the parent publication have been removed.

    Figures retain their original numbers

    (7.01 to 7.45).

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    CONTENTS

    INTRODUCING THE TWIN PIVOT GRASSHOPPER ESCAPEMENT................................................................4

    MECHANICAL ARRANGEMENT..........................................................................................................................4

    COMPONENTS AND THEIR BEHAVIOUR...........................................................................................................7

    CONVENTIONS.........................................................................................................................................7

    SETTING THE ESCAPEMENT IN MOTION........................................................................................................13

    CYCLE OF OPERATION........................................................................................................................................18

    TWO MINUTE ESCAPE WHEEL...........................................................................................................24

    NO FRICTION, LUBRICATION OR WEAR..........................................................................................24

    PERFORMANCE STIPULATIONS........................................................................................................................24

    GEOMETRICAL REPRESENTATION...................................................................................................................26

    TORQUE ARMS, RATIOS AND CIRCLES...........................................................................................................30

    GRAPHICAL DESIGN PROCESS..........................................................................................................................31

    SUMMARY...............................................................................................................................................40

    CONCLUSIONS........................................................................................................................................40

    MATHEMATICAL DESIGN PROCESS.................................................................................................................41

    OBJECTIVE AND BASES OF ANALYSIS.............................................................................................41

    MATHEMATICAL MODEL....................................................................................................................43

    MATHEMATICAL ANALYSIS...............................................................................................................47

    SUGGESTED SEQUENCE OF CALCULATIONS.................................................................................49

    MATHEMATICAL MODEL OUTPUT....................................................................................................50

    OBSERVATIONS.....................................................................................................................................................57

    IMPULSE VARIATION............................................................................................................................57

    ACHIEVING HARRISON'S 2 TO 3 MEAN TORQUE RATIO..............................................................57

    ESCAPE WHEEL SIZE.............................................................................................................................57

    TORQUE TO THE ESCAPE WHEEL .....................................................................................................58

    PALLET NIB LENGTH............................................................................................................................58

    ESCAPE WHEEL TEETH CIRCULAR PITCH.......................................................................................58

    ESCAPE WHEEL TOOTH PROFILE......................................................................................................58

    ESCAPE WHEEL RECOIL.......................................................................................................................59

    PALLET LOCKING CORNER LIFT UPON RELEASE.........................................................................59

    CHOICE OF ESCAPE WHEEL TOOTH COUNT...................................................................................59

    ESCAPEMENT SIZE................................................................................................................................60

    MECHANICAL CONSIDERATIONS......................................................................................................60

    ELIMINATING ERRORS........................................................................................................................................61

    RECOIL.....................................................................................................................................................61

    CIRCULAR ERROR.................................................................................................................................61

    ERRORS DUE TO WEAR.......................................................................................................................62

    LUBRICANT ERROR...............................................................................................................................62

    ESCAPEMENT ERROR...........................................................................................................................62

    CAUSES OF ESCAPEMENT ERROR.....................................................................................................63

    DEFICIENCIES........................................................................................................................................................64

    CONCLUSIONS.......................................................................................................................................................65

    BIBLIOGRAPHY.....................................................................................................................................................66

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    INTRODUCING THE TWIN PIVOT

    GRASSHOPPER ESCAPEMENT

    The first grasshopper escapement created by John Harrison was fitted retrospectively to a tower clock created

    by him for stables at Brocklesby Park, Lincolnshire, due to a failure of his anchor escapement interpretation to

    operate reliably. Two pallet arms were mounted upon separate pivots, from which the name 'twin pivot grasshopper'

    has been derived. As far as we can be certain, the Brocklesby escapement was the only twin pivot version of the

    grasshopper ever made by Harrison, although he may have applied the arrangement to some of his early longcase

    wooden movement regulators and subsequently substituted single pivot grasshopper escapements.

    An inquisitive mind is compelled to understand why Harrison apparently abandoned the twin pivot escapement

    after such limited use. The Brocklesby grasshopper has delivered efficient, almost continuous service since the day

    it first came to life, almost three centuries ago. There can therefore be no doubt that it is, at the very least, a reliable

    escapement, offering excellent longevity. Unfortunately, Harrison offered no clues. He was, without doubt, a

    scientist and engineer who applied ruthless logic to every one of his actions. If a deficiency in a device or method

    was exposed, he would either attack the problem until solved, or dispense with it altogether, regardless of the

    personal consequences or time and effort already invested. A study