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Straps for vintage fixed wire lug trench or officer's wristwatches.

Blog: Watch Mainsprings

Date: 29 November 2017

Copyright © David Boettcher 2006 - 2020 all rights reserved.

I make additions and corrections to this web site frequently, but because they are buried somewhere on one of the pages the changes are not very noticeable, so I decided to create this blog section to highlight new material. Here below you will find part of one of the pages that I have either changed or added to significantly.

The section below is from my page about Mainsprings.

If you have any questions or comments, please don't hesitate to contact me via my Contact Me page.

Mainspring in Going Barrel
Watch Mainspring in Going Barrel

The mainspring is the power source for the watch movement. It is a spiral strip of metal contained in a circular enclosure called the barrel. The outer end of the mainspring is hooked to the inner wall of the barrel. The inner end of the spring is hooked onto the arbor, a shaft around which the barrel can turn, or which can turn inside the barrel. By turning the arbor or the barrel the spring can be wound up, storing the energy that makes the watch run.

In English the part that the spring hooks onto seems to have always been called the arbor. Strictly speaking an arbor is simply a shaft or axle. The barrel arbor is made larger in diameter where the mainspring hooks onto it so that the spring is not bent too tightly at its inner end. This larger diameter element does not have to be made as part of the barrel arbor; it can be made as a separate part so it is sometimes called the arbor collet to distinguish it from the arbor itself. In Swiss French this is called the ‘bonde’, which literally translates as bung or plug but would in engineering terms be called a boss or hub.

In watches with "going barrels" the barrel has teeth on its outside that drive the pinion of the centre wheel. To wind the mainspring the arbor is turned so that the spring is wound around the arbor. In watches with fusees the outside of the barrel is smooth and one end of the fusee chain is hooked onto it. Teeth on the bottom of the fusee drive the centre wheel. As the watch runs and the fusee turns, the chain is drawn off the fusee onto the barrel. To wind the watch the fusee is turned, drawing the chain back and making the barrel rotate about its arbor, which is fixed, which winds the mainspring. In watches with "motor barrels" the arbor drives the centre wheel and the barrel is turned to wind the spring.

The image here shows a mainspring in a going barrel. The inner end of the spring has a hole which catches on a hook on the barrel arbor. The middle part of the arbor is increased in diameter so that the inner end of the spring is not overly stressed, and the hook is recessed so that the second coil of the spring can lie on top of the first without the hook causing a bulge. The outer end of the spring has a ‘resilient hook’ which bears against a step formed in the barrel wall. The resilient hook is usually made by riveting a short length of material to the end of the spring. In this case, a watch repairer has formed a hook in the end of the spring by annealing it and bending it over, catching a short piece of spring in it.

How to determine the correct dimensions, width, thickness and length, of a mainspring? It might be thought that this is a problem for the designer and manufacturer of a watch movement, and indeed when designing a new calibre a manufacturer will have determined the correct strength and length of mainspring for it, by calculation or more likely from experience, and have tested that it worked as expected. The mainspring barrel and arbor would then have been made the correct size to accommodate the spring and the hard work was done.

But springs need to be replaced when they get tired, and you need to know what size to use. For more modern movements it is possible simply to look up the correct spring in tables of data, but for older watches this is often not possible. If there is a spring in the barrel it can be measured to establish its dimensions, but how do you know that it is the correct one? The mainspring will most likely have been replaced at least once over the life of the watch, and repairers sometimes used whatever they had on hand that would fit.

I have seen springs in identical old movements that were significantly different sizes – different width or "height" and different thickness. For instance; a blue coloured carbon steel spring from a Marvin 13 ligne wristwatch movement measured height 1.4mm, thickness 0.18mm and length 320mm. A brown coloured carbon steel spring from an identical movement of similar age measured height 1.5mm, thickness 0.15mm and length 345mm. The 1.5mm high brown spring looked correct for the internal height of the barrel, so perhaps the blue spring had been chosen slightly thicker to compensate for its lower height. But the strength of a spring depends on the product of its height and the cube of its thickness. The slight extra thickness meant that even though it was not as high, the blue spring was 1.6 times stronger than the brown spring.

HJ article about Watch Mainsprings. S1862  I1156
Download Spreadsheet: MainspringCalculator.xlsx
Download: Spreadsheets – A Quick Introduction

This means that it is a good idea to determine what is the correct size of mainspring. The manufacturer will have determined the size of the mainspring barrel and arbor to accommodate the mainspring, so we can use the dimensions of these to determine what size the spring should be. The correct height can be determined by measuring the barrel, but determining the length and thickness is not quite so straightforward. There are two different ways of doing this depending on which book you read. The most common description says that the radius of the barrel should be divided into three equal parts which are occupied by the arbor, the unwound spring and the space between the arbor and the spring. This is not correct. The correct method is based on the spring occupying half of the area between the arbor and the barrel wall.

Mainspring Spreadsheet

An article by me about calculating dimension of mainsprings was published in the September edition of the British Horological Institute's Horological Journal. The article was based on the information contained on this page, and a spreadsheet that replicates several of the special purpose mainspring calculators from this page was made available. The article itself is only available to BHI members who receive the HJ, but the spreadsheet can be downloaded from this link: MainspringCalculator.xlsx. Please let me know if you find it useful or have any comments.

I have updated the spreadsheet to include a calculation of Turns, Fill% and other ratios of the spring to barrel and arbor from the dimensions of the barrel, arbor and spring. This can be used to analyse and evaluate an existing mainspring. If you don't have Version 1.2 of the spreadsheet dated September 2019, you can download the updated version from the links here. The spreadsheet is in Excel format. If you don't have Microsoft Office Excel spreadsheet software, then Libre Office contains an excellent alternative that can open Excel format spreadsheets and is available absolutely free from Download Libre Office

Spreadsheets were created to simplify and automate business models originally created with chalk and blackboards. They are a powerful tool, easy to use and incredibly useful but, like all complicated tools, if you have never been shown how to use one they can be initially daunting. I have created a quick introduction into how they work to get you going. Download it from this link: Spreadsheets – A Quick Introduction. NB: Updated to Rev. 2.1 September 2019.

Mainsprings and Pendulums

The methods described on this page are valid for pendulum clocks – provided that the correct number of theoretical turns are specified. For a watch going barrel, the number of turns is easily established from the required length of run, e.g. a watch that is manually wound every day is usually given a run of 30 hours or so, which gives a 25% margin in reserve. This also works for clocks with fusees, and with platform escapements. However, the number of turns required for a pendulum clock with a going barrel cannot be established in this way.

Circular error means that a pendulum is not isochronous. To reduce the effect of circular error, the pendulum amplitude must be kept as near constant as possible. The pendulum amplitude depends on the impulse, which therefore must also be as constant as possible. A pendulum suspended from a leaf spring has low friction and requires only a small impulse to keep it swinging in a narrow arc. A pendulum timekeeper with a going barrel therefore needs a long thin spring which will produce a small but sensibly constant impulse during the period from when it is fully wound until it is wound up again. This results in mainsprings that have many more theoretical turns than are required to run for the period between windings. E.g., an the mainspring of an eight day going barrel clock may be sized to have enough turns to run the clock for 24 days or more.

If you have any questions or comments, please don't hesitate to contact me via my Contact Me page. Back to the top of the page.

Copyright © David Boettcher 2006 - 2020 all rights reserved. This page updated December 2019. W3CMVS.