How to Size Any Watch Mainspring to a Barrel

if you work on vintage watches long enough it won't take long before you run into a mainspring that's either completely the wrong size or the mainspring is too weak and it doesn't provide enough power to the escapement which is resulting in low amplitude whatever the case is if you're going to be in this game you're going to need to be able to figure out for yourself what size mainspring that Barrel should have in it and that's what we're going to be learning about today [Applause] look ever since covet started there's been a boom in watch repair interest and you're going to buy a watch where the movement has been worked on by someone who knows less than you do now that's not a knock against you but I want you to think about that for a second home watchmakers will sell their movements that they can't fix or they can't get to run they'll sell them on eBay so if you're buying non-runners or vintage movements you can never trust that things are going to be original or correct in the movements that you're buying if you work on pocket watches that pocket watch literally may have been worked on five or six times before it even got to your bench now look there's tons of scenarios and reasons that an amateur or even a working watchmaker may have put an incorrect mainspring in a barrel and there's all kinds of problems that could be caused by it if it's too short it may have a shortened power Reserve which can affect your daily rate if the mainspring is too strong the amplitude may be too high and you may be getting knocking which can cause damage to the movement itself if the main spring is too weak the amplitude will be too low now if the mainspring is too wide it may not fit into the barrel properly causing friction and wear on the inside of the barrel and if it's too narrow the spring may be twisting a bit inside the barrel causing wear again now I'm going to bet that the majority of you watching this video probably assume that mainspring sizes are an absolute well if you're working on Modern movements you probably should stick with what the manufacturer is calling for but it's important for you to know that you do have some leeway when sizing a mainspring in vintage watches and you don't really have to follow the exact Factory sizing in most of these cases let me give you an example you might have a seven jewel watch that's suffering from low amplitude because there is where in the angel pivotals now you could certainly fix that problem by rebushing the holes but what if that's not an option because of either your current skill level or maybe you have the skill but the value of the movement doesn't really justify the amount of work that needs to be put into it to fix all the jewel Halls well another thing that you can do is just to increase the strength of the mainspring which is going to increase the available power coming out of the barrel that's going to compensate for the wear in the movement that's eaten up all your power before it even gets to the Escape how many times have you heard me say that 70 of the mainsprings power is actually lost by the time it gets to the escapement by increasing the power of the mainstream this means that there's going to be more available power at the pallet Fork if you were looking into mainspring sizing Theory the Carl uses what's known as the rule of thirds which simply says that if you took a barrel and split it in half one third of that space should be the arbor one third of that space should be the Unwound mainspring and a third of that space should be just open as you can see in this diagram the Unwound spring is slightly less than one-third now in George Daniel's book watchmaking he says that the spring should take up exactly half the area between the arbor and the barrel wall and then the Swiss theory of horology it says that the mainspring should always occupy always occupy 50 percent of the open space in the barrel no matter if it's at rest or if it's fully wound now in the general Resort days catalog there are around 150 different Springs just for 11 millimeter barrels alone which range the thickness from uh 95 thousandths of a millimeter up to 16 hundredths of a millimeter with lengths from 300 to 400 millimeters so why all the differences of opinion and why are all these different sizes out there well I don't know the exact answer to why there's so many sizes but what this is clearly saying is that mainspring sizing is not an exact science and you do have some leeway when fitting a mainspring as long as you stay within certain parameters okay so now I'm going to share how I learned to size mainsprings and we're going to compare this formula that I'm going to show you to a couple of known mainspring sizes first to determine the strength or the thickness of the mainstream you measure the inside dimension of the barrel okay so you measure the inside of the barrel with your calipers and divided by 87. to get the length of the mainspring you use the inside Dimension again and you multiply it by 30 for American movements American pocket watches or 35 for Swiss movements now to get the width of the mainspring you just measure inside the barrel from the bottom of the barrel to the top Edge and then you subtract the thickness of the lid plus another uh one tenth of a millimeter for clearance now let's test this formula against a couple known mainspring sizes okay so first we're going to measure the barrel out of an ATA 2824 now the inside dimension of the barrel is 11 millimeters so we're going to take 11 millimeters divided by 87 and that equals 126 thousandths of a millimeter so that's going to be our strength our thickness when we multiply the barrel ID by 35 to get our length we come up with 385 millimeters now to get the width or the height of the mainspring inside the barrel we measure from the bottom of the barrel to the top Edge which is 1.64 millimeters and then we're going to subtract the tenth of a millimeter for clearance along with the thickness of the lid which is 0.31 millimeters all right now in this case we came up with a width of 1.23 millimeters by a strength of 0.126 millimeters by a length of 385 millimeters and the ETA specs are 1.23 by 0.125 by 400. so as you can see it's almost identical now let's do another one this time we're going to do an American pocket watch which is the Elgin model 315. now I can look up the part number in my original Factory parts book and I'll cross reference that part number in my Swagger catalog to get a factory mainspring size of 2.05 millimeter by a strength of 0.196 millimeter by a length of 508 millimeters when I measure the barrel I get an inside dimension of 15 and a half millimeter and if I divide that by 87 I get a strength of 0.179 which is off by less than 200th of a millimeter now when I multiply the barrel ID by 30. I get a mainspring length of 547 millimeters which is almost 40 mil 40 millimeters longer than the factory spec uh but it would easily fit into this big barrel which has an inside circumference of about 48.5 millimeters so there's plenty of room for one extra coil when I measure the barrel depth I get 2.59 millimeter so then I just subtract 0.1 millimeter for clearance and then I subtract the thickness of the lid which is another 0.5 millimeter leaving me um 1.9 millimeter which is only 15 hundredths of a millimeter smaller in width than the factory size of 2.05 so that'll work fine so what's the takeaway here well if you're having a problem with low amplitude you have to start troubleshooting at the power source you have to verify that the mainspring is correct or at least that it's not too weak only then once that's verified can you start working your way through the powertrain to the Escape and guys if you start checking your mainspring size it will make you a better watchmaker foreign

Channel: Watch Repair Tutorials

Views: 14,044

Keywords: watch repair, watch repair tutorials, watch repair for beginners, watch servicing, watch repair lessons

Id: BEfB8ZWoeII

Length: 11min 40sec (700 seconds)

Published: Tue Mar 07 2023