Springs break for a myriad of reasons. Fundamentally, all things break because you place physical stress on the bonds, causing them to separate . The stress can create fissures, or fractures and a vibrating tattoo machine will exploit them more quickly. Metals have a certain amount of ductility, or flexibility, that allows the bonds to shift among atoms, but if you keep working the material, you will break more and more bonds until the fractures spread right through the system and the two sides separate.
It has been my experience that most rear springs break because of being overworked. They just get bent up and down too much or over bent when springing the machine. This causes the spring to weaken and then the vibration on the stress point causes the micro fissures to spread and then the metal to fail. This will usually happen quickly with really overworked springs, although it can happen anytime.
The other popular reason is too much tension on the spring. It takes more magnet to pull it down and then returns with more force than needed and will again weaken at the stress point and break. Both of these problems will be hastened when using cheap springs as well.
There are many other reasons springs can break, and most of these will cause habitual breakage.
Small burrs in the rear spring deck or debris will cause fractures to a spring. Always make sure the deck is flat and clean. Sometimes running your finger over the surface won't tell the whole story. Run a fine flat file to make sure something small doesn't exist. Also check the leading edge of the deck (see arrow). I have seen burrs on the edge that create problems and you couldn't feel them when running your finger over them. Always go slow with a file, you can create all new problems if you just go hacking away at the rear deck.
Another culprit can be what you are using as a rear deck clamp. Nothing adds more interest and character that that exotic coin or washer, but many times they are not as cleanly cut and sanded as they should be and can be hiding those burrs and and spurs that will damage a rear spring. Many of their under-surfaces are also not flush so they can create a problem down the road.
Chemicals will do a number on your springs, and your machine in general. Most of what you use to clean your stations and your tools can be highly corrosive to metal. Be aware of what you are cleaning your machines with and steer away from things that can corrosion. Also be aware of water. Metal and water are not friends, oxidation can happen overnight and jeopardize a spring quickly. I have seen a few rigs in my time that were rusted just on the underneath of the spring. Always make sure you take some time each week to look over your machines and have a maintenance schedule to clean them. Don't just wipe them down, and never be afraid to take them apart and clean them thoroughly. You will always learn something putting them back together and getting them tuned back up to where they were. Trust me the more confidence you have in your machines it will completely transfer to your tattooing.
Some more deck problems I have seen are frames that have chemically treated finishes. Those chemicals, and the process to get some finishes contain acids and corrosive compounds. These can eat into springs, so always make sure that the decks are cleared out of that finish. Steel wool and emery cloth will usually take you down to the steel and save your springs. The above pic shows a deck that has been chemically treated with something corrosive. You can see the oxidation on the frame and it will quickly affect your spring.
The deck in the lower picture is a super uneven platform to place your flat spring onto and then clamp it. All those transition are going to cause a lot of flex under your deck clamp that will put undue stress on different parts of the spring.
Here is a sneaky one that can be a definite spring breaker. A-bar is hitting on the rear coil first. I know this is a tattoo machine 101 no-no, but a machine will run hitting on the rear coil. This can also happen if parts are swapped out (new a-bar, springs, coils) and the machine is not re-set up and tuned to accommodate the new parts. Again hitting on the rear first and having the flex over the front coil will cause all sorts of different stress point to form. This one will break springs consistently. The picture above is exaggerated for my point, many times this can happen because people are trying to get that back coil right as close to the a-bar as possible. This is one of those builder things, people love to hold your machine up and judge you by how close you can shim that rear coil up. Judge it on how it puts a line in, not how much daylight is peeking through the rear coil.
I love hand made tattoo machines. They are works of art and I can appreciate something that is crafted as a piece of art that is going to be used to create art. That being said, I love machine made springs that are punched out on a die. Way back in the day I used to hand punch / hand cut springs for our early machines. My hands were always cut up, and I once smashed my foot because I threw the spring punch after it pinched me. I would do a batch of springs and after all the work some of them would be jagged and uneven and downright ugly. When you cut one right it felt good, but not all hand cut springs are created equally. Cutting, bending, punching, sanding and filing can create all sorts of micro fractures and potential fissure points. So hand made springs that break can often be traced back to the method of making them.
Back when I got started everyone I worked with used to run their new machines for days at a time, straight because they needed to "break in" the springs. Then some old time tattoo oracle told them to hit the back spring when it was running with a knife to really weaken it up for black and grey. Come to find out that old wizard was wrong and you shouldn't hit your back spring with anything, ever. You also shouldn't run your machine for days on end, a tattoo machine is not a 350 crate motor that has a break in schedule. Now and days if you want a weaker spring, replace that 20 ga. spring with a 16 ga. spring. Always have some extra springs, especially when you travel. There is not quick fix for a broken spring, be prepared.