How long does a mainspring last? – Part 1
by B.B. Pelletier
Here’s a question all spring gun owners have asked. We got this question from an anonymous reader last week, but I know it’s one many of you will be interested in. This will take some time to explain, so please bear with me.
What happens when a mainspring fails?
Before answering how long a spring lasts, we first have to know what a failed spring looks like. Mainsprings fail in several ways, and for a couple of reasons. The most common failure is when the steel in the spring relaxes, allowing the steel structure to shift. I will come back to this later, but for now, let’s learn what happens when a coiled mainspring is made.
Turning a wire into a spring
I’m not going to explain steel structure, but think of a spring as a straight rod that has been twisted into a coil – because that is exactly what a coiled steel mainspring is! This rod is actually a thick wire, and that’s important to know. When wire is made, the steel structure aligns itself along the linear axis of the wire and is then twisted into a long coil. The steel structure is stressed in the same way a tree is stressed when it is bent over to one side. There are ways to relieve the stress in the steel, but let’s first consider where that stress is located.
Mr. Wizard time!
If you’ve ever served in the military or played in a marching band, you know that when a column of marching people turns a corner, the people on the outside have to move faster than the people on the inside. So it is for a steel wire being twisted repeatedly in a circle. The steel on the outside of the wire tries to move more than the steel on the inside of the wire. Except that the outside of the wire wraps COMPLETELY around the wire, while the marchers are a flat ribbon, by comparison!
The shaded portion of the wire has been work-hardened during the winding process and is very brittle.
If the stress is left in the wire, the surface remains brittle and subject to cracking. Further stressing, through spring compression and decompression, will cause the spring to fracture like glass! The most common way to relieve the stress is to massage it out with the impact of millions of tiny balls – commonly called shot-peening. A spring that has been shot-peened has a microscopically rough surface that looks like millions of tiny craters around the entire surface of the wire.
Close examination of the break point of a mainspring reveals a stattered structure, typical of an over-stressed steel that was not properly stress-relieved. The shading seen in the steel is real – not caused by shadows.
A well-made mainspring
When a mainspring is made right – with the proper steel, the correct manufacture and the proper stress-relief treatment, it doesn’t shatter like glass. But it does fail! However, the time it takes to fail is orders of magnitude longer than the bad mainspring, so it is considered acceptable for that application. When a properly made mainspring fails, the steel structure around the circumference of the wire shifts, allowing the spring to bend at that point. The bent spring then puts uneven pressure on the inside walls of the powerplant when it decompresses and that results in vibration. The more vibration a gun has, the more you can determine something is wrong inside – often a bent mainspring.
This old mainspring served an honorable life in a military trainer, which means tens of thousands to hundreds of thousands of shots. Even if it is a replacement, it still shows lots of service. The coils are bent AND collapsed.
Tomorrow I’ll pick up at this point.