by B.B. Pelletier
When the mainspring starts to fail (bend), it rubs either against the inside of the piston or the outside of the spring guide on either end (inside the piston or at the other end of the compression chamber/spring cylinder). That rubbing slows the piston, causing a power loss, and it transmits vibration to the metal parts of the gun. If you know your gun very well, you can feel this increasing vibration, just as the owners of older Diana spring guns often notice the easier cocking and softer power stroke of a broken mainspring.
A few more factors
We still need to know a few more things. Some mainsprings are under a tremendous amount of pre-load. They are compressed 3-4″ while at rest! The HW77 is like that, as is the FWB 124. The TX200, on the other hand, is only compressed about half an inch at rest. That affects how long the springs last. So does dieseling.
The more a gun diesels, the shorter its spring life will be. Every diesel smacks the mainspring with a rapid compression stroke. There are cases where the gun has recocked itself from a detonation! That force abuses a mainspring something awful. As does a sloppy powerplant.
A gun that has a lot of slop between the piston, mainspring, spring guides and cylinder wall is abusive to a mainspring. When a spring gun fires, the spring expands and rebounds so fast that at one point it doesn’t touch either end of the gun! If there is lots of room all around, the spring will kink up in all sorts of nasty ways and get hit by other moving parts in the powerplant until it starts to degrade. A buzzing, vibrating spring gun is tearing itself to pieces, with the mainspring being the first victim.
So how LONG does a mainspring LAST?
Tom Gaylord published the only report I know about on the subject of mainspring life. In his R1 book, there is a chapter called the Mainspring Failure Test. He tested a factory R1 spring, a Beeman Laser spring, a Venom spring and a Maccari custom spring by cocking them all and leaving them cocked for ONE FULL MONTH! That’s 735 HOURS of being cocked. Throughout the test he took shots at intervals to see how the springs were holding up, then recocked them until the next test shot. Each spring was test-fired this way 23 times during the test.
The R1 book by Gaylord has a whole chapter devoted to mainspring life.
The mainspring that lost the most power was the Beeman Laser spring. After being cocked for 735 hours, it had 93.25 percent of the power it had at the start of the test. The factory spring retained 93.89 percent of its original power. The Maccari spring retained 94.65 percent of its original power and the Venom spring retained 96.93 percent of its original power. The Vemon spring was slightly bent and had begun to vibrate – something Gaylord stressed it did not do before the test. The factory spring was ever-so-slightly bent and both the Maccari and Laser springs were still perfectly straight.
A full MONTH of being cocked is more abuse than anyone can heap on a spring gun in ten years of normal use. However, (and this is a long list of “howevers”) a Chinese spring rifle MAY have an improperly stress-relieved mainspring that can fail in less than 1,000 shots. An older Diana from 1986 to 2000 will often have a broken mainspring from improper stress relief. That is not 100 percent guaranteed, however. An FWB 124 will often have a bent mainspring from failure due to excessive pre-load. An HW77 with a factory spring may fail after about 12-15 years of little or no shooting for the same reason. An older Russian spring rifle may fail due to the spring wire being too weak for the application. Any airgun that has been subjected to repeated dieseling will probably have a bent mainspring.