A mainspring’s purpose is to provide motive power for the wheels to move in a clock train. All mechanical clocks require a power source and the power source may be by weight or spring. Mechanical clocks that have mainsprings will have one, two, or three winding points which, once wound provide power for a clock’s designed cycle.
Each winding point or arbour is located on the dial face and requires a key (one key fits all arbours) to wind the clock. The mainspring is rated to provide power for a clock’s designed cycle, whether it be 1-day, 8-days, 15-days, and so on.
Mechanical clocks are machines and over time machines wear out. When a clock will not wind up, it is time for a service. The service can be done if you have the skills and equipment otherwise you must depend on a professional clock repairer.
Should mainsprings be replaced during routine servicing?
Though not in the clock repair business I am aware that some repairers routinely replace mainsprings and, of course, the cost is passed on to the customer. Should they be replaced on a routinely or only when necessary?
As a general rule, I do not replace mainsprings when servicing a mechanical clock unless there is something fundamentally wrong with the spring(s). Obviously, there are situations where a mainspring must be replaced when it has failed in some catastrophic manner.
There may be occasions when there is too much surface rust or stress cracks on the mainsprings which will make them susceptible to failure and/or seizing and the prudent course of action is to replace them.
However, repairers reason that new mainspring(s) will extend the period between servicing and provide additional insurance for the repairer so that if the clock stops soon after servicing, issues other than the mainsprings are suspect.
New quality mainsprings (American or German) also provide more power than the springs they replace and more power means they will continue to provide sufficient power for the clock through its rated cycle even if very worn.
The original mainsprings are often of better quality than a new spring. They may not have as much power as a new mainspring but many clocks, despite the fact that they are over 100 years old have springs that have more than sufficient power.
Once the movement is thoroughly cleaned, the pivots are polished and pivot wear is addressed there is less friction on the moving parts and therefore less power is required to drive the movement through it rated cycle.
What if there are minor issues with the mainspring?
There are certain situations where a broken mainspring may be salvaged if the break is at the hook or loop end but not if it is broken in the middle. The mainspring can be shortened by two or three inches and after the repair, the spring will continue to provide sufficient power for the clock to run through its cycle.
What to do if your clock requires servicing by a professional
Should you bring your clock in to be professionally serviced ask plenty of questions. If it is recommended that the mainsprings be replaced, ask why, specifically. Once you receive a satisfactory explanation proceed with the repair.
New mainsprings, when the cost is upwards of $25 for each mainspring, will add substantially to the repair cost, something to be considered if the clock has little value. Of course, if it is to be repaired for sentimental reasons the cost is immaterial.
Mainsprings need not be replaced simply because they are old and “tired” and it does not require a lot of power to run a mechanical clock if serviced correctly.
Your choice!
Antique and Vintage Clocks 
Also sometimes mainsprings become “tired” and don’t provide as much power (torque) as they used to. When removed with a spring winder “tired” mainsprings don’t expand as much as a new mainspring and need replacing,
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That is true, but would you say a clock that has been serviced will run its cycle, despite a “tired” spring. However, it becomes a judgment call. If I were in the clock business I would definitely replace a tired spring.
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