Torsion springs are devices that lift most of the weight of a garage door so that the door can be opened manually or by an electric opener. The torsion principle is applied via an efficient and economical apparatus consisting of a torsion shaft under spring torsion, which turns lift drums, which wind cables attached to the door near the bottom. All of this apparatus is mounted over the top of the door. The energy stored is sufficient, in an uncontrolled release, to break things, hurt you, or perhaps even kill you. The same could be said of jacking up your car to change a tire, or mowing your lawn, or raising children, so it is not crazy to want to do this yourself.
Whether you're planning on moving in the future or you're fixing up your forever home, replacing a garage door can provide an excellent return on investment and boost your home's resale value. According to Remodeling magazine's 2016 Cost vs. Value report, garage door upgrades provide a nationwide average return on investment of over 90%, making it one of the top five home improvement projects.
You might genuinely need some extra parts when you thought you simply needed a broken spring replaced, and a good serviceman will perform a simple inspection to identify such parts. Nor is it unreasonable for a business to charge separately for a service call versus repair work actually performed. But the best protection for you as a buyer, being somewhat at the mercy of whomever you decide to bring on site, is to understand what is being done, and ask intelligently for a clear explanation or demonstration of why extra parts are required.