Inside coil spring compressors are wonderful tools. They can safely do what is unsafe using any other method. Utilizing a threaded central rod, they use the principle of the inclined plane to exert hundreds of pounds of force to compress the coil spring into a short-enough length to remove or install.
Things can go wrong, however, and this tip is intended to help prevent that from happening to you. The threads on the spring compressor are strong, of course, and the tool-steel central threaded rod can easily withstand the tension forces put upon it during spring compression. The problem is friction...
When the spring compressor undergoes the tension/compression forces, the threads are all that is holding things from slipping. As they are turned, great amounts of friction are generated. This friction causes metal-to-metal wear that can quickly damage the threads. Turning the rod further amplifies the forces and, given the right circumstances, catastrophic failure can result.
To minimize this risk, always grease the rod's threads each time you use the spring compressor. Use bearing grease, and spread it liberally over all the threads on the rod. After using the compressor, wipe the threads clean and store it in a plastic bag until the next use.