The ornamental design for a spring clamp, as shown and described.
The nature and intended use of the article in which the design is embodied is that of a spring clamp having jaws that are biased towards each other by a spring. The spring clamp may be used to clamp an object by squeezing the handles to open the jaws, and releasing the handles to clamp the object.
FIG. 1 is a perspective view of a spring clamp showing my new design;