A clamp for mounting a structure on a support. The clamp includes a fixed jaw, a base, and a moveable thruster plate between the fixed jaw and the base. The clamp includes a thruster cylinder configured to be rotateably received through the clamp body and having threads that engage a screw so that when the thruster cylinder rotates, the screw is advanced or retracted to move the thruster plate into and out of engagement with the support. A knob is used to rotate the thruster cylinder. The base includes an annular opening for storing a spring and receiving the screw and the thruster cylinder. The clamp is configured so that when the thruster plate engages the support, further axial movement of the screw is prevented and further rotation of the knob and thruster cylinder compresses the spring thereby causing the thruster plate to apply additional force to the support.
