Image Number 4 for United States Patent #7214902.
The present invention provides a weld cylinder having a dual piston arrangement. The cylinder has a middle separator separating the cylinder into two cavities. A retract piston assembly is arranged within one cavity. One of the pistons is arranged in the retract piston assembly and the other piston is arranged in the cavity without the retract piston assembly. The pistons support a rod that is moveable between home, intermediate, work, and advanced work positions. The rod moves rapidly from the home position to the intermediate position. However, the rod moves more slowly from the intermediate position to the work position to reduce the impact force. A cushion valve and cushion trip valve cooperate to increase the rate at which the rod moves from the work position to the advanced position so that the weld force increases rapidly to minimize increases in cycle time. The cushion valve is supported by the retract piston assembly, and the cushion trip valve is supported by the middle separator. With both the cushion valve and cushion trip valve moved from closed positions to open positions, the rate of fluid flow through the retract piston assembly increases to increase the rate at which the piston moves within the retract piston assembly.