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.