Tom Gardner wrote:
Our newest machine has a logic problem. The machine has two nail guns that
assemble a handle onto a wooden block brush. The operator loads a handle
and brush into the fixture and presses two palm switches. The palm switches
energize a relay that latches and energizes the solenoid valve that controls
an air cylinder that lifts the fixture to the nail guns until the fixture
hits a limit switch. The NC contacts on the limit switch then open and the
relay unlatches. The NO contacts close and energize a solenoid valve that
supplies air to the triggers on the nail guns. (There is also a limit switch
at the bottom of the stroke that powers the palm switches only at the bottom
position.)

The problem is that when the fixture hits the limit switch it reverses
direction immediately. The gun trigger air solenoid only gets that quick
hit and doesn't have enough time to build up enough pressure to trigger the
guns. I need to delay the fixture at the top of the stroke long enough to
fire the guns.

A few tenths of a second should be enough. An off-delay timed relay should
do it, right? As I look through the Grainger catalog and McMaster-Carr I
get a bit confused.

What is a "Off delay Retrigerable, one shot" relay? I don't think that this
is what I want but I'm not sure. I think I want just an "Off Delay" relay.



Yes, an off delay would do the job, I read Off delay, retriggerable,
one shot to mean: after the trigger is removed, the relay waits a
period before releasing, this period can be reset by supplying a pulse
to the relay before it has reached off time, and one shot means the
relay only needs a pulse, not a continuous trigger.

Other options might include using a limit with a long flexible stick to
trigger the air guns just before the lift tops out, thereby letting the
last tenth of a second of lifting be your delay. Or triggering the down
action by a pressure switch triggered by the exhaust port of the nail
guns, thus ensuring that the guns have cycled before letting the
completed assembly down.

Stuart