Pneumatic Tandem Pin Retractors

I'm curious about the mechanics behind the tandem pin retractors in this video - specifically the air-powered type. At the time stamp, the pins are retracted and he goes into the cab to show a shortcut on extending the pins to lock the slides.

In the cab he pushes and pulls the tractor protection valve a few times. He seems to indicate that supplying air to the trailer will push the pins out. Isn't that the opposite of how it should be? Wouldn't needing air to push the pins out be a more dangerous design if the trailer were to lose air pressure? The pins could retract with the loss of air pressure, the tandem spring breaks would engage and that could rip the tandems off the end of the trailer.

Or does it work the opposite way where the pins operate on a similar safety principle as the spring breaks? I.e. without air pressure the default position of the pins would be for them to be pushed out?

There's a check valve built in-line that keeps the pins extended, so any loss of air doesn't affect anything.

First off, we need to use the same language to describe the position of the tandem locking pins. I was taught that "out" means the pins are pushed out through the rails fully locking the tandem from sliding. "In" means the pins are retracted so that the tandem can now slide.

On the video he points to a switch on the trailer. That switch is a push/pull that is connected to the air system. Pulling the switch will retract the pins IN, pushing extends the pins OUT. You can only pull the switch when the Trailer Air Supply valve(RED) in the tractor is pulled, also setting the trailer brakes. When air is supplied, air forces that switch in, locking the pins OUT. You have to use that trailer switch to move the pins IN to unlock the tandems to slide them.

On the video, the driver pushed in the Trailer Air Supply valve. That supplied air to the trailer. Which in turn released the trailer brakes and pulled the switch on the trailer inward causing the pins to push OUT. Then he pulled the Trailer Air Supply valve which reapplied the trailer brakes. He pushed the Trailer Air Supply valve in a second time and pulled it back out. He then pushed in the Parking Break valve (YELLOW) and slowly moved a little bit to align the holes. Air already in the system was constantly providing a pushing force on the pins as he moved. When the holes aligned the pins pooped OUT locking the trailer tandem. Then set the brakes, get out and do a visual inspection on all 4 pins ensuring they are al locked in the OUT position.

Thanks Dan67! That was beautiful - a perfect and definitive answer to the question. You get a gold star and a cookie today!

Thanks, Dan. If I followed you correctly, then what you describe is what I thought was happening. From an engineering and safety perspective wouldn't it be a better design of the tandem locking pins to push out automatically in the event of a loss of air pressure?

Amicus, the pins are always out or locked unless you pull the button by your tandems when your trailer brakes are set.