L.G. downlock mechanism
 

Best PPRuNers,

The picture below is from my current type, 737. I am trying to fully understand the downlock mechanism. Am I right in thinking the springs "pull" the downlock over-center? (I actually can't tell for sure if the downlocks are over-center geometrically, are they?) In between the springs I gather there's an hyd actuator? With the gear extended is there full system pressure holding the downlock over-center as well? Hence, spring+hyd pressure keeps the downlock from folding?

The opposite then, am I right in thinking when the gear is retracted the hyd actuator extends, working against the springs (extending them) and the downlock folds to the left in the picture?

[IMG]http://i307.photobucket.com/albums/n...n/Downlock.jpg[/IMG]

AYes, the springs assist in pulling the downlock over-centre. The overcentre is only a few degrees.

Yes, the dark cylindrical object between the springs.

Yes, with the gear lever down, the downlock actuator remains pressurised (shortened).

Before full hydraulic pressure is applied to the main retract actuator, the downlock actuator extends and pushes to the left, extending the springs and breaking (bending) the downlock.

I think your downlock explanation is absolutelly right. Downlock strut is overcentered only few degrees, so it is not clearly visible, but neccessary for transfer structural load. Stops for overcentered position are visible in mid lower part of image. Hydraulic pressue for down(lock) side of actuator is present all the time if B pump(s) runnnig, L/G lever in DOWN and access door for free fall extension closed. Spring main task is provide positive locking (overcentering) in case of free fall extension.

And sorry for too late answer :)

Both of you, thanks very much!

Case closed :)