Im in a high school engineering class and we are having a reoccuring issue that we cant seem to resolve.
We are working on walking sequences for our Wireless Brat biped but the screw on his inner right thigh (i cant recall the exact servo #) keeps falling out, we checked to see for stripping, and replaces both the nut and the screw and even the washer in between, yet the problem persists.
If anyone has any thoughts or possible silutions it would be VERY much appreciated, thx for your time :o
To make it easier to illustrate what you are talking about please refer to the assembly guide, step no. and or figure no. There’s not a lot I can say about a screw, washer, and nut. I mean… 
It’s likely we will need you to post an image of the problem area so we can look for something unusual.
I will take a blind stab. I had an issue with the screw coming out of a few of my ball bearing connections to “C” brackets. I hadn’t noticed there was a difference in length between the servo mounting screws and the ball bearing screws and had thrown them all into the same parts container. Later, I was reassembling a bot and noticed there were .750" and 1.000" screws (yeah, it helps to READ the instructions). I had accidentally put some of the .750" screws in the ball bearing connections to the C clamps instead of the 1" ones. I have no idea if this is your issue but it’s something to double-check
Also, make sure you use lock washers on each connection.
I can’t think of anything that would cause a nut and screw to become loose in this manner. The screw should be tight to the center of the bearing hub and held from backing off when a lock washer is used.
Are you referring to the screw holding the servo brackets together and not the joint?
If we are talking about the 3mm screws that attach the ball bearing to the ASB-04 then there is a difference in the length. The diagrams show it’s an 8mm long screw, but it’s old. The screws are actually 9mm long. All this aside, There does exist the possibility that a 16ish year old may not have the physical strength, or a fear of breaking something, and is just not tightening the screws down tight enough. If this is the case…
Volunteer the strongest person in class. Find a good quality Phillips screw driver. It will fit the screw perfectly and will not be sloppy or loose fitting. Now put pliers or a nut driver on the nut, screw driver on the screw head, tighten it down like there’s no tomorrow. It may help if you growl like a wild animal. 
Grrraaawwwwwllllleh! 
If the washer is not a lock washer, then the nut may work lose. If the nut is a lose fit on the screw, you might try running a piece of thread thru the nut before tightnening it on the screw. Once the nut is in place, a drop of removable glue could be put on the exposed screw threads extending out of the nut to keep the nut from baking off.
