Absolutely, in the building of any 88 note Keyboard that is designed to be moved, there is typically a large piece of wood (ply wood, or reinforced wood type product) that acts to give the instrument stability. When lifting and placing the keyboard, over time the screws will definitely loosen. The wood is very important because it 'gives'... it can flex just enough to be the ideal material to support the keyboard throughout its life cycle. This includes lifting and moving it while in the box... the box it comes in is cardboard- lifting the keyboard even in the box the rigidity still comes from the keyboard itself.

An object made the size and shape of a slab piano must deal with torque (the tendency for a slab shaped object like a weighted action keyboard to twist, rotate... obviously not a good thing - twist too much the keys will pop out. It is this tendency when lifting and moving that would cause a completely rigid item to eventually snap. The flexibility of the big plank of reinforced wood is ideal ... rigid enough, but has some play and ability to snap back and a piece of metal that resisted the torque as well would be far, far to heavy in comparison... take it from a piano maker.

It is completely normal for the screws on the bottom to loosen, (and this goes for almost every larger keyboard), take any piece of wood and hammer on it for hours; the screws are holding on in the opposite direction and working against gravity - inevitably they will loosen. it is highly recommended to check on them on a regular basis... even if you never move your instrument.

I recall the KX88 (those that remember this keyboard controller 1984-1996) one of the 'spare no-expense', 'best of its kind' early 88 Note weighted action MIDI Keyboard Controllers, (it was the King of MIDI Keyboard Controllers... it weighed a ton). Seriously, it was HEAVY, no, it defined heavy. I joined Yamaha in 1987, and the gig came with a KX88... my questions to engineers was about why this product was what it was. By that time, you saw it on stage with everyone. Very heavy, but everybody's (and I mean everybody's) favorite weighted action. That's when I learned about torque and the pounding (literally) the pounding a professional keyboard takes. And how Yamaha goes about researching and building keyboards. It was enlightening, to say the least. Stuff I had never considered.

The KX made no sounds, so why all the weight?... well that has every thing to do with how it felt to play... and how it was going to be used. For back then, the early days of MIDI, it was the controller.

But eventually lugging the weight means it would not have very wide appeal in the mass market. It stayed a current product for more than a decade (highly unusual). But if you had your pick of gear back in the eighties and early nineties, your choice keyboard controller was a KX88...

Anyway that's when I learned a bit about what goes into keyboard manufacturing... which Yamaha has been doing since 1887... a hundred years before I asked a similar question.