Some items are sturdily assembled initially but loosen over time. If people are used to using these items in their loosened state, it is not prohibited to assemble or disassemble them. As noted, assembling implements loosely is not considered Boneh. Furthermore, since people are used to using them this way, we are not concerned that they will end up fixing them with nails or glue (see Rema 308:16; SA 313:6).
Thus, if a wheel falls off a baby’s crib and people are comfortable using the crib after replacing the wheel loosely, it may be replaced. Similarly, rubber chair tips and table tips may be replaced if they are only loosely attached. This is also the case if the leg of a child’s doll has come off: if the leg is attached to the doll tightly, it may not be snapped back into place, but if it is connected loosely, it may be replaced.
If a temple fell off a pair of glasses, it may not be screwed back into place, since generally it is screwed on tightly. One may not even attach it loosely, because one may forget and tighten it, thus transgressing a Torah prohibition. However, if the screw is lost, there is no longer a concern that anyone will tighten it. Thus, one may reattach the temple using a safety pin, as this is a weak attachment that is permitted for implements (Shulĥan Shlomo 314:11:2).
If a lens fell out of the glasses’ frame and replacing it requires screwing it back in, it may not be replaced at all, even if the screw is left loose. This is because we are concerned that later on one will forget and tighten it. But if there is no screw, but rather the frame has expanded a bit so that the lens sometimes falls out, then the lens may be replaced in the frame. We are not concerned that one will permanently reattach it, as that would require expertise (Menuĥat Ahava 3:23:35; Orĥot Shabbat 8:49-50).
One may not straighten a spoon, knife, or glasses temple on Shabbat by hand, because this is considered Makeh Be-fatish (MA 340:11; MB 509:1, 7; see Harĥavot).
The prohibition of Boneh does not apply to compressing a spring on a toy car so that the car will move forward. One may do so as long as the car does not make noise or light up (see SSK 16:14).
One may inflate an air mattress, air pillow, or rubber ball that had been previously inflated. Since this is permissible, it may be done in the usual fashion, using a manual pump. However, one may not inflate these items for the first time on Shabbat, because many poskim maintain that inflating them for the first time makes them into viable implements for the first time, which is prohibited. One may not blow up a balloon even if it had previously been inflated, lest one tie a knot at the bottom. However, if the balloon is sealed with a valve instead of a knot, and it had previously been inflated, it may be inflated on Shabbat (SSK 15:89; 16:7; 34:24).