Why did the ‘ball bearings drop behind the roller bearings’?
Have you been struggling with a difficult problem all day to no avail? The only result is a complete fog in your head. That’s because your “шарики за ролики заехали” (“shariki za roliki zayekhali” or "ball bearings dropped behind the rollers").
This expression predictably originated in mechanics. In a properly functioning device, all parts operate correctly and nothing disrupts their movement. But if, for example, you were to combine two types of bearings – a ball bearing and a roller bearing – the device would begin to move erratically, eventually breaking down or jamming.
The analogy stuck and the phrase came to be used when someone is confused, unable to think clearly, or has lost their mind.
An English equivalent would be: “To lose one’s marbles.”