Why won't my ps3 take discs?

I am not a pro ps3 technician, but I tried to fix my ps3 that won't take discs. I took it apart, got to the blu ray drive, and took off the metal housing on the drive. I plugged it in to see what was going on with the gears (with the metal housing gone) and surprisingly it took the disc just fine. So I put it back together but once again it wouldn't take the disc. Take the metal housing back off and it works just fine again. So my question is, why is that metal housing around the blu ray drive preventing my ps3 from taking discs?