That is because the poster above gave the wrong information. This is a user forum and sometimes people think they are helping when in fact they give bogus or wrong information. The part number he got from hp for was a different motor. If you notice in his post he did not use that motor but had a spare non working machine to use the motor from. The motor is available as indicated in the link below.
http://www.amazon.com/Q3948-60186-HP-LaserJet-Scanner-Stepping/dp/B00IPNXJAC
http://www.amazon.com/Q3948-60186-HP-LaserJet-Scanner-Stepping/dp/B00IPNXJAC