We are not allowed to have camera at work (get caught with one and you are probably going to lose your job). I will see if I can get some dimensions when I go back to work (work 7 on/7 off) but 18inches sounds about right.
if you are around the Edmonton area you could see these guys
http://www.heavymetalequipment.ca/Home.aspx they rent them. Or just get them to deliver one to you and take some measurements, its only $32,000 a month plus delivery.