The new D series designated "ND" is a superior lock to the previous generation. Alot of what people are having trouble with is the way it feels when it is not installed on a door. The previous generation felt much more stout off a door because of the way the two halves were connected to each other. I will say the ND series is not without it's problems however it is definetely an upgrade from the last generation. We stock quite a few of these, our techs install these locks in a fraction of the time of the last generation and I haven't had one come back yet.
The problem with the older D series was that it used the same knob chassis from the first generation D lock. The original D series was a knob lockset which was eventually adapted to use levers. The problem with this was that the spring cage that was originally designed to work with a knob did not have enough tension to hold the lever in the horizontal position. The result was common lever sag. To quickly fix this problem Schlage decided to add an exterior spring cage. Thus the 3" rossette. This took care of lever sag but they still never redesigned the chassis to handle the increase load from the leverage that a lever could generate.
The most recent rendition of the ND locks which in itself has been redesigned three times since it's original induction about two years ago has an upgraded exterior spring cage. An upgraded chassis and a redesign which no longer requires for the installer to center the lock using the castle nut and threads. I can't tell you how many times I had to send techs out to reinstall uncentered D series lever locks that had been installed by a locksmith.
Because the ND series lock can be easily disassembled out of the box it gives of the impression of being inferior. Actually this lock has been tested to meet the BHMA standard at 3,000,000 cycles. I believe that's almost four times the industry standard. I think the ANSI Grade 1 requirement is 800,000 cycles.
The door range has also been increased to work from 1 3/8" to 2 1/8". I believe the older version worked from 1 5/8" to 2". For people not in the door industry, the majority of commercial doors are 1 3/4" thick. While residential doors are 1 3/8". At least here in the U.S. In order to accomidate this before you needed a shim spacer. No longer with the ND series.
There's a few more things you can check out but that's just my opinion. No, I don't work for Schlage...lol. But, I've been in the industry for almost 12 years now.
Hope that helps.
If anyone is interested you can feel free to download the ND series service manual here.
http://professional.schlage.com/pdfs/pc ... Manual.pdf
