BigAl,
AS SLOW AS YOU CAN!
I put in about 120 posts this summer with a Ferguson TO20 - trans driven PTO & all. The holes were all different - some were soil all the way down (3'), some I hit stones after a foot. Generally here there are many smaller stones on top of the clay layer. When you hit those it binds up the auger and jams it - that is when you break the shear bolt. Of course, you will have gone to TSC and bought the entire bin of bolts, so you'll be prepared! It took a while to develop a technique, but here's what worked for me:
1. Run it as slow as you can with out boging the tractor - this should be a heck of a lot easier with a DK45 than a TO20!
2. Drop it in a bit, then pull it almost out and let it clear. Then put it back down (no jokes /forums/images/graemlins/grin.gif). Lifting it up will sometimes pop the stones up so they don't jam.
3. If you hear it hit stones, keep pecking at it - you'll eventually learn to tell when you are close to breaking a bolt. Then you have to decide how lucky you feel! You won't be able to avoid using the digging bar all the time /forums/images/graemlins/mad.gif.
All of this is to keep the forece down on the shear bolts - if you are going slow the impact will be lower when you hit something. You don't need a lot of speed to dig a post hole - if the soil is soft, it'll go down fast enough, if it isn't more speed and power won't help.
Lastly, I had trouble one time when I helped a friend dig a few holes in some sandy soil - I was used to my place, with clay under the soil, but under his it was sand. As soon as it dropped a foot, it got into the sand and started heading for China - really cought me off guard. With the slow hydraulic response on my old Fergie, I couldn't stop it in time. I didn't want to put in the clutch, because if it stops all the way down, with the TO20 I can't run the lift with out running the PTO, and when you start, before you can get any lifting force it's started turning and pulling down more.....then I have to disconnect the PTO shaft. It pulled down so far it bent & broke the PTO adapter - another trip to TSC! You won't have most of those problems with a modern PTO and lift system, but make sure the PTO shaft won't bind when it is all the way down. I'm keeping the old Fergie, but I'll never dig another post hole with it!