Hammer Drill vs Impact Driver

[Dan2312]

Hello,

I am wondering if someone could tell me what are the differences are between a 18 volt hammer drill and a 18 volt impact driver; both functional (or mechanical) and uses (recommended, and non-recommended). I would like this information to use in my decision towards purchasing one or both of these tools.
I understand the basics of hammer drilling is to ‘hit’ the drill bit, rotate the bit, then hit it again, and repeat the ‘hit’ and rotate pattern; as well as that a hammer drill does this at a rapid rate. How does the impact driver differ from this? How would they differ in their use on tasks like driving screws into and out of hard wood, tightening and loosening nuts and bolts, etc.?
Please provide me with this information and/or links to where I may find these answers. Thank you for your time and assistance.

Sincerely,
Dan

[Velosapien]

The hammerdrill function is designed to make holes specifically in concrete. It hammers the drill bit back and forth at a very fast rate to chip away at the concrete. The bit rotation is mainly for clearing the debris out of the hole through the bit flutes.

An impact driver is a tool specifically for driving screws and bolts with very high torque. It is a miniaturized version of the large impact wrenches you see at a car repair shop for example. The advantage of this is that the torque is applied to the fasteners in a non-linear fashion. It hammers at it in the rotation direction with extremely high torque thousands of times a minute. This causes most of the energy to be trasfered directly to turning the bit in very short sharp burts. Even at hundreds of foot pounds you can hold it with one hand and feel nothing but a small vibration. Doing the same on a linear torque tool like a drill will snap your wrist.