trip hammer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A heavy, power-operated hammer that is lifted by a cam or lever and then dropped.

from The Century Dictionary.

• noun A tilting-hammer or machine-hammer operated by a cam or other device, which trips the lever and allows the hammer to fall. It is essentially the same as the tilt-hammer (where see cut).

from the GNU version of the Collaborative International Dictionary of English.

• A tilt hammer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A powered machine used in a forge which repeatedly strikes work placed on an anvil.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a massive power hammer; raised by a cam until released to fall under gravity