The principle of board or gravity hammer is illustrated. The ram is fastened to the lower end of a vertical hard-wood board ; the upper part of this board is placed between two counter revolving rolls. When both rolls are pressed against the board between them, they drive upward, lifting the ram. When the tools are released, the ram falls down producing a working stroke. As long as the operator continues to hold down the treadle, the hammer will continue to strike, but when the treadle is depressed, the ram will return to and will remain in its position. The board hammer is generally driven by electric motor and this requires no steam or high pressure air.
Ratings are given by the weight of the falling parts and range from 250 to 2700 kg ram weight with heights of 1 m for smaller sizes and 2 m for the larger one. The force of the blow can be varied by changing the distance of the fall, by unclaming and moving the dogs on the front rod. The anvil, which must absorb the blow, is usually 20 times as heavy as the hammer. Wooden beams or special pads under the anvil help absorb the force. A board hammer, which works rapidly, gives over 300 blows a minute. Board hammers can do a wide variety of work and they are less expensive of all types to operate.
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