All cylinders drive on one crank-axle with three cranks at 120°.
The horsepower available at the driving-axle, conveniently called the brake horse-power, is from 20 to 30% less than the indicated horse-power, and the ratio, B.H.P./I.H.P. =E, is called the mechanical efficiency of the steam engine.
This disk is carried on an axle inclined to the line of draught, and also to a vertical plane.
When portable cranes are fitted with springs and axle-boxes, drawgear and buffers, so that they can be coupled to an ordinary railway train, they are called " breakdown " or " wrecking " cranes.
There are two soft iron tongues, n, s, fixed upon and at right angles to an axle a, which works on pivots at its ends.