When the ordinary spindle with stiff driver is put in tram to the face of the runner, the miller has no assurance that it will remain so, the chances, indeed, being constantly against it. The heaviest spindle is liable to spring from its true position by the pressure of the gear or belt in driving it; the face of the runner stone changes, and the best driver, or its bearings, will from unequal wearing of the metal or in other ways cause it to get out of tram. The results are uneven grinding, inferior flour and diminished yield. The trouble necessary to take the spindle out, turn over the runner, make a staff and file the ends of the driver, is generally sufficient to deter the miller from performing the disagreeable job, and the bad grinding is conveniently attributed to some other difficulty.
Among the advantages offered by this improvement are increased grinding capacity with a given amount of power, more even grinding and better yield, and lastly, but not less important, increased facility in obtaining a perfect running balance. It will be observed that the runner is supported upon a steel seat secured in the stone permanently--being substantially the same in this respect as when the ordinary balance-iron is used.
The power being transmitted to the stone by the adjustable part of the iron and no weight upon any part of it, with the entire structure of the form to give the greatest attainable firmness and durability, combine to make it perfection as a driving iron, and it is accordingly in extensive use, although no especial effort has been made until late for their manufacture and sale. They are made in the best manner by skillful men and machinery adapted for the purpose, the parts well fitted and turned true inside where the grain enters and passes.
They are made of the following sizes: 8½, 10 and 12 inches diameter.
To order for attaching to stones with other irons already in, give diameter of the eye in stone at the face.
Distance from the cock-head point to the lower side of the driver.
Shape and exact size of spindle where your present driver goes on.
In getting the shape as well as size of place where your present driver fits, it is a good plan to oil the inside surface of the hole in driver (in which the spindle fits) and fill it with plaster, then take out and send the cast by express. The shape of cock-head may be got by similar process.