Fig. 123.
The principal and most essential dimensions necessary to be considered in setting turbines are indicated by letters, each size having its own particular dimensions.
In setting the wheel in the ordinary penstock it is necessary in the first place, to have the floor exactly level, and it is generally more convenient to lay down a ring of soft wood around the hole in the floor, as it will be much easier to dress off with a plane than the plank floor. The floor should be supported by posts under the timbers around the hole, so that there will be no settling of the floor after being once made level. As the flange on which the wheel rests is turned true, the wheel will, when placed on this level floor, stand in the exact position required, i.e., the shaft will be exactly vertical.
If the wheel is a large one and was taken apart for shipment, the draft tube is first erected in position, then the wheel is placed on its step, the other parts being put on in their order. The step and other bearings are adjusted before leaving the shop, but it will sometimes happen that they will in some way get shifted, and as the wheel is being put together, they should be inspected and readjusted, if necessary. The only change that can occur in the step is its vertical adjustment, which is regulated by screws. When the right height is found, the broad flange around the lower part of the wheel should stand about one-sixteenth of an inch below the under side of the base of the guide rim where it rests upon the draft tube. The adjustable bearing on the top of the cover plate should be fitted up closely around the shaft, but not screwed so tightly as to bind it.
All these wheels above fifteen inches in diameter are provided with chains and weights to counterbalance the weight of the gate, so that it will move easily. It is best, when it can be done without much trouble, to carry the weights outside of the flume, but they can be used inside where the height is sufficient, although it will require a little more weight to be as effective. When the wheel is not likely to be started up at once, it is a good plan, when putting it together, to smear the step and the shaft at the bearing with tallow, as a protection against rust while it remains idle.
It is sometimes necessary to use a draft tube longer than is ordinarily attached to the wheel. If properly constructed and applied there will be no sensible loss of power, but it must be air tight, and when of considerable length it is better enlarged gradually toward the lower end, especially in cases where it may be necessary to carry this tube near the pit bottom.
Iron cases for Turbines. [Fig. 124] shows the setting of a Hercules wheel within an iron case.
Fig 124.