A few images and terms that may help clarify the dizzying variety of water turbines used in the 19th and early 20th century (some types of which are still used today). The glossary is Tyler-centric reflecting the fact that the Ledyard Up-Down sawmill is powered by an original circa 1870 John Tyler turbine.

Apron Platform upon which the turbine case is set and fastened.

Backwater Increased levels of water below the mill can back up into the tailrace and impede operation of the water wheel. Many types of water wheels are susceptible to backwater during spring freshets or other times of flood; that is, they don't operate as well as with lower levels of water in the tailrace. A common claim for many turbines is that operation is not affected by backwater.

Bucket The multiple separate chambers in a turbine runner through which water flows. The runner vanes define the buckets. 19th century turbine manufacturers continually modified and patented “improved” forms and shapes of the runner buckets, and used these new features as a focal point in their marketing.

Curb The static enclosure around a turbine runner is called the curb (or case). Provides for mounting the turbine assembly and controls the flow of water through the turbine via one or more gates. The cover for the case is referred to as a dome in some of the Tyler literature.

Gate A sliding or pivoting panel that controls the entry of water into a turbine. Some turbine types have multiple gates, or a cylindrical gate that regulates flow into multiple compartments of the turbine case. Turbine and waterwheel power systems typically also have at least one other gate: the head gate controlling water from the headrace or sluiceway into the turbine penstock

Guide Many turbine types have guides – fixed partitions in the turbine case that direct the flow of water into the runner. The compartments in the case that are defined by the guides are called chutes. Tyler turbine cases, and most scroll-type turbines, do not have guides per se; rather, the case consists of a single compartment.

Head The height difference between the water source upstream (typically a pond) of the mill and the water in the outflow, or tailrace. The Ledyard Up-Down sawmill has a head of 9 - 9.5 feet when the pond is full.

Penstock The pipe or tank that supplies water directly to a turbine. The turbine inlet can be attached to a separate penstock, or set completely within the penstock. In the latter installations, a sealed draft tube at the water exit of a turbine allows placement of the turbine above the level of water in the tailrace for easier access. Sometimes spelled penstock.

Step bearing Also called a foot-step bearing or simply a step. The thrust bearing that supports a vertical shaft. In water turbines there are examples of foot-step bearings of conical and half-spherical shape – the bottom of the runner shaft is turned to match. A common material for turbine foot-step bearings is lignum vitae. (The Ledyard Up-down Sawmill has an oak step bearing that was fabricated and installed in the early 1970s - it's still in good shape about 50 years later.) Some turbine designs, like the Tyler turbine, are manually adjustable to raise the foot-step bearing in order to compensate for wear.

Runner The main revolving part of a water turbine.

Turbine pit Or tail-pit. The excavation in the base of a mill in which the turbine discharges. The difference in height of the water in the tail-pit and the water reservoir supplying the turbine is the maximum available head of water.

Vane The partition in a turbine runner that separate and form the individual buckets. See the figure in runner that show the large convex vanes in a Tyler turbine; that is, curved away from the flow of water into the runner. This is relatively unusual – in many turbine types the vanes are concave. Also called a float.

See Sash sawmill glossary for more mill terms

See turbine references on Water turbine references page of this website.