The most important part of a turbine engine is the propeller shaft, which is actually the part of the engine that connects the propeller to the engine. All a propeller shaft needs to do is follow the contours of the propeller and create a hole through which the shaft will push itself through.
But the final design for propeller shafts is often much more complicated than that. The shapes that propeller shafts take in jet aircraft designs may include places for bearings and air and hydraulic shock absorbers, while some are constructed from extruded aluminum.
The longer the shaft, the greater the amount of pressure that it will exert on the bearings, as well as the higher the tension. The opposite is true for the short shaft.
Another factor that will affect the balance of the propeller shaft will be the presence of the vanes, or slots, which are arranged just under the propeller blades. The slot arrangement can be one with an even number of vanes, or a series of highly designed slots that are spaced every other tooth.
Because the propeller shafts can also act as an elevator, the alignment of the shaft and vanes will need to be carefully controlled, and the elevating features tested before the shaft is manufactured. A shaft mounted on a moving elevator can become loose and come off its base, with disastrous results.
In most cases, bearings and hydraulic systems will be integrated into the shaft design, along with mechanical parts such as a gearbox and drive shaft. Although the shaft will be integral to the entire propeller, some bearings must be specially designed and produced.
To keep the shaft tight, it is a good idea to use crankshafts of solid beryllium and lead, which can be rotated at very high speeds without the stress being transferred to the shaft. Also, to keep the shaft properly aligned, a special drive shaft guide, which uses stainless steel lugs and bolts, can be used to correctly adjust for its alignment.
Airplane engines are almost always built with bearings between the various portions of the engine, but this doesn't mean that they will all be free from bearing damage. As long as the shaft is properly balanced, its bearing won't become damaged but bearing failure can occur due to overheating, and oil starvation.
If the propeller blade-mounted bearings fail, the blades can fail to function properly, and may need to be replaced. There are basically two ways to replace a propeller blade-they can be ground off, or else they can be replaced by means of impellers.
However, if the blade is the result of grinding or crushing, the damage is usually quite extreme and may have caused the engine to work with much less force. To keep the blade aligned, it will be necessary to ground off the top surface, which can be done with the help of a diamond saw.
The uppermost surface will be ground away, and a new section of the shaft will be produced, but the top surface will need to be ground away further, and the entire shaft will need to be resurfaced. This can be done with a grinding wheel, or it can be done with hand tools using a large block of sandstone to create a perfect surface.
An airfoil propeller shaft has a big advantage, in that the shape of the propeller blade doesn't make it difficult to find the proper shaft. However, a propeller shaft cannot be used on a twin-engine plane, because it will interfere with the cooling systems.