Split Gear.

Using a split gear design can help to improve strength, driving efficiency and reduce cost of different gear drives.

Split gear has an advantage of a helical gear but it does not create axial force.

Split gear design allows lower transversal contact ratio on the each slice for higher tooth strength and increased driving efficiency. Driving efficiency of gear depend on sliding. Sliding velocities are the highest on the tip and on the root of the tooth. Tooth has no sliding action on the pitch diameter.

It would be nice to have the tooth contact only close to the pitch diameter in order to avoid friction losses on sliding. However a conventional spur gear must have the contact ratio >1.0 in order to operate correctly. A helical gear can have transversal contact ration less then 1.0 because of the overlap of helical teeth. However the helical gear generates an axial load that requires more complex design. Each slice of the split gear can have transversal ratio less than 1.0 for higher strength and higher driving efficiency.

The split gear can be designed by many different ways but the key element of any design is face-toothed coupling and a connecting element.

The face coupling connection has to be present on both members: gear and pinion.

The correctly designed face coupling can provide the precise alignment and tooth spacing.

The connecting elements can be as simple as bolts.

The connecting element does not need to be made with high accuracy because it only holds the slices together and the face coupling provides accurate alignment.

The number of slices can be from two to as many as it seems to be practical.

More slices would provide smoother transmission.

One of the design advantages of the split gear is an opportunity to use identical parts for making different gear drives.



Different size and load capacity drives can be assembled from identical parts. Each gear slice has a face coupling on each side and can be assembled with as many gears as necessary to achieve the required load capacity.

The split gear slices have to be thin so it can be assembled more slices in a given face width. Thin gear may be easier to manufacture by forging out of metal sheets. Thin gears are also more adopted for sintering process because it will be easier to achieve a higher density and strength.

It is critical to have an accurate angular shift of each slice. Practically the accurate angular alignment can be achieved by selecting a correct number of teeth on the face coupling so the tooth surface can be finished on a package of all the slices connected together.

For example, if the gears need to be shifted for 1/3 of the pitch, the face coupling may have three times more teeth. However, sometimes the pitch of the gear is too small and it may not be practical to triple the number of the teeth on the face coupling. So the different variations of numbers can be used. It is not difficult to understand that many different numbers of teeth on the coupling can provide the same 1/3 pitch shift.

The face coupling does not need to have very accurate pitch in order to provide precise angular alignment of the gear teeth because the pitch error averages between many teeth. The face coupling can be forged on thicker gears and stamped on sheet-metal thin gears.

Using split planets on a compound planetary gear drives allows to locate ring gears closer to each other and reduce skew of the planets.

Also, compound planetary gear drive will benefit much from increasing of driving efficiency in the gear mesh. The driving efficiency in a single mesh of the split gear can be 0.995 compare to 0.985 on a regular gear. The improvement does not look high until you calculate the total driving efficiency of a compound planetary gear drive. For example, for ratio 140:1 the total driving efficiency will be:


Compound planetary gear drives are commonly used in aerospace for actuators: flight controls, doors and other devices. With higher driving efficiency an airspace compound planetary actuator can provide a higher ratio in a single stage and it can be driven directly by an electric motor or by a low torque drive shaft. Using split gears on airplanes can reduce the size of Power Drive Units and reduce the weight with low torque drive shafts.

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