Page 213 - Gear Technology Solutions

15.2 Misalignment Compensating Splines

It is desirable if the basic design of a spline is made by applying the common
national and international standards. The modification versus a standard un-
corrected spline has to be easy to dimension and manufacture. Regarding the
functionality, it has to be considered that the misalignment rotates around the
internal spline and the spline shaft, and therefore causes a load fluctuation.
Between the 12 o’clock and the 3 o’clock position (based on Figure 1) the con-
tact moves on the top flank from center to the left spline end. Then it moves
back to the center in the 6 o’clock position and from there to the right end of
the spline in the 9 o’clock position.

The idea of giving the spline connection some of the abilities of a universal
joint or a constant velocity joint seems obvious. However, the friction on the
spline teeth has to be compensated just with a permanent grease lubrication,
or with oil. Both will certainly cause problems if the speed is higher than
10,000RPM. Grease is pushed to the outside and eventually has completely
left the spline connection. If seals are used to prevent the departure of the
grease, then the oil drops will leave the soap matrix of the grease and concen-
trate on the major diameter of the spline, which is the tip-root-clearance gap. In
the case of oil lubrication, the high-speed centrifugal force unmixes the addi-
tive molecules, which then also concentrate on the major diameter of the
spline where they become ineffective. A constant lubrication flow should be
provided for high-speed applications, especially if a misalignment has to be
compensated [6].

15.3 Barrel Design

It is easier for manufacturing to apply spline surface corrections to the spline
shaft instead to the internal spline. If a function similar to a spherical joint
should be accomplished, then the outside of the shaft has to be turned spheri-
cal with a radius that accommodates the expected misalignment.

The obstacle in this case is that the pivot point of the barrel shaped spline is at
the middle of the face width at the centerline of the spline shaft (Figure 2). The
center of the spherical radius however is far away from the centerline. Alt-
hough this geometry is often used, it leads to an increase of interference be-
tween the teeth of the spline shaft and the internal mating teeth. The interfer-
ence effect is later discussed in more detail.

Barrel shaped splines are acceptable for small misalignments and for lower
speed. In case of high speed applications, a significant amount of heat can be
generated due to the friction and the high normal forces, coming from the added

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