**4.3.2 Quill shafts based on elastomeric elements**

Elastomeric elements are frequently used in quill shafts given their low elastic modulus. For example, one system (Isabelle et al., 1992), based on using elastomers (Fig. 13), consists of *"an annular cylindrical elastomeric bearing (14) and several rectangular elastomeric bearing pads (16). The elastomeric bearing (14) and bearing pads (16) have one or more layers (60); each layer (60) has an elastomer (62) with a metal backing strip (64) secured by conventional means such as vulcanization, bonding or lamination"*.

Fig. 13. Elastomeric load sharing device (Isabelle et al., 1991)

The annular cylindrical elastomeric bearing (14) absorbs possible misalignments between shafts resulting from defects in assembly. The rectangular elastomeric bearing pads (16) are responsible for providing torsional flexibility to the shafts of the possible gear paths in order to ensure equal torque transmission.

Another elastomer-based system (Kish & Webb, 1992) (Fig. 14) consists of an assembly with *"a central shaft (21) and a pair of bull pinions (22) and (23). The shaft (21) is supported by the bearings (24) and (25); a gear flange (26) at the end of the shaft has bolt holes (27) and teeth (28) on the outer circumference. A spur gear (29) is held to the flange (26) using upper and lower rims (30) and (31), consisting of flat circular disks (32) with bolt holes (33) and an angled outer wall (34). Gussets (35) between the wall and the disk increase rim stiffness to minimize deflection. One or more elastomer layers (36), bonded to the outer surface (37) of the wall (34), act as an elastomeric torsional isolator".*

Split Torque Gearboxes: Requirements, Performance and Applications 69

Fig. 15. Load sharing gear in combination with a double helical pinion (Gmirya & Vinayak,

ensuring manufacturing according to strict tolerances and correct assembly. Krantz (1996) proposed the use of the clocking angle as a design parameter to achieve adequate torque split between paths. This author has studied the effects of gearshaft twisting and bending, and also tooth bending, Hertzian deformations within bearings and the impact of bearing

Krantz (1996) defined the clocking angle as β and described the assembly prepared for measurement (Fig. 16): *"The output gear is fixed from rotating and a nominal counter-clockwise* 

2004)

support movement on load sharing.

Fig. 16. Assembly for measurement of the clocking angle

Fig. 14. Gear assembly using an elastomeric torsional isolator (Kish & Webb, 1992)

This assembly was tested in the Advanced Rotorcraft Transmission project (Kish, 1993a), by comparing it with conventional quill shafts. It was concluded that the torque split was excellent and also had other advantages such as lower transmission of force to supports, less vibration and less noise during operation.

The main problem with using elastomers to achieve proper torque split is their degradation over time, especially when used in high-torque gear transmissions where temperatures are high and there is contact with oil. Some authors therefore propose the use of metallic elements to achieve the same effect as the quill shaft.
