Advantages of Helical and Double Helical gearing

SINGLE HELICAL	DOUBLE HELICAL
Comparative simplicity in grinding No gap, low helix angles - 15 '	Longer grinding times , normal gap normal helix angle - 30 '
Complete absence of pinion shuttling obviates the use of sliding couplings Apex wander due to different composite pitch errors cause shuttling.Gear tooth couplings do not respond because of the high frictional loads. Best compromise is to use axially flexible couplings. With highly accurate gear manufacture this effect is small
Axial thrust on primary high speed pinion unless taken by turbine thrust bearing can lead to high losses if flooded thrust pads are used. The use of brown boveri thrust cones can be used to overcome this problem.(see Below)	No axial thrust and no high speed thrust bearings required. Final reduction wheel located by propeller thrust bearing
Ball and roller bearings may be used to take end thrust
Quill shafts can be solidly coupled to primary wheels and secondary pinion. The helix angle on each being arranged to balance the axial thrusts.
Simple side bearings serve to locate the shafts . The axial thrust of the final reduction wheel being carried by the propeller thrust bearing.
Axial tilting moment on wheels generally negligible.	No tilting moment
Small helix errors can be perfectly corrected . Allows tooth helix angle adjustment to negate bending , torsional and heating effects and hence balance loading across the teeth. Helix errors can be adjusted in a similar way, but not so perfectly as for single helical
Summary The main advantage is that the double helical gear does not have end thrust However they do take more time to manufacture and are slightly heavier

Brown Boveri Thrust Cone

This is a method of absorbing end thrust in single helical gears without resorting to large thrust bearings. This design is seen insmall steam turbine generator sets.
With the cone system there is a line of contact and a very large relative radius of curvature with a large oil entraining velocity of 220 ft/s .There is thus considerable axial resilience with the large radius of curvature, a small radial width of cone is sufficient to take the thrust