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Helical Gear Planetaries

Helical Gear Planetaries

Helical Gear Planetaries

Question 1.
In a helical gear simple planetary arrangement, with the sun driving, and the arm output, is there a thrust on the individual planets or is the thrust inflicted on the carrier(arm)only??

Qustion 2.
In a multi-stage helical gear simple planetary, will the thrust from question 1 be transfered to the previous stage's planet gears or to the carrier(arm)?

Question 3.
Do the planet gears need thrust bearings or just the carriers?

Any thoughts?


Edson Campos


RE: Helical Gear Planetaries

You always post interesting questions!
Any contact of the gear teeth imposes a
thrust action on both members.  Depending
on the forces, bearings may be needed
if you are looking for long life.  You
probably know all of this so maybe I am not
understanding exactly what you are asking.
I assume that with greater pressure angles
you have greater forces acting towards
their centers.

RE: Helical Gear Planetaries

Let me ask this differently, I know that thrust exists on the gears between their respective meshes.  The question is, since the arrangement is such that are three gears in contact (sun, Planet, Ring), are the thrust forces from sun-planet necessarily equal and opposite to the thrust forces from ring-planet?

Two helical gears meshing will cause equal and opposite thrust.

Will three helical gears meshing cancel out the thrust?


Edson Campos


RE: Helical Gear Planetaries

Example 3 idler system.
I think if the sun is the driver and the
force is outward, it would be 1/3 outward
on each idler which would in turn be resisted
by the carrier. All of the force is outward
so I think bearings or bushings would be
necessary on the idlers if the forces were large.
I do not think it would be necessary on the
carrier or the sun. The circumferential forces
have to be considered as well to keep them in
position on their centerlines.  This may be
the greater force on the bearings. I think
the amount of backlash would make a difference
as well as to how the load is imparted to the

RE: Helical Gear Planetaries

I think your last conclusion about the direction of forces may not be correct. A simple free body diagram may help. The sun (due to the helix angle) imparts an equal axial force on each of the three planets, all in the same axial direction. These three planets impart this same force onto the annulus but the direction of the reaction on the planets cancels out the axial force imparted by the sun. Net thrust on the planets is zero. The axial force on the sun will be equal and opposite to the axial force on the annulus....Do you agree??

RE: Helical Gear Planetaries

The tangential forces are all the same
at each interface so they are in balance.
The axial movement is the smaller force.
The force acting towards the pitch line
is of a greater magnitude wanting to move
the idlers around the sun.  The bearings
act to restrain these idlers from moving
radially around the sun.  Possibly large
bushing would keep these in place without

RE: Helical Gear Planetaries

please inform me about the calculations of the damping and the stiffness effects of the lubricant between two mating gears on the dynamic response of gears.

RE: Helical Gear Planetaries


SAP explains the thrust forces well. In a helical system, the planet thrust cancels out, and the sun thrust is the total of the thrust from all of its meshed teeth.

Another consideration is that the planets will have an overturning moment equal to pitch diameter times the thrust force. If there are clearances in the bearings, this will lead to the planets cocking, which is to be taken into account as a derating factor. The additional load must also be accounted for in bearing calculations.

Sun pinion thrust and the overturning moments are a couple of reasons why spur systems are an overwelming majority in planetary applications.

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