Theory of Machine
To obviate axial thrust, following gear drive is used

Double helical gears having identical teeth
Single helical gear in which one of the teeth of helix angle a is more
Double helical gears having opposite teeth
Mutter gears

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Theory of Machine
Two heavy rotating masses are connected by shafts of lengths l₁, l₂ and l₃ and the corresponding diameters are d₁, d₂ and d₃. This system is reduced to a torsionally equivalent system having uniform diameter d = d₁ of the shaft. The equivalent length of the shaft is

l = l₁ + l₂.(d₁/d₂)⁴ + l₃.(d₁/d₃)⁴
l₁ + l₂ + l₃
(l₁ + l₂ + l₃)/3
l = l₁ + l₂.(d₁/d₂)³ + l₂.(d₁/d₃)³

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Theory of Machine
A rigid body, under the action of external forces, can be replaced by two masses placed at a fixed distance apart. The two masses form an equivalent dynamical system, if

The sum of mass moment of inertia of the masses about their center of gravity is equal to the mass moment of inertia of the body
All of these
The sum of the two masses is equal to the total mass of body
The center of gravity of the two masses coincides with that of the body

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