EuroWire July 2016

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Technical article
July 2016 53
When each simulation runs, all loads are
automatically combined together along
the transmission chains, leading to a
precise estimation of the power demand
at all motor shafts.
As active parts of the machine, the electric
motors, are modelled taking into account
the real inertia of the rotating parts and
using the real constructive curves (torque
and speed) of modern induction motors.
Otherwise by using ideal motors (very easy
and simple in RecurDyn®) there would be
the risk to obtain an imprecise answer. In
fact, such an approach would generate
unrealistic torque peaks in the simulated
signals; motors with unlimited torque
simply do not exist.
Figure 5 shows an example of motor laws.
Dynamic simulation
and results
A lot of dynamic simulations are run, and
more than 60 cases are analysed, based
on the possible different load cases
preliminarily defined.
Each dynamic simulation is composed
of three phases: acceleration (from zero
to the maximum speed), a steady state
condition at the maximum speed, and the
emergency braking (deceleration from
maximum speed to zero in a few seconds).
From the large volume of data collected
it is possible to define all the information
necessary for the design; in particular the
maximum power required to the motors
and the maximum torque and speed on
each part.
This data is fundamental for the right
choice of motors and for a good structural
design of the parts (rotor, cradles, joints,
and so on).
Figure 6 shows the results in terms of
rotation speed and torque on each part of
the transmission chain.
Figure 7 shows a typical torque output on a
gear. The peaks, clearly visible in the curve,
are due to spools unbalance.
Dynamic results as
structural input
As previously explained, the results
obtained from the dynamic simulation are
the input of the structural simulation.
By using the CAE structural software
ANSYS Workbench®, that is directly
linked with RecurDyn®, MFL performs the
simulation of the mechanical behaviour of
the most important parts of the planetary
machine.
The goal is to verify that all parts meet the
strength and deformability specifications.
On a planetary machine, all parts are
under fatigue (Figure 8 shows the load
on the main frame of a cradle during
a rotation around its axle); so that the
engineers use specific methods for the
verification of welded structure under
fatigue as hot spot, Radaj methods and so
on.
Figure 9 shows deformation and
equivalent Von Mises stress on a cradle in
two positions.
▲ ▲ Figure 4: Planetary back-twist
▲ ▲ Figure 5: Induction motor power and torque curves
▲
▲ Figure 6: Speed and torque on each back-twist shaft
▲ ▲ Figure 7: Torque curve on a gear
▲
▲ Figure 8: Load on cradles
Torque (Nm)
Speed (rpm)
Power (kW)
Speed (rpm)
Back twister 1 – wheel 2 –
torque mag (Nm)
Time (s)
Horizontal Force
Vertical Force
Deformation
Vertical position
Vertical position Von Mises
stress
Deformation
Horizontal position