EuroWire July 2016

Technical article

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.

Horizontal Force

▲ ▲ Figure 4 : Planetary back-twist

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.

Vertical Force

▲ ▲ Figure 8 : Load on cradles

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. and equivalent Von Mises stress on a cradle in two positions. Figure 9 shows deformation

▲ ▲ Figure 6 : Speed and torque on each back-twist shaft

Figure 5 shows an example of motor laws.

Power (kW)

Back twister 1 – wheel 2 – torque mag (Nm)

Time (s)

Speed (rpm)

▲ ▲ Figure 7 : Torque curve on a gear

Vertical position

Figure 7 shows a typical torque output on a gear. The peaks, clearly visible in the curve, are due to spools unbalance.

Deformation

Dynamic results as structural input

Torque (Nm)

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.

Vertical position

Von Mises stress

Speed (rpm)

▲ ▲ Figure 5 : Induction motor power and torque curves

Horizontal position

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.

Deformation

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July 2016