# Simscape use of inertia and mass blocks

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Kai Hansmann on 31 May 2021
Answered: Yifeng Tang on 14 Jun 2021
I am trying to build a simplified model of a piston compressor in Simulink. I started with an ideal velocity source with 265 rpm, added the intertia of the crankshaft to it, converted the rotation to a linear movement with a wheel and axle block, added the mass of the piston and connecting rod to it. The translation mechanical converter block compresses the air. The goal is a suction volume of 2500 l/min. After running the simulation the suction volume is at 7e-6 l/min and I get a warning regarding the inertia and mass blocks. I struggling to find out where exactly the problem is with these blocks and how to solve it. Have I used the blocks correctly and set the references properly?
Yifeng Tang on 2 Jun 2021
Hi Kai,
I suspect there is something else going on in your model. It's currently set up with wheel and axle as the conversion mechanism between rotational and translational motion. There isn't any reciprocal mechanism that exists in a piston compressor. As the constant angular velocity is driving the wheel, the cylinder will be extended or compressed. Because it's an ideal velocity source, the hardstop you have in the model won't stop the motion. Instead, the velocity source will apply huge torque to overcome the very stiff spring from the hardstop.
Looks like you need to consider using the Slider-Crank block instead: https://www.mathworks.com/help/physmod/simscape/ref/slidercrank.html

Yifeng Tang on 14 Jun 2021
I suspect there is something else going on in your model. It's currently set up with wheel and axle as the conversion mechanism between rotational and translational motion. There isn't any reciprocal mechanism that exists in a piston compressor. As the constant angular velocity is driving the wheel, the cylinder will be extended or compressed. Because it's an ideal velocity source, the hardstop you have in the model won't stop the motion. Instead, the velocity source will apply huge torque to overcome the very stiff spring from the hardstop.
Looks like you need to consider using the Slider-Crank block instead: https://www.mathworks.com/help/physmod/simscape/ref/slidercrank.html

R2021a

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