# Loads and Inertia

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#### Mike Little

I need to verify something: If I have an electric cylinder pushing against a horizontal load, say a box or pallet, which is not physically attached to my cylinder's tooling, it will increase in the torque requirement, but not factor into inertial calculations? The additional load does impact the system inertia if the load is dropped onto a tooling fixture, the load is vertical, or it is a pick-&-place operation, correct? -- Mike Little Application Engineer Motion Science 864-647-5454 x318

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#### Tom Bullock

It must be counted during acceleration. If the programmed deceleration rate is greater than the deceleration rate of the box alone when decelerated by the friction torque only, then they will separate. The cylinder will decelerate based on the programmed rate and the box will decelerate based on the friction torque. Torque equals inertia X acceleration. If you're looking at it from a servo point of view, you've got a real problem because it will be a non linear function based on the speed you're moving and the frequency of any excitation. You don't want to go there. Tom Thomas B. Bullock, President Bull's Eye Marketing Incorporated Industrial Controls Consulting Division 104 S. Main Street, Suite 320 Fond du Lac, WI 54935 PH: 920: 929-6544 FX: 920: 929-9344 E-mail: [email protected]

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#### BJ_Johnson

If you accelerate (or decelerate) the load then you must account for its inertia whether or not it is coupled to the tooling.

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#### Bill Sturm

The mass of the load is still contributing to the system inertia. What you do not factor in on a horizontal application is the weight of the load. The mass is weight divided by gravity constant. Gravity constant varies with altitude, but is considered to be around 32.14 feet/sec^2 or 386 in/sec^2. Depending on whether you are using ft/sec or in/sec for your velocity. This is why you can push a car but not lift it. You are accelerating the mass but you are not lifting against gravity. Bill Sturm

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#### David Kane

----- Original Message ----- > I need to verify something: > If I have an electric cylinder pushing against a horizontal load, say a > box or pallet, which is not physically attached to my cylinder's > tooling, it will increase in the torque requirement, but not factor into > inertial calculations? The inertia always factors into the situation, but it may not adversely affect your situation. TRUE: The standard servo motor sizing calculations don't account for your situation, as described. For sizing think of it this way: The starting inertia is both the cylinder and the pallet. (assuing they are in contact) The stopping inertia depends on whether or not the pallet is still in contact with the cylinder. By the way, the contact point is also a coupling. Think of is as a spring. Both inertia (kinetic energy) and spring (potential energy) are energy storage. Everything we do in controlling motion is relative to controlling energy storage. That spring (contact point) could cause as much problem as inertia. > The additional load does impact the system inertia if the load is > dropped onto a tooling fixture, the load is vertical, or it is a > pick-&-place operation, correct? Inertia in a vertical plane is no different than a horizontal plane. Inertia is a way of mathematically representing energy caused by a change in speed, in the direction of motion. Gravity (vertical plane) works as potential energy. EXAMPLE: A pendulum: kinetic and potential energy A pendulum in a zero friction system would swing (oscillate) forever Constantly transforming energy from potential (maximum potential is where is reverses at each side) to kinetic (inertia) energy (maximum at center-the highest speed) Assuming minimum oscillation is desired, we keep spring low by using stiff components. And we manage the inertia by planning for how the system is going to react. Typically this is done with inertia matching guide lines. Hopefully this helps Dave Kane [email protected]

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