A Kinetostatic Analysis1 calculates the resultant forces at joints from the motion that is imposed on the mechanism by an idealised power source2. MechDesigner will do the Kinetostatic-Analysis of each kinematic-chain on a Mechanism-Plane.
A Dynamic Analysis is the opposite. It calculates the resultant motions of the mechanism from a force or torque that is imposed on the mechanism. MechDesigner does not do a 'dynamic-analysis'.
Notes:
1. | Force: when we use the word force, it will refer to a generalised force, which will include moments. There are also schools that call these Dynamic-Forces. See Dynamic Forces. |
2. | An Idealised Power Source (also Fictitious Power Source) has an infinite capacity to move the parts exactly as given by the planned motion. A simple example of an 'Idealized Power Source' would be a cam-shaft that does not deviate from constant velocity even though the torque required to drive the cam-shaft is changing rapidly. |
Kinetostatic-Forces:
Kinetostatic-Forces are a function of:
Kinetostatic-Forces do not include:
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Kinetostatic-Forces are for Ideal Kinematic-Chains:
IDEAL KINEMATIC-CHAINS:
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REAL KINEMATIC-CHAINS:
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You may think what is the point in doing a model, because it will deviate from the Real Kinematic-Chain by so much! However, |
It is important that you Configure the Power Source correctly for each kinematic-chain before you analyse forces. That is, you must select from which joint (or cam, or spring, or gear) each kinematic-chain gets its 'power'. Note: The Moment Vector we show at a joint is the Torque a Servomotor needs to provide to the mechanism. When you add a Servomotor and Gearbox, you must also accelerate their inertias, and overcome their friction. To find the Servomotor and Gearbox combination for your application, see Kinetostatic Motor Torque and Speed Data. In MechDesigner, each kinematic-chain has three different possible sources of 'Power':
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The Torque and Force values at each joint are a function of the joint you select for the Power Source. To illustrate this, we model a four-bar kinematic-chain, arranged as a simple a mechanism (see images below). The angle between each part is 90º. (See the images below). The Mechanism is stationary. The four-bar includes:
This is a Rocker. It is the vertical Part on the left. The Rocker has a 1kg mass that is 100mm horizontally to the left of the Pin-Joint made with the Base-Part. Its weight is ~9.81N (1kg * 9.81m/s/s=9.81N).
There are two Parts: one horizontal, the other vertical and parallel to the Rocker, on the right. The position of the mass does not change when we move the Power Source from the left to the right. However, when we move the Power-Source, the resultant Forces at each Joint are completely different. |
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![]() Simple Mechanism : Power Source at Bottom-Left Joint. |
Configure Power Source 1 If we put the Power Source Resolve Forces:
Resolve Moments:
We can imagine that the Drive Torque keeps the Rocker in its vertical position. |
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![]() Simple Mechanism : Power Source at Bottom-Right Joint |
Configure Power Source 2 If we put the Power Source Resolve Forces:
Resolve Moments:
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