﻿ Getting Started Tutorials - MechDesigner > Tutorial 13: Forces: Introduction  > Step 13.0: Understand Forces in MechDesigner

# Step 13.0: Understand Forces in MechDesigner

### Kinetostatic Forces

A Kinetostatic Analysis1 calculates the forces at joints that are a result of the masses, inertia, and motion imposed on the mechanism by an idealized power source2. MechDesigner will do the Kinetostatic-Analysis of each kinematic-chain.

A Dynamic Analysis (one definition) is the opposite. It calculates the resultant motions of the mechanism from a force or torque that is imposed on the mechanism with masses and inertia. MechDesigner does not do a dynamic-analysis.

Notes:

1.Force: is a generalized force, which will include moments.

2.An Idealized Power Source (also the term Fictitious Power Source) has an infinite capacity to move the mechanism exactly as planned by the motion. A simple example of an Idealized Power Source would be a cam-shaft that does not deviate from constant-velocity even though the Application Torque is changing rapidly as it rotates.

 Kinetostatic-Forces: •Forces at joints •Forces at anchor points of Springs •Forces at the contact point between gear teeth flanks, and between a cam-profile and cam-follower. •Motion-Force or Motion-Torque to move each kinematic-chain with a predefined motion. Kinetostatic-Forces are a function of: •The Motion of Parts: Inertia force, Centripetal force, Coriolis force. We assume the motions of all Parts move exactly as planned. •The Mass distribution: Center-of-Mass, Inertia •External Forces: Spring, Drag, Coulomb Forces •Gravitational Force – when the Mechanism-Plane is not horizontal •Which joint gives the 'Power' to the kinematic-chain. Kinetostatic-Forces do not include: •Forces that result from impact between colliding Parts   •Forces from the impact after traversal of backlash in Joints, Gear-Pairs or Cam-Tracks. •Forces from friction at Joints. •Forces from magnetism, electricity. •Forces in kinematic-chains that are not kinematically-defined chains •Forces that are not on the Mechanism-Plane – all forces are made to be coplanar with the Mechanism-Plane

Kinetostatic-Forces are for Ideal Kinematic-Chains:

 IDEAL KINEMATIC-CHAINS: •Rigid Parts: do not bend, twist or stretch •Rigid Parts: do not expand with temperature •Joints: do not have play (backlash) •Joints: do not have Friction •All Parts follow the motion design exactly •Power Source: idealized - it moves exactly as planned •Contact surfaces at Cams and Gears: do not deflect •The kinematic-chain: is 100% efficient REAL KINEMATIC-CHAINS: •Real Parts: do deflect, twist and stretch •Rigid Parts: do expand with temperature •Real Joints: do have backlash, play •Real Joints: do have friction between each Parts. •Real Parts: do not move exactly as planned •Real Motor:s do not move exactly as planned •Real contact surfaces: do deflect •Real kinematic-chains: are not 100% efficient. You may think what is the point in doing a model, because it will deviate from the Real Kinematic-Chain by so much! However,

### Configure the Power Source.

It is important that you Configure the Power Source correctly for each kinematic-chain before you analyze forces.

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 Application Load or Load Torque that the output-shaft of a Servomotor and Gearbox must drive. When you add a Servomotor and Gearbox, you must also accelerate their inertia, and overcome their friction in addition to the Load Torque.

In MechDesigner, each kinematic-chain has three different possible Power Sources.

 A Motor When a motor, rotary or linear, drives a kinematic-chain, make sure the Power-Source is at the joint. By default, the Motor is at the joint with the Motion-Dimension FB. However, the Motor may drive a different joint. A 2D-Cam Usually, a cam is a child to a Part in one kinematic-chain, and the cam-follower that is in continuous contact with the cam is a child to a Part in a different kinematic-chain. In the default case, when you Display Forces, you will see two motors: one motor to drive each kinematic-chain. However, the cam is most often the cam-follower's Power Source and not a motor. Hence, you must Configure the Power Source so that the Power-Source for the cam-follower is the cam, and not a motor. A Conjugate-Cam is a special 2D-Cam - it allows the Contact-Force to switch between 2D-Cam. A Spring that acts as a Linear Motor When you add a Spring FB between two Points in a kinematic-chain, or different kinematic-chains, it simply exerts a Force. However, it can be a Linear Motor. You must configure the power source to make the Spring FB the Power Source.
 Example: Configure the Power Source