Step 13.0: Understand Forces in MechDesigner

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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. See: Why Configure the Power Source

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.

btn_delkeywordExample: Configure the Power Source