Step 13.0: Understand Forces in MechDesigner

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Step 13.0: Understand Forces in MechDesigner

Kinetostatic vs Dynamic Analysis- a definition

A Kinetostatic Analysis1 calculates the forces at joints that are a result of the motions imposed on the mechanism with mass and inertia by an idealized power source2.

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 with masses and inertia. MechDesigner does not do a dynamic analysis.

MechDesigner does the Kinetostatic Analysis of each kinematic-chain.


Notes

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

2.An Idealized Power Source (or the term Fictitious Power Source) moves the mechanism exactly as planned. A simple example of an Idealized Power Source would be a cam-shaft that rotates at constant-velocity even though the torque on the cam-shaft changes as it rotates.

About Kinetostatic Forces

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 follower-profile

Force or 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: Mass, Center-of-Mass, Inertia

External Forces: Spring, Drag, Coulomb Forces, ...

Gravitational Force – when the Mechanism-Plane is not horizontal

Which joint has the Power-Source that drives the kinematic-chain. See: Why Configure the Power Source

Kinetostatic-Forces do not include:

Impact Forces of Parts that collide

Impact Forces after traversal of backlash in Joints, Gear-Pairs or Cam-Tracks

Friction Forces

Magnetic, Electrical Forces

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

Rigid-Part have an exact length, without a tolerance

Joints do not have play (backlash)

Joints do not have Friction

Power Source is idealized - it moves exactly as planned

Surfaces at Cams and Gears do not deflect

REAL KINEMATIC-CHAINS:

Real Parts: do deflect, twist and stretch

Rigid Parts: do expand with temperature

Rigid Parts: do have a tolerance to their length, plus other geometric constraints (GD&T) that need to be applied

Real Joints: do have backlash, play

Real Joints: do have friction

Power Sources do not move exactly as planned

Real contact surfaces do deflect

Real kinematic-chains: are not 100% efficient

Real kinematic-chains vibrate

You may think “Why do a model when it can deviate from the Ideal Kinematic-Chain by so much”?

However, the better your design, the nearer it will be to the Ideal Kinematic-Chain.

Configure the Power Source.

It is important that you Configure the Power Source correctly for each kinematic-chain before you do a Kinetostatic Force Analysis.

You must select at which joint (or cam, spring, gear) is the Power-Source of each kinematic-chain.

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 Follower-Profile 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 Follower's Power Source and not a motor. Hence, you must Configure the Power Source so that the Power-Source for the 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 to show you why you must Configure the Power Source