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.


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.


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:


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 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.


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