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 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'.


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 called 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.


Reaction forces at joints

Reaction forces at anchor points of Springs

Reaction forces at the contact point between gear teeth flanks, and between a cam-profile and cam-follower.

The 'Motive Force' 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: Centre-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 Load Torque at the output-shaft of a Servomotor and Gearbox. When you add a Servomotor and Gearbox, you must also accelerate their inertias, and overcome their friction in addition to the Load Torque.

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

A Motor

If you drive a kinematic-chain with a motor (rotary or linear), make sure the Power-Source is given to the correct joint.

By default, MechDesigner gives the Motor to the joint with the Motion-Dimension FB.

However, it is possible, especially when you use inverse-kinematics, that the Motor drives a different joint.

A 2D-Cam

Usually, a cam is a child to a Part in one kinematic-chain, and the cam-follower 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 of the motor that drives the cam-follower's kinematic-chain so that its Power-Source is the cam. The remaining motor drives the cam.

A Spring that acts as a Linear Motor

When you add a Spring Force 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 Force FB the Power Source.

btn_delkeywordExample: Configure the Power Source