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

The Torque and Force values at each joint are a function of the joint you select for the Power Source.

To illustrate this, we model a four-bar kinematic-chain, arranged as a simple a mechanism (See the images below). The Mechanism is not moving.

The four-bar includes:

•the Motion-Part.

This is a Rocker. It is the vertical Part on the left.

The Rocker has a 1kg mass that is 50mm horizontally to the left of the Pin-Joint made with the Base-Part. Its weight is ~9.81N (1kg * 9.81m/s/s=9.81N).

•an R-R-R Dyad.

There are two Parts: one horizontal, the other vertical and parallel to the Rocker, on the right.

The position of the mass does not change when we move the Power Source from the left to the right.

However, when we move the Power-Source, the resultant Forces at each Joint are completely different.
Power Source to the Left

Power Source to the Left

Configure Power Source 1 - Power Source at the grounded Pin-Joint on the LEFT.

Resolve Forces: Force at the Pin-Joint of the Rocker of 9.81N, because of the weight of the mass.

There Forces at all of the other Joints are ZERO.

Resolve Moments: 0.49Nm Torque at the Pin-Joint of the Rocker.

The Motor must apply a Torque of 0.49Nm to balance the Moment of 0.49Nm from the 1kg mass, at 0.05m from the Pin-Joint.
Power Source to the Right

Power Source to the Right

Configure Power Source 2 - Power Source at the grounded Pin-Joint on the RIGHT.

Resolve Forces: Vertically and horizontally to give forces at the Pin-Joints.

The force due to the mass is transmitted through the kinematic-chain to the Power-Source

Resolve Moments: Resolve Moments gives the same Moment (Torque), but at the Pin-Joint on the right.