We have used the Piggyback Sliders to specify the motion. However, it is not always necessary to include them in the 'physical' design.

Background

Question 1: Why would we want to remove the Linear Sliders?

Answer Part 1a: Slide-Rails and Slide-Blocks are more expensive than rotary bearings.

Answer Part 1b: It is easier to seal rotary Bearings than Slide-Blocks on Slide-Rails. Often, this is important for food production machines in the FMCG industries.

Model with Sliders 'Virtually' Removed

In the model so far, we use the Piggyback Sliders to move the Parts in the Dyads.

Can we configure Dyads to give a Part that moves in the same way as the Y-Slider of the Piggyback Sliders?

That is, can we find a Part that copies the XY-Path given to the Y-Slider.

QST13-1-7

 

For information, all Points in the Y-Slider Part move with the same 2D Planar Motion. Different Points in the Y-Slider move with parallel motions.

This is a definition of a 'Translating Part'.

 

 

QST13-3-6

Note: I have joined the Parts directly to the Y-slider. The Pin-Joints on the Y-Slider are slightly apart.

 

None of the Parts in the Dyad 'translate'. They all rotate, even slightly, as they move.

 

Question 2: Can we add new RRR Dyads so that one of the Parts in a new Dyad 'Translates'. That is, it does not rotate.

Answer 2: Yes, we can.

See below.

New Dyads in Mechanism

In the image to the left, we have added two more R-R-R Dyads.

 

The length of the Parts in the Dyads and the Position of the Pivot Points are important.

The orange Part does not rotate – it 'translates'.

 

Hence we can remove the Sliders in the real design.

 

Here, we use the Sliders only to specify the XY-Path of the Translating Part.

The Video shows the mechanism movement.

All the Points in the Translating Part move with the same 2D path. We do not use the Sliders in the physical machine. They only give a useful means to force the translating part to move with the XY-Path.

For completion, I have added two 2D-Cams and Cam-Followers to move the translating part with the 2D Planar Motion.

Add 2D-Cams to drive the Cam-Followers with the required motions.

 

MechDesigner derives the motions of the Cam-Followers.

Compare to a Model with the Sliders 'Actually' Removed.

Degrees-of-Freedom of Mechanism with Sliders removed

Degrees of Freedom of Mechanism with 3 RRR Dyads.

Compared to Step 11.2, we have:

removed two sliding-parts
added a Part (4P) to join the end of the Dyads together (to make eight Parts in total),
removed two Slide-Joints (to make nine Joint in total).

Gruebler Equation.

F = 3(P-1) - 2J  : P = # Parts ; J = # Joints

F = 3 * (8-1) - 2 * 9

F = 21 - 18 = 3.

There are three Degrees-of-Freedom.

QST13-3-2B

Here, there are two Motion-Dimension FBs, and Rockers.

 

Not all of the Parts have Green Part-Outlines.

Three Parts have Blue Part-Outlines. These can belong a 4-bar kinematic-chain when all the other Parts fixed.

To make the new 4-bar kinematic-chain, imagine all the Green Parts are the same as one Base-Part.

Therefore, one more Motion-Dimension FB kinematically defines the complete kinematic-chain.

Use the Gruebler Equation to make sure.

QST13-3-4

Here, there are three Motion-Dimension FB3s and Rockers.

 

Note: the Green Part-Outlines - to indicate that they kinematically-defined chain.

However, it is not easy to use the power of 'inverse kinematics'.

It is not easy to design the motions for the three Rockers so that the Part we want to translate actually moves with a Translating Motion.

 

Translating Motion = Does Not Rotate as it moves in a Plane.

Help Files for MechDesigner and MotionDesigner 11.1 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd