﻿ Step 11.3: 'Remove' the Piggyback Sliders

# Step 11.3: 'Remove' the Piggyback Sliders

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.

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

#### Degrees-of-Freedom of Mechanism with Sliders removed

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.

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.

Here, there are three Motion-Dimension FB 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.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd