Cleaning up confusion of naming of linkages

VEX V5 General Discussion General Forum
1

Hopefully I can clear up some of the confusion on naming of linkages…
A 4 bar linkage is called that because there are 4 separate links (or bars), each of which is connected to 2 pivots, one at each end. The bars are counted as they appear in the plane in which movement occurs. For example:
https://vexforum.com/gallery/files/1/2/6/6/3/4/4_bars.jpg
Two of the links are the black arms.
The third link is made up from part of the vertical support that is attached to the robot’s chassis; this is the ground link.
The fourth link is the yellow part which the robot’s manipulator (claw, conveyor, whatever) is attached to.
The pink diamonds are pivots (pin joints).

Many “4-bar” vex robots in fact have 2 of these linkages, one on each side of the manipulator (claw, ramp, conveyor). This is what I think of when I hear “double 4-bar”.

Now for the 6-bar:
https://vexforum.com/gallery/files/1/2/6/6/3/4/6_bars.jpg
Astute observers will note that this is made of 2 four-bar linkages. The first is made up from bars 1, 5, 3, 4. The second is made up from bars 1, 6, 2, 4.
There are 7 pivots, but only 6 bars.

Bar 1 is the longest part of the arm, and has 3 pivots.
Bar 2 is the top part of the arm, and bar 3 is the bottom part. Both have 2 pivots.
Bar 4 is a vertical bar with 3 pivots.
Bar 5 is made up from part of the vertical support that is attached to the robot’s chassis; this is the ground link.
Bar 6 is the yellow part which the robot’s manipulator (claw, conveyor, whatever) is attached to.

Again, most robots that use these mechanisms will have 2 of them, side by side. Our Fush and Chups used this configuration.

It is not the same as a scissor lift on its side because a scissor lift normally uses sliding joints at the top and bottom, although it does share some similarities. The 6 bar uses only pin joints.

Hopefully this goes some way to explaining why we decided to call this mechanism a “6-bar linkage” :slight_smile:

-Chris

2

Thanks for the clarification!

3

Though extended 4-bar is apparently more correct, I personally like “6-bar link” better for the specific purposes of VEX bots. It is more descriptive, as well as I think one of the first names given to it by AURA in describing their bot for Round Up. There are six linkages and that makes sense.

I can understand the concerns of the mechanical purists in maintaining that extended 4 bar is more accurate as most “n-bar” links are complex links with little relevance for VEX robots. However, I have never understood how some people persist in referring to them as seven bar links. If someone who likes to call them 7-bar links could post a photo with labeled bars like AURA has with 4-bar/6-bar, that would be great to explain how they have been counting ಠ_ಠ.

4

There are actually 7 Linkages, But only 6 bars. That is why some people call it 7 bar and others call it 6 bar.

5

I really don’t care about saying 7 Bar, because my team is used to 6 Bar… I might switch over.

6

A six bar linkage is in fact a six bar. Two of the links are ternary links (3 pivots,) but there are only six bars total, 3 vertical, and 3 horizontal.

7

There is some HEAVY confusion going on here. Would a pro tell us the exact terms?:confused:

8

I just did a little Googling and didn’t trip over an answer; but I did see some interesting material.

One possibility is that the mechanism can’t be described only in terms of bars.

Another is that it’s proper name might simply be a type of 4-bar, because that describes its net behavior, regardless of the internal shenanigans (that raise the tip).

Another answer might be that more than one popular classification system is in vougue.

I too would appreciate reading a well-researched answer that comes with some citations of authoritative sources.

I think a student or two should be able to clear this up with a little research - Who is going to do that before some old fogey like me beats them to an answer? :slight_smile:

Blake

9

I agree this should be cleared up. I cannot find anything right now…

10

I’m taking a college course in linkage design right now. I can guarantee that this arrangement is a type of six bar linkage. Specifically, it is a Watt’s Six Bar Linkage.

How can you tell? Simple. A “link” is defined as a rigid body connected to other links, by pin joints in this case. This mechanism has 6 of them. The links labeled “1” and “4” in the OP’s drawing are what are known as Ternary links. Ternary links have 3 pin connection points, rather than the more common Binary links of a four bar linkage. But they are still rigid bodies, and therefore, single links. Since the two ternary links are connected to one another, this particular geometry is termed a Watt’s sixbar, after James Watt, the originator of this arrangement.

Another way to check these things is through the use of Gruebler’s equation. Gruebler’s equation predicts the number of degrees of freedom of a linkage, given a number of links and a number of joints. It is written as:

3(L-1) - 2(J1) -J2 = DOF

Where L is the number of links, J1 is the number of full joints, and J2 is the number of half joints. Pin joints are full joints…an example of a half joint would be a cam sliding against a follower.

In this case, the degree of freedom of the linkage is obvious. Spinning one link with a gear train drives the entire thing through a predictable motion, meaning that the system has 1 degree of freedom. We can verify that this is the case for a six bar linkage of the given geometry

Links: 6
Full joints: 7 (just count the pins in this case. If more than 2 links pivot around a single pin, you count that pin multiple times, for each extra link)
Half joints: 0 (not typically found in linkage mechanisms)

3(6-1) - 2(7) - 0
15-14
DOF=1.

If it were a 7 bar…

3(7-1) - 2 (7) - 0
18 - 14
DOF=4???

It doesn’t take 4 independent motors to drive this mechanism. The degree of freedom is 1, therefore the link count must be 6.

Linkage synthesis and analysis is a fascinating topic. Their applications go far beyond creating parallel motion arms for robots.

That’s actually a common misconception I’ve noticed, and probably the root cause of a lot of the confusion on this topic. The name “four bar linkage” says NOTHING about parallel motion. A four bar linkage is simply a linkage chain with four binary links. Nothing more, nothing less. It can do parallel motion, if designed to do so. But it can do a lot more…it can be arraigned as a crank rocker…provide approximate straight-line motion…provide motion along a specified path…and so on. “Four bar” being synonymous with “parallel motion arm” in the robotics world is incorrect. Four bars can create this motion, but so can just about any other more complex linkage. Don’t confuse bar count with description of motion.

For the same reason, although this mechanism is a form of 6 bar linkage, I don’t think “six bar” is the best thing to call it. “Parallel 6 bar” maybe?

Source: Design of Machinery, 5th ed, R.L. Norton. (Fantastic book…)

11

Thank you…

12

lol, even so…
if anyone says extended 4 bar/ 6 bar/ 7 bar, we’ll all know what they are talking about…

13

Agreed. Nice discussion.

14

You have our everlasting thanks.

15

And if you call it a Watt’s 6-Bar Linkage, the judge with the mechanical engineering degree will move you up a notch on her short list of award nominees (or her list of possible interns at her company)… Food for thought.

Blake

16

however, ours is not :wink:
its a slight variation of the 6-bar

17

Thanks for cleaning this up! :smiley:

18

Then I will call yours Extended 4 bar Until I see it.

However now that the name for the other variation has been finally proven. (even though its still just a 4 bar) I will call it a 6 bar.

  • Andrew
19

Cool beans :smiley:

20

Ditto.

~Jordan