# What are the correct terms for linkages and lifts

In the past I’ve built robots that have an arm made of 4 pieces of metal that form a parallelogram. The right side is fixed, and as the arm is raised the left side moves up, but stays parallel to the right side.

I’ve used the term 4 bar lift for this, but have heard others write/say “4 bar linkage”. While I agree it’s a 4 bar linkage, it is a single case. 4 bar linkages can have each bar be a different length. With different lengths, the motion is quite different. Which term is correct, or is there a better term / phrase to use.

Moving on to 6 bar linkages. When I Google these, an entire universe of really cool motion possibilities is shown. I’ve built 6 bar lifts, where it’s two parallelogram 4 bar lifts superimposed over each other (the left vertical bar is the right vertical bar of the next linkage, the upper bar becomes the lower bar in the next linkage).

Lets go back to 4 bar for a second. There is a set of lifts where the bars cross (make an X). You see these in “high lift platforms”. I’ve heard these referred to as “4 bar lifts” or in some cases 8 bar lifts, since there are 4 bars on each side. But then I’ve heard that stacking them makes that an 8 bar lift. And more stacking is a 12 bar lift.

So I’m looking for a little terminology help here. I’d like to be using the right terms (that mentor thing) to be understood / understand what others are trying to tell me.

I’m good if we limit these to moving in the same plane. I was watching a video of a “4 bar linkage” where the points can move in 3 dimensions and that appears to open up a much bigger can of worms.

I always refer to it as a 4-6 bar linkage. Not all linkages are built to “lift” so I would stick with calling it a linkage.

The “X” lift you are talking about sounds like a scissor lift.
http://www.sunstateequip.com/static/equip-gallery/1/2/27/Genie%20GS3268RT.jpg

When first refers to a “4-bar” or “4-bar linkage” they typically are referring to the system you described, where one side moves parallel with respected to the fixed side. Like you mentioned, this is only one particular case of a 4-bar linkage. The lengths and orientations of the bars and pivots can be adjusted to create a wide range of motion.

If you are interested, you can do some research on Kinematics (I took two full quarters of classes in college on this topic), and specifically 2 and 3-position synthesis. These are design problems where you specify 2 positions of motion and determine the lengths and mounting of the other links.

The “stacked 4-bars” you are referring to are known as scissor lifts. They seem like a simple concept, but can be very difficult to execute well. They require a high degree of precision as the links need to lie in the same planes as well as have the same lengths to prevent the lift from leaning.

I’d like to point out that fore-bar linkages need not nessacarily have 4 bars.

The other term for a scissor lift, at least in FRC

, is “What were you THINKING???” In addition to the degree of precision for the manufacture of individual links, they all have to go together precisely, and one end of the stack needs to be free to move horizontally, but not vertically. (This on top of the lift being rather unstable when fully extended–even a typical 26’ lift can sway a bit when an average person walks from one end to the other while it’s near the top of its travel.) Speaking as someone who uses one on a minimum of a weekly basis (and spent most of today on top of one), if someone successfully uses one in FRC
, I will be quite surprised. (Current count of scissor-lift robots I’ve seen in elims: 1, maybe. On Einstein, 0.)

Technically, 254’s ball lift this year was a scissor lift… Just a very short single stage one.

If we want to explore a linkage system quickly, we use a program called Linkage 2.0, developed by David Rector. Here is a link for a free download.

It is very easy to use and it is a fast way to explore a variety of systems. There are even examples that you can modify.

Does a scissor lift require the two parts to be connected in the center? IIRC, and please correct me if I’m wrong, but their pinniped was two separate mechanisms that acted individually or at the same time (so they could eject the ball out either end). If they were connected, they would never be able to remove the ball through their intakes.

In 2013 842 used two small single stage scissor lift to raise frisbees into their shooter

Correct. The claptrap was two separate arms essentially, pneumatically toggled independently. No scissor.

I have heard of the four-bar linkage with equal-length opposing edges referred to as a “parallelogram linkage”, though I don’t recall that I’ve ever read it.