The Vexmen of Brandywine Robotics

Blog Post

Skyrise Lift Discussion For New Vex Teams

This post is geared towards our eleven new middle school Vex teams who need to design their first robot to play Skyrise. One of the keys to this year’s game is to reach dizzying heights in order to score cubes on posts and build skyrises.

For those who have never built a robot before, this will seem quite daunting. So we’re here to help.

First, let’s talk about the problem set in a bit more detail. The starting size constraint of 18″ and the goal heights of 40″ and 48″ mean we need to build something compact that can expand. The second is we want to pick up cubes on the floor level and bring them above 40″ so that means we may need the bottom of the lifter needs to get that high. This means your lift may be even taller than 48 inches – up to 66 inches.

I encourage mentors and student roboteers to go through the Vex design curriculum: http://curriculum.vexrobotics.com/curriculum

It has some really good information about how to build solid robot designs and principles of engineering accompanying these designs. However, some items are not battle hardened. So we will diverge from some of their build processes. Please look at these but be wary.

The most egregious bad build practice in the curriculum are cantilevered wheels without outside support. Since you will be building a pretty heavy robot, your wheels and axles are handling the complete load of the robot. If your wheels are not supported on both ends of the axle, you will place a stress on the axle resulting in bending of the axle and not turning very well. It also opens up the robot for incidental damage from opponents and even partners.

You will then notice simple arms are primarily used in the examples. Even using 17.5″ elements, a single stage arm will not reach 40″ goal let alone the 48″ goal. This is where we need to decide on a lift style.

Three years ago the game was Gateway and they introduced a 30″ goal and this was a big challenge versus previous years games. We described the various lift types here. Now with this year’s game,only a few viable options remain. Unfortunately these are some of the more complex lift types. So that narrows down our viable lift types to the following:

1) Double reverse four bar linkage
2) Double reverse six bar linkage
3) Eight bar linkage
4) Scissor lift

Discussions on the Vex forums (we’ll get to that in a bit) have centered around scissor lifts and double reverse four bar linkages. From early season videos from Asia, these lifts are the most popular with scissor lifts winning out in China as they use them year after year. Also popular are double reverse four bar linkages. So we will look at two lift designs in some detail – the double reverse four bar, and the eight bar linkage.

We will not do a dive on scissor lifts. For new teams, the lifting process of the scissor lift is generally too complex for a new team. Teams 80N in Gateway made a really tall scissor and it took over three months to develop. Last year, team 81J built a scissor but had issues with chin ups from that game and will try it again this year. This year, team 91C has started a scissor over the summer and is now working out the lifting kinks – again three months in the build cycle. These are experienced teams and it takes that long, so I want to dissuade new teams from tackling the scissor lift.

The post images are courtesy of team 323Z in Indiana. They have posted some example designs of these very lifts. You can click to their CAD Library page to view some other designs as well.

Standard Four Bar Linkage

Before we can describe a double reverse four bar, it may be helpful to know the basic four bar linkage. With this arm style, we power the arm by driving a gear through reduction of a 12 tooth gear to a 60 tooth gear. This gives a 5:1 ratio for assisting the lift.

Notice the c-channel at the non-driven end of the four bar linkage. By the linkage shape being a parallelogram, the c-channel on the end of the linkage will remain perpendicular to the floor. If you attach a forklift to this, the cube will remain parallel to the floor and not tilt. This helps with control immensely.

Four Bar Linkage

Taking a closer look at the connection points, you will notice we use bearing blocks wherever there is something rotating about an axis. The next thing to notice on the bearing block is the choice of nut. This is critically important because we need to minimize friction on lifts at all costs. This one uses the nylon locknuts which locks in the screw and is very difficult to come undone. It is also more difficult to attach so you have to use an hex wrench and a crescent wrench together to ensure they screw together properly. Additional nylon washers are not shown in the pictures but they are the next key to reducing friction. They ensure plastic on plastic sliding action which has a lower coefficient of friction than metal contact. So if we can have slippery plastic rubbing together, we will have lower amounts of friction.

Four Bar Linkage Driver

This is the backside view which has two driving gears on the bars. Lots of bearing blocks, and the gears drive the arm up and down. What will stop the arm is the four bar linkage pars will pinch each other at about 75-80 degrees in this configuration. So this lift will not get us high enough to score beyond the 24″ goals so we need to look further. But it is a good primary lift type to understand in order to build its cousin, the double reverse four bar linkage.

Double Reverse Four Bar Linkage


This linkage has two four bar parallelogram linkages that work together to raise itself from a compact starting position. This one is driven from the bottom, but other popular variants like team 80Y power theirs from the middle.

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Double Reverse Four Bar

These are cut away views so they are not attached to the towers on both sides. You will need to do that for a complete lift design. On the picture above you can see the placement of the motors on the lift. This will have a total of four motors once complete – two for the left side, two for the right side. Notice how the top and bottom gears fit together? That helps keep the upper and lower motors working together.

The next element to inspect is how the bars are laid out. You need to count the holes and ensure it is a rectangle/parallelogram and not just a four sided shape. Making the holes in different positions per side can make it bind together and not work the way you want it to.

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Double Reverse Four Bar view 2

This particular linkage is driven from the lowest part and is mechanically driving the top linkage. If we extended the middle set to 17.5″ and the top linkage to a full set of 17.5″ bars, then we will have a really tall lift!

Eight Bar Linkage

This is a variant on the four bar lift in a different way. Instead of ending the lift at the end of the parallelogram with a bar with our gripper, we extend the linkage by using that bar as part of the next section of the linkage. Do it once like this and it’s called a six bar linkage. Do it twice, it’s referred to as an eight bar linkage.

Eight Bar view 1

The eight bar linkage gets extra height based upon the compaction and difference in height to the bars. You will be constrained as the bars collapse upon themselves and fold tightly together. Once fully extended, it will reach fairly high. But as you do so, the horizontal distance along the floor gets further back. This can be a problem for your robot as the front of the robot may be banging into the wall and not allowing your cube to be placed on the post. So using this kind of design needs to be used in conjunction with the robot base to ensure you have proper clearance. Maybe a ten bar will work better?

Eight Bar view 2

The lifting mechanism is not shown in great detail, but what was shown for the four par or double reverse four bar linkage can be used in the same manner. So use those plans to assist in this lift. The last element to notice in this lift are all of the connection points. As we looked at initially, friction is our number one enemy in lifts. Adding all of these connection points can increase friction to the point of binding.

Now if you combine the two lift types here, you can make a double reverse six or eight bar linkage. That may save that forward motion problem with a single eight bar lift.

Next time we will look at base designs with these lift types in mind and how you join them.
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