View Full Version : Sideways Hanging - Yes or No
Swampdude
10-01-2010, 20:17
Pretend he's a robot (http://www.youtube.com/watch?v=gM6Nhdo3eeE)... This was our first thought on how to hang (due to it's efficiency), so I assume many of you are thinking the same thing. But we're preemptively ditching the idea as I'm sure it will knock the tower over and also point load the vertical member beyond yield. Not to mention if your hanging like this and a couple more bots climb aboard. Actually I haven't studied the footing of the tower but it seems like 3 bots piled up on each others back could get pretty far from center on the tower. Seems like it could go over pretty easy. Perhaps the GDC didn't see this coming, but I recommend a ruling on it soon.
FYI - I'm not planning on asking if this is acceptable on the GDC forum thing.
thefro526
10-01-2010, 20:27
There was a team in the 2007 FVC game that did the exact same thing. It was a really efficient way to hang, and I thought of applying it to this year's game...
I'd like to think that the GDC designed the tower with this in mind but, I guess if they haven't we'll know soon enough.
karatekid
11-01-2010, 11:08
I think you definately need to consider the balance problem-maybe 2 on 1 side and 1 on the opposite side?
Brandon Holley
11-01-2010, 11:15
Pretend he's a robot (http://www.youtube.com/watch?v=gM6Nhdo3eeE)... This was our first thought on how to hang (due to it's efficiency), so I assume many of you are thinking the same thing. But we're preemptively ditching the idea as I'm sure it will knock the tower over and also point load the vertical member beyond yield. Not to mention if your hanging like this and a couple more bots climb aboard. Actually I haven't studied the footing of the tower but it seems like 3 bots piled up on each others back could get pretty far from center on the tower. Seems like it could go over pretty easy. Perhaps the GDC didn't see this coming, but I recommend a ruling on it soon.
FYI - I'm not planning on asking if this is acceptable on the GDC forum thing.
Have you interacted with the actual field elements? The tower is pretty rigid in my opinion. The steel piping that is used is very rigid. I would make sure you fully explore this option before determining the vertical member will be stressed beyond yield.
-Brando
Kims Robot
11-01-2010, 13:55
The tower is very rigid. At one point at the NH kickoff there were three kids hanging on it, and not gingerly.
Its really hard to tell from the photos/drawings, but there are solid metal plates below the tower & the bumps that help keep them solidly in place. They are mentioned in the rules:
The BUMPS are fixed to base plates that are secured to the carpet of the FIELD to keep them from moving. The base plates are covered with the same carpet as the FIELD. Note that this forms a small (approximately ½-inch tall) transition from the FIELD surface onto the base plates.
I dont see it mentioned, but the same style plate is underneath the Tower. You can sort of see it here (http://rollingthunder.smugmug.com/2009-2010-FIRST-Robotics/Build-Season/Kickoff-NH-1-9-10/10900594_fkcVX#760952050_cZ5Mh-A-LB).
Swampdude
11-01-2010, 14:12
I guess my big concern is the column in compression and then a bot hanging sideways as mentioned shock loading that column in bending. Plus if the bot is hanging out like that and maybe sticking out 4', then another bot climbs on the back of bot 1, or even 2 bots. That could put the bots collective cg as much as 4'-6' away from the side of the tower. That's a lot of force on that 1 column that's now in compression and bending. bad.... And if the collective cg gets out past the edge of the floor plate in any direction it could pull it over.
Brandon Holley
11-01-2010, 14:34
I guess my big concern is the column in compression and then a bot hanging sideways as mentioned shock loading that column in bending. Plus if the bot is hanging out like that and maybe sticking out 4', then another bot climbs on the back of bot 1, or even 2 bots. That could put the bots collective cg as much as 4'-6' away from the side of the tower. That's a lot of force on that 1 column that's now in compression and bending. bad.... And if the collective cg gets out past the edge of the floor plate in any direction it could pull it over.
I totally agree that its a valid concern, however there may be ways to avoid that. I think its a creative idea, and I didnt want you to dismiss it quickly thinking the tower was not robust enough to handle the load.
-Brando
GaryVoshol
11-01-2010, 14:35
Is whatever mechanism you will use to grab able to be released without power?
I don't see why it wouldn't be legal. It doesn't have to lift that high, only enough to get the wheels off the gound, so it won't put that much side stress on the tower.
Brian
Chris is me
11-01-2010, 14:58
While there doesn't seem to be anything wrong with it... Why would you hang this way other than the "normal" way? Or other non-normal ways?
Jeff Rodriguez
11-01-2010, 15:00
I don't see why it wouldn't be legal. It doesn't have to lift that high, only enough to get the wheels off the gound, so it won't put that much side stress on the tower.
Brian
It has to go above the plane of the platform.
From the definitions in 7.2:
ELEVATED: A ROBOT that is completely above the plane of the PLATFORM and in contact with the TOWER shall be considered ELEVATED.
Either way, once you are off the ground its the same amount of force no matter how high you go.
Swampdude
11-01-2010, 15:24
While there doesn't seem to be anything wrong with it... Why would you hang this way other than the "normal" way? Or other non-normal ways?
Instead of making an extending arm that goes 7+ feet up to the hang bar you only need to go up 3'. and if you do it right it takes less force to get your bot up in the air. Not to mention you don't have to share the hang bar with anyone. And you could help other bots up easier from this position. And you could in theory get connected faster and elevated faster than extending 7' and pulling the full distance... so lots of reasons. I just don't want the field breaking so I threw it out there before it became a problem later on, or to verify if it's ok (not that this is the place to do that - but I don't have a tims login and I can't ask team questions, plus the answers traditionally take too long).
And yes you could design a gripper that could let go easy enough.
Kevin Sevcik
11-01-2010, 15:32
I don't see why it wouldn't be legal. It doesn't have to lift that high, only enough to get the wheels off the gound, so it won't put that much side stress on the tower.
Brian
Inaccurate, sir. You have to lift your wheels above the platform to be ELEVATED.
As to why do this.... Well this way, you don't have to reach 7 ft in the air to lift. And you can just as easily lift from under the ball return.
The towers shouldn't flip unless you're doing something very wrong. The base plate is an 80" piece of aluminum plate, so it'd extend about 18" out from the tower on each side. A single robot should have difficulty tipping the tower. Front to back, anyways.
As far a strength.... A 150lb robot with an 18" CoG will put a bending load on a tube of about 50% yield, if you only grab one pole. I don't have a convenient calculator at hand for point load crushing force on tubes, but you can reduce the crushing force significantly by simply spreading your "hands" out father.
Finally... It's dirt simple to make a grasping mechanism for this that releases without power. The dude in the video could have managed just as well with a hook for a left hand and a bunch of friction tape.
Ken Leung
13-01-2010, 08:16
The Team Drawings in the document section: http://www.usfirst.org/roboticsprograms/frc/content.aspx?id=16097, specifically TE-10012.pdf within 2010 Team Field Elements.zip, show a version of the tower using 4X4's as the vertical posts.
If these drawings are official, then it seem the GDC did not intend for robots to attach to the towers using the vertical posts. If they did, then these drawings are invalid for the official game.
Someone need to post a clarifying question in Q&A about this.
Daniel_LaFleur
13-01-2010, 08:38
The Team Drawings in the document section: http://www.usfirst.org/roboticsprograms/frc/content.aspx?id=16097, specifically TE-10012.pdf within 2010 Team Field Elements.zip, show a version of the tower using 4X4's as the vertical posts.
If these drawings are official, then it seem the GDC did not intend for robots to attach to the towers using the vertical posts. If they did, then these drawings are invalid for the official game.
Someone need to post a clarifying question in Q&A about this.
Ken,
That drawing is for the low-cost field for teams to practice on and in this case does not accurately portray the official field due to different material (1-1/2" steel tubing vs 4x4) .
we are definatly looking at this option. It was brought up by a couple of students day one as a way to hang. I'm sure the GDC built the tower strong enough to support it. It says the tower was built to support many robots. and I shall trust that.
Also... could you imagine the reaction if a robot broke the tower????:ahh:
Brandon Holley
13-01-2010, 09:18
The Team Drawings in the document section: http://www.usfirst.org/roboticsprograms/frc/content.aspx?id=16097, specifically TE-10012.pdf within 2010 Team Field Elements.zip, show a version of the tower using 4X4's as the vertical posts.
If these drawings are official, then it seem the GDC did not intend for robots to attach to the towers using the vertical posts. If they did, then these drawings are invalid for the official game.
Someone need to post a clarifying question in Q&A about this.
I do not believe it is our job to guess what the GDC intends. If there is no rule explicitly stating you cannot hang from it, it would appear you can.
Chris is me
13-01-2010, 09:47
I do not believe it is our job to guess what the GDC intends. If there is no rule explicitly stating you cannot hang from it, it would appear you can.
On the other hand, if the tower isn't strong enough to hold a robot hanging sideways (though I personally highly doubt it) than obviously that wasn't an intended part of the game, and if the tower breaks, it's breaking a ton of rules.
Ken Leung
13-01-2010, 10:13
Ken,
That drawing is for the low-cost field for teams to practice on and in this case does not accurately portray the official field due to different material (1-1/2" steel tubing vs 4x4) .
Then I would say the low-cost field is an ineffective tool for teams to practice with, no matter how cheap it is.
I do not believe it is our job to guess what the GDC intends. If there is no rule explicitly stating you cannot hang from it, it would appear you can.
I agree we should not guess the GDC's intend. That is why we should ask them to clarify. The bottom line is I am not going to design any robot to hang sideways, nor will I advice anyone else to do so, until the GDC specifically clarify it is allowed.
One thing I learned is never take any strategy for granted, until FIRST specifically say you are allowed to use it. I have seen a few robots too many that were built, shipped, and had to be completely rebuilt at the regionals, many of them with innovative strategies that came as a surprise to the GDC.
hi can someone tell me how to post a message in the forum please
i can only relply to other peoples, can't figure out how to post a new thread, where to go after i sign in??? thanks
coldfusion1279
13-01-2010, 10:47
Listen, I am going to assume the GDC thought of teams hanging from the vertical poles.
And they are steel bars affixed at both ends. Maybe if there were 3 robots (~500 lbs) hanging 6 feet directly out from the tower, the thing may bend, but (I don't think) brittle fracture would occur. But if you have 2 robots hanging off your robot in that orientation, your robot's arm will bend way before the steel support (unless your arm is a firmly planted pole of equal size).
If we know the phase, thickness, and modulus, we can calculate yield stress and whether the bar will fail catastrophically. Maybe the K1c value too ;)...
Coolness factor outweighs riskiness IMO.
Daniel_LaFleur
13-01-2010, 10:48
i can only relply to other peoples, can't figure out how to post a new thread, where to go after i sign in??? thanks
On the orange bar at the top of each page is a set of links.
The top left one should say either forums or portal:
If it says portal then click on it and it will take you to the portal anf the top left one will then be forums.
When it says forums, click on it ... that'll take you to the forums list.
Then click on the forum you wish to add your thread to.
Once there there will be a blue button that says new thread (looks like the post button)
Daniel_LaFleur
13-01-2010, 10:51
But if you have 2 robots hanging off your robot in that orientation, your robot's arm will bend way before the steel support (unless your arm is a firmly planted pole of equal size).
Don't be too sure of this. The pole is in bending (shear) while the robot is in compression/tension.
coldfusion1279
13-01-2010, 10:51
Edison go to the forums in the orange bar at the left top of the screen. Pick a topic and at the top of the screen it will say "new thread" or something like that. Also I would pm (private message) you but I am afraid you wouldn't know how to read it. PM's are in your user cp and you can message anyone on the forum privately. There are rules etc. that you should read before posting
coldfusion1279
13-01-2010, 10:54
Don't be too sure of this. The pole is in bending (shear) while the robot is in compression/tension.
Whatever is attached to the bar on your robot (assuming you are hanging next to it) has a moment on the joint attached to the robot.
Think of it this way, force applied to the vertical support is applied parallel to the floor, but the weight of the robots is applied perpendicular to the floor.
This is assuming a design like the original posting.
Kevin Sevcik
13-01-2010, 11:12
A preliminary Inventor model suggests that a 150 lb robot with a CoM 18 inches off the center of the pipe will load a single pipe to 50% of yield.
This is using the cheesy "Stress Analysis" Inventor package we get with the kit, so it's somewhat less trustworthy for the coarse mesh it's limited to. But it's also assuming the pipe is a cantilevered column with support only at the bottom, which is clearly not the case. The rigid connections to the other side of the tower will mean some load will be carried through the top of the pipe as well. So I think one robot per vertical pipe is a reasonable load for the tower.
However.... I was assuming a 12 inch separation between the "hands". The hands generate two equal and opposite forces to create a moment to balance the moment generated by the robot's CoM. The closer the hands, the greater the point force required to generate this moment. At around 6" of separation, this point load starts to become significant. At this point you have to worry about crushing the pipe between your hands in addition to bending the pipe at the base. This crushing type failure will NOT be improved by rigid connections at the top and middle of the platform. The only way to improve it is to increase the separation of your hands.
Something for to think about for teams considering this approach: This normal loading on your hands increases linearly with the weight of your robot(s). And you can customize magnitude of this normal force by choosing the separation between your hands. I wonder how this could be useful.....
MrForbes
13-01-2010, 11:24
Something for to think about for teams considering this approach: This normal loading on your hands increases linearly with the weight of your robot(s). And you can customize magnitude of this normal force by choosing the separation between your hands. I wonder how this could be useful.....
Interestingly as you reduce the stess on the TOWER by moving the "hands" further apart, you also reduce the force necessary to be able to swing the robot up to it's sideways position.
Definitely something to consider.
Kevin Sevcik
13-01-2010, 12:01
Interestingly as you reduce the stess on the TOWER by moving the "hands" further apart, you also reduce the force necessary to be able to swing the robot up to it's sideways position.
Definitely something to consider.
Well okay, I was thinking it was a useful factoid if you're planning on using friction to maintain your Z-level on the pipe. But yes, spreading your hands apart also reduces the stress on the hands, as well.
To address some posts that came up while I was playing with inventor, if you're planning on supporting the entire moment of your robot at one rotary joint... Well good luck to you. 225 ft-lbs is pretty significant for a single joint to support.
Also, as I thought my post made clear, don't assume that the tower is indestructible. It is certainly rather sturdy, but it's not nearly as reinforced as it could be. Front-to-back loads will mostly be carried by the joint at the base, with only a little load carried by the frame at the top. So.... I think it's safe(ish) to hang a robot off the side of the tower, but I don't recommend bouncing or anything when you do it.
I would not let concern about the tower post failing or tipping over stop you from pursuing this or any other climbing method. The field apparatus is designed to take whatever methods the challenge has invited. If you are clever enough to do this, go for it.
MrForbes
13-01-2010, 12:18
Yes, go for it, but before you get too far along, you might want to make sure it will not cause damage to the TOWER.
The pipe should kink before the tower tips over. A word of warning though. You kink the tower pipe, consider how long it will take to disassemble the field and find a replacement pipe (it's likely there will not be a spare shipped with the field, meaning shop time) and reassemble the field. Then consider how many teams are waiting, and that they know you are why they can't play.
Do math.
Wetzel
its going to be difficult hanging sideways since rules state the robot has to be hanging 12 inches off the ground and side ways would require more lift
I'm sure anyone designing this type of hanging would wanna have a wide grip anyways... and the wider the grip on the bar, the less likely the bar would bend or break.
Our team has talked about this hanging method and started design a system with a 28" wide grip. Far more then what was calculate by Kevin
EDIT: our system is similar to the boy on the bar, but uses mechanical leverage and should be much much safer then a "hook and lift" method
its going to be difficult hanging sideways since rules state the robot has to be hanging 12 inches off the ground and side ways would require more lift
For SUSPENDED or ELEVATED robots? Can you cite what rule you are talking about? Not to mention that 12 inches is 12 inches, no matter how you lift. (And 12 inches is not the 18 to ELEVATE)
Wetzel
Swampdude
13-01-2010, 13:20
To re-iterate my point of having a 250lb + cg that's as much as 4-6' away from the SIDE of the tower. If you have a 130lb bot 4' tall but in horizontal position (Meaning your base has over 120 lbs in it and its suspended 4' from the side of the tower) Then another bot hooks on to your base adding another 130lbs and also shifting the cg even further from the tower (then possibly even a 3rd bot hooks on), I can almost guarantee the tower will fall over and it could bend the pipe depending on design and how the bots climb on (bouncy etc.) I doubt the tower structure weighs more than 50lbs and only has an 18" plate sticking out? But 250-370lbs of bots hanging 4+ feet away will topple it. I'll be surprised if they don't nix the idea. Maybe I'm wrong.. we'll see...
The Q&A is now up. Can anyone ask about this? I cannot because I do not have access to my teams "official" login.
EDIT: One thing that I forgot to mention, its would be much harder to topple the top if you hanging sideways at a low elevation (just about the platform say) also, the platform would supply an increased resistance to bending and breaking the bar. I think this is totally doable
I'd agree that if the rules don't prohibit it, it is allowed, but I'd hate to spend a lot of time designing, building and perfecting it, just to have it thrown out by a rules clarification in week 5 of build.
It wouldn't surprise me to see that happen, especially if there was concern about damage to the field.
The Q&A is now up. Can anyone ask about this? I cannot because I do not have access to my teams "official" login.
EDIT: One thing that I forgot to mention, its would be much harder to topple the top if you hanging sideways at a low elevation (just about the platform say) also, the platform would supply an increased resistance to bending and breaking the bar. I think this is totally doable
What would you like asked? Hanging off the side is allowed, no one is debating that. If you ask if it will damage the field, the response is going to be "we can't comment on specific designs" or "if you break the field we break you."
Wetzel
"Is the tower strong enough to support the weight of three robots hanging from a vertical bar as oppose to the standard horizontal bar?"
and asking them if your allowed to hang off the vertical bar is a completely good question to me.
sanddrag
13-01-2010, 16:45
To re-iterate my point of having a 250lb + cg that's as much as 4-6' away from the SIDE of the tower. If you have a 130lb bot 4' tall but in horizontal position (Meaning your base has over 120 lbs in it and its suspended 4' from the side of the tower) Then another bot hooks on to your base adding another 130lbs and also shifting the cg even further from the tower (then possibly even a 3rd bot hooks on), I can almost guarantee the tower will fall over and it could bend the pipe depending on design and how the bots climb on (bouncy etc.) I doubt the tower structure weighs more than 50lbs and only has an 18" plate sticking out? But 250-370lbs of bots hanging 4+ feet away will topple it. I'll be surprised if they don't nix the idea. Maybe I'm wrong.. we'll see...This is the specific scenario I'm worried about. You could in theory have one SUSPENDED robot (call it a Blue Alliance robot), outside of the tower, say on the midfield side. Then another blue robot hanging off of it (toward the red side), than yet anotherblue robot hanging off of it, toward the red side. At potentially 465 lbs, this is getting to be a somewhat large cantilevered load, imposing a fairly large moment on the tower structure. Perhaps the ball return would keep the tower from toppling? Is it tied in? And remember, it's not just the weight of these robots. There is extra force required to accelerate them to these positions, and overcome stuck conditions that may occur.
Kevin Sevcik
13-01-2010, 16:49
EDIT: One thing that I forgot to mention, its would be much harder to topple the top if you hanging sideways at a low elevation (just about the platform say) also, the platform would supply an increased resistance to bending and breaking the bar. I think this is totally doable
The platform does not provide any structural reinforcement to the tower. It's a wooden box that rests on the tower's base plates with no connection to the vertical pipes. So don't expect that to save the tower.
For SUSPENDED or ELEVATED robots? Can you cite what rule you are talking about? Not to mention that 12 inches is 12 inches, no matter how you lift. (And 12 inches is not the 18 to ELEVATE)
Wetzel
18 inches isn't the 20 inches needed to elevate either. :D Don't confuse the tunnel height with the platform level.
To re-iterate my point of having a 250lb + cg that's as much as 4-6' away from the SIDE of the tower. If you have a 130lb bot 4' tall but in horizontal position (Meaning your base has over 120 lbs in it and its suspended 4' from the side of the tower) Then another bot hooks on to your base adding another 130lbs and also shifting the cg even further from the tower (then possibly even a 3rd bot hooks on), I can almost guarantee the tower will fall over and it could bend the pipe depending on design and how the bots climb on (bouncy etc.) I doubt the tower structure weighs more than 50lbs and only has an 18" plate sticking out? But 250-370lbs of bots hanging 4+ feet away will topple it. I'll be surprised if they don't nix the idea. Maybe I'm wrong.. we'll see...
I will agree with you that hanging lots of robots off of other robots can be a dangerous endeavor in many ways. I'm having difficulty believing that anyone is going to design a robot that is capable of suspending 250+ lbs of robots at a 6' lever arm from the side of the platform. That's a very serious moment to put on any part of one of our robots.
More to the point, I'm seriously doubting that you can flip the tower before you fail the vertical pipes. Flipping the tower is going to require applying that huge moment to the tower somewhere. Which is going to be transmitted through the vertical pipes. Which are going to break if you put that kind of moment on them. Plus, there's the ball return which is going to resist your efforts at flipping the tower. So... I think worries about flipping the tower are a bit less immediate than worries about breaking it.
Swampdude
20-01-2010, 08:47
FYI (http://forums.usfirst.org/showthread.php?t=13845)
Guess that means we're good to go!
Ken Leung
20-01-2010, 09:24
FYI (http://forums.usfirst.org/showthread.php?t=13845)
Guess that means we're good to go!
Yup! My only question is, in the event of damage (gripper causing damage to the pole, 3 robots hanging at the same point causing damage to the structure, etc), who is responsible?
Let's say the teams hanging all went through reasonable measures in preventing their robots from creating excessive forces and causing damages while hanging. In the event of a tower failure, is it FIRST's responsibility to make the tower stronger, or is it team's responsibilities to change their design such that failure will not occur again (like redesigning to hang from the horizontal bars)?
Interesting thing to think about.
I think a team that is going to do this is not so much thinking about holding other bots,but that they can hang from their home zone and open up more space for other teams. If this team can climb in 10 seconds they would be able to score up to that point, then head over to the pole. Now does the other team leave a defender back or does he leave early to hang. Oh, so many variables this year.
If the ROBOT does damage the TOWER by hanging sideways, the team could be excluded from any future matches until the problem is corrected.
<G24> ARENA Interaction – With the exception of the BALL RETURN and RETURN BARS,
ROBOTS may push or react against any elements of the ARENA, provided there is no
damage or disruption of the ARENA elements. With the exception of the TOWER, ROBOTS
may not grab, grasp, grapple, or attach to any ARENA structure. Violation: A warning will be
issued when a ROBOT violates this rule. If the referee determines that the TEAM is
disregarding the warning, their ROBOT will be disabled for the remainder of the MATCH.
<G26> ARENA Damage - ROBOTS may not damage any part of the ARENA or BALLS. For
ROBOTS that violate this rule, the TEAM may be required to take corrective action (such as
eliminating sharp edges, removing the damaging MECHANISM, and/or re-inspection) before
the ROBOT will be allowed to compete in subsequent MATCHES. Violation: Potential
Disablement if the Head Referee determines that further damage is likely to occur.
I'd say that if three robots somehow manage to hang from a vertical/hang from each other off a vertical, and the tower collapses, and none of the three has previously shown a tendency for field damage, IMO, there's nothing that can really be done other than to issue a Team Update saying that you can't do that and repair the tower.
Under <G24>, you can get a warning. As it's kind of hard to continue that kind of damage, no red card (unless the tower was already visibly starting to fail when the third robot climbed on). Under <G26>, the team has to take corrective action, but the only real corrective action in this case would be to not have every robot hanging at one point, assuming that none of the robots had previously done any damage to the tower.
Kevin Sevcik
20-01-2010, 11:09
The trick is that there's two potential failure modes for the tower. There's buckling/bending failure (whichever comes first) from the overall moment and axial loading on the tower. As EricH says, this can't really be remedied beyond "Don't do that!"
The second possibility is a point load failure if a team doesn't sufficiently spread out their "hands" that are transferring the moment load to the pipe. This would end with crushing the pipe at the location of the gripper. This is a less likely failure mode, as the gripper must also withstand a load great enough to crush the pipe, which seems a little unlikely. If someone does manage to make this mistake, however, it's an issue that IS resolvable with a redesign of the gripper or the addition of something below the gripper to provide a separate point to support the moment load.
Also, the one word answer is somewhat dissatisfying, as it really doesn't address the issue of whether the tower will stand up to this load. Which is worrisome, as I'm pretty certain it IS possible to fail the tower if you're trying something really foolish.
MrForbes
20-01-2010, 11:13
It'll be interesting to see the inspection checklist.
fuzzy1718
20-01-2010, 11:18
Forget about the tower, how strong does the end effector have to be to repeatidly hang that weight??:eek:
MrForbes
20-01-2010, 11:20
It depends on how you build it. If you have the "hands" pretty far apart, then the strength requirement is similar to just hanging the robot from the horizontal bar.
Draw a free body diagram, see how it works.
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