Moving the Arms of a Mobile Suit

[doghunter1]

As seen with this picture of the MS-06 Zaku II, there's a screw connecting both halves of its arms. Here's the problem, how are the arms bent for purposes like holding a 120mm machine gun, given that the screw is located on their elbows' top and bottom, which helps with bending the forearms to the sides, as opposed to the sides of the elbows, as seen in this picture?

[Seto Kaiba]

As seen with this picture of the MS-06 Zaku II, there's a screw connecting both halves of its arms. Here's the problem, how are the arms bent for purposes like holding a 120mm machine gun, given that the screw is located on their elbows' top and bottom, which helps with bending the forearms to the sides, as opposed to the sides of the elbows, as seen in this picture?

As the cutaway scan from the Gundam Wiki is way, WAY too small to see anything on... I've got a larger, higher-quality scan here. It is a good idea to remember that these ancient cutaways are not always a perfect representation of a mecha's capabilities.

Near as I can tell, the Zaku II's arm has nearly the same range of rotation and flexibility found on human arms... just the necessary articulations to make it happen are in different places.

The Zaku's upper arm (where a human's biceps and triceps would be) helps emulate some of the lateral range of motion that would come from a ball-and-socket joint. Both the top and bottom of the upper arm appear to have fifteen or so degrees of lateral flexibility in addition to being able to rotate.

[doghunter1]

As the cutaway scan from the Gundam Wiki is way, WAY too small to see anything on...

Sorry about that. Two reasons for posting it:

1. It was the only one I found about the "parts of the Zaku II" from MS Encyclopedia.

2. Did not know Macross2.net did Gundam too.

Anyways, enough of that...

Near as I can tell, the Zaku II's arm has nearly the same range of rotation and flexibility found on human arms... just the necessary articulations to make it happen are in different places.

The Zaku's upper arm (where a human's biceps and triceps would be) helps emulate some of the lateral range of motion that would come from a ball-and-socket joint. Both the top and bottom of the upper arm appear to have fifteen or so degrees of lateral flexibility in addition to being able to rotate.

But what about screw positions in arms? In Zakus, the screws make it easier to move the forearms sideways, like if you want to do a hook punch, but as seen with the RGM-79 GM and the RPI-11 Glasgow Knightmare Frame from Code Geass: Hangyaku no Rurūshu (Lelouch of the Rebellion), the screw positions make it easier to base an arm like the Letter L, but moving it sideways isn't possible. So where to position screws into a mecha's arms anyways?

[radioactive28]

But what screw positions in arms? In Zakus, the screws make it easier to move the forearms sideways, like if you want to do a hook punch, but as seen with the RGM-79 GM and the RPI-11 Glasgow Knightmare Frame from Code Geass: Hangyaku no Rurūshu (Lelouch of the Rebellion), the screw positions make it easier to base an arm like the Letter L, but moving it sideways isn't possible. So where to position screws into a mecha's arms anyways?

I think you shouldn't take those images too literally, they were drawn with major simplifications for use on TV. Even the big Zaku cutaway line-art is simplified by real-world engineering standards, but they do show the important parts.

The big actuators are usually more obvious, but there are also less obvious actuators to help provide a full range of motions and extra degrees of freedom.

For example, an MS' shoulder joint typically has 3 orthogonal axes for rotations and the wrist also has 3 for limited rotations. Further, linear actuators are also used at the shoulder-torso connection to provide 3 additional axes for limited translation. Most of these can be seen in the Zaku cutaway, but they just don't have the outward appearence ('screw') of the rotary actuators used for the elbows and knees.

All these are sufficient to mimic the mobility that we see in our own arms.

[doghunter1]

But what screw positions in arms? In Zakus, the screws make it easier to move the forearms sideways, like if you want to do a hook punch, but as seen with the RGM-79 GM and the RPI-11 Glasgow Knightmare Frame from Code Geass: Hangyaku no Rurūshu (Lelouch of the Rebellion), the screw positions make it easier to base an arm like the Letter L, but moving it sideways isn't possible. So where to position screws into a mecha's arms anyways?

I think you shouldn't take those images too literally, they were drawn with major simplifications for use on TV. Even the big Zaku cutaway line-art is simplified by real-world engineering standards, but they do show the important parts.

The big actuators are usually more obvious, but there are also less obvious actuators to help provide a full range of motions and extra degrees of freedom.

For example, an MS' shoulder joint typically has 3 orthogonal axes for rotations and the wrist also has 3 for limited rotations. Further, linear actuators are also used at the shoulder-torso connection to provide 3 additional axes for limited translation. Most of these can be seen in the Zaku cutaway, but they just don't have the outward appearence ('screw') of the rotary actuators used for the elbows and knees.

All these are sufficient to mimic the mobility that we see in our own arms.

Thanks, and sorry for not writing "about."

[Seto Kaiba]

2. Did not know Macross2.net did Gundam too.

We don't... 'cept for a handful of designs from War in the Pocket, Stardust Memory, Zeta Gundam, and Char's Counterattack that Mr March is especially fond of.

Every now and then I just abuse my position as the site's server admin to throw visual aids for discussions in the temp directory.

But what about screw positions in arms? In Zakus, the screws make it easier to move the forearms sideways, like if you want to do a hook punch, but as seen with the RGM-79 GM and the RPI-11 Glasgow Knightmare Frame from Code Geass: Hangyaku no Rurūshu (Lelouch of the Rebellion), the screw positions make it easier to base an arm like the Letter L, but moving it sideways isn't possible. So where to position screws into a mecha's arms anyways?

Like radioactive28 said, the cutaways tend to be somewhat simplified... they're more there to look pretty than anything. If they ever do a Master Archive Mobile Suit book for a Zaku or Zaku-derivative we'll probably get a much, MUCH better and more comprehensive look at the arm's articulations.

Such as it is, the Zaku in that cutaway has a couple articulations that aren't immediately obvious unless you look at "action poses" from the same period. You've got the articulations made at the shoulder, which is a compound joint combining a single-axis "knuckle" type actuator and revolute actuator to cover the shoulder's up-down and forward-back degrees of freedom. The joint which provides the left-right rotation is a revolute joint between the shoulder and upper arm, which is what's used to rotate the arm inward toward the body for something like holding a rifle or using one of the hands as a platform for the pilot to stand on. The wrist also has a revolute joint that provides the rotational functions that would be performed at the elbow on a human arm.