Universal joints question for Technic experts!

[Out of Sight]

Guys, after spending (playing to be exact! ) almost 2 weeks with the 8043..I noticed there was a wiggle/vibration when extending/retracting the dipper arm and bucket. The wiggle was quite excessive especially on the dipper arm, I check every single U-joints, make sure all the axles are not binding or misplaced. I could spin each LA freely by hand. Got me frustrated for a while. I did swap position of all 4 LA's in case there's a defective LA..nope same thing!

I then disassembled the whole boom,dipper and bucket assembly and took off all the U-joints and axles. Here is what I noticed, the vibrations seemed to be coming from the U-Joints. I did a simple diagram here:

as per Lego instruction booklet, I attached the U-joints in a perpendicular orientation on both ends of each axle.

I then repositioned all u-joints so they're now in parallel orientation on both ends of each axle.

To my surprise, this actually fixed the problem..this almost completely eliminated the wiggles especially when the LA's are under load (expanding). My question is to those Tech experts, should the u-joints ideally be installed in this manner (parallel on both ends of each axle)? Well at least I can sleep with ease tonite, knowing that my 8043 is smooth as butter

Edited December 26, 2010 by Out of Sight

[DLuders]

It could be that the Power Functions motors were inducing a "harmonic vibration", much like strumming a guitar string. Bridges (like the Tacoma Narrows Bridge in Washington State, USA in 1940, and London's Millennium Bridge in 2000) have had SERIOUS vibration problems when the wind or people induced the natural harmonic vibration of the bridge deck. Many materials can have natural harmonic vibrations if "strummed" at JUST the right frequency, and the the structure is JUST the right length.

When you turned one of the U-joints around on your 8043 Motorized Excavator, you changed the arrangement of MASS along the length of the assembly (U-joint + axle + U-joint). The motors were then not able to strike the harmonic frequency, and the "wiggles" went away.

I doubt anybody (besides you) has documented a problem with Lego Technic like this. It would be interesting to hear Blakbird's perspective on this matter.

Edited December 26, 2010 by DLuders

[me00rjb]

  On 12/26/2010 at 2:34 PM, Out of Sight said:

To my surprise, this actually fixed the problem..this almost completely eliminated the wiggles especially when the LA's are under load (expanding). My question is to those Tech experts, should the u-joints ideally be installed in this manner (parallel on both ends of each axle)? Well at least I can sleep with ease tonite, knowing that my 8043 is smooth as butter

Yes, universal joints should be in-line, as is standard practice for vehicle drive shafts with a u-joint each end. Universal joints are not constant velocity joints, therefore the rotational motion is not uniform when transmitted through a u-joint, but varies sinusoidally. By putting the yokes in-line, this variation is cancelled out (for each pair of joints, at least).

Richard.

Edited December 26, 2010 by me00rjb

[Out of Sight]

@DLuders @ Richard

Thanks for sharing your perspectives on this matter! I also believe that the wiggle and the notchiness of the LA is caused by the non-constant speed of the U-joint when positioned at an angle, as the speed of the input shaft on the LA varies and causes intermitent quickening and slowing down. Maybe Lego should make CVD's (Constant Velocity Drive) for future PF equipped models? Could be a new part in Technic inventory as well

Edited December 26, 2010 by Out of Sight

[dhc6twinotter]

This is good info! Thanks for posting this! I never even paid attention to how my u-joints were placed when I built 8043.

My current project will have around 25 u-joints, so this topic is perfect timing. Thanks!

[Zerobricks]

Problem with 8043 there are either 3 or 5 universal joints in line, so you cannot totally remove the vibration. Aligning the joints which are at the highest angles will be the best option.

[Out of Sight]

  On 12/26/2010 at 6:44 PM, Zblj said:

Problem with 8043 there are either 3 or 5 universal joints in line, so you cannot totally remove the vibration. Aligning the joints which are at the highest angles will be the best option.

There are 5 u-joints for the bucket and 3 for the dipper arms. I made sure all the u-joints on each axle are in-line. The operations improved a lot, yes there's still a very slight vibration, but almost unnoticable,and almost completely gone on fresh alkalines. I would suggest everyone do this fine tweaking to improve the whole operations of their 8043 or any PF equipped Technic set which utilizes U-joints.

[Splat]

I noticed that when I first put together my 8043 that one of the axles wobbled a bit, causing a weird vibration when operating the bucket. One of my universal joints wasn't aligned how you suggested. Now that I think about the universal joints, and the fact that they aren't constant velocity, your solution makes perfect sense. I have now rotated the universal joint on the axle, and the vibrations are gone. Out of Sight, thanks for posting this.

Lego already has CV joints (eg. used in the drive-train of 8880 and 8297), but I've only seen them used in 4WD steering.

[bord4kop]

Now this is good and handy info!

Thanks for sharing

[Jetro]

Interesting! Will have to check my model, but the information is great!

[Buffalo]

That does make sense. Since the motors that work the stick and the bucket on my excavator are in the boom, I have fewer U-joints, but I have noticed some vibration, so I will have to look into this. Another possibility, in addition to aligning the joints, is putting one or more brick, 2x2 round on one of the intermediate shafts to dampen some of the vibration. It works for big trucks, so why not an 8043? The M motors generate sufficient speed to set up vibration in the driveshafts, so a bit of extra mass might help. I will be doing some "research" later on.

EDIT: I did rotate my universal joints so they are parallel. I replaced one 8L axle with one made of 2L axles and axle joiners to add some mass. It made a pretty big difference.

For my bucket driveshaft, which has 3 U-joints, I made the U-joints on each intermediate shaft parallel, like:

(Credit to Out of Sight for original image)

The only noise audible when moving the bucket is from the motor, and it's perfectly smooth.

*EDIT, again:

I also took her out to play in the snow for a bit. The snow is really wet and heavy so it didn't sink out of sight right away. The treads packed the snow down really nicely, and I was able to move a bit of snow with the bucket. It also didn't get any water where it doesn't belong.

Edited December 28, 2010 by Buffalo

[Out of Sight]

  On 12/28/2010 at 6:59 PM, Buffalo said:

That does make sense. Since the motors that work the stick and the bucket on my excavator are in the boom, I have fewer U-joints, but I have noticed some vibration, so I will have to look into this. Another possibility, in addition to aligning the joints, is putting one or more brick, 2x2 round on one of the intermediate shafts to dampen some of the vibration. It works for big trucks, so why not an 8043? The M motors generate sufficient speed to set up vibration in the driveshafts, so a bit of extra mass might help. I will be doing some "research" later on.

EDIT: I did rotate my universal joints so they are parallel. I replaced one 8L axle with one made of 2L axles and axle joiners to add some mass. It made a pretty big difference.

For my bucket driveshaft, which has 3 U-joints, I made the U-joints on each intermediate shaft parallel, like:

(Credit to Out of Sight for original image)

The only noise audible when moving the bucket is from the motor, and it's perfectly smooth.

*EDIT, again:

I also took her out to play in the snow for a bit. The snow is really wet and heavy so it didn't sink out of sight right away. The treads packed the snow down really nicely, and I was able to move a bit of snow with the bucket. It also didn't get any water where it doesn't belong.

Great! It's nice to see everyone doing their part in making their 8043 a 'smooth operator' I mean this is one of the best and most playable "look, I can do this with no hands" type Technic set, every bit is worth the effort! Combined with the new LAs, everything works like it's supposed to.

[davidmull]

After doing this mod it's better but I'm having a problem where the 2 bottom univer joint that are beside each other near the bottom of the arm seem to hit each other and makes a clicking sound,anyone else have this too?..

[Out of Sight]

  On 12/30/2010 at 10:56 PM, davidmull said:

After doing this mod it's better but I'm having a problem where the 2 bottom univer joint that are beside each other near the bottom of the arm seem to hit each other and makes a clicking sound,anyone else have this too?..

That's weird. You mean those near the square grey beams? Those u joints shouldn't contact each other..even though there's only less than 2mm of spacing. Make sure you've inserted each u joints on each axle at maximum depth, this will ensure proper length of the axles assembly.

[Zerobricks]

The joints dont touch each other, the black connector thath holds axle has too much room and the upper u joint slides sideways, touching the 5x7 frame brick, at least thats by my excavator.

[davidmull]

No mine do touch some times cause they are right beside each other, if I work the bucket and centre arm at the same time they hit every time too and make a enoying noise ;(

[davidmull]

Does anyone else have this problem?

[Sam42]

Also worth paying attention to their alignment in drivetrain applications too... i've snapped u-joints on my landrover truck trialler before now through not paying attention to their alignment....

alignment alignment alignment, i need to stop posting while i'm not paying full attention....

Edited January 4, 2011 by Sam42

[davidmull]

I'm still having this problem where my 2 bottom u- joints are hitting each other 50% of the time ,it happens even worse if I use the bucket and centre dipper arm at the same time they click like he'll till I stop one of the movements,iv checked everything over a few times and it's still the same,the 2 bottom shafts also seem to have play in them ,it's very frustrating at this stage as u have to operate it a certain way to stop this noise , anyone shed any light on this ?

[Out of Sight]

@davidmull

You might have defective joints..try swapping the u-joints. (eg from the base of the LA to the middle and so on). The only thing that the u-joints on my 8043 touch is a slight rub on the grey square beam, but not enough to cause any noise or interference. Operation remains smooth.

Check all the pins around the boom assembly..if one or few are not fully inserted or had been slightly pulled apart due to playing etc, they might cause the beams to misalign and cause altered geometry hence the issues. The boom and dipper assembly is rather flimsy when lateral force (side to side) is applied, excessive force can twist the beams and causing the pins to be slightly pulled apart.

Better yet, just disassemble the whole boom and arm, and reassemble. I mean this problem occurs just recently right?

Edited January 5, 2011 by Out of Sight

[rien]

  On 1/5/2011 at 3:24 AM, Out of Sight said:

@davidmull

You might have defective joints..try swapping the u-joints. (eg from the base of the LA to the middle and so on). The only thing that the u-joints on my 8043 touch is a slight rub on the grey square beam, but not enough to cause any noise or interference. Operation remains smooth.

Check all the pins around the boom assembly..if one or few are not fully inserted or had been slightly pulled apart due to playing etc, they might cause the beams to misalign and cause altered geometry hence the issues. The boom and dipper assembly is rather flimsy when lateral force (side to side) is applied, excessive force can twist the beams and causing the pins to be slightly pulled apart.

Better yet, just disassemble the whole boom and arm, and reassemble. I mean this problem occurs just recently right?

Try to take a pict of the joints maybe we can see something.

I understand the frustration belive me.

[davidmull]

It is annoying tho,I might disassemble the arm and rebuild again,iv tried swapping around joints but it's still the same,my joints are right beside each other all the time and sometimes it's smooth but if I reverse movement then of dipper arm or bucket both joints seem to meet each other and start clicking ;(

Edited January 5, 2011 by davidmull

[davidmull]

I'm just wondering can other people check there 8043 for the same rubbing of uni- joints that I'm getting ,u notice it some times under normal operation but more so if u operate the bucket and dipper arm at the same time,i think iv heard similar noise on YouTube but can't be sure,it's the 2 u-joints that are beside each other near the grey block on the boom and basically want to know are anyone else,s touching or very near each other,would appreciate it thanks.also them shafts start to move from side to side which makes them even hit worse and click like hell.

Any imput would be great as iv checked everything over and all looks ok.

[Zerobricks]

I checked my 8043, the u joints are close, but they dont touch.

[davidmull]

Anyone else check this for me?

Thanks,

David.