# 3 Axis Skull Weight Issue



## goosta (Jul 24, 2011)

I built a 3 axis skull and am running into an issue with the weight of the skull. It's front heavy, so it drops down and leads to strain on the nod servo causing it to buzz. I added a spring on the back for support but am still getting buzz.

Any suggestions on a fix for this? I'm planning to try a stronger servo. Currently using Hitec HS-425BB servos.

Here are some pictures

If anyone has any ideas, I'd really appreciate it! Thanks!


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## goosta (Jul 24, 2011)

Forgot to mention that I am using a "Lindberg Human Brain and Skull Combination" model. It's my understanding that skull is pretty much the same as the pirate skull that was popular to use while it was still available.


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## halstaff (Nov 18, 2009)

I now use the Hitec 485HB servos in my 3 axis skulls. It has 72 oz-in of torque at 4.8V versus 46 oz-in for the 425. The stronger servo is not that much more expensive and eases the load on the servos. 
The nod servo can be a problem in our skulls, especially when we have sculpted on the skull and added weight to the front. I have had to change out the nod servo for a HS-5645MG in our witch.


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## goosta (Jul 24, 2011)

halstaff said:


> I now use the Hitec 485HB servos in my 3 axis skulls. It has 72 oz-in of torque at 4.8V versus 46 oz-in for the 425. The stronger servo is not that much more expensive and eases the load on the servos.
> The nod servo can be a problem in our skulls, especially when we have sculpted on the skull and added weight to the front. I have had to change out the nod servo for a HS-5645MG in our witch.


Good to know - thanks!

I switched to a HS-645MG servo, as suggested by ServoCity and it's much better, but I still get buzz at certain positions. I may play around with additional springs to ease the weight on the front and see if that helps.

Any thoughts on digital vs analog servos for this application?

Thanks!


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## halstaff (Nov 18, 2009)

Analog servos are fine for this application. 
I do like some of the programmable features of digital servos to be useful in other projects. You will need a programmer to take advantage of all the features that digital servos have to offer.


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## Ktays (May 13, 2017)

I'm also in a similar situation with my 3 axis talking skull build this year. I'm using the 485HB servos and added some 470uF capacitors across each servo to see if that would help. Minor improvement but overall still get some buzzing. I've considered adding a weight balancing spring as you did, but have not tried it yet. I also thought of adding a spring over the u-joint to see if that might help with overall balance and strain for all the servos. I'm still thinking about that one.

I've also turned it into a talking skull using Steve Koci's (halstaff) and Steve Bjork's Wee Little Talker (http://www.haunthackers.com/). A fantastic board for the purpose and highly recommend it. The servo sound coming from the talking motion drowns out the buzz and when it's talking you don't notice it much. I'm using the HS-422 servo for that motion.

Here's a link to a few pics of my build for this year. I'm going to try the HS-645MG and see how that works on mine. For now I don't think it will be noticeable when it's in the yard. Best of luck and look forward to seeing how it turns out.

https://photos.app.goo.gl/QsErCKE92e4d346d7

Kevin


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## xronos (Sep 25, 2013)

Agree 100% that the _minimum_ in a regular sized servo is a 485HB which has become a sort of de facto standard over the years. If you need more torque or speed, go up in specs from there. Another thing you can do on the nod mechanism to increase torque is to adjust the linkage. You can get more applied torque by varying the ratio of the servo arm side of the linkage to the control side. Specifically, you would shorten the servo arm and/or lengthen the "control side;" i.e., increase the distance between the nod pivot point (often the middle of a universal joint or somesuch) and where the linkage attaches, either by moving the attachment point lower beneath the u-joint or further out perpendicular to the neck. It's obviously much easier to shorten the servo arm side. If the servo arm is half as long as the control side, you'll get twice the power. Note, you'll also get half the speed, but it's not usually a big deal on the nod mechanism, because the total angular distance on nod is only around 20% or so of what a directly connected pan servo would be, for example. Because of the relatively short distance traveled and the front-heavy nature of the application, you should optimize nod for torque, either via the servo type, linkage ratio, or both.


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## goosta (Jul 24, 2011)

xronos said:


> Agree 100% that the _minimum_ in a regular sized servo is a 485HB which has become a sort of de facto standard over the years. If you need more torque or speed, go up in specs from there. Another thing you can do on the nod mechanism to increase torque is to adjust the linkage. You can get more applied torque by varying the ratio of the servo arm side of the linkage to the control side. Specifically, you would shorten the servo arm and/or lengthen the "control side;" i.e., increase the distance between the nod pivot point (often the middle of a universal joint or somesuch) and where the linkage attaches, either by moving the attachment point lower beneath the u-joint or further out perpendicular to the neck. It's obviously much easier to shorten the servo arm side. If the servo arm is half as long as the control side, you'll get twice the power. Note, you'll also get half the speed, but it's not usually a big deal on the nod mechanism, because the total angular distance on nod is only around 20% or so of what a directly connected pan servo would be, for example. Because of the relatively short distance traveled and the front-heavy nature of the application, you should optimize nod for torque, either via the servo type, linkage ratio, or both.


Thanks so much for the information! Since this is basically my first experience using servos, please bear with me.

So if I am understanding your post correctly, it looks like the control side length on my nod mechanism is 25mm (from the pivot point of u-joint to the point where the clevis connects). From what points is the servo side measured? Mine is 14mm from the center of the servo spline to the connection point on the arm.

I should be able to move the attachment point lower beneath the u-joint by redesigning my collar part. How far would I have to move it down to notice a difference? Any other suggestions you have would be very much appreciated. Thanks!

Here a side view of the nod mechanism



Front view


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## xronos (Sep 25, 2013)

Correct - center of the servo spline to the connection point on the arm. I wasn't able to get an accurate measurement on any of mine without pulling them apart, but if I had to approximate on the control side, I'd say it's about 35mm. The ratio (control to servo arm) on mine is maybe just a bit more than 2 to 1, and it looks like you're in the ball park -- you're at 1.79:1. (Increasing by 10mm on control side would be 2.5:1, a 40% power increase if I'm doing the math right.) Could be worth experimenting, but it still might be preferable to upgrade the servo first. As far as how much you'd have to drop it if you go that route, it's pythagorean theorem -- it's a right triangle -- imagine a vertical line that's the distance vertically from the u-joint pivot point to an imaginary horizontal line even with where the clevis attaches, the horizontal side is the distance from the center of the neck rod to where the clevis attaches, and the imaginary line at an angle between the two (the hypotenuse) is the measurement you need. So it's the square root of ( vertical distance squared + horizontal distance squared), and you can plug different numbers into the vertical distance to see how it affects the outcome. I'm not sure you'd even need to redesign the part -- it looks like you could just slide it down the shaft and leave a gap between the upper rod clamp and the u-joint. It looks adjustable and a very nice design as long as the bearing stays secure in the plastic piece.


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## bfjou812 (Aug 15, 2008)

I'm trying the same setup with the same skull. I have removed a lot of the interior for my eye mechanism so that has helped on the weight issue a little. What I've done in the past is try to mount the skull to the support rod so the skull is balanced. I've accomplished this by mounting the skull on the servo plate, moving it forward or backward , to achieve the results I'm looking for. You won't get it perfectly balanced but you can get it fairly close.


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## goosta (Jul 24, 2011)

xronos said:


> Correct - center of the servo spline to the connection point on the arm. I wasn't able to get an accurate measurement on any of mine without pulling them apart, but if I had to approximate on the control side, I'd say it's about 35mm. The ratio (control to servo arm) on mine is maybe just a bit more than 2 to 1, and it looks like you're in the ball park -- you're at 1.79:1. (Increasing by 10mm on control side would be 2.5:1, a 40% power increase if I'm doing the math right.) Could be worth experimenting, but it still might be preferable to upgrade the servo first. As far as how much you'd have to drop it if you go that route, it's pythagorean theorem -- it's a right triangle -- imagine a vertical line that's the distance vertically from the u-joint pivot point to an imaginary horizontal line even with where the clevis attaches, the horizontal side is the distance from the center of the neck rod to where the clevis attaches, and the imaginary line at an angle between the two (the hypotenuse) is the measurement you need. So it's the square root of ( vertical distance squared + horizontal distance squared), and you can plug different numbers into the vertical distance to see how it affects the outcome. I'm not sure you'd even need to redesign the part -- it looks like you could just slide it down the shaft and leave a gap between the upper rod clamp and the u-joint. It looks adjustable and a very nice design as long as the bearing stays secure in the plastic piece.


Thanks for the help! You're right - I can just move down the collar part.


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## goosta (Jul 24, 2011)

bfjou812 said:


> I'm trying the same setup with the same skull. I have removed a lot of the interior for my eye mechanism so that has helped on the weight issue a little. What I've done in the past is try to mount the skull to the support rod so the skull is balanced. I've accomplished this by mounting the skull on the servo plate, moving it forward or backward , to achieve the results I'm looking for. You won't get it perfectly balanced but you can get it fairly close.


Thanks for the tip! On my next version, I will have to experiment with that.


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## xronos (Sep 25, 2013)

If you do change the mounting (i.e., by moving the pan servo/vertical center line forwards), there are a few things to be mindful of. First and most important is range of motion. As it is, the nod range is quite limited in the downward direction due to the fact that the jaw when fully open will collide with the neck. (It might seem fine on the bench with only the threaded rod or d-shaft, but when you put an actual neck on you might have clearance issues.) Moving the center line of the neck too close to the front could further aggravate this. Second, if you add an eye mechanism, there's very limited real estate to work with as it is. Third, it starts getting further away from what is anatomically correct (though I'm not sure it really matters all that much in a skull if it's "close enough"). You're kind of boxed in by all these parameters. I'm currently working on a character that's a good bit larger then a Lindberg, and it was so nice to finally be able to get the underskull better balanced, but I still ended up having to front-weight it somewhat, solely due to clearance and anatomical reasons, especially given the weight/position of the eye mechanism.


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## goosta (Jul 24, 2011)

So I decided to change the servos out. Moved to a HS-5645MG for nod and Hitec 485HB for tilt. Also reduced the servo arm distance a small amount. I'm still getting servo strain unfortunately, which is frustrating. It's not too bad when I hook up springs to help, so I guess I will just go with that for now.

This is for a haunted photo booth project I'm working on for this year. I have SO much other stuff to do, I think I will have to save future iterations for a later date. I think for the next version I am going to try out a geared version.


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