Author Topic: Crust crawler arm design  (Read 1131 times)

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Offline RyanSTopic starter

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Crust crawler arm design
« on: June 09, 2009, 07:01:12 AM »
 I have just signed up to this forum. I have about 11 servos around from my RC plane days.   I want to use what I have to design a 5 DOF arm, it doesn't need to lift a huge amount of load, and the servos i have,  do not have a particularly high torque. This is what I have:

4  x   JR     NES-511     3500g p/cm     0.24sec p/60
4  x   Futaba     FP-S148     3024g p/cm     0.22sec/60
3     Hitec     HS-300A     3500g p/cm @ 6v     0.19sec/60

Don't have much other information, the specifications I assume are reasonably accurate. I also have an Arduino duemilnove which I will use as the controller.

As this is my first project I essentially want to copy a design from a small commercial hobby arm.  this is the one I was looking at:

I have drawn up an FBD based on the information from their Web site: (larger)

I chose this one because it has a great detailed assembly manual online, which will be a great help in designing.

Obviously my servos  are not as strong.  What modifications would you recommend to the design? Could I for example use 2 servos for the bicep. is it possible to add gears ( externally to the servo) in order to get more torque  at a reduced speed?  I used the robot arm calculator, I doubt mine would be able to  lift much more than 100g, but I guess that's okay for experimenting with. I think the  crust crawler arm uses 2mm thick aluminium sheet, perhaps I should go with something lighter like 1.5mm.  I have a mill & lathe so building is not a problem. I'm going to design in solidworks,  if I can work out  how to stress test assemblies   Rather than just individual parts. I am definitely no engineer, I just like to mess with various software  ;D

 I look forward to hearing from you.


Offline SeagullOne

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Re: Crust crawler arm design
« Reply #1 on: June 10, 2009, 12:31:53 AM »
Hey there, Ryan! Welcome to SoR!

I'm kind of inexperienced with engineering myself. I've had to tinker a lot and read a ton of books, but it looks like you're doing your homework too.

I think your idea about using two servos for the bicep would work okay. Also, you can make the linkages a little shorter. That would help you move more weight.
I think the chauffeur did it.


He did.

Offline Admin

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Re: Crust crawler arm design
« Reply #2 on: June 27, 2009, 10:54:07 PM »
I'm going to design in solidworks,  if I can work out  how to stress test assemblies   Rather than just individual parts.
If you know the forces on each part, you won't need to do assembly analysis. But yea, I can see how assembly analysis would be more useful for a robot arm . . .

The bicep motor always needs to be strong. I recommend looking around for a motor with a worm gear on it.

You could also replace components on the arm with carbon fiber to reduce the weight.

Offline madchimp

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Re: Crust crawler arm design
« Reply #3 on: June 28, 2009, 12:25:46 AM »
Another thing you can do to help deal with the weight of the arm and the servos is use helper springs. This way the servos don't have to support much of the weight of the arm or the other servos and most of their torque goes into lifting objects. The springs would add a little complexity and bulk but you should be able to pick some up pretty cheap and allow you to use what servos you have to lift a little more weight without the speed penalty of gearing them down. As far as carbon fiber for material look into cheap carbon arrow shafts they are pretty light and very strong and the biggest difference in the price is the straightness so just get cheap ones you don't need shafts that are perfectly straight.


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