3 wheels vs 4 (potentially dumb question)

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3 wheels vs 4 (potentially dumb question)

Postby crispyfry » Sat Mar 24, 2012 7:51 pm

On the Quantum and Hadron, the Y carriage and X carriage have 4 roller wheels each, while the Z plates have only 3. At first glance, it would seem that 3 wheels would center and guide just as well as 4. Is there something I'm missing that led to the decision to use 4 wheels on X and Y?
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Re: 3 wheels vs 4 (potentially dumb question)

Postby IPvFletch » Sat Mar 24, 2012 10:31 pm

My "potentially naive answer" is on the X and Y, the carriage could sit unlevel to one side, so 4 are used. On the Z that can't happen since it's bound to the threaded rods, so it isn't necessary... *shrug* ??
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Re: 3 wheels vs 4 (potentially dumb question)

Postby crispyfry » Sun Mar 25, 2012 1:14 am

IPvFletch wrote:My "potentially naive answer" is on the X and Y, the carriage could sit unlevel to one side, so 4 are used. On the Z that can't happen since it's bound to the threaded rods, so it isn't necessary... *shrug* ??


If the carriage is not level with the extrusion then one of the wheels will have jumped the rail, in which case all bets are off anyway, right?

I'm hoping bdring will chime in here and say if this decision was driven by a design constraint I'm not aware of.
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Re: 3 wheels vs 4 (potentially dumb question)

Postby daveczrn » Sun Mar 25, 2012 3:10 am

with the z plate not moving nearly as fast or as often i could easily see where having 3 with a not perfect adjustment could move around on you on the x and y axis. Could just be a little safety engineering built into the machine.
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Re: 3 wheels vs 4 (potentially dumb question)

Postby r691175002 » Sun Mar 25, 2012 7:44 am

You are correct, three wheels are all that is needed.

However, with only three wheels, if there is significant load, the side with only a single bearing has half the load bearing capacity. It is plausible that the wheels/plate of the build table could deform under enough load. Using four bearings at least ensures the deformation will be symmetrical, and even a few thou of bending can affect the quality of prints.
Using three bearings for the table would also be asymmetric and wouldn't look as cool.

The X axis could just as easily have only three wheels, but then the wheel mounting might interfere with the extruder since it would be in the center instead of the corners.

I'm just guessing though.
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Re: 3 wheels vs 4 (potentially dumb question)

Postby WhiteB0rd » Sun Mar 25, 2012 9:16 pm

Not a dumb question at all.....

But consider, the Z rides on two makerslides.

Read the assembly stuff on the wiki. I get the impression that we're getting the 4-bearing stability of the X and Y by virtue of the 7 bearings on two slides in the design.


Just my .02 $MONETARY_UNITs



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Re: 3 wheels vs 4 (potentially dumb question)

Postby crispyfry » Mon Mar 26, 2012 5:35 pm

Thanks for all of your responses. The reason I asked is I am planing on designing a slight variation on the ORDbot, and printing the flat plate parts on a reprap (yes, it's a silly way to make 2d parts, but it's what i have available). Based on the responses here I will plan to use 3 wheels on all axes.
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Re: 3 wheels vs 4 (potentially dumb question)

Postby IPvFletch » Mon Mar 26, 2012 6:29 pm

We really need Bart to reply, with his thinking on this! :D
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Re: 3 wheels vs 4 (potentially dumb question)

Postby WhiteB0rd » Mon Mar 26, 2012 11:03 pm

We really need Bart to reply, with his thinking on this!


Yeah, you're right......

But this week, I would really rather have Bart boxing and shipping ORDbots!



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