3 wheels vs 4 (potentially dumb question)

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3 wheels vs 4 (potentially dumb question)

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?
crispyfry

Posts: 60
Joined: Tue Mar 20, 2012 2:03 pm

Re: 3 wheels vs 4 (potentially dumb question)

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* ??
IPvFletch

Posts: 127
Joined: Thu Feb 16, 2012 4:37 am

Re: 3 wheels vs 4 (potentially dumb question)

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.
crispyfry

Posts: 60
Joined: Tue Mar 20, 2012 2:03 pm

Re: 3 wheels vs 4 (potentially dumb question)

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.
daveczrn

Posts: 21
Joined: Sun Mar 11, 2012 10:08 am

Re: 3 wheels vs 4 (potentially dumb question)

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.
r691175002

Posts: 242
Joined: Mon Dec 13, 2010 5:05 pm

Re: 3 wheels vs 4 (potentially dumb question)

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

WB
WhiteB0rd

Posts: 60
Joined: Thu Feb 16, 2012 2:04 am
Location: South Central PA, USA

Re: 3 wheels vs 4 (potentially dumb question)

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.
crispyfry

Posts: 60
Joined: Tue Mar 20, 2012 2:03 pm

Re: 3 wheels vs 4 (potentially dumb question)

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

Posts: 127
Joined: Thu Feb 16, 2012 4:37 am

Re: 3 wheels vs 4 (potentially dumb question)

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!

WB
WhiteB0rd

Posts: 60
Joined: Thu Feb 16, 2012 2:04 am
Location: South Central PA, USA