Belt length comp for 3 leadscrews and stepper motor

[Dan Newman]

[Edited 7 Dec 2014: revised sketch/image to clarify gap, G.]

Needed to compute the belt length for a three leadscrew + driver motor setup.

While I did this for the leadscrews at the vertices of an equilateral triangle,
it's trivial to generalize the math to other configurations (e.g., the 3*l1 term
becomes l1 + l2 + l3).

The diameters of the pulleys and idlers ideally should be pitch diameters: the
diameter of the belt center line when mounted on the pulley or idler.  SDP/SI
provides that diameter with their pulley specs.  Likely other major suppliers
as well.

Dan

[Dan Newman]

P.S. Add a fourth leadscrew, locate the leadscrews at the vertices of a square, and you have the configuration used on the Cupcake.

[Ryan Carlyle]

G is the axis-axis distance between the two small idlers?

[Dan Newman]

On 07/12/2014, 8:45 AM, Ryan Carlyle wrote:
> G is the axis-axis distance between the two small idlers?

Yes. On the side, I've set

G = D1 + 2 * (D2/2) = D1 + D2

However, it's not necessary to use that particular spacing.

And, of course,

l2 > D1/2 + D2/2

otherwise the two will interfere with one another.

Dan

[Vernon Barry]

Double V arrangement for maximum belt wrap on every pulley and still only uses 2 idlers.

608 skate bearing idlers.

Motor and overall arrangement.

Moving stage plate detail from the bottom side.

[Ryan Carlyle]

Vernon, how do you tension the belt? Or is it just precision-preloaded?

[Vernon Barry]

Use one of the idlers in a slot. It's not obvious in the picture because I use big fender washers to spread the load so the idler bolt stays 90 degrees and doesn't bend with tension (M8 bolts and nuts to match the 8mm ID of the 608 skate bearing). FYI, fender washers are from Ultimachine (the best place to get metric hardware in bulk). I order a few bags every time I buy filament since it doesn't matter shipping wise.