GT2 pulley and belt

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cvoinescu
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Re: GT2 pulley and belt

Post by cvoinescu » Wed Feb 19, 2014 1:31 pm

tm america wrote:By my caculations the tr8*8 has a 1.5mm pitch
How could it be 1.5 mm? The only reasonable pitch for that screw is 2 mm (a four-start screw).

This is how it works. Say you have:
Tr 8 x 12 (P2)
This reads:
  • Tr -- metric trapezoidal screw (30°);
  • 8 -- diameter: the screw is 8 mm thick across the peaks of the threads;
  • 12 -- lead: one turn advances the nut 12 mm;
  • P2 -- pitch: the spacing (along the axis of the screw) of consecutive threads is 2 mm;
  • The lead and the pitch together give the number of starts: 12 / 2 = 6 starts.
If the pitch and lead are the same (i.e., if the screw has a single start), the part in parantheses (the pitch) is omitted.

So for Tr 8 x 8, one turn moves the nut 8 mm; for Tr 8 x 12, one turn moves it 12 mm -- and that's regardless of the pitch. In both cases, the pitch is almost certainly 2 mm (and should be specified), so you have a four-start screw, and a six-start screw. Both of them would be much faster than single-start 1/2-10 Acme screws.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

tm america
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Re: GT2 pulley and belt

Post by tm america » Wed Feb 19, 2014 4:18 pm

I seen in the specs it said thread pitch 1.5mm or 2mm.but if it moves 8mm or 12mm per turn that would mean it wouldn't have enough power to raise and lower the z axis.Nor would it have the ability to hold the z axis when the motor is not powered?You would have to go with a tr8*4 which would be around 6.35 tpi or a tr8*2 which would be 12.7 tpi?Seems like the tr8*4 might be on edge of what it takes for anything other than a hf knock off demel.The tr8*2 would be slightly slower than a 1/2-10 acme screw but would still be nearly twice as fast as the m8 rod ..One of my biggest concerns with using one of these motors would be the loads places on the bearings in the motor when used on a z axis ..I'm thinking you would need to find two more nuts and use a bearing and mounting plate at the bottom of the z axis ..I would like to give it a try as it would make the z axis much shorter and you would have less parts to source ..

cvoinescu
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Re: GT2 pulley and belt

Post by cvoinescu » Wed Feb 19, 2014 5:26 pm

You are correct, Tr 8 x 8 is probably not a good idea for the Z axis, and Tr 8 x 12 is worse.

The pitch can't be 1.5 mm for the Tr 8 x 8 screw because the lead (8 mm) must be a multiple of it. Only 2 mm works. The correct, complete designation of the thread is Tr 8 x 8 (P2). For the Tr 8 x 12, 1.5 mm would work (with eight starts), but it's practically unheard of. It's always Tr 8 x 12 (P2).

Tr 8 x 4 (P2) should work, and Tr 8 x 2 definitely works, and works very well.

I'm afraid I don't understand how adding a bearing at the bottom would make the Z axis shorter.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

tm america
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Joined: Tue Feb 04, 2014 5:58 pm

Re: GT2 pulley and belt

Post by tm america » Wed Feb 19, 2014 5:41 pm

Adding the bearing to the bottom of the z axis would not make the z axis shorter it would just take the load off the bearings in the motor..using the motor with the lead screw built in would shorten the z axis by around 1.75 inches.. as you would not have the spacers required due to the flexible coupler and nut on top of the bearing at the top of the z axis.The z axis extrusion would not be effected by this change.. Just the total length from the bottom of the z axis extrusion to the top of the motor

cvoinescu
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Re: GT2 pulley and belt

Post by cvoinescu » Wed Feb 19, 2014 6:53 pm

Okay, I see what you mean. I would not worry about the motor bearings too much -- unless you use the machine a lot every day, they'll last a long time. For moderate wear, the threaded shaft motors I've seen have a pressure nut at the back that you can tighten to compensate (regular steppers have a much weaker spring washer with no adjustment, because they're not designed for axial loads).
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

tm america
Posts: 84
Joined: Tue Feb 04, 2014 5:58 pm

Re: GT2 pulley and belt

Post by tm america » Wed Feb 19, 2014 7:32 pm

That's good to know the motors have an adjustment built into them..Now i just gotta get the motor ordered and design a mount for the nut..I will probably just machine two flats into the nut they give you and drill one more hole then mount it to a new block that is cut to fit their nut.That way it will be a bolt in install

janapriya
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Re: GT2 pulley and belt

Post by janapriya » Fri Feb 21, 2014 3:54 pm

@robotdigg, any plans to include 20 tooth 6.35mm bore, GT2 pulley? I only see the 18 tooth one. Furthermore, would you offer dual bearing V wheels in the future? I see you already have idlers.

Nickies08
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Joined: Mon Dec 23, 2013 5:12 pm

Re: GT2 pulley and belt

Post by Nickies08 » Fri Feb 21, 2014 5:03 pm

Their motors are amazing! I'm not sure how well they would be on the Shapeoko (I'm not experienced enough) but on my Printrbot Plus 3D Printer, they rock. I ordered custom Nema 17's with T8*8 x 500mm rods. The ones at 450 and up are usually T8*12 but I ordered custom ones so I could use their anti-backlash nuts. They have a 2mm pitch and are 4 start.


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