GT2 Belts

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  • 5 thousandths, or .12 mm - is enough to cause trouble. Depth of cut is .12mm for circuits.
  • See Shapeoko discussion - [1]
  • Latest Shapeoko uses GT2 9 mm
  • Most RepRaps use GT2 6 mm


  • Using stepper motors to provide accuracy, and further using geardown to increase accuracy - it should readily be feasible to obtain sub mil positioning accuracy
  • Using 3D printed rubber or rubber-nylon belts, it should be possible to get the required holding strength for high force systems
  • Using 3D printed planetary gears, we can get the necessary force
  • Longevity of Herringbone, axial thrust-stabilizing gears is >>1000 hrs.

These 3 elements combine to the feasibility of precision machining systems at low cost.


  • Positioning accuracy of stepper motor using GT2 belt - Say 1/2" pulley. Circumference about 40 mm. 200 steps per revolution = .2 mm positioning - but with 16 microstepping RAMPS standard - we get down to 0.01 mm positioning accuracy, or 0.5 thousandths. Positioning Accuracy does not stand in the way of a successful build - therefore only backlash and belt stretching need to be resolved. For belt stretching, the solution is proper tensioning. So this looks optimistic, gien a solid frame."
  • Chart at [2] shows positioning accuracy of GT2 belt to be 0.0003 - about half that of the RAMPS 16 microstepping calculation above - meaning that GT2s can accept up to 1/32 microstepping.
  • The question is - how much does the belt stretch upon acceleration (force)?
  • Why is there no resource that states belt stretch as a function of force? It seems this is irrelevant because ratings are given, and one doesn't go above ratings. But to engineer the system more highly, this stretch is required to understand the performance in extreme or unintended applications


  • Note that on single-side drive- we effectively use one strand of belt. To double the drive capacity, we can use double-sided drive
  • Based on 9 lb tooth jump force (see graph in document) for a 20 tooth pulley - 1/2" pulley - if that is for 6 mm of belt width - we could do up to 3 15 mm belts per 2" universal axis. That is 45 mm of belt - or 7*9 lb force = 63 lb of force per axis prior to tooth jump as the absolute limit. Doubled axes mean that we have about 130 lb of tool force for the super heavy mill. That is not a lot, but sufficient for many jobs including milling with up to 1/2" mills.
  • It is actually a little better than this - looped belts have 2 strands of belt, s0 can effectively go up to 260 lb force. Double sided drive is needed in this case.. Actually no - if you see how belt is attached, 2 motors are pulling on the same side of the belt, so it's effectively as if we have the limit of 1 belt width for holding the force.
  • This is if our limit is the tooth jump force. But - what is the real limit of practicality?
  • Cutting force - take a lathe - and let's see how much force is required for a cutting operation. If steel is 50,000 psi strong, then cutting a 0.01 by 1/8" sheath of metal takes that fraction defined by the surface area of the cut - 1/8"*.01" - where it's the width of the cut * depth of cut. So 1/1000 of 50,000 psi is 50 lb force on the tool. If we are doing 50 lb of force on the tool tip - then we should be ok. But catching peak loads would be 500 lb - insufficient for our system to hold. Possible solutions are workpiece cooling and higher tool speeds. In the limit - we can do grinding operations, which require very little force. So precition grinding would be at least one application of a lower force system.
  • Actual torque rating - 7 in-lbs rated torque for 2" diameter pulley - see after page T-147 for low RPM. This is per 6 mm width of belt. And appears outside of pre-tensioning force.
  • Double this for double-sided drive - 14 in-lbs for double sided drive per 6 mm width of belt.
  • Per inch of belt - this goes to 56 lb of force.
  • Thus, with 45 mm of belt (3 times 15mm belts) = 45/25*56 lb = 100 lb of linear drive force
  • With dual axis drive - this goes to 200lb of linear drive force. 6 belts total. 100 lb limit for 2 axes. The only way to expand this is with more belts. Tool Force in Milling

Belt Drive Cost

  • Design cost for 200lb drive force - means 6 belts, 2 feet long, per foot of travel. That is $12 per drive-foot of travel of dual axis on xyz, for 200 lb of linear force. This price point is outstanding. It does not include motors, just the precision belt.

SMEs (Subject Matter Experts

  • Contact Application Engineering Department - of SDP-SI - this belt bible - [3]- Tel: 516-328-3300



  • 20 mm GT2 belt AliExpress $23 for 5 meters (16ft) [4], or about $1/foot for 15mm
  • about $1/foot for 15mm [5]
  • EBAY 15 MM - $60 FOR 30 feet -[6]
  • Ebay - 15 mm belt - $1/foot at 100M quantity - [7]. 500 lb per inch breaking strength. Operate at 10% of breaking strength?
  • SDP-SI has up to 9 mm belts - [8]
  • $40 for 10m of 9 mm - [9]
  • $32 Amazon prime for 9mm, 10m - [10]


  • 15mm pulley, 8 mm bore $4 ea - [11]
  • 15/8 $10 ea Amazon - [12]
  • $1 for 10 mm 8 mm bore Aliexpress - [13]
  • 10mm pulley - [14]
  • 15 mm pulley $8 - [15]