Belts and Pulleys
The Shapeoko uses power transmission/timing belts, pulleys, and idler wheels for linear motion along the X- and Y-axes.
Timing belts have teeth that prevent slippage so that the pulley and the belt always move together. They are made of rubber or other flexible polymer, reinforced with very tough tensile fibers, usually glass, running lengthwise. The tensile fibers are what gives timing belts their resistance to stretching. Toothed rubber belts are manufactured as a long tube the diameter of the belt (if a closed belt). The rubber is extruded. Many diameters are made, and the closed belts are made by cutting a tube of the right diameter into slices of the desired width. Open-ended belts are made by slicing one such tube in a spiral (helix), so very long lengths of belt can be made in one piece from a reasonably-sized tube. The tube is large enough that the slight slant of the teeth is unnoticeable. Timing belts were first commercially introduced around 1949.
There are many different types of timing belts, suitable for different applications. For a Shapeoko, forces are normally low and the most important consideration is registration (positioning accuracy). As users seek stiffer machines for more demanding materials and speeds and/or larger work areas, one of the upgrades to look at is stiffer belting. This can mean changing the basic design of the timing belt, the width, the pitch, and/or the materials.
TL;DR for this page: The GT2 belts that come with the machine are very accurate if properly tensioned and the only other belts you should consider to handle more force are either the same 2M GT2 in wider width (+55% modulus @9mm wide) or 3M GT2 for maximum stiffness at the expense of precision (+325% modulus @15mm wide, 4X-5X backlash).
No matter what belt is used, performance will be poor without proper tension. When the belts are installed with high tension, they stretch less with the machine's motion and cutting forces, giving better accuracy and results. Good tension is also essential to prevent ratcheting (skipping/jumping teeth at the pulley). For regular power transmission, Gates recommends 4.0 lbs tension in .25" width MXL belting and 5.3 lbs tension in 6mm width 2M GT2 belting. These figures sacrifice precision for long life, and should be considered minimum values for the Shapeoko. Tensioning beyond these values can reduce backlash in the pulley teeth because the belt pitch stretches to become slightly larger than the pulley pitch. Larger pulleys (more precisely, more tooth engagement) increase this effect and reduce belt wear. For the best accuracy no matter what tension is used, actual machine travel must be measured using a dial indicator to calibrate the controller to the true pitch of the stretched belts.
When using wider, coarser belts that require greater installation tension, the tension could become too much for the rest of the machine to handle. The idlers, motor plates, and even the rails could bend.
Tension: Z-axis belt wants ~20 lbs. (total) --- by inference the X- and Y- axes should have ~10 lbs.
Using wooden shims to tension Shapeoko 3 Z-axis (incl. photograph): http://www.shapeoko.com/forum/viewtopic.php?f=37&t=8249
Belting of any tooth design is generally available in various widths. Belt tensile stiffness increases approximately linearly with width, so doubling belt width doubles tensile stiffness. Tensile cords at the edge of the belt are ineffective, so wider belts are actually 5-10% stiffer than the linear relation would suggest. Despite the increase in tensile stiffness, increasing the belt width does not require a larger bend radius, so the same pulley diameter can be used. However, wider belts will require new, wider pulleys and possibly idlers to fit the new belt(s), as well as possibly new motors w/ longer shafts or spacers to reposition them so as to allow the pulleys to be fully placed on the shafts so that the belts run true.
9mm perform better and will last longer.
9mm wide belts for SO3 X- and Y-axes
- 2 mm (GT2) 20 Aluminum Alloy 9mm 0.25 2 Flanges / With Hub
Nomad 883 parts specification
New Pro model doesn't use belts save for the spindle mechanism: 162-2GT-60 or https://www.amazon.com/RobotDigg-162-2GT-6-Endless-Length-Synchronous/dp/B07P3ZZM7G
Nomad 883 (Original): 18-2P09-6AF2 pulleys, a 742-2P-09 belt and a 994-2P-09 belt 
Tooth pitch is the distance from the middle of one tooth to the middle of the next. Fine pitches like MXL (2.032 mm) and 2M GT2 (2 mm) cannot carry a large amount of torque because the small teeth will skip, but with proper belt tension they are adequate for the torques a Shapeoko is capable of producing. Belts with coarser tooth pitch are designed to handle more torque, so they also have more tensile cords and are stiffer for a given belt width than fine pitch belts. While the better tensile stiffness of a coarser pitch timing belt is attractive, the poor accuracy of coarse-pitch belting is insufficient for precise machining.
Tensile Cord Materials
Tensile cords in timing belts are usually fiberglass, and this is the case in the MXL and GT2 belting used by the SO1 and SO2. Other cord materials are widely available and some are stiffer, but the stiffness increase is marginal compared to simply increasing the width or pitch of a fiberglass belt.
- Aramid (Kevlar) cords have slightly higher tensile stiffness and higher breaking strength than fiberglass and are used in high-shock applications.
- Steel cords have higher tensile stiffness, but can't be bent as tightly so they may require larger-diameter pulleys and idlers.
- "The steel core have a much smaller range of tension compared to stock and the y axis needed to be set the exact same side to side or they can fight each other."
- stretch of fiberglass and steel belts measured: https://community.carbide3d.com/t/belt-stretch-and-stepper-holding-measured/18480/22
- Carbon fiber tensile cords are a new development. Belts using carbon fiber are very strong and can carry high power and torque with a thin belt, but at present they are only available in 8 mm and 14 mm pitches, too coarse for use with a Shapeoko.
A small selection of the belt designs available are as follows:
- MXL/XL/L -- The original trapezoidal-tooth timing belt from when synchronous belt drives were first developed. Designed as general-purpose belts, these have significant backlash. The different abbreviations, standing for Mini Extra-Light, Extra-Light, and Light, denote the tooth pitch. MXL belting in .25" width is stock fitment on the original SO1.
- GT/GT2/GT3 -- Gates PowerGrip® GT® industrial synchronous timing belts. GT stands for "Gates Tooth". GT belts have a special tooth design that allows them to have low backlash despite high torque capacity, so the positioning accuracy is an order of magnitude better than trapezoidal-tooth belts. 2M GT2 has about 4X better positioning accuracy than 3M GT2. GT/GT2/GT3 refers to versions of the same profile, which come in a variety of standard pitches. You may see 2GT/3GT/5GT used to refer to 2mm/3mm/5mm pitch original GT belting. 2mm pitch GT2 belting in 6mm width is stock fitment on the SO2.  and was used on the SO3 before it transitioned to 9mm wide belts.
- T2.5/T5 -- Metric trapezoidal-tooth belt.  --- possibly too stiff for some usages
- HTD -- Stands for High Torque Drive. This is a legacy belt type designed for high torque, but has worse registration than some newer belt types. 
- AT --- 
Considerations with Stepper Motors
When upgrading the motors on a Shapeoko, new timing belt pulleys will be required with a larger bore to fit the new motors' larger shafts. Normally, NEMA 17 motors have 5mm shafts, but some rare motors will have 1/4" shafts. NEMA 23 motors typically have 1/4" shafts. See Stepper Motors: Shaft Sizes for further details.
Increased belt tension may exceed the overhung load rating of the stepper motors. Ratings for NEMA 17 steppers range from 5-10 lbs., and ratings for NEMA 23 steppers range from 10-20 lbs. Note that those numbers vary by manufacturer, and that they are for long life in continuous use. Shapeoko users may be able to get away with higher loading because of the low cycles involved in non-industrial usage. 
- Re: just ordered a shapeoke 2 mechanical kit
- Re: Belt Tension and Eccentric Nuts
- Upgraded to 9mm wide belts
- Re: Shapeoko Belt Drive --- discussion of the actual numbers for cutting forces and the amount of stretch which they induce in the belts.
- Re: Belt tensioning --- discussion of causes of problems w/ timing belts
- http://www.cnczone.com/forums/momus-design-cnc-plans/219166-belt-driven-z-axis.html --- belt-driven Z-axis example.
- Re: GT2 pulley, 20 tooth (part # S3023) are a tight fit! --- notes on installing pulleys and cautions to take
- Any suggestions for small Pulley puller?
- https://www.reddit.com/r/Reprap/comments/4dgeau/maybe_you_guys_can_settle_an_argument_cable_vs/ --- cables vs. belts in a 3D printing context
The Shapeoko 2 and 3 use Gates GT2 2mm pitch belts, 6mm wide on the 2, 9mm wide (for X and Y) on the 3 (but upgrading to 9mm belts is an excellent upgrade for an SO2). Z-axis on SO3 uses 6mm wide 520 or 524mm endless loop (528mm was originally specced).
Metric pulley set screws are usually M3, typically ~4.75mm o.a.l.
124 lb / 56kg Breaking Strength
6.25 lb / 2.8kg Working Tension
Please see the Parts Overview page for sources.
Note that the set screws used in pulleys are a commodity item and should be available at any hardware store.
Note that the stock for pulleys is also available: https://shop.sdp-si.com/catalog/?cid=p345&filter=a0:2:2%20mm%20(GT2) and may be cut (w/ a bandsaw) and turned down and drilled for the correct bore (on a lathe) if available parts do not suit.
- Handbook of Timing Belts and Pulleys (pdf) (also available at sdp-si.com (html))
- Forum discussion: Rated torque on pulleys?? Why is there not a problem here?
- Gates: Belt Tensile Properties
- http://www.bbman.com/assets/files/pdf-library/Engineering/Timing%20Belts/BeltTensileProperties.pdf 
- https://www.engineeringtoolbox.com/thermal-expansion-pvc-d_782.html --- figures for expansion/contraction of fiberglass
Gates Belt Drive Design Software --- Quickly Design Industrial Synchronous and V-Belt Drives
Rack and pinion, ballscrew, gearbox
- Rack & pinion for X-Axis
- threaded rod (used on the SO1/2 Z-axis) https://old.reddit.com/r/hobbycnc/comments/bh92gw/using_threaded_rods_for_x_and_y_movement/
- Acme / Trapezoidal rod (a frequent upgrade)
- pulley reduction / gear system: https://github.com/shapeoko/Shapeoko_Experimental/tree/wduembeg_PulleyDrive4to1
One suggestion is to loosen the pulley set screws while the belts are tensioned, allow them to rotate, then tighten: https://community.carbide3d.com/t/laseoko-pro-build-thread/28073/97
Removing pulleys --- a plumber's tool will work: https://community.carbide3d.com/t/psa-check-your-x-axis-pulley-before-assembly/30182/18
Discussion of Gates-branded versus 3rd party: https://reprap.org/forum/read.php?1,842583,842665#msg-842665