Timing Belt Guide¶
Timing belts are toothed rubber belts used to transmit motion between rotating shafts or create light duty linear drives. We commonly use them to couple DC motors or stepper motors to driven mechanisms. The primary benefit is the ability to separate the motor and load shafts without needing complex gear trains, while also providing an opportunity for transmission reduction. If a small motor pulley drives a larger pulley, the transmission reduces the speed and increases the torque. Timing belts also add drive compliance, can be low-backlash, and are tolerant of shaft misalignment and mechanical imprecision.
Timing belts are commonly sold as endless belts selected from a catalog of specific lengths defined by the number of teeth. A common design problem is choosing the pulley diameters and relative center positions so the matching belt is available within the standard lengths.
There are different sizes of miniature timing belts and pulleys, primarily described by the tooth-to-tooth distance. For this course, we recommend the ‘XL’ series with 5 teeth per inch; these belts are easily available, and the teeth are coarse enough to cut well on the laser cutter. MXL pulleys can also be laser cut but the smaller size means the laser kerf distorts the tooth profile noticeably.
Sample SolidWorks models for belts and pulleys for both XL and MXL sizes may be found in the SolidWorks/timing-pulleys folder. The belt models are helpful for visualizing placement. The pulleys include tooth profiles which can be laser-cut.
Two-Pulley Drive Design¶
One of the most common design cases is transmitting motion between two parallel shafts, often a motor and a load. A general procedure for choosing parts is as follows:
Select a belt series; for most of our course applications, the XL series with 0.200 inch pitch is recommended.
Select a drive ratio. E.g. if the motor speed needs to be reduced and torque increased, choose a ratio such as 1:3 between a small motor pulley and a larger load pulley. A single stage reduction might typically range between 1:1 and 1:6. With larger ratios, the larger pulley can get quite large; a 120 tooth XL pulley is about 7.6 inches (193 mm) in diameter.
Select specific pulley sizes. Commercial pulleys may be selected from a catalog set of sizes specified by the number of teeth, e.g. McMater-Carr stocks 22 sizes of XL pulleys ranging from 10 to 72 teeth. The SolidWorks models linked above include a design table for generating any size pulley, and a number of sizes are pre-defined. Just a few of these sizes are pre-rendered as DXF files.
Select a hub and shaft design.
If you are laser-cutting your own pulleys from flat stock, the SolidWorks models include features for a plain bore (e.g. for a shaft), a D-shaped bore to press-fit onto a motor shaft (marginal), or a bore-hole with keyway for adding an elliptical key (cut separately). You’ll need to consider the contact length along the shaft and possibly laminate an additional hub part at the center for stability.
If you are purchasing commercial pulleys, the hub design is often linked to the material choice, but commonly includes plain bore and set screw options.
Select pulley material. Commercial pulleys are often available in aluminum or Delrin. Laser-cut pulleys may be plywood or acrylic. Plywood is marginal for the smaller MXL size, the features may be too imprecise for good belt traction.
Calculate a center-to-center distance between the two parallel shafts. This is a function of the belt series and the pulley diameters; please consult one of the online calculators below.
Select a belt tensioning strategy. Timing belts are tolerant to misalignment and shaft distance error, but will function best if calibrated for a lightly snug fit with minimum backlash. The simplest way to achieve this is to mount one component such as a motor in slots so it may be lightly pretensioned during assembly.
Select a belt assembly strategy. An endless timing belt often represents an assembly constraint, especially in tight spaces or if other components run through the opening or nearby. Frequently a belt needs to be partially assembled over shafts early in an assembly process, then wrapped around the pulleys only after other components are mounted.w
Resources¶
GT2 timing belts and pulleys are used in many DIY 3D printer designs and have become available at low prices via Amazon. Some common pulley sizes with a 5mm bore to match our stepper motors are 16 or 20 tooth. The belting is most often sold open-ended which is suitable for long linear or reciprocating drives, but endless belts of specific lengths can be found (e.g. SDP has lengths from 27 to 1109 teeth).
McMaster-Carr has a large stock of standard miniature timing belts and pulleys. Many sizes are available from the Cleveland warehouse with one day lead time via ground shipping:
Stock Drive Products has an 86-page technical guide online describing the use of their timing belt products:
Several companies have online calculators for computing pulley placement or belt length:
Common Timing Belt Profiles¶
series |
units |
pitch |
tooth height |
thickness |
|---|---|---|---|---|
L |
inch |
0.375 |
0.075 |
0.140 |
XL |
inch |
0.200 |
0.05 |
0.090 |
MXL |
inch |
0.080 |
0.02 |
0.045 |
GT2 |
mm |
2.00 |
0.76 |
1.52 |
Standard Endless Belt Sizes¶
The following belt sizes, specified in the number of teeth, are commonly available. Open-ended stock is also available for linear drives.
XL (0.200 inch pitch) Common Lengths (teeth)¶
25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 140, 145, 150, 155, 165, 170, 175, 185, 190, 195, 200, 210, 225, 240, 250, 285, 315, 385
MXL (0.080 inch pitch) Common Lengths (teeth)¶
40, 45, 50, 55, 60, 70, 80, 85, 90, 95, 100, 105, 110, 114, 120, 130, 140, 144, 150, 153, 155, 165, 175, 184, 190, 200, 210, 212, 225, 250, 260, 295, 300, 400, 498