Prusa Mendel Wide Belts
Contents
Modification: 10mm wide belts for driving the X and Y axies=
Description
The X and Y axes of a Prusa Mendel are positioned by timing belts driven by stepper motors. On a standard Prusa Mendel, these belts are 5mm wide, and are often made of rubber with glass fibre tension members.
This modification replaces the standard belts with 10mm wide AT5 belts made of Polyurethane with steel tension members.
Advantages of modification
Availability
10mm wide belts and pulleys are easy to find as standard items (unlike 5mm) for many tooth pitches of timing belt. This avoids the need to split belts and allows the use of machined pulleys (instead of printed ones).
Using 10mm wide belts allows the constructor to choose from a wider range of belt materials and tooth profiles.
Performance
Stiffer (less stretchy) belts tend to allow more accurate positioning.
Other things being equal, belt stiffness is proportional to width. Using wider belts also makes it possible to use heavier types of belt, which are stiffer still.
Stepper motors apply enough torque to stretch drive belts by a non-trivial distance (I calculate a maximum of ~0.8mm for a typical 5mm glass fibre & rubber belt). This decreases the accuracy of the print at corners and holes, and can cause "ripples" in prints after as the print head bounces back and forth on the belt after a sudden acceleration. Stretch errors are proportional to belt stretchiness.
Non-zero friction on the axis guides leads to stick-slip positioning errors, which also decrease as the stiffness of the belt increases.
Disadvantages of modification
Lower Acceleration
Wider belts are more massive ("heavier"). For a given force, acceleration is inversely proportional to mass. If the belt is thicker, then it also has a larger minimum bend radius, which means larger drive pulley on the motor, and so lower force for a given motor torque.
The extra mass is only a very small disadvantage, because the belt is only a very small part of the moving mass (most of it is the build bed or extruder), so doubling the mass of the belt only increases the total moving mass by a few %.
Additionally, larger drive pulleys allow a higher maximum speed, and more torque at any give speed, which offsets the reduction in acceleration due to reduced leverage.
On my prototype, X and Y speeds of >250mm/s are possible, with acceleration to full speed over a few mm.
Cost
More belt material tends to cost more money. However, the belts are a small part of the cost of the printer, and the extra few $ is a small price to pay for better performance. Also, time and effort are worth something, and avoiding the need to split belts saves both.
