Prusa Mendel Wide Belts
Modification: 10mm wide belts for driving the X and Y axies
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.These are much less stretchy.
Advantages of modification
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.
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. The 10mm AT5 belt used in the prototype should stretch at most ~0.2mm.
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
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 (since stepper torque drops off fast with rpm) more torque at any given speed, which counteracts 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.
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.
The belt idler pulleys need to be made wider in order to support the wider belts. This can be achieved with a printed idler on a single 608ZZ bearing, or by putting two bearings side by side.
Note that the Y belt is slightly shorter than the X belt.
Because the belt is wider, there is no room on most motor shafts to have the motor away from the idler. Follow the the standard build instructions for the Y axis, except:
- omit the optional nut and two of the small washers between the idler and motor mount.
- place two 608ZZ bearings between the mudguard washers, instead of one.
Additionally, if you have them, you can improve the belt geometry by adding a stack of stand-offs between the Y carriage and the open belt ends. This means that the belt moves parallel to the carriage guides, and keeps the steps per mm of travel constant along the entire travel of the carriage.
The X axis belt mountings are too short on the standard carriage, so the printed carriage needs to be modified or belt mount extenders must be used.
In order to take best advantage of the stiffer belts, they should be quite tight. It also takes more tension to pull them snugly against the pulleys. For best results, tightly clamp one end and thread the other end of the belt loosely through its clamp. Then pull the loose end with mole grips or pliers to get the tension and tighten the clamp.