Designing Small Belt Drives

[RKW]

I working on a couple of small MXL timing belt drives and looking for suggestions on determining which belt to use. My center to center distance between pulleys is not currently fixed yet and I'm trying to figure/approximate/calculate which belt(s) to use. I know I want to have enough slack to be able to mount or replace the belts and to get over the flanges of the pulleys but not so much that an external idler causes the belt to rub on itself. Drive and driven pulleys will remain stationary so all adjustment will be through the idler. Almost any tooth count is available in increments of 2-4 teeth.

I will figure out what I need through trial and error and some CAD modeling but I was curious what others may have done. I'm thinking primarily working in reverse with standard belt sizes and my known pulley diameters to determine the C-C distance and then going up a size or two to get the needed slack.

Thoughts?

[strokersix]

If you are going for maximum power or torque you want to get as much wrap as possible on the small sheave and locate the idler on the slack side. But you probably already knew that.

Often flanges are used on the smaller sheave only. This allows the belt to be removed without as much slack.

Polyurethane belts are much more durable than rubber belts in my experience.

[Evan]

A few questions:

What size pulleys?

Will the belt be run in both directions?

What rpm or linear belt speed?

How much load on the belt maximum ( inch-oz) ?

Must the centre to centre distance be fixed or is it just desirable?

[RKW]

The pulleys are 1/2" and will run in both directions. Low torque, low RPM. The fixed centers on the pulleys are just desirable. One could be movable, but it is a stepper motor and I didn't want to get into additional parts or slots for belt tensioning. I was hoping the idler would be sufficient and easier to implement in a limited space.

[Evan]

I recommend that you make one of the pulleys movable for tensioning. Using an idler will cause a directional drive error that will show up as a small amount of backlash that cannot be removed without over tensioning the belt. It really isn't difficult to make the motor mount adjustable.

Polyurethane belts are much more durable than rubber belts in my experience.

WD-40 and similar solvents degrades them rapidly but doesn't harm rubber belts.

[RobbieKnobbie]

Determining center distance, capacity and so forth is pretty straightforward. Check out the section in Machinery's Handbook for all the relevant equations. In the 26th edition is starts on page 2372

[RKW]

Evan,

Even if it is a fixed idler and not a spring loaded one? I had hoped to have the idler run directly in the middle of the two pulleys from a swing arm. I have not seen anything on directional drive error before but completely understand backlash, can you elaborate?

I can reconfigure my mounting options without difficulty, I would just like to understand more of what you are referring to.

Keith

I recommend that you make one of the pulleys movable for tensioning. Using an idler will cause a directional drive error that will show up as a small amount of backlash that cannot be removed without over tensioning the belt. It really isn't difficult to make the motor mount adjustable.



WD-40 and similar solvents degrades them rapidly but doesn't harm rubber belts.

[Evan]

With an idler between the pulleys the tension on the belt will be slightly different depending on which way it is driving. Any time a belt/pulley system is asymmetrical and driving both ways there will be a small difference in how the belt tension is distributed depending on direction, even with a fixed tensioner.

How much this matters depends on your application.