Belt Drive configuration - moving or fixed?

[robertbrenton]

Hello,

I’m new to belt drives and have a basic configuration question. I searched the forum but could not find the answer
I want to use a belt drive for a 3.6m long X axis. I’ve done some calc’s and I’m going with a 32mm wide AT10 belt on either side of the axis (can’t source a GT2 system economically). To be driven by a servo motor with a 1:5 or 1:10 gear head (haven’t decided).

What I’m confused about is what belt configuration to go with in terms of simplicity of build, efficiency, accuracy etc

The two configurations I’ve seen around are:
Option 1: Fixed Belt
Belt is fixed and is clamped on both ends.
2 idler bearings and timing gear travels with Y Axis gantry.
One configuration is to couple both sides of the X axis with a Jack shaft and driving the jack shaft with one motor.
Example1

Option 2: Moving Belt
Timing pulley on either end of the axis. Jackscrew on one end couples both sides of table to one drive.
Example1

I’ve listed my scattered thoughts below

Cost
The moving belt option is more expensive for me as you need two more timing pulleys and double the length of belt. I have not factored the cost of the idle bearings in Option 1 as I already have these (they are big as 100mm OD required for the outside of AT10 belt).
For a moving belt do you need the low backlash pulleys on both ends or can you get away with them at the jackscrew driven end only?

Build Complexity
A moving belt seems simpler, mainly because a fixed belt would require the machining of a couple of plates to mount the idle bearing etc on. Then again a moving belt may require idler brackets like this . Both systems would require a jack screw. I have read on a post here where someone went with the fixed belt option and couldn’t get it to track correctly and then went with the moving belt.
One complexity of the moving belt option is that I may need to pass the return belt though the X axis T-Slot support beam (the profile does not have much room). Another option is to run the moving belt off to one side of the table as shown here . This requires a cover for safety.


Efficiency/ Accuracy
The fixed belt option would make the Y axis gantry at least 20kg heavier. This is extra inertia, more stress and I’ve got less breathing space with the 32mm AT10 belt.
I’m not sure about accuracy/repeatability. Would there be twice as much “give” in the moving belt example as you have double the length of belt?

Other Considerations
I like the idea of having an open table, where you can feed a work piece in from either end. With the fixed belt option the jackscrew would be part of the Y axis gantry however with my table configuration the jackscrew for a moving belt could be in the way. This configuration gets around it but means that the drive is farther away from the Y axis plane.
I noted that Belt manufacturers tend to only show the moving belt option (perhaps so they can sell more belt). Most linear stages use the moving belt option.
I’m leaning to the moving belt option as it appears to be a more common option and there looks to be less build complexity although it will be pricier. Also if it goes wrong I can always cut the belt and try the fixed belt option.

Any input/advice would be appreciated.

Cheers
Robert