- The two part construction of the motionco 12 tooth gear means that when bored out to 5mm there is very little metal left on the inner sleeve that joins the hub to the pulley itself. Its probably worthwhile ordering a spare 12 tooth gear just in case anything goes wrong in the boring process.
- Order spare belts – if you don't get the tension right the belts will slip and quickly the teeth will wear.
- I lengthened the flat on the worm shaft that accepts the pulley grub screw. This gives a little more flexibility in positioning the pulley and allows you to get it away from the worm bearing.
- Extend the slots on the motor mounting plates. On the DEC motor plate I filed back one of the motor support pillars just to get a little more play when setting the tension.
- Position the pulley on the motor shaft so that the distance from the top of the pulley flange to the motor plate is 9mm. Note that this means the top of the motor shaft will be recessed inside the pulley.
- With the worm carrier removed fit the motors and check the motor pulley position by looking through the slot through which the belt will pass.
- With the motors removed, fit the worm carrier and make sure the belt has plenty of clearance passing through the mount body. I widened the casing slot slightly and rounded off all the edges - may not be necessary but I'm just cautious.
- I used epoxy to to secure the pulley to the motor – with extra applied to the top of the flange as it isn't being held on by much metal (and I don't want it coming loose and falling into the worm compartment). I know some folks will be concerned that doing this makes the mod irreversible but even if the epoxy can subsequently be weakened by heat it really shouldn't be too hard to locate some suitable replacement 200 step bipolar stepper motors.
- Make yourself a wire hook to help fit the belts on the motor pulleys – once you get the knack its quite easy.
- When putting the motors in use a strongly magnetised screwdriver or a dab of grease to keep the washers in place in until you've got the bolt safely located. You can bet that if a washer does drop off it will head straight into one of the worm drives or down onto the polar scope (I had both before I learned my lesson!)
- To adjust the belt tension I made up the little device shown in the accompanying attachments. It simply sits on the top corner of the motor and the set screw is tightened against the casing side wall using an allen key. This pushes the motor inwards tensioning the belt. When your happy you just tighten the mounting plate bolts and slacken set screw to remove the tensioner – patent pending, every belt kit should have one!
- Run the motors at a fast speed for a few minutes then check inside the motor housing for any signs of belt dust – if there is remove the motors and inspect the belts (you might as well remove the DEC axis totally as you'll find it hard get the belts back on otherwise.
The belt mod is rather fiddly. The belts are not easily accessible and its hard to monitor what is going on with them once the mount is reassembled. Changing the belts will require complete disassembly. Be prepared to disassemble the DEC axis if you remove the motors – it hard to get the belts back on with the DEC axis in place. Should I ever have the mount in pieces again I may well drill and taping some holes in the rear of the motor housing to make the process of belt engagement and inspection easier.
I would recommend limit protection to be enabled as there is the possibility that the belt might start to slip with resultant damage to the belt teeth if movement is obstructed. Although the belts are cheap the hassle involved in belt replacement (particularly RA axis) means you really want to take all steps to avoid this if possible. Cable snags also present a situation where damage to the belt might occur.
All in all I'm happy with my belt mod. The only real concern in recommending the belt mod to others is the marginal nature of the machining required on those 12 tooth pulleys. An alternative solution worth of consideration would be to source a pair of 0.9 degree/step bipolar stepper motors and fit 24 tooth motionco pulleys instead (these are of a solid construction).