Belt Tensioning Tool, iOptron CEM25 Mount

thingiverse

Improved belt tracking and backlash reduction have been achieved by upgrading motor mounting and reducing slip and chatter. Disengaging and reengaging the worm without adjusting tension settings has also become possible. After purchasing a used CEM25P telescope mount, which was returned by a customer, I discovered that both RA and Dec axes would occasionally skip, causing the mount to lose sync. However, turning the tension adjustment knobs did not resolve the issue. Upon disassembling the covers and tension knobs, I found that the tension springs were cut too short and lacked sufficient strength. The tension knobs were also bottoming out on the worm blocks due to this reason. Furthermore, one of the screws fastening the Dec stepper motor had stripped and was loose, resulting in belt slippage. Since these screws only engage by 3-4 threads into soft aluminum, this problem needed immediate attention. The necessary parts include: Four M2.5 x 40mmL fully threaded screws Four M2.5 washers (the existing four washers can be reused on the screws) Four M2.5 nylon insert hex locknuts (requires a 5mm hex socket) Two 1/4"OD x 1/2"L 11.6lb compression springs (available at https://www.amazon.com/gp/product/B008RFGNGA) To balance both axes, slew the mount to Zero Position if it's not already there. Then, turn off power and unplug all cables from the mount while taking precautions against static discharge. - Remove the top covers, revealing the Dec and RA stepper motors and circuit boards. - To access the motor screws, remove the eight screws holding both circuit boards. Unplug the Dec motor from the Dec circuit board. Set aside the Dec board and screws. The RA board does not need to be unplugged and can be moved slightly to one side to access the motor screws. - Remove a motor mounting screw, washer, and corresponding long housing screw. Replace the housing screw with the M2.5 x 40mmL screw and tighten it. On the opposite end of the motor, loosely place an M2.5 washer and locknut over the stud protruding from the mounting bracket. Do not tighten yet. Repeat this process for the remaining three motor mount screws and three housing screws. - Insert the tensioning tool between the curved portion of the motor bracket and the motor shaft and rotate it towards the + mark to increase belt tension. You want just enough tension to remove visible backlash, but not so much as to cause excess wear and tear. Tighten the two nuts, remove the tool, and repeat for the other stepper motor. - Reinstall circuit boards and cables. Remove the springs from the tensioning knobs and replace them with new springs. Replace the knobs making sure that the ball bearing sits between the worm block and knob. Do not tighten the knobs yet. - Manually move the mount to Zero Position. Turn the lock levers to the locked position and turn the tension knob until the worm engages and noticeable play is gone. - Turn on the mount and slew both axes through their full range of motion. Verify that the belt is centered, not skipping, and not exhibiting excess backlash. Readjust as needed. If the tests are successful, turn off power, verify that the wiring is tucked away properly, remove the tension knobs, replace the covers, and replace the tension knobs. At this point, you can now manually move the mount by turning the lock lever to the unlock position without having to unscrew the tension knob first. Turn the lock lever to the locked position and verify that there is no play and the tension knob does not need adjustment. Slew the mount through the entire range of motion to verify proper operation.