THE SINGER MODEL 15-91 IS DRIVEN BY A DIRECT DRIVE MOTOR WHICH DRIVES THE HAND WHEEL WITH A FIBER GEAR. 

If the motor is spinning without driving the mechanism, you may suspect the fiber gear immediately.  Unfortunately, the Singer 15-91 has a unique direct drive motor (0.7 Amp) and no other motor of any kind can be fitted on it.  Below is a 15-91 broken hand wheel gear:

THIS GUIDE WILL BE HELPFUL ALSO TO CLEAN AND LUBE YOUR ACTUAL GEAR !

REPLACING THE FIBER GEAR

STEP 1:  Unscrew the hand wheel stop motion knob screw.  Then unscrew the stop motion knob and remove the position washer.

STEP 2: Remove carefully the hand wheel assembly.  Check all the fiber gear teeth and clean them as well.  Check for any damaged areas, cracks and worn out teeth.

STEP 3: Remove the fiber gear by loosing the 3 gear collar set screws.  You should be able to remove the gear collar by applying an equal pressure all around.  The fiber gear is deposited in place, check for the stud facing inside the hand wheel.

STEP 4: Install the new gear with the stud facing inside the hand wheel.  This stud will be in contact with the damping spring (already inside the hand wheel).  Once the new gear is properly fitted, install the gear collar and tighten the 3 set screws.

STEP 5: Prepare to install the hand wheel back to the machine.  Clean the machine shaft end as well as the motor worm gear.  Apply a few drops of sewing machine oil to the shaft prior to insert the hand wheel assembly.

I RECOMMEND YOU TO APPLY EITHER ON THE MOTOR WORM GEAR AND ON THE FIBER GEAR A GENEROUS COAT OF SINGER 2129 LUBRICANT.

STEP 6: Once the hand wheel assembly is back to the machine, insert the stop motion washer.  Note there are 2 positions for this washer.  Be sure the 3 small recess dots are facing outside.  If the stop motion knob does not disengage the hand wheel properly when the washer is installed in one position, it needs to be installed in the other position.

STEP 7: Install the stop motion knob and tighten its screw.  You should disengage the hand wheel (the machine mechanism is not moving) and engage it (the machine mechanism is moving).  If this is not working, you need to reposition the washer.  Turn the hand wheel by hand first to feel the correct meshing between the motor's worm gear and the hand wheel fiber gear.  DO NOT FORCE ANYTHING !!!  If you are not able to move the hand wheel freely, please repeat steps 1 and 2.  If the meshing looks good when turning the hand wheel by hand, you can try the machine with electricity.

THANK YOU FOR YOUR TIME!  Please do not hesitate to e-mail me for any help.  I will gladly put over 30 years of experience at your service!

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Andre (alcnational)