Antique Phonograph Spring Motor Restoration Using 3D Printed Parts

Parts for any antique mechanical device are difficult if not impossible to find. This project will describe the design and fabrication of 3D-Printed missing and failed parts in the restoration of an antique phonograph wind-up spring motor.

I acquired a Brunswick-Blake photograph that would not crank correctly. The problem was a broken spring. The previous owner did discover the broken spring and in the process of reassembling the motor left out a primary drive gear assembly. I easily found the bad spring but had no idea about the missing gears. I found a similar motor on eBay with photos of all the parts including the missing gear. So I proceeded to design the gear and an adapter to repair the failed spring and 3D printed the necessary replacement parts.

The images above show the 3D-printed drive gear and the spring spacer.

3D Printer (Prusa used for this project)

SketchUp Design Software with gear.3a.rb Ruby Script plug-in

PETG Filament

PETG Carbon Fiber Filament (can be used for all parts of this project)

Hand tools for motor overhaul

Whitney Punch or drill for attachment hole in spring

Scientific calculator or spreadsheet for gear calculations

1/8" - 3.5mm shaft for the drive gear

The inner end of the primary wind spring was broken off and no longer attached to the motor shaft linking the two motor springs. I designed an adapter cylinder to fit over the 13.2mm shaft with an od of 26mm and a height of 30mm (same as the spring case). The id of the adapter has a 7mm x 4mm grove x 24mm tall to make use of the original spring shaft hook as a stay for the 3d printed adapter. The spring will now wrap around the 3D-printed adapter when the motor is wound.

A 3mm hole for a 3mm x 6mm self-taping screw to attach the spring to the adapter. Cut the broken end of the spring using metal hand shears. I used a Whitney Punch for the 3/16" hole in the center of the spring (a 1/8" drill can also be used).

The photos:

#1 3D printed adapter attached to the spring using 3mm x 6mm self-tapping screw

#2 Dual spring drive mechanism removed for repair

#3 Illustration of the secondary spring with the factory attachment to the central shaft

#4 thru #6 26mm dia x 30mm high 3D printed adapter with a 13.2mm hole and 6mm x3mm x 24mm deep notch for shaft lock. Note the 3mm hole in the center of the cylinder for spring attachment using a 3mm x 6mm self-tapping screw.

#7 Repaired spring assembly ready for the winding gear to be attached.

The STL file for the cylinder adapter below is rendered at 20% fill using PETG (or PETG carbon fiber).

It took some trial and error to get the gears the proper dimensions for a good mesh. I designed the gears using the Involute Gear plug-in in SketchUp, plugging in several values until they agreed with my measurements from the motor. The issue is the measurement of the "Pitch Radius", OD (and ger radius) are easily measured, but the Pitch radius entry in the gear tool took some guessing. See the final table in the photos with the parameters needed to get 13.48mm and 63.98mm OD for each gear.

Measure the following from the mating gears:

The pitch of a gear is defined as the distance between two identical points on two adjacent gear teeth.

Radius from the center of the new 3.5mm shaft to the inside of the tooth for both gears (2*Radius=OD).

Circumference = 2 Pi R, divided by the gear pitch = Number of Teeth (nearest integer).

Photos:

#1 Involute Gear Parameters.

#2 The printed gears.

#3 3.5mm shaft spacer and spacer washer, fit tight to the shaft, to hold the assembly in place.

#4 Spacer-Gear-Shaft Spacer assembly details.

#5 Assembled gear-shaft assembly ready for installation.

#6 Image of the actual gear-shaft assembly found on eBay for reference.

Below are the STL files for the Drive Gears and Shaft Spacers.

Gears are printed with PETG Carbon fiber, PETG for other assemblies is fine.

References:

Wiki Involute Gear: https://en.wikipedia.org/wiki/Involute_gear

Sketch-Up Involute Gear Ruby Script Plug-in:

Here is a link to the SketchUp Gear plug-in that will work with current versions of SketchUp.

http://kansascitykitcompany.com/explorations-and-speculation/sketchup-gears-plugin

gear.3a.rb: https://www.dropbox.com/s/2nsk80r6v7cv1lb/gear.3a.rb?dl=0

Installation Instructions: Drop the Ruby Script file in...

users/user/AppData/Roaming/SketchUp/SketchUp2021/SketchUp/Plugins. Involute Gear then shows up in the TOOLS menu.

Only 4 screws hold the motor together.

Photos:

#1 The motor's robust lower frame.

#2 Spring motor assembly in the lower frame.

#3 The 3D-printed drive gear and spacers are ready for final assembly.

#4 Close-up of the drive gear mating with the spring drive.

# 5 Motor fully assembled.

The 3D-printed carbon fiber gears are amazingly strong.

The spring motor will run through several 78 rpm records even with the diminished capacity caused by the 26 mm spacer.

Video Below Completed Motor running with governor control.