Kenmore 158: Flipping the Pulley

The sewing machine motor drives the handwheel through a double pulley on a jackshaft:

Kenmore 158 - handwheel - jackshaft pulley
Kenmore 158 – handwheel – jackshaft pulley

I’ve been figuring a 10:1 speed reduction, based on counting revolutions and ignoring belt slip. The correct answer also depends on belt tension and whether you turn the motor or the handwheel.

Measuring the pulley diameters isn’t straightforward, because the belt runs deep in the handwheel pulley (the brown smudge is near the rim, above) and high on the motor pulley:

Kenmore 158 - NEMA 23 stepper - on adapter
Kenmore 158 – NEMA 23 stepper – on adapter

Measuring across the tops of the belt ribs on the pulleys gives these diameters:

  • Motor: 18 (or 16.6 at the belt midline = pulley OD)
  • Jackshaft large: 48
  • Jackshaft small: 24
  • Handwheel: 75

The end-to-end ratio is either 8.3 or 9, depending on what you call the motor pulley. Either of those are close enough to 10:1 to allow for a turn or two of motor pulley slippage.

Flipping the jackshaft pulley doesn’t quite work, as the pulley ends aren’t symmetrical, but I think it can be forced to align with the handwheel if I add a lathe-turned hoodickie. If so, then the end-to-end speed ratio drops to a little over 2:1 and the original belts fit just fine:

Kenmore 158 - reversed jackshaft pulley
Kenmore 158 – reversed jackshaft pulley

The maximum handwheel speed ran a bit under 1000 RPM, so the reduced ratio lets the motor turn at 2000 RPM. That’s well within range of a NEMA 23 brushless DC motor, but it must also satisfy the other non-obvious requirements:

  • Acoustic = no squeals, not even a little bit
  • Physical = a scant 100 mm from mounting plate to edge of the frame casting for a 57 mm diameter cylinder

Measuring the torque required to drive the sewing machine would go a long way toward finding the proper motor. The Leadshine BLM57050 would drop in, the BLM57090 might barely fit with some filing of a rib in the machine’s base, the and the BLM57130 isn’t in the running. The OEM motor dataplate says it’s 110 – 120 V @ 1 A = 110 – 120 W, but that surely doesn’t mean the same thing as the 130 W rating for the BLM57130.

I should just buy a motor and driver brick to see what it’s like … [sigh]