A few years ago, after my plastic portable machine broke, I bought a Singer 20u73, light industrial sewing machine. I was pretty tired of slow, weak machines with low torque. Going through 4 layers of fabric was a struggle so of course I went all out and got this nice machine.
Unlike little portables which turn on the DC motor when the foot-lever is pressed, industrials use a clutch motor. The motor is running the moment you turn it on, and when the foot-lever is pressed, it moves the clutch towards the motor, engaging it. This means higher torque when you turn it on, thus getting rid of the frustrating “have to help the machine along” stuff necessary with small sewing machines. Great. only problem is that the motor runs at a perky 1750 RPM and the clutch is very sticky. This means it requires some practice to get used to the foot pedal: instead of being linearly related to stitch speed, its much more ‘exponential’. Seems like either nothing is happening or its going at a ridiculous 2000-2500 stitches a minute — much too fast for someone out of practice.
The solution? Basically everyone says “you’ll get used to it with lots of practice.” Which is another way of saying “this design really sucks”. (There’s DC servo motors that have no clutch because they can give high instantaneous torque but I’m not 100% sure they solve the speed-control issues)
Another solution is to change the pulley, which will bring the max speed down, and tweak the clutch setup for better response. For $20 it’s a nice simple fix. I spent a few days figuring out how the hell one does this. I figure if I post all of this now it will possibly save someone else the hassle.
p.s. You could screw this up and hurt yourself – 1/3 HP motors don’t stop for you or your hands. If you aren’t comfortable with this sort of mechanical assembly and disassembly, maybe have your sewing machine repairman do this for you?
Click “more” for the full article
Step 1. Go to this website on pulleys and belts. Its the best thing ever. My motor is 1/3 HP and 1750 RPM. The center distance on my machine (the distance between the center of the pulley on the motor and the center of the pulley on the sewing head is about 16.5″. Pulley 1 (on the motor) is about 3.8″. Pulley 2 (on the hand wheel of the sewing machine) is 74mm according to the manual, which translates to 2.9″
Plug this in and press Calculate. Look for “R.P.M. Pulley 2″ which is basically stitches-per-minutes. On my machine it was 2200 spm. I’d like to crank this down a bunch. By getting a smaller pulley you can change the gear ratio to your favor. Change Pulley 1 from 3.8″ to 1.75″ and press Calculate. Stitches-per-minute goes down to 1000, more than half the speed!
Step 2. Buy a pulley. You can get a 1.75″ pitch diameter one at Sailrite. Basically make sure its a pulley for a motor with 3/4″ keyed shaft and a 3L size v-belt. Should be able $8-$10. I was lazy and spent more money for an adjustable pulley from mcmaster-carr (they didn’t have a fixed pulley in the right size). Having an adjustable pulley is probably not what you want but it is kinda neat. You can separate the plates of the pulley to get a different pitch.
Anyways, if you get an adjustable one, just set it to max width, and tighten the set-screw. Make sure its not -too- wide that the v-belt binds. The two halves should be ‘flush’ with no threads showing on the outside or inside of the outer edge.
Step 3. Remove old pulley. Take the guard off, remove the pulley and make sure you don’t lose the tiny metal key.
Step 4. Install new pulley. I had to put this one backwards, but if you get a non-adjustable pulley like the one from Sailrite it should go on just like the old one, with the key to the right. Make sure the key is in. I had to file mine down just a bit to make it fit. My setup is not ideal (its less safe), but it works for illustrative purposes:
Step 5. Get a new v-belt. Look on the pulley calculator page, under belt length it says “40.3” which is the ideal length. You can only get then in full-inch lengths so get the closest value. Say 40″ in this case. Note that you’ll have to measure your setup’s center-to-center length to get the right number here! Mcmaster has these too, they’re 3L sized fractional-HP v-belts, $5 each.
Step 6. You’ll probably have to adjust your motor to get good tension. The way to do this is to screw/unscrew the nuts on the long, vertical bolt shown in the center this photo:
Move the motor up or down so that your new belt is well-tensioned. Not knowing the best way to verify this I guessed that the belt should be able to wiggle 1/2″ at the center when tensioned right. YMMV.
Once the tension is good you can adjust the rest of the clutch motor. First of all, make sure the belt has clearance, and move the motor on the mount to get this right. I doubt it’ll be a problem if you just get the non-adjustable pulley.
If you want max-control, put the foot-pedal bar on the left-most mounting hole. Adjust the length of the bar so that the pedal is pretty much at the angle you want to use it (you’ll adjust it again later).
Now adjust the 5/8″ bolt that is right in front of the pulley, it has a nut that you’ll have to loosen first. This bolt sets where the clutch sits. Basically you can use it to eliminate ‘deadspace’ in your pedal travel. I found that the ideal setup has it so that when then pedal is not pressed at all, its a bit hard to turn the hand wheel. This means the ‘brake’ is engaged. When you lightly press on the pedal it should release and you can turn it easily. This way you can use your heel to brake the machine. There’s also a wingnut to adjust the spring return of the clutch.
I don’t have a magical formula for adjusting these but basically mess around with the pedal-rod length, the clutch adjust bolt and the spring adjust bolt until you have a comfortable pedal angle (rod length), amount of travel (pedal rod mounting holes), break-release-engage capability (bolt) and its not too hard or two easy to press the pedal (wingnut). Do all of this with the machine unplugged, for crissakes.
When you’re done, make sure the pulley setscrew is tightened, the clutch bolt nut is screwed on all the way to prevent the bolt from vibrating loose and you have v-belt clearance. Put the pulley guard back on and power up your machine. Mine hums along at a nice pace and once I get better at dealing with the clutch I may move back to a large pulley. Have fun!
Have an amazing project to share? Join the SHOW-AND-TELL every Wednesday night at 7:30pm ET on Google+ Hangouts.
Join us every Wednesday night at 8pm ET for Ask an Engineer!
Learn resistor values with Mho’s Resistance or get the best electronics calculator for engineers “Circuit Playground” – Adafruit’s Apps!
Sorry, the comment form is closed at this time.