When I first started trying to use my Juki industrial machine, I had a major problem with the machine's speed. My Juki has a clutch motor, and it was set up to sew too fast for me. The treadle on a clutch motor equipped machine doesn't provide for much speed control at all - it's more like an on-off switch. The treadle basically just engages or disengages the clutch - that's it. While you can "slip" the clutch a bit to get you started, that builds up a lot of heat and can eventually damage the clutch.
I quickly found out that I wasn't the only one who had this problem. DanTwoLakes, Hotrodders' resident upholstery guru and moderator to the stars, told us how to solve the problem by doing a simple motor pulley swap.
Probably the most commonly suggested fix for this type of thing is to replace the clutch motor with a servo motor that has variable speed control. That's fine if you have the money to invest (and it is a good investment,) but what if you're a mope like me who doesn't have the $180 to spend on a new motor right now? The answer is to change out the pulley on the clutch motor for a smaller pulley.
Yes, a smaller one. I know that some are saying that a smaller pulley will make the machine run faster. People immediately think of a 10-speed bicycle - you move the chain down to the smaller pulley on the rear sprocket to make the bicycle go faster. That's true - but when you do that, do you suddenly start pedaling faster or slower? That's the point. Think instead of the FRONT sprockets on a 10-speed. When you move the chain down to the smaller front sprocket, you pedal faster, but move slower.
To sum up this point, it's all in where you provide the power, not in where the power ends up. If the chain on our imaginary bicycle is on the smaller front sprocket (where we provide the power) the bike moves at a slow speed. If we move the chain up to the larger sprocket and pedal at the same speed, the bike moves faster. Conversely, if we move the chain down to the smaller sprocket and pedal at the same speed, the bike moves slower. The same is true with the pulleys on the sewing machine. The motor spins at the same RPM no matter what size pulley it has on it. By switching down to the smaller pulley, the machine will run slower.
Keep in mind that when you swap pulleys, you'll also have to get a smaller belt. In my case I swapped out the 3 1/2 inch diameter motor pulley for a 1 3/4 inch diameter pulley, and swapped from the 45 inch belt to a 42 inch belt. It slowed down the machine considerably, and gave me better control all around. You most likely won't get the exact belt you need on the first try. Luckily the hardware store I went to let me exchange the belt with no problem. The motor can be adjusted to take up some slack, much in the same way you would tighten the belt on an alternator or similar, but it won't take out much more than an inch of slack.
Here's a video I found that demonstrates how to slow down the speed on your industrial sewing machine with a clutch motor by swapping out the pulley on the motor, changing the belt, and adjusting the belt tension.
The guy who shot this video is in Mexico, and he's done several upholstery videos. He uses subtitles to communicate, and there are a few typos, but you will get what he's saying. I've found that I have to pause the video in spots to read the subtitle - you'll see what I mean.
I got my pulley and belt at my local hardware store, and the whole modification cost less than $20. The two things you need to know going into the store is the shaft size on your motor (most are 1/2 inch or 3/4 inch) and the belt width. I took my old belt in with me and matched the pulley width to the old belt, then matched the width of the new belt to the new pulley. Have some idea as to how small a pulley you want to get, but remember that the smaller you go, the slower your machine will run. I went with a 1 3/4 inch diameter pulley, but I probably could have gone smaller - maybe 1 1/4 or 1 1/2 inch diameter. I'm used to this set-up now, so it doesn't matter to me anymore, but your mileage may vary.
No matter how small of a pulley you swap down to, keep the old pulley and belt, just in case you want to change them back some time down the road.
In summary, a smaller pulley on your clutch motor will slow down your industrial sewing machine, giving you better control, and improve the quality of your work - all for about $20. It's a great modification to your machine for those who are learning to sew or just starting out and can't immediately afford to upgrade to a servo motor with variable speed control.