Advice for speed reducer for Seiko CW-8B

[WMages]

I just recently picked up a Seiko CW-8B looking to fill couple of niches in my sewing tools and am on a budget. I currently have a CB797 Flatbed that performs very well. I have a Weaver Cub for heavier sewing. (I'm still learning with the Cub). I hope when money permits, to buy a Cobra Class 4. Regarding the Seiko CW-8B: I have it sewing beautifully with 138 thread top and bottom. It's equipped with a servo motor (China made) but its sort of a pita to adjust speeds etc unlike my Cowboy which is quite easy. Although the Seiko will sew slow enough I would like to really slow it down and give it a little more torque to sew up to about 9 or once in a while 10mm of thickness. I would like to find a speed reducer pully set for it and if possible, to order a really good servo motor with a speed reducer together if that's practical. IF it is practical, could someone tell me the best dealer or place to order?

Also, I've ordered plenty of correct size bobbins for the Seiko but is there an upgrade where a slightly larger bobbin case can be installed to accommodate slightly larger bobbins?  Thanks in advance.  

[GerryR]

I don't know anything about your Seiko but would recommend you look at some AC drives (VFDs - Variable Frequency Drive) and AC-3 phase gearmotors.  The VFD will convert a single phase input (like regular household power) to 3-phase power that the motor requires.  AC gearmotors can be bought with different ratios; I use one that is a 5:1 reduction. The gearmotors are much more compact than speed reducers made with pulleys and belts. The VFD requires a potentiometer input to vary speed.  I found a TIG welder footpedal, new, for $35.00, which had the included pot I needed to control the VFD.  Just another option.

CowboyBob, on this forum, I believe offers DC servo systems and good support from the comments I have seen. 

[dikman]

There are plenty of posts on here about fitting a speed reducer and there's no reason why it shouldn't work using your existing motor. If you're uncertain then as Gerry said the easiest way is to contact one of the dealers who advertise on here.

[kgg]

On 4/4/2025 at 11:45 AM, WMages said:

I would like to really slow it down and give it a little more torque

Most of the commercially available speed reducers are in the range of 6" (large pulley) and 2" (small pulley). The servo motor belt will connect to the large pulley and the smaller pulley will connect to the sewing machines hand-wheel. You didn't mention 1) the type of servo motor you have brush or electronic brushless. A speed reducer will reduce the existing speed and increase the torque (punching power) 2) the size of handwheel 3) the size of the pulley on the servo motor. The brush servo motors typically have a lower startup speed then a brushless servo motors. 

An example of the speed reduction:

i) size of servo motor pulley: 2" (assumed for calculations)

ii) size of sewing machine handwheel: 4" (assumed for calculations)

iii) size of speed reducer: 6" and 2"

iv) brushless servo motor startup speed: 500rpm

v) brushed servo motor startup speed: 200rpm

Results:

1. For brushless servo motor: The handwheel speed without a speed reducer would be 250rpm versus with a speed reducer the handwheel speed will be 83 rpm.

2. For brushed servo motors: The handwheel speed without a speed reducer would be 100rpm versus with a speed reducer the handwheel speed will be 33 rpm.

kgg

 

Edited April 9 by kgg

[WMages]

Kgg thank you very much for your informative reply. 

[WMages]

Thanks to each of you who responded. Regarding the TIG controller, I never even considered that application in operating sewing machines. I resisted the urge to use my Ohmite (VariAC) to tinker with the speed knowing low voltage would be detrimental to any AC motor but I'm not sure what would happen if a DC Servo motor would do with its mains power looking at lower AC voltage. I've ordered a larger servo motor with almost twice the torque than what what was on the machine when I bought it. If that fails to give me smooth low speed operation, I will order the speed reduction pulleys from the same dealer. At least that's my plan. Thanks all.

 

[dikman]

Lowering the AC voltage input to an electronic servo motor won't slow it down, if that's what you're thinking, as many of them are designed to run from 110v to 230v. If you want really low speed then a speed reducer is the only way to go, the trade-off is that it will reduce your top speed but for most of us that is an acceptable loss.

[WMages]

11 hours ago, dikman said:

Lowering the AC voltage input to an electronic servo motor won't slow it down, if that's what you're thinking, as many of them are designed to run from 110v to 230v. If you want really low speed then a speed reducer is the only way to go, the trade-off is that it will reduce your top speed but for most of us that is an acceptable loss.

Thanks. I can't ever envision I would want or need even half of the upper end  speed when doing leather work. If I make canvas covers or sails, it might be different but that's not at all in my future plans.