Check spring range of motion

[Wyowally]

Wiz and Uwe, I'll try those ideas. Might be able to get a picture today. Thanks again. Still wonder if that 3/32" rise from bdc should make the needle bar bump into the presser feet when they are raised. Mine does that, so I can't manually turn the wheel with the feet up!

[Wizcrafts]

  On 8/4/2021 at 2:14 PM, Wyowally said:

Still wonder if that 3/32" rise from bdc should make the needle bar bump into the presser feet when they are raised. Mine does that, so I can't manually turn the wheel with the feet up!

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So, lower the presser and vibrating foot bars until they clear the needebar.

[Wyowally]

I can lower them. Think I was going for maximum lift, which I don't need. Thinking the 3/32" is priority over manual lift height.

[Wizcrafts]

  On 8/4/2021 at 2:27 PM, Wyowally said:

Think I was going for maximum lift, which I don't need.

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You can probably find lower profile feet, or grind down and polish the bottom of a standard set.

[Wyowally]

I think this might have some sort of aftermarket Brand X hook. It has no markings from Simanco or anyone else. I'm trying to figure out where the entire upper thread is at each stage after the hook catches the loop at the needle.

[Wyowally]

I'm continuing this thread because I'm trying to cure check spring bounce. Thinking I've narrowed it down to problemIs with the hook. In the bottom side pic, is the thread supposed to get under here where it could catch on that gib screw tip?  The thread is hanging up on the corner of the gib plate (red arrow, other picture). Yellow line representing hung up thread. That opening at the corner is not quite as exposed as it is in the picture with it timed the way it is. Something seems to be holding the loop back so the thread goes to that corner, instead of moving freely. How/when do you decide the hook shaft bushing is worn? It can be adjusted up and down. Don't know if that's a factor in my problem or not. Thanks.

Edited August 5, 2021 by Wyowally
More text

[Constabulary]

Believe it or nor when I was tinkering on my Singer 51w I had a similar problem. I installed a Durkopp brand hook cause the OEM Singer hook was badly worn. The top thread kept hanging and I could not figure why. I tried different brand hooks, even ground down the hook body, tried different brand bobbin cases and nothing worked until I figured the "rail" on an OEM Singer 111 bobbin case is a bit shorter and so I replace the Durkopp bobbin case with the OEM Singer bobbin case and it worked! So the thread kept hanging on the slightly longer "bobbin case rail" when the top thread has been pulled up

I have used not the OEM throat plate and not OEM hook so this "special parts situation" can probably not copied to other sewing machines but I just want to give a hint.

I cannot put the parts in a good angle to show the difference but the rail on the left bobbin case is approx 1mm longer. Picture shows a sample but the rail on the OEM Singer bobbin case # 233764 (at least on my bobbin case) is even shorter (2mm I think - compared with the Durkopp bobbin case) and that shorter rail solved the thread handing problem - at least in my special case.

maybe this helps - maybe not.

Edited August 5, 2021 by Constabulary

[Wyowally]

Constabulary, thanks for the effort and the idea. If I understood Witz and Uwe correctly, advancing the timing just a little might accomplish similar to a shorter rail. My hook looks identical to your right hand sample. Tip of the rail is involved here somehow, I believe.Wondering if bottom thread wants a gap between that tip and the squared end of the gib plate.

[Wyowally]

Apologies for kinda hijacking this thread. Think I'll start a new one.....

[Wyowally]

My problems are solved, I did start another thread. I learned a lot here. The check spring range of motion and tension allow for some leeway, but can be fine tuned. For me anyway, if the check spring does not cycle smoothly without jerks and hopping up and down, then there is something wrong underneath causing those. Whatever it is creates a false increase in lower tension as well. There are variations in hooks and bobbin cases, for sure they are not all the same and must be compensated for. The last thing I learned before - this forum and the people here are the best!

[Gymnast]

I guess the basics regarding timing of needle, take-up-lever and hook are almost the same for most sewing machines. But perhaps some variation between machines with oscillating hooks and rotating hooks.

I made a study of my Singer 201k with rotating hook regarding how much the check spring was engaged when the thread go around the far side of the shuttle. I tried 4 different situations:

1) 1 mm thick fabric and 4.5 mm stitch length.
2) 7 mm thick webbing fabric and low tension, so the stitch lock of the threads is near bottom surface.
3) 7 mm thick webbing fabric and average tension, so the stitch lock of the threads is expected to be in center of fabric.
4) 7 mm thick webbing fabric and high tension, so the stitch lock of the thread is near the upper surface of fabric.

Here is a photo of the check spring positions:

I do not think that the differences are that significant, but I did not know that before I tried it.

I looked at some details of movement of the parts as shown in this diagram:

You got max check spring deflection at this far shuttle point, when the take up lever is on its way up. I did a calculation on a change of hook timing, so it catches the thread when the needle had moved 0.5 mm less up than before. It is a 3 degree change that makes about 3.6 mm more thread to be used when the thread comes around the shuttle. So if you have got a problem with wrong check spring deflection at this point, it can be right to look at hook timing, because it is sensitive to that.

Edited August 11, 2021 by Gymnast

[captainsquidhead]

Fabulous comprehensive thread, thanks to all those contributed.

[Gymnast]

I have studied a Pfaff 230 regarding the check spring motion. It is actually engaged four times each stitch.

Can it be right? 

I think the machine works with no problem, and it sews with a max speed of 1825 stitches/min with a Tex 70 polyester thread with 4 mm wide zig zag in a 2 mm thick cotton webbing. The needle size is 120/19 with this task.

The check spring stroke is set so it correspond to 11 mm motion of the needle thread. The tension of the check spring is set to 0.25 N (1 oz) at the point it engages the thread. This check spring tension setting is significantly higher than it was when I got the sewing machine, but this higher tension "feels" right to me. The clearance at the position finger bracket have been checked.

The check spring is engaged when you have some tension in the needle thread and no thread slack. I made a video with the handwheel very slow turned, so you can see how and when the check spring is engaged:
https://youtu.be/3vZr0fmwNsU

The four times the check spring is engaged, seems to be caused by this:
1) The thread has been pulled down in max length and is around the shuttle.
2) The thread may be catching something at the hooks or shuttle, that shortly require some tension to pass.
3) The thread require some tension to pass the position finger bracket.
4) The take-up lever tightens and forms the stitch with the higher thread tension from the tensioner.