Problem with hook gib (Gegenspitze) on a rotary hook Dürkopp 291

[Gymnast]

I know the German term, "Gegenspitze" for a point part of the hook, where the upper thread leaves the area, when pulled the last part up by the take-up lever. It is the point you see here:

For some machines like the Singer 211W this "Gegenspitze" is part of the hook gib.

My problem is, that the thread is pulled hard by the take-up lever on this Gegenspitze before the thread finally is pulled off the end of the tip. I have read, that the reason for this tip is to avoid, that the hook may catch or damage the remaining thread when it comes around the second time. I know that only a few rotary hooks got this tip. 

I have of cause checked timing and some other matters in order to fix this problem. One matter was to grind and polish this tip. It do get worse with thicker thread (I have tried Tex 70 and Tex 135 Amann Serafil). I know another user with another Dürkopp 291, that have used the machine without this tip and with no problems. And I have tried the same with some threads and fabrics, and I have not seen problems either. But I also have got some warnings about that from a sewing machine repair professional. I have also been told, that this hook have been made with specific differences, that may relate to subclasses. The original subclass of this machine is E1 and it is one of the early versions of the machine.

I have tried to measure the length of this tip when taken off in order to possibly compare the length with other users:


Do you know about the issue with this "Gegenspitze" and possible ways to fix it?

Edited Thursday at 11:14 PM by Gymnast

[friquant]

 

2 hours ago, Gymnast said:

My problem is, that the thread is pulled hard by the take-up lever on this Gegenspitze before the thread finally is pulled off the end of the tip. I have read, that the reason for this tip is to avoid, that the hook may catch or damage the remaining thread when it comes around the second time. I know that only a few rotary hooks got this tip. 

Can you post an up-close slow motion video of the hook area as this is happening?

My usual approach to preventing the hook from catching on the thread when the hook comes around the second time is to retard the hook. It's quite predictable on the two machines I've tried this: Advance the timing and the hook grabs the loop of thread on the second time around. Retard the timing and the loop of thread is too far out in front of the hook point to be grabbed.

Rather than follow factory timing on those two machines, I found my own timing specification that is advanced enough that the thread can be pulled around the bobbin case without too much tightness, but retarded enough that the hook does not grab the thread on its second time around.

See this post for some background: https://leatherworker.net/forum/topic/131646-new-way-to-measure-and-visualize-hook-timing/

 

[Constabulary]

afaik the "counter hook tip" is meant for thinner thread at higher sewing speed. But I do not know how thin and how fast. So if the problem is solved w/o the tip then take it off al long as it does not cause other problems.

[nejcek74]

This is explanation from Cerliani:

which follows what your experience is and what Constabulary said, that with thicker threads the full gib is not always used. On my Pfaffs I have some hook variants with short and long gib for the same machine. They are variants for either A/B and C/D thickness classes and different gib goes with different hook.

Of course you can try and take this part away, but I am not sure exactly why. You wrote: "My problem is, that the thread is pulled hard by the take-up lever on this Gegenspitze before the thread finally is pulled off the end of the tip." Does it cause any real problem, like skipped stitches, broken thread or similar? Maybe you just need to make a little bit longer throw of the spring before the take-up lever? Have you compared hooks for different subclasses? Find if the differences is only in the gib or also in the other parts.

This is for DA N291, gib looks always long.

Durkopp also names it "Greiferspitze"

[Gymnast]

Many thanks for your good and helpful answers. I will try to comment and answer questions.

This machine is owned by a Fablab (community workshop) in Copenhagen, and it was donated from a previous leather shop. It got the subclass E1 designation, and according to specifications, it is for max thread size of Tex 70 (V69) and 110/18 needle size. The machine is a tripple feed machine, and it was made to sew very fast (4000 stitches/min) within some limits of stitch length and materials. It came with a clutch motor and it was geared for fast sewing.

According to specifications and from my look on them, it is not the best machine for a leather shop, but perhaps they got it from somewhere else. The Fablab intent is to use the machine for light and medium heavy fabrics and up to Tex 135 thread and likely not below Tex 70. So I have given the machine another motor drive, and the speed is now limited to 1100 stich/min. For normal use, the electronics limit the torque on the sewing machine shaft to 3 Nm, but it can be increased to 8 Nm. The Fablab got many users, so the machine will be used for various jobs and settings. The users will need to attend a course before they use it. I should like to know about experience on such kind of sewing machines on this kind of community driven workshops.

This is a table of the subclasses:

This is the drawing and part list of the hook for the Dürkopp 291. It seems like alle the subclasses got this "Gegenspitze":

 

1 hour ago, nejcek74 said:

Durkopp also names it "Greiferspitze"

5 hours ago, Constabulary said:

afaik the "counter hook tip"

A German author, Wilhelm Renters wrote a (to me) very well written 3-volume book on sewing machines in German language. The last edition is from 1957. I think it have been used for education of sewing machine mechanics in German language countries. He named this tip "Gegenspitze", and this term can be found used more places. As you see above, the previous Dürkopp translation is "counter tongue". But I have not found this name elsewhere. I guess "counter hook tip" would be a good name.

 

5 hours ago, Constabulary said:

afaik the "counter hook tip" is meant for thinner thread at higher sewing speed. But I do not know how thin and how fast. So if the problem is solved w/o the tip then take it off al long as it does not cause other problems.

 

2 hours ago, nejcek74 said:

This is explanation from Cerliani:

Thank you very much for this information. I was not aware of this change in the length of the counter hook tip dependent of thread thickness. 
 

9 hours ago, friquant said:

Can you post an up-close slow motion video of the hook area as this is happening?

My usual approach to preventing the hook from catching on the thread when the hook comes around the second time is to retard the hook. It's quite predictable on the two machines I've tried this: Advance the timing and the hook grabs the loop of thread on the second time around. Retard the timing and the loop of thread is too far out in front of the hook point to be grabbed.

Rather than follow factory timing on those two machines, I found my own timing specification that is advanced enough that the thread can be pulled around the bobbin case without too much tightness, but retarded enough that the hook does not grab the thread on its second time around.

See this post for some background: https://leatherworker.net/forum/topic/131646-new-way-to-measure-and-visualize-hook-timing/

 

2 hours ago, nejcek74 said:

Of course you can try and take this part away, but I am not sure exactly why. You wrote: "My problem is, that the thread is pulled hard by the take-up lever on this Gegenspitze before the thread finally is pulled off the end of the tip." Does it cause any real problem, like skipped stitches, broken thread or similar?

Yes, I made this video with my attempt on German language comments  But I hope you get the Idea. A Serafil Tex 135 thread and 140/22 needle is used in video.

https://www.youtube.com/watch?v=5fmdezeTUKk 

The sewing machine with this counter hook tip do work OK with the Tex 135 thread at various test materials, but I don't like the click sound, and sometimes the needle is moved about 1.5 mm to the side due to the high tensions pulled in this tip. I like to educate users to be aware of the sewing machine sound and don't accept something like this for the safety of the machine.

I have tried to advance the timing of the hook, so the thread glides off the tip earlier. The nominal timing position is, that the needle have raised 2.0 mm (5/64 inch) from dead bottom position. The problem disappear when this figure is changed to only 1.4 mm (7/128 inch) from dead bottom position. I tried a timing with 1.7 mm raise, but with this timing I occasionally get skipped stitches. Just now I have removed the counter hook tip, and delayed the hook timing (as you have proposed) so the needle have raised 2.1 mm, and it seems to work with some tests of thread sizes and fabric. Of cause I did also alter the height of the needle bar with these changes.

Thank your for the link to an alternative method for timing of a sewing machine. I shall try it on a few machines and make a comment in this other thread.

2 hours ago, nejcek74 said:

Maybe you just need to make a little bit longer throw of the spring before the take-up lever?

Do you mean the throw of the check spring? Yes, I did try various positions here, and it did not change anything. You are also able to change the thread throw of the take-up lever a bit on this machine, and it did not change this either. 

2 hours ago, nejcek74 said:

Have you compared hooks for different subclasses? Find if the differences is only in the gib or also in the other parts.

I have looked the parts list and drawings I got, and they don't specify drawings of other kinds. But I now looked on the subclasses as you suggest, and for the heavy subclass E20 you got this comment: "With short hook counter-point for very resistant seams (Mit kurzer Greifergegenspitze f.extrem feste Nahtverbant)". The E4 version for not that heavy use, you don't have this comment. So it seems that some shorter Gegenspitze do exist, and I shall try to ask for it.

I have measured the clearance between the lower bobbin case and the hook body, and it is 0.93 mm on the hook I got. This will of cause set some limit to the thread size, because some slack in this clearance is needed as well.

[nejcek74]

Love the Germans 😁 so pedantic and systematic, but they still fail to be on point. In the part list of n291 tha part has name Greiferspitze, but this must be a mistake and your source is right, hook point is hook point and the counter hook point cannot have the name.

Yes, I meant the throw of the check spring. Btw watching your video, your bobbin case opener/thread release finger is not moving, you need to reposition the hook a little bit.

The finger in that opening should open to let the thread through and close after to make the opening on the other side. There are videos about this somewhere around but I cannot find that atm.

 

Edited yesterday at 01:29 PM by nejcek74

[Gymnast]

Thank you @nejcek74 . Yes I agree that the click from the thread passing there cause a too loud sound, and I shall look into the lower part of the bobbin case might in some way bind in this opening or the surfaces there might not be smooth.

Edited yesterday at 03:45 PM by Gymnast

[nejcek74]

There is a screw beside the hook assembly that after untightening allows you to set relative rotation of the hook assembly and proper function of that finger. 

[Gymnast]

Thanks. Yes you are right. I got the service manual, and in section 20 page 46-47 you got a description on how to adjust this.