Landeron 248 hammer and cam interaction

[Klassiker]

I am currently working on a Landeron 248 and have the watch reassembled and basically working OK apart from one major issue. A check of the chronograph functions showed the seconds hand reset sometimes returning to zero, and sometimes to a different place on the dial entirely.

I checked the interaction of the hammer with the heart-shaped cam, and sure enough the hammer sometimes slips off the cam profile on impact. The result is the undefined reset position.

I tried increasing the tension on the runner friction spring, which has improved things slightly, but is not a 100% solution. Any suggestions what to check or try next? 

[nickelsilver]

The faces of the hammer have to be flat and "square". Also, the two shouldered screws that hold the hammer in place must provide freedom, but not too much. There really shouldn't be enough room above the cam, even with the chrono runner wheel pushed down, for the hammer to be able to ride up over the cam. Are you sure the bridge is fully seated?

[Delgetti]

Side-view-pic would help. Like this:

[Jon]

1 hour ago, Klassiker said:

I checked the interaction of the hammer with the heart-shaped cam, and sure enough the hammer sometimes slips off the cam profile on impact.

When you say the end of the hammer slips off the heart, do you mean the hammer rides up and touches the bridge, or rides down and touches the chrono wheel?

I've posted a couple of picks of a 248 which shows the hammer is a little high, but there is clearly a gap between the top of the hammer and the underside of the bridge, as you can see in the pic. The action of the chrono sweep second hand is perfect

If the hammer is bent, you'll need to straighten it, so it hits the hearts correctly

 

[Klassiker]

Hi Jon, thanks for the quick reply. I mean the hammer rides up and touches the bridge, or maybe it doesn't quite touch the bridge, but it rests between the bridge and the cam. I can`t match the quality of your pictures I`m afraid, but I hope this shows it clearly. It is most likely to happen when the hand is at around 30s, so when the hammer strikes near the tip of the heart.

[nevenbekriev]

You have to check if the post on which the hammer sits and is pivoted has not untightened. It is on thread and often gets loose and thus lets the hammer have significant free play. The bearing on the hammer also can get loose and has to be checked for tightening, but I don't remember if in this movement it is on thread or not.

If there is no problem there, You must take care that the hammer falls on the heart not too high above the wheel surface and in the same time the wheel not to be able to go down as much as to be enough free place between the heart and the cock for the hammer to get in between. What I mean is if the cock is lifted up (like it seems looking at the picture) in order to place the wheel higher, it will not help as the wheel will go down easy. The axial free play of the wheel should not alow the hammer to get above the heart. So if the hammer hits the heart not in place, it must be solved by changing the hammer position rather than the wheel position. It can be achieved by bending the hammer or the post on which it sits, but both lead to risk of break. If such correction needed, I would rather anneal the hammer localy close to the bearing and polish it to clean the blued surface after.

 

PS Now I see that what i have written about the post is not correct for this calibre, as the hammer has different bearing type here.  But no mater, the hammer should not have significant free play

Edited April 11 by nevenbekriev

[Jon]

The 248 hammer doesn't sit on a post. There is a shouldered screw that allows the movement of the hammer once it is fully tightened.

How far up does the chrono wheel sit towards the jewel without the hammer pushing on it? If it sits too low and there is a slight bend to the hammer, add those two together and there's a recipe for disaster.

I still think it's the hammer that needs bringing back to flat. What does the end of the hammer position look like when 'return to zero' and no chronograph runner in place; is it still too high?

[Klassiker]

Thanks @nevenbekriev and @Jon. I will do the checks you suggested at the weekend and get back to you.

I didn't notice the problem when I checked the functions before disassembly, so I was convinced that I'd put something out of adjustment. It seems not, but I want to keep an open mind about that.

From a movement design point of view, is there a reason for the large amount of axial travel available to the runner? The friction spring resists the movement, but doesn't prevent it. @nickelsilver one of my first thoughts was that the bridge wasn't seated properly, so I have checked it at least three times already, but I will do it again! It's surprising how often I notice something new when an expert tells me where to look. Both you and @nevenbekriev have suggested that the space between the bridge and the cam should not be available. I think I need to investigate this very carefully on Saturday.

[nevenbekriev]

As the hammer is fixed in place by two shouldered screws, try to switch the places of the screws, as they can be a little different.

[Jon]

The two screws that hold the hammer down should be the same size, but that's not to say they are correct and they haven't been changed. The hammer does sit on a post. I initially said that it didn't, my bad! The two screws just keep it on the post and allow the movement

[Klassiker]

Once again, thanks to everyone who has offered advice so far. I read all your replies over again and made myself a checklist. This is what I found out today.

1. Faces of hammer - should be flat and square. I inspected the face, and it is slightly curved and not square. I don't have the experience to know if this is the lone cause, a contributing factor, or nothing to worry about:

 

2. Screws: should allow movement but not too much. The screws allow movement, and as far as I can tell, the freeplay is minimal.

3. The vertical gap between the cam and the bridge when the runner is pushed down should not be enough for the hammer to enter. Is the bridge seated correctly? I checked this again. The bridge is seating down tight on the mainplate, and the mating surfaces are clean and undamaged.

4. Check the post is tight in the mainplate. So, I'm not sure what is meant by the post, but the hammer assembly pivots on a kind of domed stud. This is secure and doesn't wobble. There is no discernible play between this stud and the hammer assembly when the two shouldered fixing screws are tight.

5. Check the vertical gap between the cam and the bridge with and without the runner. I did this and as long as the hammer is sitting on the cam, the gap to the bridge is the same as when the runner is not present.

6. Does swapping the fixing screws make a difference. The two fixing screws are visually identical, and swapping them makes no difference.

[nevenbekriev]

40 minutes ago, Klassiker said:

4. Check the post is tight in the mainplate. So, I'm not sure what is meant by the post, but the hammer assembly pivots on a kind of domed stud. This is secure and doesn't wobble. There is no discernible play between this stud and the hammer assembly when the two shouldered fixing screws are tight.

Well, I ment this kind of post in this kind of movement

This was because I didn't see the movement first and the problem described  is often found in such type movements...

The angle pointed on the picture is a contributing factor.

I would try to put something like a small washer under the wheel to limit the vertical free play by forcing the heart stay closer to the bridge. Will this make the resetting reliable? If such solution chosen, then take care to make the friction spring under the wheel work normally regardles of presence of the washer

 

[Delgetti]

On 4/10/2024 at 8:46 PM, Klassiker said:

 

On this picture the surface of the heart looks quite damaged? Or are these mirror effects or grease/oil?

 

[Klassiker]

1 hour ago, Delgetti said:

the surface of the heart looks quite damaged

I assumed it was grease, but I will investigate, thank you!

[Jon]

You may find with the hammer end being that profile is naturally pushing the chronograph wheel down and getting into a gap between the top of the chrono runner and the bottom of the bridge because it is like a knife edge. You may want to correct the shape of the end of the hammer and check the tension of the chronograph runner tension spring pushing the runner up towards the chrono bridge.

Ideally the hammer should be flat on the end, but a slight rounded shape won't hurt. I've seen some right rounded ends and they work fine. Focus on stoning the end down so it hasn't got that gap/angle when put against a flat surface like you've shown.

Here's a lesson I just posted regarding the chrono work on the Landeron 248, especially looking at the chrono tension spring and its setting and amplitude difference from the chrono running to not running.

Unfortunately it's a PDF so won't show the vids, but there is a link to the PowerPoint that will

 

[Klassiker]

An update, for everyone who contributed advice, and for those who come after with a similar problem.

Based on the answers received, I decided to work on the face of the hammer first. I used a square degussit stone to guarantee a vertical surface to work against, and ground the face back until it was square across 90% of the depth. I was conscious of the risk of removing too much material.* After I'd got the shape how I wanted it, I polished the surface with lapping film. To cut a long story short, it did the trick and the hammer hasn't slipped off the cam since.

Of course, that wasn't the end of my problems. Have a look at this video and tell me what you think is wrong. https://youtu.be/sgAUMIPaw98 The first four attempts show (0 to 34 sec.) the chrono seconds hand jumping forwards, the next two attempts (35 to 47 sec.) seem "normal", then on the seventh attempt (48 to 54 sec.) the seconds hand jumps to 5 sec. and the minute counter jumps to 1. The rest of the video just shows repeats of these three variants.

I solved it by rotating the minute counter finger on the chronograph (seconds) runner relative to the cam.  I'd be interested to hear your opinions on that. It seemed to be the right thing to do, but maybe I've introduced another problem I'm not aware of.

* What is the correct relationship between the two hammers and cams, by the way? Should both hammers strike the cams exactly at the same time, or is it correct for the minute counter hammer to be a bit behind the seconds hammer? In this picture, I removed the adjusting screw at 1, and the hammers are contacting the cams simultaneously at 3 and 4. I had to turn the screw down tight to achieve this condition after stoning the seconds hammer and replacing the bridge.

Edited April 20 by Klassiker
Details of the video added

[nevenbekriev]

See, what has happen is a normal consequece of the reducing the hammer size and changing it's shape by the removing metal from it. But here the hammer is adjustable and just adjustment is needed to correct, and this is what You have done. There is a rule for the adjustment and it is that the hammer must lay firmly on the seconds counter heart and there should be a litle free play in the same time of the minutes counter heart/hammer which alows counter movement of about 0.5 min on the small dial hand (+/- 0.25). Of course, there is no way to make one hammer to delay from the other, as they are one single part. What has changed too is the slope of the hammer head and thus the orientation of the heart has changed, and thus the switching finger position. This led to need of the finger position correction. The rule here is that switching of the minute counter has to start when the seconds counter hand is on '59'. Of course, the seconds hand must be positioned as so the resetting is at '0' exactly. If switching is earlier than 59, there will be no problem, but it will be wrong as reading can be not correct. If the switching starts later, the problems that You described can happen.

[Klassiker]

@nevenbekrievVery reassuring, thanks! The minutes counter starts to move at 58 and is finished at 00. If I stop at 59, the minute counter is in between the markers. To me, that's a clear reading so I'm satisfied.