Springs and leakage?

[chrisstevens]

Recently replaced the pads on an old Lachenal anglo, and the instrument is leaking a touch more than I'd like. The bellows are tight, no cracks anywhere. Pads are aligned properly. Using a strong light and looking from the bottom of the action board, I can just barely see some "muted" light coming through the holes, as if the pads are not seating fully. I constructed my pads using chamois/felt/matboard. Should I replace the springs with a stiffer material? They are the original Lachenal springs, made of what looks like brass. Thank you

[Geoff Wooff]

The chamois could be your problem. It is not IMO an 'airtight' material. Try to find a thin Pneumatic hide for the pad faces. You could strengthen the springs but that might make the 'action' too heavy.

[chrisstevens]

Thanks Geoff. Anyone else care to weigh in?

[Theo]

Thanks Geoff. Anyone else care to weigh in?

 

Agree about the chamois, its not the right material for the job. If the pads are "aligned properly" then I don't understand how you can see light round the edges, even with a chamois facing. Pads can be pushed out of alignment when you fit the top plate and the buttons move slightly. How big are the pads? You may need a larger size. If there is space I would want pads to be 3 to 4mm bigger than the diameter of the hole.

 

Make yourself a pad checker as described by Bob Tedrow, then you can determine which pads are sealing and which are not.

[Frank Edgley]

Thanks Geoff. Anyone else care to weigh in?

 

Agree about the chamois, its not the right material for the job. If the pads are "aligned properly" then I don't understand how you can see light round the edges, even with a chamois facing. Pads can be pushed out of alignment when you fit the top plate and the buttons move slightly. How big are the pads? You may need a larger size. If there is space I would want pads to be 3 to 4mm bigger than the diameter of the hole.

 

Make yourself a pad checker as described by Bob Tedrow, then you can determine which pads are sealing and which are not.

 

 

I suspect the pads are not seating properly. Try reinstalling the pads. First glue a small leather circle onto the centre of each pad. This is to provide some flexibility and enable the pad to seat itself properly. Then make sure the leather nut on the end of the mechanism arm is glued onto the small leather circle, and that the whole thing is centred on the vent hole.

Lachenal springs were also a bit stronger than what is usually used nowadays. The spring not only had to be strong enough to seal the pad, but also had to provide enough pressure to hold the arm up tightly against the fulcrum. It seems to me that the spring placement was affected by the second need, and resulted in the spring a bit closer to the fulcrum. This necessitated a stronger spring to provide sufficient closing pressure to the pad.

[chrisstevens]

Thanks lads.

 

Already fitted the leather dots and leather grommets. The pads are plenty big, same size as the old ones.

 

Will check the seal using Bob's tool. Short of measuring the actual weight on each spring, is there a good way to test if I have sufficient pressure on all springs to seal the pads properly?

 

 

 

Thanks Geoff. Anyone else care to weigh in?

 

Agree about the chamois, its not the right material for the job. If the pads are "aligned properly" then I don't understand how you can see light round the edges, even with a chamois facing. Pads can be pushed out of alignment when you fit the top plate and the buttons move slightly. How big are the pads? You may need a larger size. If there is space I would want pads to be 3 to 4mm bigger than the diameter of the hole.

 

Make yourself a pad checker as described by Bob Tedrow, then you can determine which pads are sealing and which are not.

 

 

I suspect the pads are not seating properly. Try reinstalling the pads. First glue a small leather circle onto the centre of each pad. This is to provide some flexibility and enable the pad to seat itself properly. Then make sure the leather nut on the end of the mechanism arm is glued onto the small leather circle, and that the whole thing is centred on the vent hole.

Lachenal springs were also a bit stronger than what is usually used nowadays. The spring not only had to be strong enough to seal the pad, but also had to provide enough pressure to hold the arm up tightly against the fulcrum. It seems to me that the spring placement was affected by the second need, and resulted in the spring a bit closer to the fulcrum. This necessitated a stronger spring to provide sufficient closing pressure to the pad.

[JimLucas]

Short of measuring the actual weight on each spring, is there a good way to test if I have sufficient pressure on all springs to seal the pads properly?

Very subjective, but for each pad cover the hole with your mouth and blow gently. There should be no leakage.

 

How much or how little pressure is needed to force the pad open against the force of its spring should give you an idea as to whether there's a problem, especially if there are significant differences from pad to pad.

 

Also try sucking gently. That should pull the pad tighter, so if there's any air flow at all, then that particular pad isn't seating properly.

[Ardie]

Will check the seal using Bob's tool. Short of measuring the actual weight on each spring, is there a good way to test if I have sufficient pressure on all springs to seal the pads properly?

 

Using the said tool or blowing/sucking directly over the holes is rather difficult or vague in reality. If the instrument is well constructed initially the spring force ought to be sufficient if the button resistance ("weight") is the same all over the keyboard and roughly set at the highest value ( usually 50-90g) you get from measuring. To know for sure if the pads seal properly you have to test each one individually while blocking the others with the instrument assembled and "played".

[Frank Edgley]

Bob Tedrow's "pad checker' works quite well, and will show you the problem pads well enough. With an English concertina, Checking each pad individually, by disassembling & reassembling it for each pad would result in 48 disassemblies and 48 assemblies times 12 endbolts. (if you're lucky enough to fix each difficiency th first attempt). In other words 1152 removal and replacement of endbolts. This is impractical, and puts way too much wear on what are essentially fragile and worn endbolts (with vintage instruments), as well as slipped screwdriver marks on endgrills and fatigued fingers & wrists. No, Bob's "pad checker" is quite sufficient.

[Ardie]

Bob Tedrow's "pad checker' works quite well, and will show you the problem pads well enough. With an English concertina, Checking each pad individually, by disassembling & reassembling it for each pad would result in 48 disassemblies and 48 assemblies times 12 endbolts. (if you're lucky enough to fix each difficiency th first attempt). In other words 1152 removal and replacement of endbolts.... No, Bob's "pad checker" is quite sufficient.

 

The method may work fairly OK to find out if the pads seal on pull when playing - by sucking the tool. It does not work however to find minor leaks on push when playing - by blowing the tool - and this is what we are dealing with when asking whether "springs seal the pads properly"according to this:

 

Will check the seal using Bob's tool. Short of measuring the actual weight on each spring, is there a good way to test if I have sufficient pressure on all springs to seal the pads properly?

 

As I said:"Using the said tool or blowing/sucking directly over the holes is rather difficult or vague in reality". Simply because you can not detect the relatively small change of the air stream and first of all you can not *measure* the spring pressure needed to keep the pad tight.By just blowing/sucking with your lips directly over the holes without the tool or something alike it is impossible either way.

 

Whether the "springs seal the pads properly" can be a really intricate question with instruments having much varying sizes of pad holes -as with baritones or extended wide range models - since the pressure on the pads will vary a lot and the large pads may leak on forced push without being noticed by some obviously sounding note. Anglos often have the same hole size all over which makes things easier.

 

So - there is no other way than to know that the spring force generally is OK and if necessary check possible leaks individually as I said before. If this procedure is done systematically it is a bit tedious but not that demanding at all.

[Ardie]

Lachenal springs were also a bit stronger than what is usually used nowadays. The spring not only had to be strong enough to seal the pad, but also had to provide enough pressure to hold the arm up tightly against the fulcrum. It seems to me that the spring placement was affected by the second need, and resulted in the spring a bit closer to the fulcrum. This necessitated a stronger spring to provide sufficient closing pressure to the pad.

 

I haven't managed to find much support for that according to this:

- When comparing some Lachenal and Wheatstone trebles the springs have roughly the same strength and the leverage ( the location of the springs) is about the the same too.

- Comparable Wheatstone English models - earlier ones with riveted action and later ones with a saddle connection ( "pivot action" similar to Lachenal action) have the same spring strength and leverage as well

- Some Lachenal baritones with long levers where the springs are located closer to the fulcrum show no regularity in that respect. The variation of the spring locations seems to have practical rather than mechanical reasons since the springs seem to be located at the best position for assembly and not to interfere with other levers or buttons.

- Theoretically the spring normally does not need to be extra strong to keep the lever in place ( i e with the common construction and a button resistance 50g - 90g )

Edited April 27, 2011 by Ardie

[Theo]

Bob's "pad checker" is quite sufficient.

 

Indeed it is. It allows one to check the pads very quickly and although in theory it may be imprecise, in practise it will quickly show up any pads that are leaking. It does assume that the majority of pads are sealing effectively, but this is usually the case when a new set of pads has been fitted.

Edited April 25, 2011 by Theo