Key pressure check & adjustment

[rcr27]

What device/gauge is recommended to check the key pressure? (and how?) How should any adjustments be made? My springs (brass I believe) are all in good condition but one particular key has a significantly lower pressure than the others. It’s a Lachenal non-riveted action. 

 

Thanks in advance

Roger

[DDF]

Try popping it on your kitchen scales then if you depress the key it will show you the pressure you are exerting.

[rcr27]

21 minutes ago, DDF said:

Try popping it on your kitchen scales then if you depress the key it will show you the pressure you are exerting.

Thanks, sounds easy, didn’t think about that.

[Chris Ghent]

If you are worried about a single spring and you can feel the difference just bend the top run of the spring up a little to make it stiffer.  Not a sharp bend. Feel it again and adjust accordingly. Note that brass springs do not have a very long life compared to some other materials and the offending spring may be giving up. If it breaks, order a new one and in the meantime shift one from a little used key and put tape across the underside of the springless pad hole so it doesn’t leak while you are waiting. 

[rcr27]

Thanks, all the buttons have a similar pressure (apart from one or two) but I’d like to make sure they’re all even. 

[Geoff Wooff]

I  have a hand  held  spring tension  gauge, which  would have  been used to  check  the strength of   electro /mechanical   relays.  You  can still find them  for  sale, try  amazon.co.uk  and  search   ' dial tension meter'  .  This will bring up  several  very  similar  types;  basically  a mechanical  dial  with a  protruding metal  arm  which  can be  pressed against  the  keys  to  test  how much  downward  force is  needed  to  start moving  the  button.

You'll need one  that  measures  up to   at  least 100 grams  as most people  are  happy with  a pressure  of  60 to 100 g.

 

I  spear  a small  block of  cork  onto  the  measuring  probe, so  as not  to  scratch, or  slip off, the buttons.

 

 

Edited June 27, 2020 by Geoff Wooff

[rcr27]

Thanks that sounds more accurate. 

[d.elliott]

I use a  very basic mechanical dial tension gauge with a dial in gf +/- 2.5 gf. it's range is 0 to 155 gf. I fitted a simple non slip tip made from soft plastic tube. It is not over complicated  but I can at least use it as a comparative measure to identify and rectify overly weak or strong springs.   The easiest way to make basic pressure adjustments to key pressure is to bend the spring to make it hook onto the lever arm closer to, or further away from the arm's pivot post. I agree with Geoff's figures of 60 to 100 gf for most instruments. I am currently working on a 64 key Aeola tenor treble. that is set to 75 gf when the key is in stationary in mid travel, This pressure accommodates pads 18 mm down to the 12 mm, the rest being done by spring position.

 

Dave

[Dana Johnson]

Buying anything for adjusting a single concertina seems a waste unless you are going to pass it on to someone else.  Makers or repairers or people who have a number of instruments they want to all feel the same, might get enough use to make it worth while.  Your brain is a highly sophisticated device for comparing similar things, not so good without training for quantizing things.  To adjust one spring to “feel” like the others just needs you to feel the difference which you have proved you can already do. ( if you can’t feel a difference than you are close enough ) The kitchen scale is a free way to go if you have one and is as accurate as you can possibly feel.  Beyond that, Chris’s approach is a good one to follow and need a new spring.  Some people want to change their whole concertina a button pressure they prefer.  Even there, a kitchen scale will do.  
   When I adjust my instruments, I use a dial gram gage but on the levers at the button position rather than the buttons themselves which are vulnerable to friction in the button bushing.  If buttons feel stiffer after that, I know they are not moving freely and fix that.  
Dana