Some thoughts on jack design

by admin on May 13, 2015

There are a few fundamental details that need to be considered regarding jack design. Minimising as much as possible friction and minimise the travel of all moving parts.http://michaeljohnsonharpsichords.com/wp-admin/post.php?post=419&action=edit

Showing how I limit the friction area of jack blank in the guides

Showing how I limit the friction area of jack blank in the guides

The guides that control the actual jack are helped if all touching areas of the jack are reduced to an area as small as possible and the working areas of both jack and guides must be finished to a very high quality. The finishing preferably achieved honing from a steel plane blade or a very sharp cutter, never sanded; sandpaper is death to such action parts!

The face side of the three jacks

The face side of the three jacks

Mass and size will vary from instrument to instrument but if friction is very low through the guides, lighter mass from a small jack has a better chance of returning fast and its damper working. Rigidity is another important factor; flex should be avoided as much as possible and that gives wooden jacks a major advantage over plastic!

Also guides (some refer to those as racks) will be better made in wood because the jack movement within the rack will be less noisy and there will be better control over friction! Some prefer to cover the top of upper guides with leather to reduce noise; however I have found very accurate hand work when fitting the jacks in the guide mortise can be given very fine tolerances making for a much better plectra control at the final working stage. You will need to be a little more generous with leather and that play will require allowance in plectra length.

The rear view of the tongue

The rear view of the tongue

The tongue is also a very important, if not the most important part! There needs again to be total rigidity, the material needs to such that it holds the plectra positively and length is a critical factor for balance and travel! Again it is a major advantage to keep the travel down to the minimum.

This shows how tongue placement builds in the stagger

This shows how tongue placement builds in the stagger

This shows how tongue placement builds in the stagger

This shows how tongue placement builds in the stagger

The longer you physically make that part the further it will travel at the bottom of the tongue where it beds into the blank and the top of the tongue going through the escapement. Two points that need control over noise!

Materials can and do vary from makers to makers and schools and historic periods but that is not really the point of this discussion. I have over many years used pear for the jack blank with hornbeam or holly for tongues. Over the last fifteen years or so I have found preference using steamed beech for jack blanks finding it a little more ridged than pear and have a slight preference for holly tongues. For top guides I have always used the same material as the jack blank but bottom guides work better in a softer wood such as lime of poplar.

A side view of a Johnson tongue

A side view of a Johnson tongue

The tongue pivot pin is probably the most critical detail in design! If you get the position too low you will get a clacking noise at the point of pluck and excessive activity of the tongue immediately after the pluck. Too high and you will lose control at the pickup of string stage, it has to be just right. This is best done by experiment as indeed there are other factors that can and do affect this, such as string tension from the design of the instrument. Also of course the angle and the way you have effectively positioned your return spring, usually hog bristle or peek but brass is used in some cases. We must assume you have got the return spring angle correct so the following concentration is on stabilising the tongue.

Stabilising is a good word to use for what happens at this stage. On contact with the string there must no movement from the tongue at all, throughout the pickup and flex of the plectra curve to the point of release the tongue must still remain perfectly still. That will not happen if the pin position is even a fraction out and energy and tone focus will be impaired. Of course you can, and indeed some do, control this with a strong return spring but that is bad design and prone to lead to jack hangers. The spring tension can and indeed should be very light if the pin position is correct! It is also a great help to have a slight curve upward of about 5 degree for the plectra mortise, this will also have a beneficial effect on the plectra curve shape at the pluck.

Showing the slight upward angle of plectra through its mortise

Showing the slight upward angle of plectra through its mortise

The escapement is also critical only because there is bound to be some flap back as the returning plectra touches a vibrating string fractionally before the damper has its effect. Again if the tongue is kept to the minimum require length that travel will not be excessive and a back stop will control that. I use a cotton tie but many use a staple, they require some padding to silence them and I personally hate any form of packing!

For forty years or so I have used a tongue length of 30mm and I set my plectra mortise 8mm down from the top of the tongue. That gives me 22mm space to the bottom of the tongue and the axel pin has to be positioned and calculated within that distance. What goes on above the plectra on the tongue has no effect at all on this operation other than if you have a long tongue, then balance could come into play. On my instruments I find that the point of pivot is about 10mm from the bottom of the tongue leaving 12mm to the plectra but I always make trials and adjust the small fractions within a centre point until I get the best result. Another useful tip is to square the bottom of the tongue so it beds almost parallel with the axle pin, it helps for a more silent escapement.

The photos show my jack design but the principal can be adapted to any design. A maker once remarked he thought my tongues were the most ugly he had seen. Could well be true but a jack is a mechanical action part that has to be as sensitive and articulate as the players finger, for it is their total contact with the string through the keyboard. And keyboards are another story!!!!

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