We weighoff a key to help its note function properly. With good geometry, good regulation, and appropriate friction in place, key balancing fine tunes how well a note will play. Key weights, traditionally made of lead, offset weight and friction on the back side of the balancerail to help the player make music from the front side.
Unless we are installing a new keyset, we inherit the factory leading (with possible technician additions or subtractions). This may or may not help us along. One style of leading spaces key weights across the whole front of the keystick. Some will certainly be in our way.
Assess how your proposed work will affect the existing weighoff by fully regulating at least one natural and one sharp. Confront the impact of friction (verdigris in action centers, for instance), hammers that are too light or too short after filing (see photos below for raising backchecks to catch overly short hammers), and any other factors that have changed since the original weighoff. Decide which parts or materials need to be changed and what geometry might have to be tweaked.
When strategy, validation, and budget-fit have been accomplished, remove lead as needed and plug holes, then bench regulate and weigh off. 20 grams upweight in the bass indexes the balancing: less compromises repetition, more makes the heaviest notes heavier than they need to be. Remove lead from any key that cannot lift 20g in the bass and graduate the testing to higher minimums for the rest of the notes. I like to remove as few leads as possible to save cost of plugging but clear as much space as possible to facilitate placement. The hope is to do the removal in one pass ahead of regulating and then, determining best specs, add the new weighoff's key weights (I now use copper) in one pass.
Two methods of weighoff offer very even results for the player, one producing consistent downweight and the other consistent balanceweight, both with incremental upweights that match their downweights in speed. The Grandwork Weighoff Kit makes selection and implementation of either method easy. Align upweights in the kit's front slot with pairing weights in the middle slot. Select pairing weights that descend incrementally bass-to-treble by 1g each for consistent downweight or by 2g each for consistent balanceweight. Front row upweights ascend in 1g increments from 20g in the bass to the desired treble maximum. Slide the front row below the middle row to display the best match, generally the one with the minimum upweight matching the pairing weight that results in speeds of downweight and upweight being equal, with the upweight lifted all the way up and it’s hammer entirely returned to its rest position. Test multiple notes by trial-and-error in the bass to quickly optimize the matching. Then, place the selected pairing weights on top of their matching upweights. As you proceed, notes with slower or faster symptoms than their neighbors will have more or less friction, respectively, and can be corrected.
I recommend using consistency of downweight for actions that come with a consistent downweight. Their geometry will be set up to support this configuration. Changing weighoff method might have plusses but it will likely cost more, a negative for you, and change the playability characteristics of the piano, a potential negative for your customer!
For most actions (heavier in the bass and lighter in the treble), use the balanceweight consistency method. Balanceweight is the halfway point between upweight and downweight. If you find that a bass key lifts 20g at the same speed as it’s depressed by 54g, the balanceweight will be 37g (17g less than 54g and 17g more than 20g). Expressed mathematically, it's (54g + 20g) ÷ 2 = 37g (downweight plus upweight divided by 2 equals balanceweight). If you place 37g at the front of the key, it will stay horizontally balanced without touching frontrail or backrail.
Given a fixed minimum upweight in the bass, the higher the balanceweight, the more friction there will be in the action. Ideal friction in all bushings will render balanceweights between 35g and 38g. How fast your tests rise and fall affects this calibration, of course. 20g rising quickly might be the equivalent of 21g or 22g rising more slowly. In the high treble, 25g upweight rising quickly might translate to 30g rising slowly. I like fairly quick-moving symptoms because they offer a safety margin for changes in weight and friction that may happen later and they are easy to read.
The amount of friction in the example above is calculated by (54g - 20g) ÷ 2 = 17g. The details of what this means are complicated, but experience tells us that friction added to the ideal has a cost in repetition and friction subtracted from the ideal has a cost in both stability and the player’s sense of control. Where the ideal spot lies (i.e., the best balanceweight) can be found in the middle by trial and error.
Inertia may be more difficult to optimize. Lots of lead in the keys means lots of mass to set in motion, so even though your upweight and downweight work fine, starting and stopping each note will take more effort, especially when playing louder. Hammers that are too heavy (or are effectively too heavy because of geometry) cause this problem. Regulate samples and try variations of what can be done to find a solution. You will be limited by elements that cannot be changed and elements that are too expensive to change. But improvement can generally be made, even if it is only to make what you’re stuck with more consistent. A good weighoff helps.
Pragmatics dynamically influence our work. Form a hierarchy of influencers and pay most attention to those at the top. If your hammer friction is less than 6 half-swings, you will have trouble. If more than 13 half-swings, you may also have trouble. If you have 33 to 55 half-swings, as may be found in certain reputable pianos, huge improvements can be made, and these improvements include tone as well as playability. But beware of factors hidden by the loose pinning – they need to be assessed ahead of time and accounted for.
Finding the best geometry has similar limitations. Move a knuckle or a capstan too far to make things lighter and you will have too short a blow distance or find that sharps have to be too high and dip too deep. The photo above shows a capstan move that stayed away from the filled hole of the original but reduced the effective change by converting its angle to vertical (which also matches the existing replacement whippens that have straight heels). With the whippen rail slots ground to expand an overly short spread distance (209mm to 212mm), regulation samples validated that the new configuration would produce standard specs and bring downweight in the bass down from 75g to 54g with a balanceweight of 37g.
The best manufacturers live with these constraints and each piano we work on teaches features of the hierarchy and variations on what to do when things are out of balance. Sensible weighoff is where your solutions can at least be so consistent that your customer will be happy and your work will have justified their investment.