Friction is a necessary and useful component of piano action mechanics. Too much causes trouble in one direction and too little, trouble in the other. The best balance optimizes performance and the player's control of tone. How much is just right and how is it measured? I have found that for the sum of keys and top action, 17 grams per note gives the ideal balance between control and freedom, stability and repetition. The fastest and most effective way to observe and calculate this sum employs David Stanwood's concept of balanceweight.
The balanceweight for a key is just what its name implies: the weight that when added to the key balances front and back. The closer to no friction there is in the system the more sensitive this balance will be. With the balanceweight perched at the front of the key, additional weight on either side of the balancerail fulcrum will unbalance it in that direction.
The more friction in the system, the more inconsistent will be the observable outcome. And with enough extra friction, the depressed key won't lift itself, never mind the 20 grams needed for good repetition. Extra friction simultaneously reduces the maximum upweight a key can lift and increases by the same amount the minimum downweight needed to set it in motion.
Too little friction makes the action feel "fly-away", hard for the player to sense where they are and hard for them to control what they're playing. Too much friction hampers repetition and again makes the action hard to control. For our purposes of bedding and sampling, too little friction is only a problem if spacing in our samples keeps changing. Too much, however, obstructs measuring and fogs our view of what's going on.
We need a well-bedded keyframe and stable samples of keyframe shape, key level, and strike. If these four factors are in place and maintain their relationships from piano to bench, regulating on the bench will be as if in the piano - except easier.
For bedding and leveling samples, we need backrail and balancerail to share weight of parts at rest and keys to consistently return to the same height. If key bushings or balance holes are too tight, key level will be inconsistent. If hammer or wippen flanges are too tight, overall weight on the keyframe will be the same but that transferred through capstans to keys will be impacted, again making key reset inconsistent.
A note's ideal total friction in a well-designed action will be 15-17 grams, making a balanceweight of 35-37 grams if upweight in the bass is 20 grams. So, in the bass, we add weight to one side or the other of the balancerail until the key lifts 20 grams at a moderate speed and then try downweights until we find the one that depresses the key at that same speed. Halfway between these two is the balanceweight and how far from either end to the middle is that note's friction. So, if a key lifts 20 grams at a modest rate and is depressed at the same rate by 54 grams, 37 grams is the balanceweight and 17 grams the friction.
When preparing parts for regulation, we will want to be specific in where the increments of this friction lie, but for reliable bedding and sampling, we just need the parts we're using to be unencumbered. Spacing must be addressed if anything rubs against a sample. If there is verdigris, we need to assure that each sample note's centers will not obstruct. The household cleaner Fantastik works well, both for this short-term goal and for a long-term improvement (thanks PTG Connecticut Chapter - see April 2022 issue of PTG Journal). If keys need easing, take the action apart and do them properly. For balance and front rail bushings, doing them all with a heated caul takes only slightly longer than solving just the samples. Balance hole refinements, however, can be done globally later. We just need the samples to be free.