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Brain surgery is bringing the brain up to spec by testing, adjusting, and repairing it, or modifying it for a purpose. The big difference is that we are as not certain what the purpose is.

But for the piano action the specs are published, or at least easily discoverable.

Supposedly, so are the brain's.

Published? Yes, in many incomplete and incompatible versions. Discoverable? Maybe, over a space of many years. Required tools available? Well, I have a hacksaw and a Swiss Army knife. But you better whack me with the tuning hammer first.

I'm not sure I've really made my OP clear yet in terms of what my question was about so let me rephrase it. I have read that Renner builds actions to the maker's specifications.

Does this mean that the basic design for all grand actions is the same but there are just minute differences in the sizes of various parts?

If the answer to that question is "yes", what determines the sizes of those parts?

If the answer is "no", what kinds of objectives determine the more basic differences in action design?

To answer just this particle of your question, it is the ratios and equations mentioned above.

The biggest difference of those criteria is the size of the piano. In an upright, it would be the distance from the keybed to the strike point, and similar considerations. In a grand, it would be from the keybed to the strings, as well as the location from the front of the keyboard to the strike point and the damper line.

It works both ways, as those measurements are dictated by what is possible with the action design.

What you're really asking for is a book on the subject of piano and action design. But, yes, if the piano maker is using a “Renner” action the basic working parts—the wippen and the hammershank—will be the same or, at least, very similar. (As will the parts of the many in-house action makers that have essentially cloned the Renner action.)

As I mentioned earlier, there may variations in the exact location of the hammershank knuckle along the length of the hammershank and in the distance between the hammershank center and the c/l of the hammer. Placing the knuckle closer to the action center theoretically can increase hammer velocity (for a given amount of wippen rotation) but also increases stress on the action center bushing and increases hammershank flex. Heavier hammers seem to dictate moving the knuckle some away from the action center to reduce the stress on the bushing and to reduce hammershank flex.

There may also be differences in how the repetition lever spring is adjusted.

And there will almost certainly be differences in the height of the capstan block and where it is located along the bottom surface of the main wippen rail. This will be a function of where the contact point between the capstan and the capstan block lies and this will be a function of the horizontal and the vertical offset between the key balance point and the wippen flange action center.

There may even be differences in the action bracket style and the action rails—I still prefer the laminated wood rail furnished by Renner—but the relative locations of the hammershank centers and the wippen flange centers will stay essentially the same for all Renner-style actions.

Some of these variations are interactive—giving a bit on one requires taking a bit from others—while others are necessary because of variations in how the piano itself is designed. For example, the height of the capstan block will be determined primarily by the distance between the top of the keybed and the bottom of the string plane and by the design of the height of the keyframe and the height of the keys. It will be a balance between all of these and the bore distance of the hammers.

Some makers set the keybed rather far below the string plane, others—like Steinway—set it some on the shallow side. Both have their advantages and disadvantages. Steinways approach minimizes the hammer bore distance and action mass—both are advantages, I think—but makes regulating things like the capstan a bit more difficult. It also requires a well-made keyframe and careful attention to overall action height; the distance between the top of the keybed and the bottom of the pinblock is only about 155 mm (don’t quote me, it’s been a while since I measured.) The other extreme leaves lots of room between the keybed and the pinblock making servicing easier but increasing the mass. Everything is a compromise.

ddf

For reference I measured from the top of the keybed to the string plane on two grands:

Baldwin: 195mm
Hardman: 215mm

The moment one removes these two actions, it is apparent how much smaller the Baldwin action is.

I found it interesting that the biggest difference I found between these actions was the distance between the capstan and the key balance point. If I remember right the difference was about a full inch (shorter on the Hardman). Of course this was made up for in other ways in terms of the action geometry; I think the wippen heel block was located differently in relation to the action center. The shanks are also significantly longer on the Hardman (not just a little bit: something like a half inch). And the hammer bore distance is also significantly different; again, not just one or two millimeters.

For the non-technician, I mention these to illustrate how much these dimensions can vary. I was surprised when I compared the pianos.

Yes, they feel very different. But since hammer travel is comparable and key travel is comparable, as one can see also in Del's first post, the range of possible OAR's is quite limited.

I have read somewhere else that it is common to position the hammers this way to the string that the 7th harmonic is surpressed.
That would mean the hammers are on 1/7 stringlength.

This should also somehow dictate the lenght of the action.

By how much was the capstan block moved?

This myth has been around for decades (at least). No one quite seems to be able to determine where it started; some say Helmholtz but Helmholtz never built pianos. Others say Broadwood; but so far as I know Broadwood never built a piano with this strike point configuration.

As may be, the hammer strike point along the speaking length of the key varies from approximately 1/14th to 1/20th (depending on which designer or company you are talking to—and this is a subject for a whole other article) at C-88 more or less linearly to approximately 1/8th somewhere around C-40 (give or take a half-octave or so) and then remains somewhere around 1/8th down through the bass.

If this all sounds somewhat vague that’s because piano makers have treated the hammer strike point in a somewhat vague and cavalier manner over the years.

The exact ratio and/or numbers aside, your point is well taken. I did allude to this in my response to hpeterh but it probably could have been more clearly written. The hammer strike point along the string is a more or less fixed ratio. Longer pianos generally have longer strings so the hammer strike point will be further away from the front of the piano; hence, the keys will have to be physically longer.

ddf

Uh, you want this from memory? You have a lot of confidence in my little gray cells!

I don’t remember exactly. I do have records but it would take me some time to look them up. But (from what remains of my memory) it was moved toward the wippen flange somewhere in the neighborhood of 15 mm; give or take a couple. The height of the capstan block also changed to keep the capstan/block contact point on line.

I also don’t remember the length of the front key lever arm but this was about a 5’ 2” grand and I do remember finding it was about the same a another 5’ 7” or 5’ 8” grand we had in the shop at the time.

I used fairly light Ronsen/Wurzen hammers—what he calls 14 lb—and tapered the sides down per my usual practice so there wasn’t a whole lot of mass up there. I ended up with a hammer travel of 45 mm and a key travel of 9.5 mm with normal hammer letoff and key aftertouch. The action played quite nicely when it was all finished up. One of the nicer small grand pianos around. Certainly as modified—the action was not the only part of the piano that was modified—it compares quite favorably with anything even close to its size on the market today. Price notwithstanding.

ddf

I updated my image for better clarity:


More images are here:
https://www.pianoworld.com/forum/ubb...s%20compared%20-updated.html#Post1545456

That does illustrate the principle but, while those force proportions might be accurate for keys of very short lengths, they will not be nearly that extreme in pianos with longer keys.

Also, wear patterns on real-world keys show that most playing takes place in the forward 100 mm or so of the keys length. It is not often that the finger goes all the way back to the end of the tail. It happens but, when it does the key is generally being played with such force that the change is not noticed. (And, no, I don’t know how to explain all the keycovers that get chewed up by pianist’s fingernails. Very few pianists admit to ever hitting the keycover yet there are all those marks….)
ddf

Why is it that piano manufacturers don't move the key cover back 10 mm to avoid this? Or even just carve out a slot near the bottom, if they don't want to move the whole thing...

At about 1:00 it is visible that this Petrof Grand has such long keys:


I just want to show the limits.
My final conclusion is, if you look to my other thread, that almost all digitals have shorter keys than most uprights.
https://www.pianoworld.com/forum/ubb...s%20compared%20-updated.html#Post1545456
But the advertisement compares them to grands....