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The New Touchweight Metrology

By David C. Stanwood, RPT

Boston PTG Chapter

**Introduction**

As piano builders and rebuilders, we have inherited
a crude and archaic system for measuring the balance of

the action mechanism. The weight of the hammer,
which sits out on the end of a long lever arm and has

such tremendous influence on touch and tone, is
measured in weight to the nearest pound of a sheet of felt

from which many sets of hammers are made.
We assume the proportion of key to hammer movement is

roughly 1:5, but have no reasonable means for accurately
measuring this ratio or detecting leverage

problems. The keys are "balanced" using downweight
as a primary indicator but "balance" implies a state of

static equilibrium and downweight is taken from
the moving key.

We know that when a piano is built, the weight
of the action parts sitting on the back of the key exerts an

upward force at the front of the key which is too
high without the addition of keyleads to the front of the key.

What is the effective weight of the action parts?
How does their weight translate to an upward force at the

front of the key? How much is the downward
force at the front of the key? Conventional wisdom simply does

not provide answers to these important questions.

**A New System Of Weights & Measures**

I have found answers to these and many other questions'
by inventing a new system of weights and

measures. Metrology is the science of weights
and measures so I call this system "The New Touchweight

Metrology." The units of the New Touchweight
Metrology define the balance of the upwards and downwards

static forces at the front of the key as contributed
by the weight and leverage of each action component. The

piano action may seem like a complex mechanism
but in fact it acts as a simple lever that propels a hammer

into the string. It functions as a catapult, with
a short lever arm on one side of a pivot and a long lever on the

other. The long lever arm is shortened into
what engineers call a "folded beam" by use of the wippen and

shank levers. The New Touchweight Metrology
takes the folded beam of the action and "Unfolds" it into a

simple balanced lever such as the scale you might
find in your doctors office, where:

B = Balance Weight

F = Front Weight

W = Wippen Weight at the Key Ratio radius

S = Strike Weight at the Strike Ratio radius

Figure 1

Figure 2 shows the balance of static forces at the
front of the key, where: The downward static force of the

Wippen Weight on the back of the key translates
through the Key Ratio to the upward force of the Wippen

Balance Weight at the front of the key, and: The
downward static force of the Strike Weight is multiplied

through the combined leverage of the shank, wippen,
and key to the upward force of the Strike Balance

Weight at the front of the key. The balance
of the upward and downward static forces at the front of the key

are expressed as the equation:

BalanceWt + FrontWt = (WippenWt x KeyRatio) + (StrikeWt
x StrikeRatio)

**Definition & Determination Of The Units**

__Balance Weight__- The amount of weight, placed
on the front of the assembled key that equals the upwards

static force at the front of the key. Balance
Weight is found by measuring UpWeight and DownWeight and

calculating:

Balance Weight = (DownWeight + UpWeight)/2

When measuring UpWeight and DownWeight the touch
weights are placed on the key centered on a point

13mm in from the front vertical edge of the key.
When the balance weight is placed on the front of the key it

is balanced and motionless as if it were a balanced
scale. Additional weight must be added to the balance

weight to overcome friction and start the key moving
down (DownWt) and weight subtracted from the

balance weight to start the key moving up (UpWt).

__Front Weight__- The radius weight of the keystick
pivoted on its balance point, taken at the front of the key. It

represents the downward static balancing force
at the front of the key.

Front weight is found by placing the key on a wedge
pivot so that the balance hole is centered across the

edge of the wedge. The front of the key rests
on a roller bearing which is on the pan of a digital scale. The

key is oriented in a horizontal attitude similar
to that when the key is at rest in the assembled action. The

roller bearing rests on a vertical axis through
a point on the surface of the key 13mm in from the front vertical

edge of the key (see Photo I).

__Wippen Weight__- The radius weight of the wippen
pivoted on the wippen center, where the capstan contacts

the wippen heel. The wippen heel rests on
the roller at the capstan contact point. The wippen flange rests

on the felt wedge so that the wippen center is
aligned with the vertical axis through the

center of the roller. If necessary the flange
may be wedged with a sliver of wood to prevent the flange from

rotating (see Photo 2).

__Strike Weight__- The hammer weight plus the
radius weight of the hammer shank, pivoted at the hammer

flange, taken at Strike Line Radius. The
strike line of the hammer rests on the felt wedge block and the end

of the tipped up flange rests on the roller so
that the flange center aligns with a vertical axis through the

center of the roller. The height of the roller
is adjusted so that the shank rests horizontally. Playing cards can

be helpful as shims (see Photo 3).

__Key Ratio__- The ratio of down- wards force
at the capstan to the corresponding upward force at the front of the

key. The key is set on the jig as for weighing
front weight. An amount of weight is placed on the front of the

key to make the front weight at least 70 grams.
This weight holds down the front of the key. The scale is

then tared to zero. (Digital scales have a tare
button which makes the scale read zero, regardless of what

weight is on the pan.) Two 50-gram weights are
placed on either side of the capstan so that there combined

center of gravity is at the capstan/heel contact
point. The scale will then read how the 100 grams translates

to the front of the key. For instance, if
the scale reading were -57.0 the key ratio would be 0.57 (see Photo

4).

__Wippen Balance Weight__- The upward static
force at the front of the key from the leveraged weight of the

wippen. Found by calculating:

WipBW = KeyRatio x WipWt

__Top Action Balance Weight__- The total upward
static force at the front of the key resulting from the leveraged

weight of the wippen, hammer, and shank.

Found as:

TopBW = BW + FrontWt

__Strike Balance Weight__- The upward static
force at the front of the key from the leveraged weight of the

hammer and shank.

Found by calculating:

StrikeBW = TopBW- WipBW

__Strike Ratio__ - The amount of weight to balance
one gram of strike weight at the front of the key.

Found as:

Strike Ratio = StrikeBW/StrikeWt

**Conclusion**

The New Touchweight Metrology bridges from the
old Metrology of DownWeight and UpWeight through the

Balance Weight, thereby maintaining the connection
to traditional touchweight parameters. The array of

information provided by the New Touchweight Metrology
gives a wealth of information that has heretofore

remained hidden from us. Of particular utility
is the ability to measure hammer weight "on the shank" and

the calculation of Strike Ratio. The New Touchweight
Metrology provides a useful and relevant framework for

a more complete understanding of the balance of
piano action mechanisms.

The weights and measures described above only partially
describe the units and methods of the New

Touchweight Metrology. Other units and methods
will be described in future articles. In my next article I will

show the results of studies using the New Touchweight
Metrology and discuss the correlation between strike

ratio and leverage which leads to the ability to
rate the "dynamic" feel of piano actions using methodology of

the New Touchweight Metrology.

**Notes:**

1. To the best of my knowledge, the Balance Weight
value was first described by Don Galt, RPT, in

"Resistance in Piano Action," in the April, 1969
issue of the Piano Technicians Journal. He called it Weight

Resistance. In the October, 1990 Journal
is published a method for balancing keys to a specified Balance

Weight, by David C. Stanwood, RPT.

2. For this work, a scale needs to have 150-gram
capacity and resolution accuracy of 0.1 gram. The roller

bearing shown is an "idler bearing," which can
be purchased from small parts component suppliers. In a pinch,

an edge-trimming router bit can be used.

3. In all cases it is only necessary to carry the
decimal to the nearest tenth except for the "key ratio," which is

carried to the nearest hundredth.