A Key Forces at Play: Friction, Coil Resistance, and the Challenges of Tuning

[themaximillyan]

The Key Forces at Play: Friction, Coil Resistance, and the Challenges of Piano Tuning

Re: Trouble setting the pin - ideas?
Don L#332943902/23/23 08:59 PM
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Becket
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Joined: Feb 2023
https://forum.pianoworld.com//ubbthreads.php/topics/3594341.html#Post3594341
«The coil’s turns are supposed to touch, especially the final turn where the string exits the pin. This is the location where coils open up due to technique. Impact tuning technique has its place, but when used exclusively it will open the bottom turn over time and necessitate that the coil be reset again somewhere down the road. Not all tuning techniques makes this happen. Anyone can easily test this on their own. In reality, it is exceedingly rare to find a piano in the wild that doesn’t have open coils on the bottom turn.

The whole coil doesn’t move around on a pin. The struggle of the tuner is to adjust the tension higher up on the coil. That aspect is hard to “tune” because the upper turns get locked solidly onto the pin very early on in the process.

The changing of the position of the becket in the hole, has no relation to the sound or performance of the string. The orientation of the becket in the pin block likewise has no relation to the sound. The position of the becket only limits how the coil’s angle can be positioned on the pin. If the becket is positioned too close to where the string exits at the front, the becket limits the coil from being tapped down and angled toward the string.

Setting the coil doesn’t destabilize the piano, as long as the tuner knows how to bring the string to pitch. The end result should be a tone that is straight, without false beats, and is stable. This is how we know if we did a good job.

Setting the coil is done when there is an observed problem with how the pin responds. Tuner’s know when they are fighting with the pin in terms of its response and stability. Instead of ignoring it, the problem is better dealt with directly and early on. It is like a pitch raise. It is better to deal with it in the beginning.»
Tuning a piano is a process governed by intricate mechanics and complex interactions of forces, especially in older models such as the 1977 Model 45 Steinway. The inherent challenges lie not only in managing string tension but also in combating excessive friction and spring-like resistance caused by the string coils and their interaction with the tuning pin. As Becket's insights reveal, even seemingly minor issues like improperly aligned string coils can have a profound effect on the tuning process. When we incorporate Newton’s laws into the analysis, the difficulty becomes even more apparent.
1. Excessive Friction: The Crux of the Problem
The high breakover angle created by the V-bar in certain piano designs results in significant friction at the string’s point of contact. This friction is compounded by:
The elastic deformation of the string as it bends around the V-bar.
Resistance generated by the coils wound around the tuning pin. As Becket emphasizes, poor alignment or lack of compression in the coils further amplifies this problem, increasing the force required to make even the slightest adjustment.
Why the Felt Doesn’t Fully Solve the Issue: Although the felt beneath the string is intended to facilitate sliding and reduce friction, it cannot fully counteract the effects of the steep breakover angle and the high tension pulling the string taut. The result is a system where friction and tension dominate, making precise tuning a monumental task.
2. Newton’s Laws and Piano Tuning Mechanics
First Law of Motion (Inertia)
The static friction at the V-bar and tuning pin resists the string’s movement. When a tuner applies force, the string resists this adjustment until the applied force overcomes the friction, leading to abrupt, jerky movements that disrupt the tuning process.
Second Law of Motion (Force and Acceleration)
The force applied by the tuner must contend not only with the string’s tension but also with the additional resistance introduced by misaligned or loose coils. This uneven distribution of force results in unpredictable responses, as the string alternates between static and kinetic friction states.
Third Law of Motion (Action and Reaction)
Every action the tuner takes—whether turning the tuning pin or adjusting the coils—elicits an equal and opposite reaction from the system. The reactive forces from the string and the pinblock create a spring-like effect that further complicates precise adjustments.
3. The String as a Clamped Beam
From a mechanical perspective, the piano string behaves like a clamped beam, anchored at two points (the bridge and the tuning pin) with a significant deflection at the V-bar. This configuration creates:
Elastic deformation: The string bends under tension, creating stress concentrations at the V-bar and the tuning pin.
Vibrational dynamics: The string vibrates as two segments—between the bridge and the V-bar, and between the V-bar and the tuning pin. Misalignment or improper tension can disrupt these vibrations, affecting both the tuning stability and the tonal quality.
Tension redistribution: Any adjustment at the tuning pin propagates through the string, encountering resistance at the points of contact.
4. Becket’s Solution: Coil Compression as a Key Adjustment
Becket highlights the importance of addressing the string coils wrapped around the tuning pin. Properly aligned and compressed coils play a crucial role in mitigating the effects of friction and elastic resistance. His method involves:
1. Lowering the string’s tension on both sides to relieve stress.
2. Realigning the coils by pulling up at the back of the pin and tapping the coil down at the front to achieve uniform compression.
3. Ensuring coil tightness: Tightly compressed coils minimize slippage and provide a more predictable response when adjusting the tuning pin.
By addressing the state of the coils, the tuner can reduce the cumulative friction and improve the string’s responsiveness, making tuning adjustments smoother and more precise.
5. Calculating the Mechanical Challenge
The forces at play in piano tuning vividly illustrate the substantial mechanical demands of the task. Key contributors to the overall resistance include:
String Tension Torque (Mtension): The resistive torque generated by the constant pull of the string on the tuning pin.
Frictional Torque (Mfriction): The torque resulting from the friction between the tuning pin and the wooden pinblock.
Spring Torque (Mspring): The elastic resistance exhibited by the tightly wound coils of the string around the pin.
The total resistive torque, which the tuner must overcome, is calculated as the sum of these individual torques:
M total = M tension + M friction + M spring
Based on empirical data, the total resistive torque is approximately:
M total ≈ 4.82 N·m
However, the actual torque required to achieve effective and precise adjustments of the tuning pin typically exceeds this value:
M required = 13 N·m
This implies that the tuner must provide an additional torque to compensate for the difference:
M shortfall = M required – M total ≈ 8.18 N·m
The Critical Role of Coil Compression:
Ensuring that the coils are tightly compressed plays a crucial role in reducing the frictional and spring torques. By minimizing these resistive forces, the tuner significantly lowers the overall resistance of the system. While this approach does not eliminate the fundamental mechanical challenges inherent in the design (e.g., V-bar design), it demonstrably enhances the tuner’s ability to make highly precise adjustments, leading to improved tuning stability and accuracy.
6. Conclusion: The Limits of Felt and the Role of Precision
Even with the felt attempting to reduce friction, the design of the piano string mechanism—particularly the steep breakover angle—creates unavoidable challenges. The high friction and elastic resistance at the V-bar and tuning pin demand substantial effort from the tuner, with half of their strength often spent compensating for mechanical losses.
Becket’s advice to carefully compress and align the string coils offers a practical solution to mitigate these issues. By addressing the small, seemingly inconsequential aspects of the system, the tuner can reduce the total resistance, improve the responsiveness of the tuning pins, and achieve greater tuning stability.
Ultimately, this analysis highlights the art and science of piano tuning, where every adjustment is a balancing act of forces, akin to Don Quixote’s battle with windmills. Success depends not only on skill but also on understanding the intricate mechanics that govern the instrument.
https://www.academia.edu/128611005/The_Key_Forces_at_Play_Friction_Coil_Resistance_and_the_Challenges_of_Piano_Tuning

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