The Wippen Assembly: Where Precision Lives
Part 2 of 6 in the Floating Piano Factory Piano Action Series
If the hammer assembly is the voice of the piano, the wippen assembly is its nervous system. Nestled between the key below and the hammer above, the wippen is a compact lever mechanism that does something deceptively difficult: it takes the slow, deliberate movement of a pianist’s finger and converts it into a fast, clean hammer stroke — then immediately resets for the next note, often before the finger has fully released.
Understanding the wippen is understanding why some pianos respond effortlessly to the most delicate touch and why others feel vague, heavy, or uneven.
The Wippen
The wippen itself is a pivoting wooden lever, mounted on its own flange and center pin, that sits atop the capstan screw of the key. When a key descends, the capstan pushes the rear of the wippen upward. The wippen rotates around its pivot point, and this rotation drives everything above it — the jack, the repetition lever (in grand pianos), and ultimately the hammer.
The wippen is the mechanical translator. Its pivot geometry determines the mechanical advantage of the action — how much force is required at the key to produce a given hammer velocity. This is one reason that regulation matters so profoundly: small changes in pivot height, capstan height, or wippen geometry have a magnified effect on touch weight and feel.
The Jack
The jack is the slender wooden finger that actually throws the hammer. It sits inside the wippen and pivots on its own small flange. As the wippen rises, the jack pushes directly against the hammer butt, accelerating the hammer toward the string.
Just before the hammer reaches the string, the tip of the jack encounters the let-off button (also called the regulating button) — a felt-covered wooden or plastic button mounted on the action rail above. This contact trips the jack sideways, out from under the hammer butt. The hammer, now free, travels the final millimeter or two to the string purely under its own momentum, strikes, and rebounds.
This tripping action — called let-off — is critical. Without it, the jack would push the hammer directly into the string and hold it there, completely blocking the vibration and killing the tone. Let-off must happen close enough to the string that the hammer has committed to the stroke, but not so close that it produces an uncontrolled, weak blow.
Regulating the let-off is one of the most precise adjustments in piano service. A technician sets the let-off distance — typically around 1/16 to 1/8 inch from the string, depending on the piano — by adjusting the height of the regulating button. Too early let-off produces a soft, weak blow. Too late let-off causes the hammer to “block” against the string, stopping the vibration.
The jack also contains its own jack spring — a thin wire spring that returns the jack to its resting position after it has been tripped. A broken or weak jack spring produces a key that won’t repeat cleanly; the hammer fails to reset and the note cannot be replayed until the key fully releases.
The Repetition Lever (Grand Piano Only)
This is the component that defines the grand piano’s mechanical superiority over the upright for most musical applications.
In an upright piano, once the jack trips and the hammer rebounds, the hammer is caught by the hammer rest rail and must wait for the key to fully release before the jack can reset. This limits how quickly a note can repeat — typically requiring the key to return close to its rest position before the next stroke is possible.
In a grand piano, the repetition lever (also called the balancier or repetition mechanism) solves this problem elegantly. It is a secondary lever within the wippen that catches the hammer on its rebound before the key has fully released. As the hammer falls back from the string, it lands not on a rest rail but on the repetition lever, which holds the hammer at a raised position — ready for the next stroke. The pianist can re-strike the note with only a shallow key movement, rather than requiring a full key release.
This is what allows rapid repeated notes, fast trills, and expressive dynamic control in music that demands it. The repetition lever is the mechanical reason that concert pianists overwhelmingly prefer grand pianos for performance.
The repetition lever is controlled by its own spring — the repetition spring — which maintains upward tension on the lever. If the repetition spring weakens or fails, the note loses its ability to repeat from a shallow key depth, and fast passages become unreliable. Spring replacement is a routine but exacting service.
The Backcheck
The backcheck is a small leather-padded arm mounted on the wippen that catches the hammer on its rebound and holds it in a ready position until the key releases.
Without the backcheck, a rebounding hammer would bounce freely back toward the strings and potentially produce an unwanted second note — called bobbing or hammer bounce. The backcheck arrests this rebound at a controlled distance from the string, typically around 5/8 inch, though this varies by piano.
Check distance regulation is part of a complete action regulation. Too close, and the hammer is held so near the string that repetition is compromised. Too far, and the hammer has too much travel before it’s caught, which allows bounce. The backcheck leather also wears over time and eventually becomes too hard to grip the hammer butt consistently, requiring replacement.
The Let-Off Button (Regulating Button)
As described above, the let-off button is the felt-covered button that trips the jack. But it deserves its own note because it is one of the most frequently adjusted components in action regulation.
Every note has its own let-off button, and they are adjusted individually with a regulating screwdriver or regulating tool. The felt on the button compresses over time, effectively changing the let-off distance without anyone touching the adjustment. This is one reason a piano that was perfectly regulated several years ago may no longer play consistently — the felt wear has changed the geometry.
In high-quality regulation work, a technician checks and adjusts all 88 let-off buttons as a standard step, not an occasional repair.
What a Technician Looks For
During a thorough evaluation of the wippen assembly, a technician examines:
- Jack spring condition and tension — weak or missing springs cause notes to fail repetition
- Jack-to-butt leather contact — worn leather produces vague, inconsistent blow
- Let-off distance — too early (weak blow) or too late (blocking)
- Repetition lever spring tension (grand only) — critical for fast repetition
- Repetition lever height (grand only) — affects the depth at which repetition engages
- Backcheck distance — set to arrest hammer rebound cleanly without impeding repetition
- Backcheck leather condition — worn leather fails to grip; hammer bounce results
- Wippen center pin friction — the wippen must pivot freely and consistently
- Capstan-to-wippen relationship — proper contact height affects key travel and power transfer
The wippen assembly is the most mechanically complex zone of the action. A well-regulated wippen is nearly invisible to the pianist — everything simply works. A neglected one produces the kind of uneven, frustrating response that makes a player feel inadequate when the problem is actually mechanical.
Up Next
The hammer hits the string. The wippen makes it happen. But none of this is possible without the structure underneath everything — the keys, the key bed, the balance rail, and the capstan screws that translate the pianist’s touch into mechanical movement. That’s Part 3: Keys and the Key Bed.
Floating Piano Factory is a premium piano care company serving Chicago and beyond. Our technicians approach every instrument with precision, patience, and genuine respect for the music it makes. Book a service appointment or explore our membership plans for ongoing piano care.