Steve's Corner - Correcting Double-Striking Hammers
Double-striking is the perception that the hammer has struck the tine two or more times with one keystroke. The sound produced is a "thunk" or dead, dull sound. The actual problems behind this are varied: possible causes include improper strike line, poor escapement, damper adjustment, damper tension, a dead tine, or inadequate hammer-to-key pedestal interaction. Note that unlike an acoustic piano, the key is not to hit the guide rail felt washer or strip at the bottom of the keystroke. That felt is there for excessive key travel, to protect the key from being broken. If the key hits the guide felt before the hammer/pedestal stop lock action occurs, double-striking will occur. Many of these cases (such as strike line and escapement) have been addressed in more detail in other articles.
The Action
The hammer has to rotate up on the key pedestal to the stop lock position, then (due to momentum) rotate up past the lock position and hit the tine. All while pulling down the damper so the damper pad clears the tine on its first and successive down swings. After hitting the tine, the hammer has to fall back quickly to try to avoid the tine on its first and successive down swings. Quite a task. Any failures in this activity can cause double-striking.
If the escapement (the distance between the hammer tip at upper rest/stop lock position and the tine) is inadequate, the tine will strike the tip on a downswing, producing a double-strike.
If the strike line is not correct, the tip can hit the tine too close to the generator, resulting in an incorrect transfer of its energy (which will produce a "thunk"). Or it can hit the tine in an area of greater tine swing or flexibility, and the tine will partially absorb some of the tip energy (plus the tine's greater swing arc will cause it to hit the tip on a downward swing). Up until the late 1970's, strike line was set by ear at the factory during the assembly process. Errors may exist because the operator might not have played the notes hard enough, causing minor misalignments to be overlooked. Later, the harp support mount holes were machine-punched, eliminating operator errors and ensuring consistency. I frequently reset the strike line: sometimes a small move makes a big difference.
If the damper is not adjusted correctly, it can pull away too late and be hit by the tine's downswing. If the damper arm's tension is not correct, the damper's initial downward motion will be for the arm to flex instead of pulling the pad away. This will leave the damper pad too close to the tine during initial tine downswings. Damper tension is determined by the action rail's angle where the damper mounts and the width of the arm plus the damper material. To verify the tension of one arm or module, unscrew the arm or module from the action rail. If the arms are straight or flat, they are OK. If they are bent down, the tension is weak and will result in poor damper response. If they are bent up, they will be more aggressive in stopping the tine, but they will add to the amount of energy the player will need to hit the note, giving the impression of a heavier action. It is very difficult to add tension consistently from note to note for a balanced feel just by bending the damper at the mount. Note that uniformly increased tension can be achieved by the use of spacers, but as mentioned, the action becomes heavy and less responsive to a normal or light touch.
Hammer-Key Pedestal Interaction
The back edge of the hammer cam curve is sharp, and some versions can eventually dig a line into the pedestal felt near the back end of the pedestal, especially with models that have longer hammer throw distances. If this has happened, double-striking can occur because the hammer will not snap back to the stop lock position. If the pedestal felt has been damaged only slightly through compression, you can re-fluff it using a piano tech's felt picker. If the felt is gouged or simply worn out, then replace the felt.
There were 4 generations of cam curves and 7 key pedestal configurations between the introduction of the Mark I and the Mark V. The more recent versions offer slightly more resistance at the start of the stroke and therefore need the silicone lubrication more than earlier versions. Without good lubrication, the friction can cause the hammer cam curve to heat a little during continuous playing. Some of the plastic from the hammer will come off and become embedded into the pedestal felt (the early modular hammers that look white are more prone to this). This also leaves the cam curve pitted or not uniformly smooth.
In addition, the hammer-to-pedestal interaction includes a rolling and a sliding motion. Sufficient lubrication is required to minimize friction and prolong wear. Silicone spray, its testing and application have been mentioned in numerous articles: follow those instructions. Using an air compressor occasionally to blow out the felts and then re-siliconing them won't hurt. The blowing out tends to slightly re-fluff the felt and clean out the plastic debris.
More Info
- Chapter 2: The RHODES Modular Action
- Chapter 4: Dimensional Standards and Adjustments
- Chapter 8: Early Design RHODES Pianos - Action (Prior to 1975)
- Chapter 10: Early Design RHODES Pianos - Dimensional Standards and Adjustments