John Hartman wrote: >Thanks for sharing this with us, your insights are very helpful. >Maybe you can clear something up for me. In the real word actions >and other machines have friction. Let's say I measure a note on a >grand action and find that there is 12g of friction measured >statically (D-U/2). When the action is played through its dynamic >range does the friction go up proportionally to the increased force >applied to the key? I doubt if there is a simple relationship for dynamic friction, due to the nonlinear geometry. But certainly dynamic friction will not be constant and will increase with increase key force as well as varying significanlty through the key stroke. In practice there would be no reason to isolate just dynamic sliding friction between components (if that was even possible), but also we want the overall energy lost in the bushings and contacts, i.e. any source of energy loss in the action while it is operating. Static friction obtained with the U and D method is useful for practical purposes in analysing and adjusting pianos, but only up to a point. Static friction isn't really indicative of dynamic friction, except we might use a rule of thumb that estimates dynamic friction will probably be higher in an action with higher static friction. A shop method to measure true dynamic friction would be difficult because it needs to be measured dynamically, i.e. while the key is in the process of being depressed, and forces need to be measured, or some way to quantify what is being applied to the front of the key. Something based on energy input/output might be a feasible compromise. Alternatively, the dynamic computer model under development will be useful for analysing dynamic friction. David Stanwood suggested: >Try simulating dynamic friction by attaching weight onto the hammer then >Measure U/D and calculate friction. Tie down the jack so it stays under >the knuckle when measuring UpWt. I'd use maybe a 50 gram weight.... Any method that measures U and D weights is still measuring static friction. Adding the extra weight to the hammer just increases the static weight that is common to the DW and UW formulas. Not sure what the reasoning to keep the jack under the knuckle is. Stephen Dr Stephen Birkett Associate Professor Department of Systems Design Engineering University of Waterloo Waterloo, Ontario Canada N2L 3G1 Davis Building Room 2617 tel: 519-888-4567 Ext. 3792 PianoTech Lab Ext. 7115 mailto: sbirkett[at]real.uwaterloo.ca http://real.uwaterloo.ca/~sbirkett
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