In a car you can state a precise gear ratio through a manual transmission and differential. With an automatic transmission with a torque converter the ratio is continually changing until you get to converter-lock. You can't really express a ratio for a car with an auto transmission. In a piano you could state that the ratio is 5.5 at the start of the stroke, but it might be 5.7 or so by the end of the stroke. Neither measuring system takes that into account so their answers are different. ???? dave David M. Porritt, RPT dporritt at smu.edu From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of David Love Sent: Thursday, January 14, 2010 9:52 AM To: pianotech at ptg.org Subject: Re: [pianotech] Action Ratios Recap Except that the weight ratio also changes through the keystroke. And then why would the relationship between SBR and RLA vary (assuming it's not just a measurement error)? David Love www.davidlovepianos.com From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Porritt, David Sent: Thursday, January 14, 2010 7:43 AM To: pianotech at ptg.org Subject: Re: [pianotech] Action Ratios Recap OK, I hope this is somewhat better than just a guess, but it might not be. The actual ratio changes as the keystroke goes through its motion. Yet both of these measurement systems measure weight or lever length (depending on which system you are using) as a static measurement. They will come up with different answers because neither method takes this varying ratio into account. The SBR method interpolates the weight ratio by calculating how much lift will be generated by so much down pressure. The RLA interpolates the ratio by measuring lever arms that are actually changing through the keystroke. I don't think either system can be 100% accurate with the tools we have to use, yet for all practical purposes both are probably accurate enough for our purposes. dave David M. Porritt, RPT dporritt at smu.edu From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of David Love Sent: Thursday, January 14, 2010 9:20 AM To: pianotech at ptg.org Subject: Re: [pianotech] Action Ratios Recap Nick: Thanks again for this further clarification. The next logical step will be defining the specific relationship between this particular approach (ratio of lever arms or RLA) with Stanwood's SBR. While I use both in "fixing" problem actions, the exact relationship is somewhat ill defined and I end up doing them separately. Since the distance measurement is better, IMO, at removing measurement error as a variable it would be nice if the relationship between RLA and SBR could be more clearly defined. Stanwood has recently posted some information on the RLA/SBR ratio in terms of >, = or < 1. But my question here would be why does it not equal 1 every time and what is the functional explanation as to why variations in that particular ratio might make a difference. David Love www.davidlovepianos.com From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Nick Gravagne Sent: Tuesday, January 12, 2010 12:12 PM To: pianotech at ptg.org Subject: Re: [pianotech] Action Ratios Recap To all that have followed this thread: After revisiting the recent exchanges RE action ratios it is occurs to me that too much emphasis has been placed on aftertouch (AT). To briefly recap, Pfeiffer's stated relationship of EQ 1: W / S = the product of the RAs / EAs was my beginning point. This basic relationship was also restated in reverse as: EQ 2: (keyout/key in) x (wippen out/wippen in) x (shank out/shank in) = (blow distance - letoff) / (key dip-aftertouch). Both formulas imply the same thing; that the ratio of the leverage arms as configured in the key, whip and hammer shank will always be reflected in the key dip as it relates to hammer rise at the point of letoff. We do not have a single term or expression for "hammer rise at the point of letoff". What I did was to rearrange Pfeiffer's EQ 1 and solve for S, which is dip, regardless of AT; which is what the term (key dip-aftertouch) implies. EQ 3: S = (V x Ra x N x W) / (H x Rs x K) (*See below for factor designations) EQ 3 example with assigned values was S = (245 x 67 x 18.25 x 44) / (126 x 94 x 141) = 7.89 What this states is that 7.89 mm of dip is required to raise the hammer 44 mm. AT begins the moment that the jack tender makes contact with the letoff button and completes when the key bottoms out on the punching. But for the sake of these relationships it is proper to focus on the amount of dip required to raise the hammer to the point of letoff. Thus, the ratio of the (key leverages) to that of the (engaged dip and related hammer rise) will be equal. In the case above, 44 / 7.89 = 5.58, and this equals the Action Ratio. And so, the leverage ratio = 5.58, and this ratio is reflected exactly in the ratio of hammer rise to key dip, without regard to AT. Now why bother to think this way? Because it encourages us to remember that any changes made to the key leverages are going to be reflected in the dip; and this dip is going to be required to raise the hammer the required distance to the point of letoff. RE Aftertouch: in the above EQ 3 example, factor S (or calculated dip) equals 7.9 mm. Thus if total regulation dip is not to exceed, say, 10.7 mm, then AT must fully occur by using up the difference between required dip (S) and the limit of 10.7. The difference is 2.8 mm. Clearly this is plenty; in fact the regulated dip would likely be less than 10.7. David Love wondered if an informative relationship can be found RE a useful ratio or percentage of overall hammer rise (OHR as calculated without regard to letoff) as it relates to standard blow and dip specs. Or stated another way, how many mms are required after S has been achieved to safely complete AT. I feel certain the relationship can be uncovered. RE Pfeiffer in general: no useful understanding of Pfeiffer's work can be had until the overall goal of his work is appreciated. At the drawing board, and when designing new actions from scratch, Pfeiffer sought to create a conjugate (gearing) relationship of the six levers in question (two in the key, two in the whip, and two in the shank). In brief, this conjugate relationship is intended to minimize friction by creating a "rolling" rather than "sliding" friction at half-stroke and thus "minimize the slide path" of the contacting profiles (capstan, whip heel, etc.) For most modern actions today, the layout of the balancier and knuckle prevent such conjugation from taking place, at least at that interface. But the Langer action as appears in Pfeiffer's book The Piano Hammer (page 110) admits of such an arrangement due to the flattened and re-angled top portion of the balancier; ditto RE the Bender action page 84. There is a ton more which could be said of Pfeiffer's work. *********************************************** Designations per Pfeiffer: W = (hammer travel - let off) S = (key dip required to lift the hammer to the point of letoff) H = rear key lever arm resistance (key out) Rs = whippen lever arm resistance (whippen out) K = hammer lever long arm resistance (hammer out) V = front key lever effort (key in) Ra = whippen lever arm effort (whippen in) N = hammer lever short arm effort (hammer in) Nick Gravagne, RPT Piano Technicians Guild Member Society Manufacturing Engineers Voice Mail 928-476-4143 ________________________________ -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://ptg.org/pipermail/pianotech.php/attachments/20100114/3a90dcfc/attachment-0001.htm>
This PTG archive page provided courtesy of Moy Piano Service, LLC