poids-moment

07:59 Feb 21, 2018 

Many thanks, eveyone, for all your contributions, which have beem most enlightening and have enables me to get a much better (though still fairly hazy!) understanding of what is going on here.

I'm sorry if I misled anyone: the units used do not appear in my document, they simply cropped up as a result of my own research into this and related terms; the use of / instead of . may be the result of my rushed misinterpretation of a not-terribly-clear reference.

And no, Daryo, there are no formulæ given anywhere — this just appears in a list of operations to be carried out.

Indeed, any variation in the moment weight may also be due to some quite alarmingly large amounts of machining that may be done to these blades as part of the reconditioning process. Hence I suppose why re-balancing becomes even more important than usual.

20:44 Feb 20, 2018 

there are measurements that do involve mass (in plain kg) situated at a certain distance from an axis of rotation see https://en.wikipedia.org/wiki/Moment_of_inertia which is not the same as a force (in kgf, or less confusingly in N(ewtons)) applied at a certain distance form an axis of rotation. which is a "torque" https://en.wikipedia.org/wiki/Torque

The widespread cavalier [mis]use of "kg" when "kgf" should be used (= "linguistics") can not change basic physics.

"The academic distinction between mass and weight (not to mention “kilogram-force * meter”) is somewhat irrelevant in this context."

The distinction between mass and weight is fundamental/basic physics, NOT some "academic semantics" and is extremely relevant in this context.

19:44 Feb 20, 2018 

3. At the linguistic level, it seems pretty clear that Tony’s ST is influenced by the term “moment weight” used in the US. And Tony tells us what the units are in the ST (kg/m (which should read kg.m) – so there’s no point in trying to apply different units (eg kgf.m) in the translation: units of measurement do not translate (although kg.m may be converted to oz.in, using the “1,388 and a bit” factor mentioned elsewhere on this page, if Tony’s client so requires).
4. One more for good measure: as Chris has said already, “mass moment” is somewhat meaningless – especially when the term “moment weight” is already used in the relevant literature.

19:42 Feb 20, 2018 

1. The test refers to the balancing of turbines by selecting blades with (ideally) identical characteristics to be fitted opposite sides of the shaft. The most-basic test involves pairing blades of the same “weight”, when placed on a weighing machine. The “weight moment” measurement is a refinement of that, which selects blades with the same (angular) momentum when the turbine is running. It takes account of the fact that blades which are well-paired in the first text may still be unbalanced, when running, if one has its centre of gravity further away from the shaft axis that the other (causing it to have greater momentum). This may be caused by a lack of homogeneity in the materials used to make the blades.
2. The academic distinction between mass and weight (not to mention “kilogram-force * meter”) is somewhat irrelevant in this context. Since the two blades are tested in identical conditions, the ratio of the measurements on a pair of blades (which indicates the “quality of balance”) will be the same, regardless of the units used. Superfluous factors introduced by the mentioned academic distinctions will all cancel out in the engineering context.

18:53 Feb 20, 2018 

do you have any example how this "poids-moment" is later used in calculations?

It is not entirely impossible that what they wanted to measure was really a mass (in kg, not in kgf or N), [at a certain distance form the axis of rotation] in which case it would more have to do with "rotational inertia" and/or balancing the turbine.

19:37 Feb 19, 2018 

too many people confuse mass and weight; in fact the "kg.m" in the ST is strictly speaking an abusive abbreviation for "kgf.m" (kilogram-force * meter) - measuring a force applied at a certain distance from the axis of rotation.

18:53 Feb 19, 2018 

What’s “utterly bizarre” about oz.inches? We all know there are 1,388 and 47/64ths (+/- a smidgeon) oz.in in a kg.m and convert these things in our head as we go along. Errr… don’t we?

10:30 Feb 19, 2018 

The concept of mass moment (expressed in kg.m, not kg/m) has very little meaning. However, engineers often use mass (a fundamental unit) and weight (a force, generated by gravity acting on a mass) interchangeably, which is incorrect. A torque does indeed have the same units as a moment (either the old-fashioned kgf.m or more properly N.m). Note that a moment always has to be defined about a specified point; the moment about the centre of mass is identically zero (i.e. the object balances).
The US Space Electronics company seems to be one of not many organisations to use this term. They define "Moment Weight = (Mass of the blade) x (Distance from the engine centerline to the center of gravity of the blade)" (of course they mean "weight" of the blade, not mass), using the utterly bizarre unit of oz.inches. If only engineers understood physics, there would be much less confusion!