View Full Version : Gram-inches to inches-per-second conversion

3rd Jun 2008, 09:41
One comes across both sets of units in discussions of dynamic crankshaft or propeller balancing.

What is the conversion from one to the other?

Crankshafts get dynamically balanced (by specialist US engine rebuilders) to below 1 gram-inch. They leave the Lycoming factory anything up to 50 times this value, apparently.

Propellers get dynamically balanced to normally below 0.1 IPS. Static balancing alone tends to get you below 1 IPS.

3rd Jun 2008, 10:08
Inches per second is a speed - gram-inches is something else (force??), so you cannot convert one to t'other.

3rd Jun 2008, 10:48
gram-inches smells like a measurement for torque to me.

What kind of a berk mixes metric and US units like that? As if it wasnt confusing enough as it is already.

3rd Jun 2008, 15:18
NASA do when designing the software for Mars probes, with interesting results.

And yes, they do look like two completely different measures. gram inches works out as a force times a distance, whereas inches per second is a distance/time (speed) measurement.

3rd Jun 2008, 15:41
Believe it or not :eek: Balancing machines, which measure imbalance moment (not torque, albeit with the same type units) are commonly calibrated in gram-inches. Why these bastard units, I have no idea.

Inches per second is indeed a velocity, in this case the velocity amplitude of a sinusoidal vibration waveform. To convert to imbalance moment (see above) one would have to factor in the prop mass, balancing rpm, & maybe a few more unknowns that escape me at the moment.

But both are common imbalance / vibration measures.

3rd Jun 2008, 16:16
Dynamic balancing rigs for crankshafts use gram-inches.

Dynamic balancing rigs for propellers use inches per second.

I believe the latter is not directly RPM dependent. The measuring device (which incidentally senses prop rpm and position - optically from a piece of reflective tape) will show the IPS figure and that figure then hardly varies as you wind up the RPM from idle to say 2400.

Cranks are also sometimes measures in inch-ounces :) At least the conversion from that to gram-inches isn't hard...

Capt Pit Bull
3rd Jun 2008, 16:50
gram-inches sounds like mass x distance to me, not force x distance, (S.I. units being kind enough to distinguish between force and mass).

Whereas inch-ounces could be distance x mass or distance x force, as the Imperial system somewhat unhelpfully blurs the line between force and mass.

I don't know much about balancing rotating machines though, so where to go from here I have no idea....

Standing by to be corrected!


3rd Jun 2008, 21:35
The Captain rightfully holds my feet to the fire: gram-inches is indeed moment (e.g. imbalance units), whereas torque would be in Newton-inches. :}

4th Jun 2008, 10:46
Gram-inches and inches per second are both commonly used in balance but they cannot be converted one to another as your many correspondents have said. They describe completey different parameters.

Gram-inches tell you how to calculate what to put where. For example 175 gram inches can be a 25g weight placed 7 inches from the spinner or a 7g weight at 25 inches. It says absolutely nothing about the residual vibration velocity in inches per second but merely suggests that if you do it the prop is correctly balanced within the recommended limits. It also tells you what you must remove if you are taking the prop off to fit elsewhere.

All sorts of "funny" units have come along because rather than having electronic calculators, engineer had to create a set of units that landed within a decade of each other from one extreme to another if they wanted to keep the slide rule work manageable or sometimes within measureable commonsense. There is no point in measuring with a micrometer what you will mark with chalk and cut with an axe. Thus R-R chose the coarser Centigrade Heat Unit for example in preference to the British Thermal Unit. Spark plugs had millimetric threads because the first suppliers were French or German and everyone else followed suit, even KLG and Lodge. Balance weights seem to be popular in gram sizes but all the propeller loadings I have done involved inches.

At school the purists insisted on the poundal as unit of force with the pound as unit of mass. At university commonsense prevailed and the lb-wt was the unit of force so the unit of mass was conveniently upped to 32.174 lbs [numerically "g" pounds] and called the slug. For years, even after the Marshall Plan was history, the most accurate milling machines read off in inches not microns or millimetres. Aircraft got built to inches and thousandths thereof.

Mass and Weight? Generally speaking you cannot readily tell what the mass is. The only practical thing to do is "weigh" it and trust that the acceleration due to gravity is uniform enough not to affect the outcome. Not true in space shots.

Any way, enough gabbling. gram-inches are darn useful "egg" numbers that relate directly to who makes what and how do I make it better whereas inches second relate to "chicken" of vibration wot is left over once I have dun fiddling.

Even so the two-bladed prop has certain bad gyro properties compared to the lovely three-blader so even inches per second vibration will have a value that might be "good" on a two blader but definitely "bad" on a three blader.

Then of course you cannot get three-quarter inch plumbing... it is 20mm and sold, not by the metre, oh no. Betcha it is sold ny the foot.

Brian Abraham
4th Jun 2008, 11:20
The typical balancer measures IPS (Inches Per Second) and then advises the weight in grams to correct the imbalance. In the helicopter business balancing the main and tail rotor is an oft performed task and usually achieved by adding/deleting washers. In helos the balancers are programmed to be aircraft specific and also advise the ammount of tab (trim tab on each individual rotor blade) required on a particular blade.

4th Jun 2008, 12:41
you're right of course, mass times distance is not the same as force times distance (slaps self on wrist). My shoddy thinking.

regards, OORW

19th Aug 2016, 19:45
Static Balance and Dynamic Balance are similar, but not the same. Static balance is often given as oz inches, dynamic balance as velocity in Inches Per Second. The only reliable way to convert inch oz to a dynamic measure is to measure the affect of a given weight (maximum permissible imbalance tolerance in oz in) in the dynamic condition via vector analysis. One must be careful because there are tons of caveats on the use of this method - rigid body or "live", target speed, potential for critical speeds causing non linearity and so on. It is possible to be dynamically balanced but not be in static balance limits. Since a part lives in the dynamic environment, this then becomes the driver for maintainers. I have never seen a dynamic component wear out in a static on the shelf environment - ADs notwithstanding.

Chu Chu
20th Aug 2016, 15:45
I guess if you're in the U.S, and like to measure in inches, but the weights come in grams, gram-inches is a perfectly logical unit . . .

23rd Aug 2016, 00:06
Well, its only logical to the completely illogical.