Prop balance

Areeda

Areeda

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Areeda
We got our prop balanced today and it makes a difference.

The prop had been balanced before we bought it, but enough dressing and nicks it was time.

The starting value was 0.14 ips and ending was 0.02 ips.

My question is what is moving at 0.02 inches per second.

My best guess is that it is the axis of rotation of the prop and crankshaft but I'm not sure.

Joe
 
Areeda said:
We got our prop balanced today and it makes a difference.

The prop had been balanced before we bought it, but enough dressing and nicks it was time.

The starting value was 0.14 ips and ending was 0.02 ips.

My question is what is moving at 0.02 inches per second.

My best guess is that it is the axis of rotation of the prop and crankshaft but I'm not sure.

Joe
Click to expand...

The force caused by out of balance mass?
 
clipped this from off the net..

IPS is just an acronym for "inches per second". When measuring a vibration, the velocity is the speed the object reaches as it passes through the center of the range of displacement. Similar to the highest speed a pendulum reaches at the center of its swing.

Velocity is arrived at by simply integrating the signal that an accelerometer produces. If you integrate a second time, you get displacement.
It has been found through experience that velocity is a better unit of measurement to use if you want the numbers to track with how severe a vibration is independent of RPM. In other words, a 1 IPS vibration is pretty severe no matter if the object is turning at 200 RPM or 200,000 RPM. This is not true for units of acceleration or displacement.
Now, for harmonic oscillation, the acceleration is just omega times the velocity; for, say 2400 rpm (fundamental frequency of 40 Hz), your vibration of 1 IPS corresponds to an acceleration of 20.9 ft per sec^2 or about 0.7 Gs. That does seem a fairly healthy vibration for the front end of an engine with propeller. It takes on average 65 grams to correct for a 1 IPS vibration. But this value varies greatly from ship to ship. From a low of 20 to a high of 130.
 
Areeda said:
We got our prop balanced today and it makes a difference.

The prop had been balanced before we bought it, but enough dressing and nicks it was time.

The starting value was 0.14 ips and ending was 0.02 ips.

My question is what is moving at 0.02 inches per second.

My best guess is that it is the axis of rotation of the prop and crankshaft but I'm not sure.

Joe
Click to expand...
IIRC, vibration is often expressed in peak velocity (e.g. ips) because that's independent of frequency for a given amount of imbalance. For prop balancing I believe the raw measurement is lateral acceleration at the top of the crankcase. If you know the acceleration and frequency you can calculate the peak velocity of a sinusoidal oscillation.

I guess the simple answer to your question is the front of the engine is moving around in a circle roughly centered on the crankshaft and the lateral component of that movement is .02 ips (peak).
 
Last edited:
Thanks gentlemen it makes sense now.

I really like the guy who did the work Jim Fackler, we talked the whole time and he answered all my questions.

This is one I forgot.

Joe
 
Think of it this way. you have a tetter-totter 100 inches long. you were out of balance by 1.4 inches when you went in and came out .o2 inches out of balance
 
Remember with a prop that has been dynamically balanced make sure the prop, spinner and backplate have been "indexed" after the job is complete. If during maintenance the spinner is removed and replaced 180 degrees out it will negate the balance.

On Lycoming engines the balance weight is usually placed on the starter flywheel (bolt, nut and washers). On the Continental engines the weight is typically placed by drilling a hole in the propeller backplate (again, bolt, nut and washers).
 
Tom-D said:
Think of it this way. you have a tetter-totter 100 inches long. you were out of balance by 1.4 inches when you went in and came out .o2 inches out of balance
Click to expand...
Nice analogy but you're off by a factor of ten.
 
Rotor&Wing said:
Remember with a prop that has been dynamically balanced make sure the prop, spinner and backplate have been "indexed" after the job is complete. If during maintenance the spinner is removed and replaced 180 degrees out it will negate the balance.
Click to expand...
The person who did the balance discussed that. Now I just have to figure out how to make sure anyone who may take off the spinner also know that.

... On the Continental engines the weight is typically placed by drilling a hole in the propeller backplate (again, bolt, nut and washers).
Click to expand...
What Jim did was use existing screws and bolts on the backplate and add washers of specific weights. The previous balance was done by drilling out rivets and replacing them with a bolt. Jim didn't like that but conceded it works.

Prop was in pretty good balance. Total weight added was 12 grams with about half of that due to a good nick we got when the surface of our runway was repaved.

Joe
 
Areeda said:
The person who did the balance discussed that. Now I just have to figure out how to make sure anyone who may take off the spinner also know that.
Joe
Click to expand...

I use a drop of contrasting paint, at each separating joint.

1 at the spinner to backing plate,
1 at the backing plate to crank
1 at the blade to spinner

My prop/crank has a master spline, it only goes 1 way. no spinner.
 
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