why phase rule +30/-30 degrees = same angle?

Original Post

Do you mean that, in practice, the phase angle is considered stable if its value does not change a lot? Your definition of a lot of change is greater than 30 dg plus or minus the center value of the phase?

Regards- Ali M. Al-Shurafa

()

The only "Rule" that mentions +/- 30-degrees is for Balancing, where this phase change amount is recommended for a good Trial Weight vibration response. I have never heard of it meaning the same angle. Can you clarify your question?

Walt

()

Phase alone lacks the amplitude; phase plus amplitude will give the vector.  Look at how the vector changes.  A ratio of this vector change to the average vector can be useful as can just the magnitude of the vector change.

()

"A ratio of this vector change to the average vector can be useful as can just the magnitude of the vector change."

I agree, but I have not seen a balance program display the Vector Ratio; perhaps it does calculate it; or not. I have seen the vector change guidelines described as indicated by either a sufficient phase change or an amplitude change. The underlying principle is that the Vector has a sufficient change.

I have no idea what context the OP was questioning about.

Walt

()

I agree. I have used the 30/30 Rule many times, but I have also departed in many directions and achieved a good balance result. We still don't know whether OP is referring to Balancing or something else.

Walt

()

Interesting article, but I do not believe it is great nor the 30/30 rule.

When you place a trial weight in the right location, you expect no phase change unless the weight is a little large in which case you get a 180 degree change.  Either result can be excellent.  Either result can produce valid (excellent) balance data if a trim is required.

It is the vector change that is important.

()

I am going to guess that the OP is referring to the various training courses, pocket guides, and wall charts that may state, for example, "for unbalance, the phase difference between vertical and horizontal will be 90 degrees +/- 30 degrees" - and all of the similar rules for unbalance, misalignment, etc. The OP may wonder why there is a tolerance of 30 degrees.

On the assumption I am right, the reason for the tolerance is because those rules (90 or 180 degrees phase difference) assume ideal circumstances. The phase will be 90 degrees between vertical and horizontal when the vertical and horizontal vibration is equal. The unbalance forces may be equal in both directions (it is trying to move in a circular motion) but the stiffness and restraint in the vertical and horizontal axis are not equal - thus the movement is elliptical - thus the phase is not 90 degrees (90 degrees is only true for circular motion). The phase is either higher or lower than 90 degrees based on the position of the sensors and relative motion of the bearing (greater in horizontal or greater in vertical).

If the phase difference was greater than 30 degrees away from 90 (or from 180) we have to start questioning whether other factors are at play (weakness, flexibility, resonance, etc.) instead of, or in addition to, unbalance. That's why it helps to understand the machine, its motion, the forces and restraints, the dynamics, and the failure modes.

We could go over every diagnostic rule, but enough said for now.

Jason

()
Jason Tranter posted:

On the assumption I am right, the reason for the tolerance is because those rules (90 or 180 degrees phase difference) assume ideal circumstances. The phase will be 90 degrees between vertical and horizontal when the vertical and horizontal vibration is equal. The unbalance forces may be equal in both directions (it is trying to move in a circular motion) but the stiffness and restraint in the vertical and horizontal axis are not equal - thus the movement is elliptical - thus the phase is not 90 degrees (90 degrees is only true for circular motion). The phase is either higher or lower than 90 degrees based on the position of the sensors and relative motion of the bearing (greater in horizontal or greater in vertical).

Jason

The relative phase between two transducers is also 90 deg when the major axis of the filtered orbit is directly in line with one of the probes.

()
John from PA posted:
Jason Tranter posted:

On the assumption I am right, the reason for the tolerance is because those rules (90 or 180 degrees phase difference) assume ideal circumstances. The phase will be 90 degrees between vertical and horizontal when the vertical and horizontal vibration is equal. The unbalance forces may be equal in both directions (it is trying to move in a circular motion) but the stiffness and restraint in the vertical and horizontal axis are not equal - thus the movement is elliptical - thus the phase is not 90 degrees (90 degrees is only true for circular motion). The phase is either higher or lower than 90 degrees based on the position of the sensors and relative motion of the bearing (greater in horizontal or greater in vertical).

Jason

The relative phase between two transducers is also 90 deg when the major axis of the filtered orbit is directly in line with one of the probes.

Ah yes, quite correct. I answered too hastily. The graphic (from one of our simulators) illustrates John's correction.

#### Attachments

Photos (1)
()

Imagine several "vibration experts sitting" on a park bench talking about baseball, or what ever. A small gust of wind blows a tiny piece of paper, similar to size contained in a Fortune Cookie, that reads "why phase rule +30/-30 degrees = same angle?"! The conversation shifts around to this paper phrase (not phase) as a topic. Unfortunately a real discussion cannot occur, because the author (OP) is not participating!

Nice illustrations, as always, Jason.

Walt

()
ConfMx posted:

why phase rule +30/-30 degrees = same angle?

()
John from PA posted:

Not certain what you mean; can you provide a source?

()

Hola a todos.

```I thank you for your kind comments.
The exact point of the discussion or doubt is based on the following:
An automotive manufacturer asks to perform some tests of residual imbalance in their balancing machines, asks that the resulting phases are NOT different in more than 10 degrees between each start of the balancing machine, without making any change in weights, speeds, sensor position, etc.```
`In the example of the diagram, we can see that the solid color arrows are the residual imbalance to the effect of placing 3 pesos in a rotor, the automotive manufacturer questions that the angles of these 3 arrows are NOT less than 10 degrees.`

#### Attachments

Photos (1)
()

To amplify what JohnPa said, with circular motion any orthogonal probes will have a 90 degree relation, and any probe location would be at a principal axis.

Likewise, having equal amplitude does not mean that there is a 90 degree phase difference. If the motion is such that x = -y ( with probes at x and y), then the motion is equal but the phase difference between the probes is 180 degrees - a 0 degree phase difference with equal amplitudes is created by x=y.  In fact, equal amplitudes at the probes can have any phase difference.

()

What is the weight of a rotor?  What is the weight of a peso?

()
ConfMx posted:

Hola a todos.

```I thank you for your kind comments.
The exact point of the discussion or doubt is based on the following:
An automotive manufacturer asks to perform some tests of residual imbalance in their balancing machines, asks that the resulting phases are NOT different in more than 10 degrees between each start of the balancing machine, without making any change in weights, speeds, sensor position, etc.```
`In the example of the diagram, we can see that the solid color arrows are the residual imbalance to the effect of placing 3 pesos in a rotor, the automotive manufacturer questions that the angles of these 3 arrows are NOT less than 10 degrees.`

I think that the manufacturer is asking for the repeat-ability of the balancing machine. That too is considered important. For higher residual unbalances this is quite stable and you may not get more variation in phase.

Also for shop balancing machines since there is less possibility of other problems such as alignment and rigidity phase angle is generally stable with less variation.

However, if the residual unbalance is near the tolerance limit of the balancing machine then there might be fluctuations in phase but that will be acceptable due to low amplitude.

Rohit

()
OLI posted:

What is the weight of a peso?

3 Mexican peso's = 0.12 Pound sterling

()

I agree with Rohit about the balance machine qualification.

It was a nice focused discussion by John and Bill about proximity probes!

Walt

()