I think that one of the best ways to understand a subject is trying to build something that uses somehow materials that are part of this subject. I mean, if you want to know something about the wind, you can start building your own weather-vane, and probably you would need to read about this matter before starting to make anything until you gain the confidence to put your weather-vane at the top of the roof.
That's what have happened with this application. I am not a super gear expert but I have to deal with them, I have to decide if something has to be scrapped or not, and understand quality deviations over the tooth surface to move forward with the parts, test them, rework them (not always possible).
As many others technicians I had many doubts about how to interpret the information coming from quality, from the measurements of the gear profile, so I decided to read, and collect enough information to be sure that next time I have to review a gear report, I would have the chance to see it with a different perspective.
So, here you have some notes, links and a application created for android to define the deviation values of the profile (surface) of a gear tooth.
First of all, deviation calculations are based on the standard DIN 3691, which is similar to the ISO1328 or AGMA ISO 1328.
I will try to describe the three most important measurements (or type of measurement) that are always reported in a gear inspection:
Profile Deviations:
If you hear the word "profile" of a gear, you should relate it to the arc that goes from the bottom of the tooth to the top of it, basically up to the tip or outside diameter of the gear.
In general, deviations in the profile, are only measured at the 92% of the active length, and it is called "profile evaluation range".
As it is defined by the ISO 1328, "Profile deviation is the amount by which an actual profile deviates from the design profile. It is in the transverse plane and normal to the involute profile."
The image describe how the profile is obtained from the roll angle of the gear tooth.
Image taken from :http://www.gearsolutions.com/article/detail/5565/an-elementary-guide-to-gear-inspection
Type of Profile Errors:
Three important types of errors are related to profile, Profile Form Deviation, Profile Slope deviation and Total profile deviation.
Profile Form Deviation (ffa):
Distance between two involutes of the actual base circle, that enclose the actual involute profile within the profile inspection range
Can also be defined as the divergence of the measured profile from a best fit line or curve.
Profile Slope Deviation (fHa):
Distance between two nominal profiles that intersect the average profile at start and end points of the profile range or Profile slope deviation is the amount of deviation from a nominal involute profile over the evaluation range.
Total Profile Deviation: (Fa):
Is the total amount of profile error, including slope and form errors or the distance between two nominal profiles enclosed within the profile test range.
Images taken from:http://www.geartechnology.com/issues/1194x/smith.pdf
Helix deviations:
AGMA’s current inspection handbook defines “helix deviation” (formerly tooth alignment variation and lead variation) as the difference between the measured helices to the design
helices. In practice an appropriate measuring machine aligns the measuring probe on the
test gear at the pitch circle diameter and the “lead” is traced and recorded graphically,
with a correct unmodified helix being represented as a straight line on the chart. Helix
measurement is used to determine correct face contact between mating gears. Incorrect
helix will create uneven loading and noise.
The ISO 1328 explained in the following terms:
" Helix deviation is the amount, measured in the direction of the transverse base tangent, by which an actual helix deviates from he design helix ".
Image taken from :http://www.gearsolutions.com/article/detail/5565/an-elementary-guide-to-gear-inspection
Type of helix errors:
Helix Form Deviation (ffb):
Distance between two helical lines that enclose the actual lead within the lead inspection range. Helix form deviation is the divergence from a best fit curve along the helix
Helix Slope Deviation (fHb):
Helix slope deviation is the amount of deviation from a nominal helix over the evaluation range. Distance (in transverse plane) between two nominal leads that intersect the average lead (helix) at start and end points of the lead inspection range.
Total Helix Deviation (Fb):
Distance between the two nominal leads enclosed within the lead inspection range.The total amount of error including angle and form errors.
This picture is quite helpful because it gives you an idea bout the way the tooth is measured and what is the meaning of the traces showed in the right side.
Pitch Deviations:
The notation pitch, should make you think about tooth location, and the effect that this type of deviations would have into the gear motion and torque transferring. It is related to, backlash, contact ratio variation...
It is looking at the location of a gear tooth with respect to the others, as well as the global location between all gear teeth. Measures, the difference between the actual position of the tooth and the "should be here" position.
The application calculate the values of the pitch error (fu), the single pitch deviation (fp) and total pitch deviation (Fp).
Types of pitch deviations:
Single pitch deviation (fp):
It is the difference between two adjacent teeth index values (+,-) or the algebraic difference between the actual pitch and the corresponding theoretical pitch in the transverse plane, defined on a circle concentric with the gear axis at approximately mid-depth of the tooth.
Pitch Error (fu):
It is the difference between adjacent pitches.The difference between actual dimensions of two successive right or left flank transverse pitches.
Total cumulative pitch deviation (Fp):
Maximum cumulative pitch deviation of any sector of the corresponding flanks of a gear. It is represented by the total amplitude of the cumulative pitch deviation curve.
Pitch deviations
Gear index and pitch definitions.
Transmission error deviations (tangential composite deviations):
In this case, those deviations are related to the gear motion. You need to look at the gear in touch with a master gear and how the center of the gear changes when it revolves one complete turn. That will affect load transferring, and bearing cycle load reactions.
The application gives two different values:
Two flank working deviation( f´´i):
Maximum difference between the effective and theoretical circumferential displacements at the reference circle of the gear under inspection, when meshing with a master gear, testted product gear being turned through one complete revolution.
Single flank working deviation (f´i):
Value of the tangential composite deviation over a displacement of one pitch. It is a true tangential measurement and is indicative of the functional characteristics of the gear.The non uniform motion is called "transmission error"
As a reference, here you have also a table with the quality grades comparison between DIN and AGMA standards:
Finally, the link to the Application. You can find it at Google Play.
App Images:
Links to the different documents used to create this entry:
