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Involute Spline sketching

Question asked by Joseph Pizzoferrato on Jul 31, 2013
Latest reply on Aug 2, 2013 by Jim Sculley

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Hello,

I am trying to create an involute spline curve for gear teeth on the outer profile of a part.

Spline Data is as follows:

# teeth = 128

Pitch = 10/20

Pressure angle = 45

Base dia = 9.05

Major dia = 12.9

Pitch dia = 12.8

Form dia = 12.71 Max

Minor Dia = 12.67 Min

Pin Dia = .1920

The teeth sit on a 12.9'' OD.

I calculated a few items i needed:

Pitch(teeth/inch) = 10

Tooth thickness = p/2 = .157

Addendum distances = 0.1

deddendum distances = 0.125

I have attached a picture of what i have so far. The pressure angle with the horizontal is 45 degrees.

The base dia is off the screen and drawn at 9.05'' OD.

I do not have data for the gear that will mesh with the gear I am creating. Do i need any meshing data from the other gear?

I highlighted the top of the tooth in red where the pitch dia and addendum dia meet to create the top of the tooth.

Does something not look right?

thanks,

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Involute Spline.JPG185.4 KB

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Roland Schwarz Jul 31, 2013 5:01 PM

45° pressure angle? Zoiks!

Base diameter is determined by pitch diameter and pressure angle. Involute is built from the base diameter.

For the involute profile, you don't need to know the other gear's diameter or pressure angle. However, the pitches need to match.

Part of the beauty of the involute is that it still works if the gear center distances are off. The pitch diameter and pressure angle will be different IAW the contact point.

See attached.

Attachments

Tick Gear--28Jan2013.SLDPRT.zip1.1 MB

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Joseph Pizzoferrato @ Roland Schwarz on Aug 1, 2013 9:43 AM

Why is their base dia so small? It seems like the teeth will be quite large.

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Joseph Pizzoferrato @ Joseph Pizzoferrato on Aug 2, 2013 11:22 AM

I'm having a hard time drawing the teeth. Roland, do you have any instructions for me that would help.

thanks

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Peter Biggert @ Joseph Pizzoferrato on Aug 2, 2013 3:33 PM

Do you need to draw the involute curve or do you want to approximate the teeth so they look reasonable?

To draw an involute you need to construct several lines that are tangent to the base circle. Make the next line longer than the one before. The additional length added to the next line equals the arc length along the base circle from the start of one line to the start of the next. The end points of the construction lines are points on an involute curve.

I usually approximate the teeth using circular arcs. I don't have time to go into details but I use the base diameter, transverse base pitch, transverse circular tip thickness, and tooth height. If it is a helical gear I use the lead to define the helical path.

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Jim Sculley @ Peter Biggert on Aug 2, 2013 4:00 PM

Equation Drive Curves using the 'Parametric' option eliminate the need for approximations and multiple lines and arcs. You can input the involute equation directly which generates a true involute curve that you can use to sketch the tooth.

Jim S.

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5 Replies

Roland Schwarz Jul 31, 2013 5:01 PM
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Correct Answer
45° pressure angle? Zoiks!

Base diameter is determined by pitch diameter and pressure angle. Involute is built from the base diameter.

For the involute profile, you don't need to know the other gear's diameter or pressure angle. However, the pitches need to match.

Part of the beauty of the involute is that it still works if the gear center distances are off. The pitch diameter and pressure angle will be different IAW the contact point.

See attached.
Attachments
- Tick Gear--28Jan2013.SLDPRT.zip1.1 MB
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- Joseph Pizzoferrato @ Roland Schwarz on Aug 1, 2013 9:43 AM
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  Why is their base dia so small? It seems like the teeth will be quite large.
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  - Joseph Pizzoferrato @ Joseph Pizzoferrato on Aug 2, 2013 11:22 AM
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    I'm having a hard time drawing the teeth. Roland, do you have any instructions for me that would help.
    
    thanks
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    - Peter Biggert @ Joseph Pizzoferrato on Aug 2, 2013 3:33 PM
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      Correct Answer
      Do you need to draw the involute curve or do you want to approximate the teeth so they look reasonable?
      
      To draw an involute you need to construct several lines that are tangent to the base circle. Make the next line longer than the one before. The additional length added to the next line equals the arc length along the base circle from the start of one line to the start of the next. The end points of the construction lines are points on an involute curve.
      
      I usually approximate the teeth using circular arcs. I don't have time to go into details but I use the base diameter, transverse base pitch, transverse circular tip thickness, and tooth height. If it is a helical gear I use the lead to define the helical path.
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      - Jim Sculley @ Peter Biggert on Aug 2, 2013 4:00 PM
        Mark Correct
        Correct Answer
        Equation Drive Curves using the 'Parametric' option eliminate the need for approximations and multiple lines and arcs. You can input the involute equation directly which generates a true involute curve that you can use to sketch the tooth.
        
        Jim S.
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Involute Spline sketching

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