Primitive Diameter vs. Pitch Diameter

Only a guess:

1.) For outside threads:

Pitch Diameter = diameter of a screw, measured at the narrowest part of the screw;

Primitive Diameter = diameter of the rod which is going to be turned into a screw, before the threads are cut into it;

2.) For inside threads:

Pitch diameter: the diameter of the widest part of the thread - that is, the deepest part of the valley between two thread

Primitive Diameter: the diameter that the metal must be drilled to, in order to cut a thread into it;

Tom

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tomfranpat,

Your guess is misleading. There are major and minor diameters and they are described as such.

Pitch diameter more closely resemble the contact or working area of the thread form, please refer to diagram page six.

Pitch diameter with reference to gearing has nothing to do with the spacing of car wheel studs or any other studs/ hole location. One correct term for this is "bolt circle" as in "6 holes equally spaced on a 5.25 in diameter bolt circle".

Pitch diameter is the diameter of the pitch circle of a gear. When gears are properly meshed their pitch circles will meet at a tangency point termed the pitch point. This then is the theoretical point at which the meshing gear teeth will maintain contact of their involute profiles throughout gear rotation. In a perfect world this contact point remains on the pitch circle and there is constant angular velocity of the gear train. However, due to errors in gear manufacturing such as circular pitch error, tooth-thickness variation, and profile error (Tooth-to-Tooth Composite error) the actual pitch diameter and pitch point varies resulting in variations of the gear angular velocity. Measuring this variation in the pitch point results in a minimum and a maximum diameter around the theoretical pitch diameter. The band formed by this area between the min and max diameters is sometimes referred to the primitive diameter. In the perfect world then the pitch diameter and primitive diameter would be one in the same.

The band formed by this area between the min and max diameters is sometimes referred to the primitive diameter.

Incorrectly termed at that. The term primitive diameter is a descriptive term used when discussing a "prime mover mechanism".

I can't comment on the use of the term primitive diameter when discussing prime movers but primitive diameter is also used in gear descriptions. I was incorrect in the description of the band area formed between the min and max diameters (long time since I've worked with gears) termed the primitive diameter. A more correct description from another engineering site is "useful diameter on a gear is the average diameter where there is contact between teeth. This diameter is called primitive diameter". So, measuring the pitch diameter of each tooth on a gear and taking the average is the primitive diameter. If the gear is manufactured in a perfect world where there are no errors then the average pitch diameter (primitive dia) is the same as the pitch dia. OldTechNewToys picture represents this. Good discussion!

Oh my mistake it was hidden under my "Buckeye" pipe wrench.

GA, I agree and propose the possibility the "primitive diameter is a misnomer or misrepresentation of the term "prime mover mechanism".

Never have I seen or heard the term "primitive diameter mentioned in gear tooth or thread form nomenclature

Anyone can get stuff twisted about if they don't use correct terminology

The following is taken from a patent claim for a "gear pump or motor", note claim #2:

1. Mechanism of transmission comprising two toothed wheels meshing with each other, characterized by the fact that the module of one of the said wheels is greater of 10 to 20% than this one of the other wheel, so that, when the wheel of great module is driving, the rotation of the mechanism takes place normally, the wheel of small module being duly driven while, when the wheel of small module is driving, a locking effect takes place between the teeth of the two wheels, preventing any rotation.

2. Mechanism as claimed in claim 1, characterized by the fact that the distance between the centers of the two wheels is higher than the sum of the radii of their two primitive circles.

3. Mechanism as claimed in claim 1, characterized by the fact that the end of the teeth of the wheel of small module is rounded.

4. Mechanism as claimed in claim 1, characterized by the fact that the end of the teeth of the wheel of small module is of square shape.

5. Mechanism as claimed in claim 1, characterized by the fact that each tooth of the wheel of great module has a surface substantially perpendicular to the radius of this wheel coinciding with the axis of the tooth, so that, when the wheel of small module is driving, the force exerted by a tooth of the wheel of small module on a tooth of the wheel of great module passes through the center of this last wheel, that produces the locking of the mechanism.

6. Mechanism as claimed in claim 5, characterized by the fact that the said surface of each tooth of the wheel of great module is constituted by a flat part.

7. Mechanism as claimed in claim 1, characterized by the fact that the end of each tooth of the wheel of small module is cylindrical.

8. Mechanism as claimed in claim 1, characterized by the fact that the wheel of great module comprises a number of teeth higher than the number of teeth of the wheel of small module.

9. Mechanism as claimed in claim 1, characterized by the fact that the wheel of great module comprises a number of teeth lower than the number of teeth of the wheel of small module.

10. Mechanism as claimed in claim 1, characterized by the fact that the two wheels have the same number of teeth.

2. Mechanism as claimed in claim 1, characterized by the fact that the distance between the centers of the two wheels is higher than the sum of the radii of their two primitive circles.

Excellent description of a prime mover mechanism

I've yet to use or refer to this "primitive Diameter" term. I am very familliar with the "pitdh diameter" or "PD" I did find a couple of illustrations from a few of my suppliers web pages that have useful calculators if you need them. No reference to "primitive diameter" could i find.

This one is from: http://www.precisiontwistdrill.com/techhelp/help_pages/pitch_diameter.asp

P = Pitch Diameter
D = Wire Diameter
W = Measurement Over Wires

Example:

And this one from;

http://www.rushgears.com/Tech_Tools/techupdates.php?fromDropDownMenuHeader

Hello niners12,

I believe your query can be assessed in posts #5,#6 and #7