Understanding Gear Toothing and Gear Tooth Profile Geometry
7 min
Introduction
The foundation of gear toothing lies in the tooth profile, defined by a unique and exact geometry for every tooth. The precision of this profile dictates the gear's performance, including noise levels, load capacity, and long-term wear resistance.
This article aims to explain the fundamental parameters related to the geometry of the gear tooth profile. By becoming familiar with these parameters, engineers and students (as well as more experienced technical buyers) can gain further insight into why today's gears act the way that they do, and how thoughtful design results in a quiet, long-lasting gear.
The Base of Gear Toothing: The Involute Curve
Derivation and Principle of the Involute Curve
It is convenient to derive the tooth profile equation based on the involute curve. The involute is a curve obtained from another given curve, known as the base. It is traced by a point on a taut string that unwinds from the base curve for its entire length.
The involute is a curve that ensures, as far as is possible, whether the gears are of mated axes or not, that the constant velocity ratio is true throughout meshing. This is because the LoA (Line of Action), that path along which force is transmitted, always remains straight and constant in direction. Therefore, power delivery is smoother without a speed surge.
It is this singular property that has led to involute gear tooth generation becoming the worldwide standard for all spur, helical, and bevel gears.
Importance of the Base Circle
Base Circle is the root from which the involute gear tooth profile is formed. It is shorter than the pitch circle, and it is base of generating the involute curve.
This generated base circle forms the basis of every point on the working part of the tooth flank. The base circle also establishes the pressure angle (and is proportional to it. Note that this relationship affects how the gears mesh, and if the match isn't close enough, they will tend to jam up against each other).
Ratios of the Gear Tooth Profile Key Dimension
Pressure Angle and Its Selection
The angle between the line of action and a tangent to the pitch circle is called the pressure angle. Typical working values are 14.5°, 20°, and sometimes 25°; modern gear toothing by default uses a pressure angle of 20°.
- The choice of pressure angle has a direct influence on the gear's performance:
- The greater pressure angles allow transmitting higher loads and minimize undercutting.
- Lower pressure angles also provide smoother and quieter operation
- Greater pressure angles are associated with the radial forces being larger, possibly causing bearing loads to increase.
The pressure angle is selected by designers to obtain service life, noise, and strength.
Relationship Between Addendum and Dedendum
The addendum is the portion of the tooth above the pitch circle, and the dedendum is that below. They define the operating depth of the gear tooth profile.
In standard gear toothing systems:
- An addendum is usually one module
- Dedendum is slightly increased to allow for clearance
The correct ratios maintain full contact of mating gears but prevent interference at the tooth root. Wrong addendum and dedendum will make the teeth weak, too much noise, or lead to failure in advance.
Influences on the Hertzian Contact Stress and Load Capacity
Contact Ratio and Load Distribution
The contact ratio is the number of average tooth pairs in contact during meshing. The fact that the contact ratio is larger than 1 indicates that at least one tooth pair is always in contact for continuous transmission of power.
Higher contact ratios:
- Improve smoothness and reduce vibration
- Distribute the load over multiple teeth
- Lower surface stress and wear
Conventional spur gears have a contact ratio ranging from 1.2 to 1.6, and that for helical gears often exceeds 2.0 by the effect of overlapping contact, though it is not actually used alone.
Reduction of Fillet Radius and Root Stress
It serves as the radius of an arc that is used to connect the flank and root of the gear tooth. This feature is small, but it contributes significantly to the strength of the gear.
A larger fillet radius:
- De-stressing at the tooth root
- Improves fatigue life
- Reduction of the risk of tooth fracture
These days gear tooth design gives considerable importance to optimal fillet shapes, especially in high-load or age load range.
Modification and Customization of the Gear Tooth Profile
Profile Shift (Addendum Modification)
Generating machine profile modification (or addendum) is a radial movement of the generating tool during gear cutting. This changes the gear tooth profile while maintaining a constant module.
Profile shifting is used to:
- Change the central distance of the mating gears
- Distribute the strength of the pinion and gear evenly
The positive profile shifts support small pinions; the negative ones enable specific design requirements.
Tip Relief and Crown to Ensure Smooth Operation
Two typical gear tooth profile adjustments are addendum modification and crowning.
- Helix Relief Helical or spiral tip relief removes a small amount of material from the vicinity of the tooth’s tip and prevents edge contact under load.
- The crowning is a slight curve of the tooth face width to correct for angular misalignment.
Both features minimize noise, vibration, and localized stress, providing smoother and quieter gear tooth action.
Gear Tooth Profile Inspection and Control of Quality
Measuring Profile Error and Run out
The gear tooth profile after manufacture has to be checked for accuracy. Special tools such as involute checkers and gear measuring centers are frequently employed.
These tools measure:
- Profile deviation
- Lead error
- Pitch variation and run out
- Correct checking guarantees proper functionality of the gear toothing under operating conditions.
Tolerance Criteria and Their Influence on Gear Toothing
ISO and AGMA provide tolerance grades for gear tooth profile accuracy. For most precision grades, more careful geometry and surface finish are necessary.
Precision levels directly affect:
- Noise and vibration
- Load capacity
- Efficiency and service life
The choice of tolerance grade is a trade-off between production cost and desired performance.
Conclusion
The gear tooth profile is a key factor that reflects gear tooth performance. From the involute curve and base circle to pressure angle and full-depth teeth, no other gears demonstrate such exemplary design. The only variations pertain to the finer design characteristics that impact strength, efficiency, and durability.
To guarantee the precise profile geometry for mating gears, these parameters must be carefully designed and well-controlled to meet the demands of modern mechanical systems. Whether in industrial machinery, automobile transmissions, or precision machines, the importance of fundamental knowledge in gear tooth design cannot be overstated.
JLCMC understands the importance of micrometer-level tooth profile compensation in precision manufacturing. Through stringent process control, we ensure perfect meshing of every involute curve. If you're looking for gear transmission components that precisely match design intentions, visit our platform for expert support.
FAQs
What is the fewest number of teeth that will not undercut?
For 20 Pressure Angle (non-profile shifted) standard gears, this minimum is generally 17 teeth. The bound can be lowered with profile shifting.
What is the impact of wear on the tooth profile of an involute gear?
Wear slowly changes the tooth flank, which increases backlash and noise. Notwithstanding, the involute profiles accept some wear while proper tooth meshing is achieved.
When would you use a non-involute gear tooth profile?
Non-involute shapes, such as cycloidal gears, can also be used for specific applications where minimal backlash, small size, or unique motions are needed.
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