# Gear Glossary

### A

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**Addendum**- The radial distance of a tooth from the pitch circle to the top of the tooth.**Addendum Circle**- Basically an imaginary circle that passes through the addendum of gear teeth.**Angular Momentum**- How much an object is rotating around a fixed point.**Arc of Approach**- The curve traced by the pitch point form the beginning to the end of engagement.**Arc of Contact**- The path traced by a point on the pitch circle from the starting of the engagement to the end of the engagement for a given pair of teeth.**Arc of Recess**- The portion of the path of contact form pitch point to the end of the engagement.**Axial Pitch**- The distance parallel to the axis between corresponding sides of adjacent teeth.**Axial Thickness**- The distance parallel to the axis between two pitch line elements of the same tooth.

### B

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**Backlash**- The difference between the tooth space and the tooth thickness. It measures along the pitch circle. As in the manufacturing of the gear it is desirable to keep the backlash as close to zero but in actual practice some amount of backlash is provided to prevent jamming of the teeth because of the thermal expansion or tooth errors.**Base Circle**- A circle from where involute portion of tooth profile will be produced.**Base Diameter**- The diameter of the base circle of a gear.**Billet**- A part machined from a forged piece of metal rather than a casting.**Bottom Land**- The bottom portion of a gear tooth which will be extended below the pitch circle.**Brinell Hardness Number**- A value assigned to the hardness of metals and alloys.**Brittleness**- A tendency to fracture without appreciable deformation.

### C

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**Case Hardning**- Adding carbon to the surface of a mild steel object and heat treating to produce a hard surface.**Center Distance**- The center distance of 2 spur gears is the distance from the center shaft of one spur gear to the center shaft of the other.**Center-to-center Distance**- The center distance of two spur gears is the distance from the center shaft of one spur gear to the center shaft of the other.**Circular Pitch**- The measured distance along the circumference of the pitch diameter from the point of one tooth to the corresponding point on an adjacent tooth.**Circular Thickness**- Thickness of a tooth measure along the circumference of the pitch circle.**Clearance**- It the radial distance from the top of the tooth to the bottom of the tooth in a mating gear. A circle passing through the top of the mating gear is known as clearance circle.**Compound Gear Drives**- A compound gear is made up of two gears solidly connected. Often they are machined from the same stock or keyed to the same shaft.**Contact Ratio**- The ratio of length of arc of contact to the circular pitch. It is the denotation of number of pairs of teeth engaged in one contact**Contact Stress**- The maximum compressive stress within the contact area between mating gear tooth profiles.**Cross Pin Shaft**- A hardened shaft which installs into the case and keeps the spiders securely in place. In semi-float applications, it also prevents the axles from sliding inward into the carrier case.**Crowned Teeth**- They have surfaces modified in the lengthwise direction to produce localized contact or to prevent contact at their ends. Crowning can be applied to all types of teeth.

### D

**Dedendum**- The circle drawn through the bottom of the teeth.**Dedendum Circle**- It is the circle drawn through the bottom of the teeth.**Diametral Pitch**- The ratio of the number of teeth to the pitch circle diameter.

### E

### F

**Face of the Tooth**- The surface of the gear tooth above the pitch surface.**Face Width**- The width of the gear tooth measured parallel to its axis.**Flank of the Tooth**- The surface of the gear tooth below the pitch surface.**Force**- The push or pull of an object resulting in a change from rest or motion.**Friction**- The mechanical resistance to the relative movement of two surfaces.

### G

**Gear Backlash**- The difference between the tooth space and the tooth thickness, as measured along the pitch circle. The backlash should be zero, but in actual practice, some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion. But too much backlash also should be avoided. That leads to the gear chatter in the gear boxes.**Gear Chatter**- Noise due to vibratory torque transmission through the gears.**Gear Profile**- The tooth thickness, diametral pitch and pressure angle all go into determining the gear tooth profile. These factors are determined by the desired contact ratio between mating parts of the gear.**Gear Ratio**- The number of teeth of the larger gear to the smaller gear.**Gib Head Key**- These are keys tapered on the top surface to ensure a tight fit and having a raised head on one side so that its removal is easy. Gib keys are generally rectangular or square keys.

**Helix Angle**- The tooth angle relative to the central axis. Helix angles typically range from 10° to 45° depending on the design requirements. Gears with 0° helix angles are called spur gears. Helical gears (over 0°) increase contact ratio and improve gear noise, but also produce an axial load that has to be accounted for in the bearing design.**Helix Hand**- The direction of the helix angle expressed as either right or left. Looking down the central axis of the gear, a right hand helix angles from left to right down the tooth face away from you. The opposite is true for a left hand helix.**Hooke's Law**- The amount of spring force, compressed or stretched, is proportional applied to the spring deformation.

### I

**Interference**- The contact between mating teeth at some point other than along the line of action.

### J

### K

**Key**- Usually made from steel and is inserted or mounted between the shaft and the hub of the component in an axial direction to prevent relative movement.**Keyseat**- A recess in the shaft.**Keyway**- The recess (slot or groove) in the hub to receive the key and thus securely lock the component.**Kinetic Friction**- The force opposing two objects rubbing together that are moving relative to each other.

### L

**Longitudinal Stress**- The stress imposed on the long axis of any shape. It can be either a compressive or tensile stress.

### M

**Mechanical Energy**- The change in kinetic energy and potential energy generating from the force of gravity, external forces or the movement released in machine movement.**Module**- It is the ratio of pitch circle diameter millimetres to the number of teeth.

### N

**Normal Tooth Thickness**- The tooth thickness measured normal or perpendicular to the central axis of the gear, also at the pitch diameter.**Normal Plane**- A plane normal to the tooth surfaces at a point of contact and perpendicular to the pitch plane.**Number of Teeth**- The number of teeth contained in a gear.

### O

**Outside Diameter**- The diameter of the circle that contains the tops of the teeth of external gears.

### P

**Path of Contact**- The path travelled by the point of contact of two teeth from the starting of the engagement to the end of the engagement.**Pitch**- The pitch represents the size of each tooth on the gear.**Pitch Circle**- An imaginary circle that divides the gear teeth in two portion, top lands and bottom lands.**Pitch Diameter**- The diameter of the pitch circle.**Pitch Point**- It is a common point of contact between two pitch circles.**Pitch Surface**- It is the surface of the imaginary rolling cylinder that the toothed gear may be considered to replace.**Path of Contact**- The path traced by the point of contact of two teeth from the beginning to the end of the engagement.**Polar Pitch**- The proportion of the quantity of teeth to the width of contribute circle millimeter.**Pressure**- The force exerted perpendicular to the surface of an object and is expressed as force per unit area.**Pressure Angle**- The pressure angle figures into the geometry or form of the gear tooth. The angle between the direction of contacting force on the contact point of the tooth and direction of motion of that point on the tooth profile.**Profile**- It is the curvature contained by the face and flank of the gear tooth.

### Q

### R

**Rectangular Key**- They are wider than their height and are sometimes called flat keys. The extra key width allows it to transmit greater torque without increasing the depth. An increase in depth means a weaker shaft due to a reduction in effective shaft cross-sectional area.**Root Circle**- A circle that passes through the root of gear teeth and the diameter of root circle.**Root Diameter**- The diameter at the bottom of the tooth.**Round Pitch**- The separation between a point of a tooth to a similar purpose of the adjoining tooth, estimated along periphery of the pitch circle.

### S

### T

**Teeth Number**- The number of teeth of the gear.**Tooth Depth**- It is the radial distance between the addendum and dedendum circles of a gear.**Tooth Face**- The surface between the pitch line element and the tooth tip.**Tooth Space**- The width of space between the two adjacent teeth measured along the pitch circle.**Tooth Surface**- The total tooth area including the flank of the tooth and the tooth face.**Tooth Thickness**- The width of the tooth measured along the pitch circle.**Top Land**- The top portion of a gear tooth which will be extended above the pitch circle.**Torque**- A type of force that is applied to an object that results in the object rotating around an axis.**Torque Speed**- A motor converts electrical energy into mechanical energy by the rotational motion.**Total Depth**- The radial distance between the addendum and the dedendum circles of a gear. It is equal to the sum of the addendum and dedendum.**Transverse Tooth Thickness**- The tooth thickness measured normal or perpendicular to the helix angle—also at the pitch diameter.

### U

### V

**Velocity**- The rotational speed of a gear. Is expressed as the distance a point along the circumference of the pitch circle will travel over a given unit of time.

### W

**Whole Depth**- The distance from the top of the tooth to the bottom of the tooth.**Woodruff Key**- A semi-circular disc and fits into a circular recess in the shaft machined by a woodruff keyway cutter. These keys are mostly used in machine tools and automobile shafts and cannot carry the same load as long parallel keys.**Working Depth**- The depth of engagement, when two mating gear meshes with each other.