Regardless of the style of gear that needs to be designed, there are specific criteria that need to be specified in order to manufacture the product. Some of the values are determined by the designer and others are the result of standard formulas.
One of the criteria set by the designer is the number of teeth on the gear. The number of teeth that are machined onto the gear will contribute to the calculation of speed ratio. The value for the number of teeth can be as few as eight and as many as 1,000. When the number of teeth is less than 20, the gear strength is diminished.
In order to assemble a gear to a shaft, a center hole needs to be machined into the gear. This hole is designated as the bore of the gear. The most common shape for a bore is a round hole. Other options include a D-shaped bore, a hexagon-shaped bore, a square bore, or a double D-shaped bore. Considerations for the bore include the overall diameter of the gear, its weight, and the shaft speed. Ideally, the bore should be no less than one-eighth the diameter of the gear.
When looking at a gear, the outer most point is the tooth tip. The surface along the tooth tip is called the top land. The distance across this area is designated as the face width. The face width of a gear can be set to any value; however, it should never be less than double the value of the gear tooth height.
In addition to the width of the gear at the teeth, another important width is known as the length through the bore. This value includes the face width and the projection of any hubs on the gear. A gear where the length through the bore is equal to the face width is known as a hubless gear.
Gears typically have a hub projecting from one side. Whereas gears with no hub are commonly referred to as “A” style, those with a hub are known as “B” style. A gear with a hub permits the inclusion of a tapped hole into which a screw can be inserted to fix the gear to a shaft. Alternately, the hub can be drilled through during assembly and a pin can be inserted to fix the gear to a shaft. The hub diameter is set by the designer based on the bore diameter, the root diameter, and weight considerations. The hub projection is based on space requirements.
The pitch of a gear can be diametral pitch (DP), circular pitch (CP), or module. The selection of pitch and its value is determined by the designer when calculating the loading of the gear and the space constraints of the area where the gear will operate.
In addition to the pitch, the pressure angle of the gear tooth is selected by the designer dependent on the gear strength requirements. The most common pressure angle values are 20 degree and 14 degree 30 minutes.
The desired backlash is also set by the gear designer. Backlash is a value for the reduction in tooth size that is set in order to allow for the smooth operation of a gear. If allowance is not made, then the gear will not operate as desired.
Other criteria for the gear will be determined by formulas using the information provided. The pitch and number of teeth will determine the pitch diameter (also known as the reference diameter). These values will also determine the outer diameter and the root diameter. The difference between the outer diameter and the root diameter at the centerline of one tooth is the total tooth height.
These terms are critical to the understanding of the design and manufacture of gearing. Most of the geometry of a gear is user specific and can be adjusted to suit your individual application.
