Spur Gear Construction

Machine Design

Gear Drive

Introduction:

The gear construction may have different designs depending upon the size and its application.

Construction of spur gears

• When the dedendum circle diameter is slightly greater than the shaft diameter, then the pinion teeth are cut integral with the shaft as shown in Fig(a).

• If the pitch circle diameter of the pinion is less than or equal to 14.75 m 60 mm (where m is the module in mm), then the pinion is made solid with uniform thickness equal to the face width, as shown in Fig(b).

• Small gears upto 250 mm pitch circle diameter are built with a web, which joins the hub and the rim.

• The web thickness is generally equal to half the circular pitch or it may be taken as 1.6 m to 1.9 m, where m is the module.

• The web may be made solid as shown in Fig(c) or may have recesses in order to reduce its weight.
  

Gear with arms

The hub diameter is kept as 1.8 times the shaft diameter for steel gears, twice the shaft diameter for cast iron gears and 1.65 times the shaft diameter for forged steel gears used for light service. The length of the hub is kept as 1.25 times the shaft diameter for light service and should not be less than the face width of the gear.
The thickness of the gear rim should be as small as possible, but to facilitate casting and to avoid sharp changes of section, the minimum thickness of the rim is generally kept as half of the circular pitch (or it may be taken as 1.6 m to 1.9 m, where m is the module). The thickness of rim (t_R) may also be calculated by using the following relation, i.e.

where T = Number of teeth, and
n = Number of arms.

The rim should be provided with a circumferential rib of thickness equal to the rim thickness.