DESIGN OF PLASTIC GEARS - Quality Transmission Components - #9

The allowable tangential force at the pitch circle is calculated by Equation (18-5).

Therefore, sliding speeds must be contained within recommendations of Table 18-20.

(m/s)

S liding sp ee d v_s = —

60000cosy

I— Ra — b , ,,

F = m —R-yba _bK_v

Lubrication of plastic worms is vital, particularly under high load and continuous operation.

Fastening of a plastic gear to the shaft is often done by means of a key and keyway. Then, the critical thing is the stress level imposed upon the keyway sides. This is calculated by Equation (18-10).

where:

y = Form factor at pitch point (by equivalent spur gear from Table 18-15)

^Zv=-uosir ^(18-6)

where:

R_a = Outer cone distance

8 = Pitch cone angle (degree)

z_v = Number of teeth of equivalent spur gear

Other variables may be calculated the same way as for spur gears.

The allowable tangential force F(kgf) on pitch circle of Duracon M90 bevel gears can be obtained from Equation (18-7).

(kgf/cm²)

a = Pressure on the keyway sides (kgf/cm²)

T = Transmitted torque (kgfm)

d = Diameter of shaft (cm)

I = Effective length of keyway (cm)

h = Depth of keyway (cm)

where:

The maximum allowable surface pressure for MC901 is 200 kgf/cm², and this must not be exceeded. Also, the keyway's corner must have a suitable radius to avoid stress concentration. The distance from the root of the gear to the bottom of the keyway should be at least twice the tooth whole depth, h.

Keyways are not to be used when the following conditions exist:

- Excessive keyway stress

- High ambient temperature

- High impact

- Large outside diameter gears

When above conditions prevail, it is expedient to use a metallic hub in the gear. Then, a keyway may be cut in the metal hub.

A metallic hub can be fixed in the plastic gear by several methods:

- Press the metallic hub into the plastic gear, ensuring fastening with a knurl or screw.

- Screw fasten metal discs on each side of the plastic gear.

- Thermofuse the metal hub to the gear.

where:

and y = Form factor at pitch point, which is obtained from Table 18-15 by computing the number of teeth of equivalent spur gear via Equation (18-6).

Other variables are obtained by using the equations for Duracon spur gears.

Generally, the worm is much stronger than the worm gear. Therefore, it is necessary to calculate the strength of only the worm gear.

The allowable tangential force F (kgf) at the pitch circle of the worm gear is obtained from Equation (18-8).

F = m_nybObKv (kgf)

The nature of the part and the molding operation have a significant effect on the molded gear. From the design point of view, the most important effect is the shrinkage of the gear relative to the size of the mold cavity.

Gear shrinkage depends upon mold proportions, gear geometry, material, ambient temperature and time. Shrinkage is usually expressed in millimeters per millimeter. For example, if a plastic gear with a shrinkage rate of 0.022 mm/mm has a pitch diameter of 50 mm while in the mold, the pitch diameter after molding will be reduced by (50)(0.022) or 1.1 mm, and becomes 48.9 mm after it leaves the mold.

where: m_n = Normal module (mm)

y = Form factor at pitch point, which is obtained from Table 18-15 by first computing the number of teeth of equivalent spur gear using Equation (18-9).

Zv = —^ (18-9)

Worm meshes have relatively high sliding velocities, which induces a high temperature rise. This causes a sharp decrease in strength and abnormal friction wear. This is particularly true of an all plastic mesh.