thermal rating of worm gearing:

Introduction:

In the worm gearing, the heat generated due to the work lost in friction must be dissipated in order to avoid over heating of the drive and lubricating oil.

Calculations of thermal rating of worm gearing:

The quantity of heat generated (Q_g) is given by

Q_g = Power lost in friction in watts = P (1 – η)                 ...(i)

where

P = Power transmitted in watts, and
η = Efficiency of the worm gearing.

The heat generated must be dissipated through the lubricating oil to the gear box housing and then to the atmosphere. The heat dissipating capacity depends upon the following factors :

1. Area of the housing (A),

2. Temperature difference between the housing surface and surrounding air (t₂ – t₁), and

3. Conductivity of the material (K).

Mathematically, the heat dissipating capacity,

Qd = A (t₂ – t₁) K                  ...(ii)

From equations (i) and (ii), we can find the temperature difference(t₂ – t₁). The average value of K may be taken as 378 W/m²/°C.

Notes :

1. The maximum temperature (t₂ – t₁) should not exceed 27 to 38°C.

2. The maximum temperature of the lubricant should not exceed 60°C.

3. According to AGMA recommendations, the limiting input power of a plain worm gear unit from the standpoint of heat dissipation, for worm gear speeds upto 2000 r.p.m., may be checked from the following relation, i.e.

where

P = Permissible input power in kW,
x = Centre distance in metres, and
V.R. = Velocity ratio or transmission ratio.