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Gearing Up for Hot Temperature - Thermal
Limits of Gearboxes
by Dr. Gerard Antony
Every gearbox manufacturer is familiar with calls from a concerned
gearbox user asking for advice: "My gearbox is running (or feels)
hot. What is wrong?" In most instances this concern is based on
a subjective feeling and not on temperature measurement. The general
rule is as long as you can touch the unit and hold your hands on
it for a few seconds, the temperature is in most cases not excessive.
The temperature limit for the gearbox components is generally significantly
higher than what a human touch can tolerate. The figure below shows
the common temperature limits for typical gearbox components.
Here, the temperature limit of a component is defined as the peak
temperature where the material characteristics of a component are
still within the range which allows a full satisfactory function
and structural integrity.
The temperature of a gearbox in operation will increase until the
heat balance of the internally generated heat plus the external
imposed heat reaches an equilibrium with the dissipated heat.
Sources of internal heat generation are the power losses in the
gearbox due to friction between the components in relative motion.
There is friction in the gear mesh, in the bearings, at the seals
and hydrodynamic friction between the lubricant and the moving components.
In worm gearboxes the gear friction is the dominating source for
heat generation due to the high sliding friction in the gear mesh.
Helical spur, bevel, Cyclo type gears and others have basically
rolling contact, so the heat is mainly generated by the hydrodynamic
friction (churning losses) and the friction between the seal and
shaft. Consequently worm gearboxes are less efficient and generally
have a "thermally limited power rating." Only low ratio, high speed
helical, spur, bevel and Cyclo gears would have a thermal power
limit.
External heat can be imposed by radiation such as direct sun exposure
or absorption of the high ambient temperature or other means.
The heat is normally dissipated by radiation through the gearbox
housing surfaces and through convection to the surrounding air,
surrounding structures and components. In cases where the ambient
conditions do not support an adequate cooling (heat dissipation),
additional external means are necessary such as a fan, special oil
cooler, etc.
Lubricant the Main Temperature-Limiting Component
Lubricants have important heat-related functions in the gearbox:
- The lubricant reduces the friction by separating the components,
avoiding metal to metal contact and so reducing the heat generation.
- The lubricant transports and distributes the heat from the friction
spots (gear mesh, bearing, seals) to the gearbox housing so the
surface can dissipate the heat
On the other hand the lubricant is the most heat (temperature)
sensitive gearbox component. The viscosity of the lubricant decreases
drastically with increasing temperature. Even though the lower viscosity
reduces somewhat the churning losses, it also decreases the ability
of the lubricant to fulfill its main function, namely separate the
components in contact by building an "EHD film" (Elasto Hydro Dynamic).
Without this separating film the components would score, wear out,
pit and fail within a short period.
As the figure shows, the lubricant and the seals have the lowest
thermal limit, but still far higher than the tolerance of the human
hand.
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