Gravity makes the world turn, but (to continue this horrible metaphor) gearboxes are what make manufacturing go around. But, what are their limits?
In answer to that question, Dr. Gerard Antony, Sumitomo Machinery Corp. of America, presents a paper, “Gearing Up for Hot Temperature – Thermal Limits of Gearboxes.”
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.
As you know, friction can create, namely diamonds. It also has a less desirable creation, heat which makes live miserable for technicians working with rotating apparatus dealing with subsequent equipment failure.
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.
There is one variable that Dr. Antony introduces, an obvious – yet important – one: lubrication:
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.