The harsh environmental conditions of winter make many of us think about the upcoming change of seasons. Soon, the frigid temperatures, black-iced roadways, and endless inches of snow will be gone, and temperate conditions will be the norm. Once the seasons change in June, the summer months will bring the heat and humidity that will once again make us wish for cooler weather.
Just like us, gearing is sensitive to both temperature and humidity. Detailed below are values for the coefficient of thermal expansion of common gearing materials at 100°C.
Steels and Alloys:
- Carbon Steel: 11.2-11.7μm/m • °C
- 4140 Alloy Steel: 12.2μm/m • °C
- 303 Stainless Steel: 17.2μm/m • °C
Copper Alloys:
- Brass: 18-19μm/m • °C
- Bronze: 17.5-18μm/m • °C
- Aluminum Bronze: 16.2-17μm/m • °C
Polymers:
- Acetal (POM): 85 to 110μm/m • °C
- Type 6 Nylon (PA6): 80 to 85μm/m • °C
- Type 6,6 Nylon (PA66): 80 to 95μm/m • °C
The effect of temperature in a gear system can be regulated in several ways. If a pair of gears is operating in an open environment, then the gears will be subject to thermal expansion based on the ambient temperature of the room. For some lower speed applications, typically gearing that is operating at less than 100 rpm, a grease lubrication can be applied to a gear pair. This type of lubrication helps to reduce the friction generated by the gear pair and will help to reduce the heat generated by the gear mesh. As speeds increase, it is necessary to use other methods of lubrication to reduce friction and heat generation. Tables 1 and 2 detail the best lubrication method for spur gears, bevel gears, and worm gearing based on the operating speed.
Enclosed gears operating at moderate speeds cannot use grease, as the centripetal forces on the gearing will cause the grease to be thrown from the gear mesh and stick to the inside of the housing. For these applications, oil is the preferred lubricant. It can be either splashed onto the gear mesh, or the gearing can be submerged in an oil bath. Splashed lubrication can be suitable for applications in which the used oil can be captured without affecting the environment. When oil is enclosed in a housing, it is important to add sufficient oil to remove the heat from the friction of the gearing — but not an excessive amount, as too much oil will result in frothing and a reduced capacity for cooling. Detailed in Table 3 are several diagrams of recommended oil levels for distinctive styles of gearing.
In addition to the effects of temperature on gearing, humidity can also negatively affect gearing. For polymer gears, the effect of humidity is to change the dimensions of the gearing as these materials are subject to water absorption.
Some of the effects of humidity are:
- Acetal (POM): Grows about 0.2% in length at 50% relative humidity and is capped at 0.7-0.8% at 100% relative humidity.
- Type 6 Nylon (PA6): Typically expands 0.5-0.6% in length at 50% relative humidity and expands 2.0-3.0% at full saturation.
- Type 6,6 Nylon (PA66): Typically expands 0.6% in length at 50% relative humidity and expands 3.5% at full saturation.
For carbon steel and alloy steel gearing, humidity will lead to rust formation and premature surface failure. For stainless steel, rust formation will still occur but at a slower rate. Brass, bronze, and aluminum bronze gears typically will have minimal issues in humid environments. When temperature and humidity affect our environment, we can book a flight to another climate that is either warmer or cooler, wetter or drier, all depending on the conditions that we prefer. Gearing does not have this luxury and must operate in the environment that we chose for it.
In order to maximize the life of your gear system, it is vital that you make the appropriate accommodation for temperature and humidity.
