Design engineers developing applications that rely on servo motors and gearheads must consider thermal issues. The effects of temperature changes should be a primary concern if the goal is top-notch performance and reliability. Servo motors and gearheads are heat sources, so it is important to understand the effects of both excessive heat and temperature variation.
When servo motors and gearheads are coupled together, it can be difficult determining which component is generating the heat, and what percentage of heat is dissipated rather than transferred to other components.
When considering the basic design criteria of gearheads, several factors relate directly to heat generation:
- Tooth geometry.
- Heat treatment/surface finish.
- Bearing selection.
There are several decisions engineers designing gearheads must make when choosing the tooth geometry for a gearhead. The first is usually the profile that will provide the best combination of low rolling contact and minimal sliding friction. The amount of sliding between gear teeth significantly affects the amount of heat generated. An involute tooth profile is the most common choice among gearhead manufacturers looking to optimize performance and minimize heat.
A feature on teeth helpful for heat reduction is gear-tooth tip relief, also called crowning. It reduces the amount of impact caused by one gear tooth engaging another. Optimizing the entry and exit points of the gears reduces vibration and noise and the sliding friction and heat generation.
Heat Treatment/Surface Finish
As all gears have teeth in contact, with some either rolling or sliding, it’s important to understand how heat treatment methods and the resulting surface finishes affect heat generation. (Heat generated from sliding friction is directly related to the gear’s surface finish and the ability to maintain a well-lubricated surface on the gear teeth.)
Common heat-treating processes for gears include carburizing and induction heating. Both of these processes involve high temperatures and quenching, so it is necessary to grind the gears after heat treating. The post-process grinding creates a surface finish that reduces the coefficient of friction between the teeth. The primary shortcoming of is that as the degree of surface finishing increases, the ability of the gear teeth to retain lubrication becomes more difficult as the pores of the metal become more closed.
In contrast, plasma nitriding leaves a hard surface without using excessive temperature to harden. It is a computer-controlled process where the case depth of the tooth hardness can be precisely controlled. Surface hardness is typically kept at 63 Rc for excellent wear characteristics while the core is kept at 37 Rc for excellent shear strength characteristics (see the figure below). Basically, it lets the tooth flex while under load while still having a hard surface that minimizes wear. In contrast, a brittle tooth might break.
Plasma nitriding is done at a lower temperature than carburizing or induction heating. There is no material distortion, so there is no need for post process grinding. The resulting surface finish retains lubricants and lets the meshing gears rotate smoothly. The reduction of friction between the gear meshes also reduces the amount of heat generated during operation.
Lubrication is key to gearhead performance. It reduces friction between gearhead components, which reduces wear. It also lowers the amount of heat generated during operation.
Synthetic lubricants are typically chosen based on their energy-efficiency benefits. They create a thicker lubricant film at operating temperature versus mineral oils and this reduces the extent of metal-to-metal contact and associated energy losses caused by friction. Other benefits of synthetic lubricants include:
- Better low- and high-temperature viscosity performance.
- Decreased evaporation losses.
- Resistance to oxidation, thermal breakdown, and sludge problems.
Oil or grease may also be chosen to lubricate a gearhead. Some of the pros and cons of oil and grease are shown below.
The choice of lubricant depends on other design performance requirements and constraints.
The type of bearings used in a gearhead determines how much radial load, axial thrust, and moment loads can be applied to its output shaft. It also determines how smoothly the gears rotate within the gearhead and how much torque it can generate given the side loads the bearings will need to support.
Different bearings can be chosen by a gearhead manufacturer. Deep- groove ball bearings are popular, as well as angular contact and tapered roller bearings. The choice typically depends on what the manufacturer is looking to accomplish.
Tapered roller bearings, for example, provide excellent load-carrying capabilities as they are cylindrical and the weight they support is distributed via a line contact. But this same line contact results in a larger surface area that generates additional friction and heat.
A deep-groove ball bearing supports its load via a point contact, so it cannot carry the same amount of weight as a tapered roller bearing. But it also operates at higher speeds without generating as much heat.