If you have ever sat down for Sunday dinner in a traditional Italian-American home, you know not to question the red sauce known as “Grandma’s Gravy.” This tomato-based elixir coating your pasta is a gift from heaven itself. But the ketchup served with your French fries at the local fast food drive-thru is also a “sauce” made from tomatoes. Therefore, they must be interchangeable, right? Absolute sacrilege! However, this same type of comparison occurs all too frequently with gear materials; there are certain materials that are universally suitable for gearing, but others that are only appropriate for specific applications.
Gear racks
Gear racks are most frequently produced from carbon steel. This is a very suitable material for racks in most environments and under most use conditions. The reason this material works well is due to the relative strength of the rack tooth in mesh with the teeth on the pinion. In almost every application, the pinion is inherently the weaker member in the mesh and will fail long before the rack. The material also lends itself to being able to be heat-treated for additional strength, it permits the machining of additional features such as threaded or bolt holes, it maintains dimensional stability and can be straightened if necessary. Rack systems with requirements for high loads, high speeds, or high accuracy require ground gear racks. The best material for ground gear racks in these applications is alloy steel. Alloy steels are typically carburized and ground finished in order to add additional surface durability and precision. As with carbon steel racks, threaded holes or bolt holes can be added to allow for attachment.
For machinery that will be subjected to frequent wash downs or in food-processing environments, stainless steel is frequently used for gear racks. Most stainless steels share all the attributes of carbon steel except for the ability to be heat-treated for additional strength. Some stainless steels are also non-magnetic, which is necessary in some gearing applications. In some applications, if there is a need for the rack to be self-lubricating or if there is a need to reduce system weight, a plastic rack can be used. Plastic racks are very durable and useful, but only when selected for the proper environment. For example, acetal racks will not maintain their straightness over a long length (>1 meter) and are known to have the possibility of voids within the material. The possibility of voids in the rack makes the machining of bolt holes or threaded holes a risky venture. Alternately, nylon racks will not maintain their dimensional accuracy when exposed to varying temperatures or changes in humidity, as this material reacts to both temperature and moisture. For a direct replacement, based on the relative tooth strength, a nylon gear rack will need to have a face with or pitch that is six-times larger than that of a carbon-steel rack in order to handle the same loading.
Drive gear
For gearing that operates as a friction mechanism, such as a worm gear pair, it is critical the drive gear have a surface strength (durability) greater than that of the driven gear. It is because of this requirement that most worms are made from carbon or alloy steel, and most worm wheels are made from bronze, cast iron, or aluminum bronze.
Consider a 60:1 worm gear pair. In this arrangement, the worm will be rotating one tooth in and out of mesh continuously. However, the worm wheel will be engaging just one tooth every revolution. Therefore, each tooth of the worm wheel has a duty cycle of one worm revolution engaged and 59 revolutions disengaged. This implies the worm needs to have a durability 60 times greater than that of the wheel. If the wheel was produced from a material that has the same durability as the worm, the worm will fail in an accelerated manner due to scoring. This is a condition that occurs when metal-to-metal contact causes the tooth flanks of the gears to weld together. This process pulls metal from the pair and begins to scratch the tooth surface in the sliding direction. Although proper lubricant can minimize scoring, using materials that allow for heat dissipation is the better design consideration.
Bevel gears
Bevel gears are typically used in high torque environments and are usually produced from carbon steel, stainless steel, or alloy steel. Bevel gears can be produced with either straight teeth or spiral teeth. Carbon steel and alloy steel are used to produce both styles as these materials can be heat treated and ground finished. Stainless steel is typically used only for straight bevel gears. Carbon steel bevel gears are typically used in lower speed, lower torque, and commercial applications, whereas alloy steel bevel gears are designed for high speed, high torque, and precision applications. However, there are applications where acetal or nylon bevel gears are suitable, such as R/C helicopters, hand-crank applications, or for mass-produced toys. Plastics can be used to produce straight tooth bevel gears, but they cannot be used to produce spiral bevel, zerol bevel, or hypoid bevel gears, as the material would deform because of the heat generated by the cutting process.
Spur gears
Spur gears are the most popular form of gearing and can be produced in almost any material. Gears produced 100 years ago were typically produced from cast iron. This was an economical method for producing change gears for industrial equipment. Materials commonly used today are various types of steel, plastic, and specialty metals.
Steel spur gears
Carbon steel gears are economical and suitable for most applications. Carbon steels offer moderate strength, low cost, and ease of machinability. Stainless steels also offer moderate strength and ease of machinability but at a higher material cost. Some stainless steels are also non-magnetic. Alloy steels offer superior strength and can be hardened for superior durability but at a high raw material cost. All carbon steels, some stainless steels, and most alloy steels can be heat-treated to improve surface durability. All steels are dimensionally stable and suitable for keyways and tapped holes.
Plastic spur gears
Acetal gears are typically the most economical, as they are generally produced by injection molding. The moderate cost of raw material and low cost of machining make this a common material for prototyping. Acetal is a very dimensionally stable plastic due to its low absorption of moisture. It is recommended that acetal gears are not subjected to shock loading as they have a poor resilience to impact loading. Nylon gears are common in power transmission applications due to the material’s moderate cost, the ease of machining, the material’s self-lubricating properties, and its vibration absorbing abilities. Nylon gears are sometimes made from reinforced nylon. Depending on the reinforcing material, a nylon gear can improve its bending strength by 30 to 90 percent.
Fiberglass-filled nylons maintain the self-lubricating and weight saving properties, reduce the dimensional growth due to temperature and moisture, and increase the bending strength by 30 percent, albeit at a 50-percent increase in raw material cost. Carbon fiber-filled nylons also maintain the self-lubricating and weight saving properties, greatly reduce the growth due to temperature and moisture, and increase the bending strength by 90 percent. However, the raw material cost is at least 600 percent more expensive than unfilled nylons. Unfilled nylon gears have a bending strength that is one-sixth that of a similar sized carbon steel gear. This reduced torque carrying capacity is useful in the design of safety mechanisms within gear drives.
When designing a system, it can be useful to include a nylon gear so the gear can fail purposely if the torque of the system is exceeded and in turn keep the other components of the drive from being damaged. Many complex gear systems that are not supposed to be driven in reverse will include a sacrificial nylon gear for this reason.
Aluminum spur gears
Aluminum gears are used in many commercial applications. Aluminum has a high strength-to-weight ratio, which allows for its use in the production of fine pitch gearing. This makes it suitable for low torque, low speed, instrument drive-type applications. The major disadvantage of using aluminum for gearing is that it has a high coefficient of thermal expansion when compared to steel.
Spur gears are also produced from various specialty metals including titanium, Inconel, and other exotic alloys. In most cases, these specialty metals are used when designing gear systems for classified military applications and space applications. These materials exhibit the fatigue strength, density, and surface finishes required for long life cycles with limited field serviceability.
As detailed in these examples, there are many distinct materials for each style of gearing. Each material has unique features and properties that make it suitable for the application. The application will dictate what material should be selected for the gear design. Regardless of the material you select for your gearing, please don’t put ketchup on your spaghetti.
