Gears come in a wide variety of shapes and sizes, and are used in nearly every mechanical system. Here, we introduce you to some of the most common gear types that you may come into contact with as a maker.
SPUR GEARS are the simplest of gears, and are what most people envision when thinking of gears. They are wheels or cylinders with teeth arranged radially outward from the center. When working as a system (e.g. with multiple spur gears meshed together), the shafts of spur gears must be parallel to each other. Spur gears work well at lower speeds, but are noisy at high speeds because of the gear teeth impacting each other as they engage.
HELICAL GEARS are wheels or cylinders with the teeth cut at an angle relative to the axis of the gear. The teeth are curved along a helix shape (hence the name helical gear; they are sometimes called herringbone gears). Due to this helical profile, they engage more gradually than spur gears, thus providing smoother and quieter running gears. Helical gears can be arranged with their shafts parallel or at various angles relative to each other, depending on the teeth configuration. When in operation, helical gears tend to have higher friction due to the “sliding” of the gear teeth against each other. Also, helical gears have an axial load or thrust load on the gear shaft, since the gear teeth engage along a curve instead of straight on; in other words, the gear tends to “screw in or out” as it rotates. Double helix gears eliminate this issue by having a double set of teeth oriented in opposite directions. The axial thrust developed by one side of the gear is negated by the axial thrust generated by the other side in the opposite direction. Double helix gears are difficult to make and are very expensive, but they are necessary in certain instances.
BEVEL GEARS are wheels or cylinders with teeth arranged around a cylindrical cone with the tip lopped off. When two bevel gears with the same diameter and number of teeth are working together in a system, they are known as miter gears. Miter gears only change the gear shaft axis angle or axis of rotation; there is no change in rotational speed relative to the input or output gear. Similar to spur gears, bevel gears run well at lower speeds, but they get noisy at higher speeds. Engineers have combatted this issue by developing a bevel gear with a curved tooth profile. This type of gear is known as a spiral bevel gear.
HYPOID GEARS look very similar to spiral bevel gears, but they are designed so that the axis of the gear does not intersect the axis of the mating gear. The curved teeth are shaped along a hyperbola (hence the name hypoid gear). Hypoid gears produce much less noise than other gear types and also run more smoothly. They almost always have an axis of rotation 90° to that of the mating gear, and since the shaft axes of both gears do not intersect, it is possible to support both ends of the shafts of both gears. Hypoid gears are commonly used in automotive axles, where a gear reduction and right-angle change of direction are required. The disadvantage to hypoid gears is that their curved tooth profile creates an axial thrust force (similar to helical gears) that must be handled by support bearings. Also, they tend to be relatively expensive.
A WORM GEAR set consists of two gears: a worm and a worm wheel. In general terms, a worm gear looks like a screw and a worm wheel resembles a spur gear. A worm can have a few teeth, or one long tooth that is wrapped continuously around its base like a screw. Worm gears are typically used when a large gear reduction is required. The gear ratio of a worm drive is simply the number of teeth of the worm wheel divided by the number of teeth of the worm. So, for example, if a worm wheel has 40 teeth, and the worm has only one, then the worm has to rotate 40 times for every one revolution of the worm wheel (40:1), producing a 40:1 gear reduction ratio. Due to the large gear ratios of worm gear sets, they can also transmit a very large amount of torque. For example, when set in an ideal, frictionless state, our 40:1 ratio worm gear provides a torque 40 times greater than the input torque to the worm. Finally, worm gear sets typically cannot be back-driven, meaning that the worm wheel cannot drive the worm when a torque is applied to it. The worm gear set can only be moved by rotating the worm.
A winch is a great application of a worm gear set. The high gear ratio and subsequent increase in torque in only two gears makes this an excellent gear choice for a winch. Also, since the worm gear set cannot be back-driven, the winch’s load does not fall when no torque is applied to the crank handle (or worm). It is important to note that a worm bears an axial load directly related to the load on the worm wheel. The bearings used to support a worm must be able to handle this axial or thrust loading along with the rotation of the worm.
RACK AND PINION SET: So far, all of the gear types presented transmit only radial or rotational motion. A rack and pinion gear set, however, is used to convert rotational motion into linear motion. The pinion is a gear that looks like a regular spur gear. The rack is a straight bar with gear teeth cut along one edge of the bar. In a rack and pinion system, the rack is constrained such that it can only translate back and forth along a linear axis. The pinion gear engages the rack, and when it is rotated, the rack is driven in one linear direction. The rack can be driven in the opposite direction by rotating the pinion in the opposite direction. Car steering uses a rack and pinion gear set to translate the rotational torque from the steering wheel to a linear force applied to the tie rods, causing the wheels to turn.
PLANETARY GEAR SETS (also known as epicyclic gear trains) contain four main elements. The first is an outer ring gear with inward-facing teeth. Engaged with the ring gear are multiple planet gears. These planet gears are linked together via a carrier. A single, central gear known as the sun gear is engaged with all of the planet gears. The ring gear, carrier, and sun gear rotate about the same axis. Each planet gear rotates on an axis constrained by the carrier. As the carrier rotates, the planet gears all rotate about their individual axes while simultaneously “orbiting” about the sun gear. Planetary gear sets are used to transmit a large amount of torque in a relatively small, compact package. They are also able to behave differently, depending upon which part of the gear train is held stationary. They can work as a reduction gear, an increasing gear, or even a reverse gear. This makes planetary gears well suited to applications like automatic transmissions. In fact, this type of gear set is used in many applications ranging from heavy construction equipment to bicycle gear hubs.
General Gear Nomenclature
Before we get into any gear train design specifics and analysis, we need to discuss some general gear terminology. To accomplish this, let’s look at our simplest and perhaps most common gear example: the spur gear.
• ROOT: This is the bottom-most part of the gear tooth. The tooth height of two engaging gears is cut such that it is slightly less than the root of the gear. This is so that the tip of each tooth does not hit the root of the gear with which it is engaged.
• OUTER DIAMETER: This is a circle describing the outermost extent of a gear. The outer diameter circle lies on the tip of each tooth.
• PITCH: The pitch of a gear is the distance between the same point on one tooth and that of the next.
• PITCH CIRCLE: The pitch circle of a gear runs roughly through the center of the gear teeth. When properly aligned, the pitch circles of two engaged gears touch at a single tangent point.
• PITCH POINT: The point where the two pitch circles of two engaged gears touch. This is the point where the gear teeth make contact.
• CENTER DISTANCE: This is a critical part of gear system design that has to be correct for the gears to engage properly. It is the distance between the axis of rotation of one gear to that of a second gear. To find the proper center distance between two gears, add the pitch circle diameter of both gears and then divide by two.