Industrial Reduction Gears | Part I
by Jonathan Sowers, CLS
Senior Diagnostician

Gears are mechanical devices used to transfer power and speed from one shaft to another. This transfer may be rotational or linear in its output. Rotational energy output can be varied in regard to speed, torque and direction of rotation. For example, a large gear driving a small gear will produce a high speed in the output, but a lower torque than the input. However, a small gear driving a large gear will have a lower speed than the input, but a higher torque in the output. In either case the small gear is called a pinion and the large gear is called a bull gear, ring gear, or gear wheel. For this article we will focus on gear sets used to reduce high rotational speed from the input shaft to a useable lower speed in the output.

Gear types vary in design and function. Spur, helical and herringbone gears are used where the rotating shafts are parallel. When shafts are at right angles with intersecting centerlines, bevel and spiral bevel gears are used. Right angle shafts that have nonintersecting centerlines require worm and hypoid gears. Rack & Pinion and Internal Spur gears are also specialized designs.

Materials commonly used in manufacturing gears are include low carbon steel alloys with 0.25 % carbon to 1.7 % carbon; cast iron alloys with higher amounts of carbon, running approximately 1.7 % carbon to around 4 % carbon content; non-ferrous metals of which bronze (90% copper + 10% tin) is the most common and is widely used in worm gears. Metallic properties are enhanced with small amounts of additional alloying elements such as chromium, molybdenum or nickel. There are non-metal materials, primarily thermoplastics, and more exotic metal alloys that are also used.

Manufacturing process uses various heat treatments to produce the desired strength and ductility in the finished gear. Two types of common hardening processes for steel are through-hardened and case-hardened. In case-hardened gears the surface layer is harder than the interior of the material, while through-hardened refers to hardening so that the material hardness is the same throughout the piece. Heat treatments typically used to produce the desired hardness are Anneal, Normalize, Quench, and Quench & Temper. Surface hardening is accomplished through Induction/Flame and Carburizing. Surface finish of gears has significant impact on life of the gear and the lubrication requirements. Gear finish method – Hobbed, Shaved, Lapped, Lapped Run-in, Ground, Polished – effects surface roughness. Surface roughness of gear teeth is related to the lubrication regime that exists in a set of running gears.

It is the gear teeth that transfer power along the pitch line of contact when they are in mesh. Gear teeth slide into mesh, then roll, then slide out of mesh. The combination of sliding and rolling is common to all gear teeth; however the tooth design will determine the amount and direction of sliding and rolling contact. With spur and bevel gears only one pair of teeth is in contact at one time, with the entire load against that pair of teeth. These gears are used with moderate loads and moderate speeds. Other gear designs have multiple teeth in contact at one time sharing the load which allows for higher load application and faster speeds. The meshing of gear teeth also prevents slippage of the gears and synchronizes the rotating shafts.

Reduction gears are necessary to reduce high rotational speed from a power driver input shaft to a useable lower speed in an output shaft. This also results in increased output torque. Reduction gears are able to reduce rotational speed due to the difference in size of the pinion and the gear. For example, if you have a pinion that is 6 inches in diameter driving a gear that is 12 inches in diameter, the pinion gear shaft must rotate twice in order to rotate the driven gear shaft once. This produces a gear ratio of 2:1. Another way to determine the gear ratio is to count the number of teeth in each gear. If the larger driven gear has 80 teeth and the smaller pinion has 20 teeth, the gear ratio is 4:1. So, if you have a power drive rotating the input shaft at 1000 rpm (revolutions per minute) and need an output of 200 rpm, a gear ratio of 5:1 will be necessary to reduce the speed to the desired rate. This reduction also supplies higher torque in the output. For reduction ratios higher than approximately 10:1, multiple stages of reduction are required.

Figure A (below) shows a parallel axis single reduction gear set, and is comprised of a pinion (driver) and bull gear (driven) as seen in the double helical gear set here. This arrangement will have a speed reduction of approximately 10:1 with a horsepower range up to 30,000 hp.

Gears are classified as double reduction when reduction in speed occurs through two sets of gears. The cut-away image (Figure B) shows a view of parallel axis double reduction herringbone gear unit. This arrangement will allow speed reduction ratios of 5:1 up to 40:1, with a horsepower range of approximately 10,000 hp.

The triple reduction spiral bevel gear unit is a left-hand spiral bevel gear triple reduction gear set with a right angle output. When looking at the face of the spiral bevel pinion gear, the teeth are curved in a counterclockwise direction. This is referred to as a left-hand spiral bevel gear because of the direction the teeth are curved away from the axis. The reduction in speed is transferred from the pinion and drive gear through two sets of helical gears. This arrangement will allow reduction ratios of 6:1 up to 500:1, with a horsepower range of up to 3,000 hp.

Our last example is a worm gear right angle reduction gear set. The output shaft is at 90 degrees to the input shaft. This unit is a double reduction helical-worm gear set, because the input drive is through a helical gear set and the output is through the right angle worm gear set. The helical gear set provides the first reduction in speed, with the larger gear of the helical set connected to the worm. The second reduction is accomplished through the worm gear set with a change in the direction of output. This arrangement can produce speed reduction ratios or 6:1 up to 290:1, with a horsepower range up to 25 hp.