What is a Gearbox?
A box or case containing gear sets has many different names: speed reducers, reduction gearboxes, gear speed reducers, and gearboxes. Gearboxes are devices for the transmission of movement (precisely torque and speed) between two shafts, those shafts can have axis parallel, perpendicular, or cross to each other. In industry, gearboxes are used to transform the speeds and torques produced by the prime mover to ensure they are appropriate to the machine. The speeds and torques required by the machine are dictated by its use. Prime movers can generally only meet these requirements when combined with gears.
Gearing
It is easy to understand gearboxes and how they operate if we understand their internal mechanisms and their relationships. Gearbox components consist of the box (or housing), gears, bearings (or bushing), shafts, seals, and lubricant. Of course, the gears are the main component of any gearbox. Consequently, the rest of the components are there to support the function of the gearing such as housing will support the bearing, the bearing will support the shaft, and the shaft will have gears mounted on them. Seals prevent the leakage of lubricating oil.
From increasing torque and decreasing speed to changing rotational axes, gears are designed for numerous functions. But before we look at how it all goes together; we will delve into exploring the basics of gearing.
Circumference
Gears transmit rotational energy from one shaft to another. To do this, the gear teeth of one gear mesh with the gear teeth of another gear. Energy is transmitted at the point of contact, which lies on the circle (imaginary circle!) called the “pitch circle.” The diameter of the circle is known as the pitch circle diameter. By changing the diameter of one gear to a larger or smaller size, the surface contact area changes (circumference). It is this change that causes the speed of the output shaft’s rotation to change.
Gear Ratios
The easy way to translate circumference distance with gears is to calculate the gear ratio. To do this, we assume the gears are mechanically designed to work together (same gear pitch, face width, etc.), and then count the gear teeth. If the input (driver) gear has twenty teeth, and the output (driven) gear has twenty teeth, then it is said to have a “one to one ratio” (1:1). This means, that for every one revolution, the input gear makes, the output gear also makes one revolution. In most cases, gears are used to increase or decrease output speed. To increase a gear ratio, the gear circumference (number of teeth) of the input or output gear changes. If the input (driver) gear has ten teeth, and the output (driven) gear has twenty teeth, then it is said to have a “two to one ratio” (2:1). This means, that for every one revolution the input gear makes, the output gear make one-half a revolution, thus reducing the speed of the output shaft. Conversely, if the gears are transposed, so the input (driver) gear has twenty teeth, and the output (driven) gear has ten teeth then the output shaft speeds up. This means, that for every one revolution the input gear makes, the output gear makes two revolutions, thus increasing the speed of the output shaft. This configuration is referred to as a “speed up two to one ratio” (2:1).
Compounding Gear Ratios
Speed reducers incorporate compound gear sets in order to multiply the overall output gear ratio. Gearboxes can also run in tandem to multiply the overall speed output. For example, if an electric motor (prime mover) provides 1750 RPM and is affixed to a 5:1 speed reducer, the output shaft speed is 350 RPM. Other configurations include the addition of a belt-driven, chain-driven, or mechanical variable speed drive. Again, the speed reduction is a factor of the ratio. In the V-belt example below, the driver sheave is three inches in diameter and the driven sheave is six inches in diameter, thus producing a 2:1 ratio. By adding another gearbox to the configuration, the output speed is reduced even further. Using the example provided above, we add another 5:1 ratio speed reducer. As can be seen in Figure 7, the 350 RPM output becomes 70 RPM. Adding another reduction (2:1) by the V-belt drive reduces the 350 RPM output from the gearbox by half, making the final drive output of 175 RPM.
Torque v. RPM
A 1 hp motor operating at 1750 RPM produces a torque rating of 36 lbs./in. By reducing the RPM of the motor shaft through a 30:1 speed reducer, the torque produced becomes 1087 lbs./in. Consequently, every speed-reducing device added to the overall drive increases torque. In the example below, the initial shaft torque of the 1 hp motor is 36 lbs./in. After the first reduction (5:1 speed reducer) the torque increases to 180 lbs./in. Finally, in the V-belt drive speed reduction (2:1) the shaft torque is 360 lbs./in.
Reverse Rotational Direction
In combination with any speed-up or down ratio configuration, the direction of rotation is adjusted by the quantity of gears. Normal output gear rotation is always opposite that of the input gear. Apart from changing the direction of the input source, an intermediate gear can be added to ensure the same input and output gear rotation.
Synchronize Two Axes Rotation
The same basic theory is used to synchronize the rotation direction of two parallel shafts. With one input gear, two output shafts rotate in the same direction. By adding an intermediate gear, the two outputs rotate in opposite directions.
Change Rotational Axis
Depending upon the desired engineering configuration, it is sometimes necessary to change the output shaft’s axis. This means the output shaft is not parallel to the input shaft. While ninety-degree (right angle) speed reducers are most common, angles of varying degrees are also used.
Gearing Terminology
Like everything else, speed reducers have their own terminology. This next section provided a brief overview of common “need to know” industry phrases.
Efficiency – Important to any power transmission component is the efficiency rating. Usually stated as a percentage, speed reducer efficiency is the amount of the reducer’s output power compared to the amount of input power.
Axial Movement – Expressed in thousandths of an inch, axial movement is the distance measured through the input or output shafts – commonly referred to as endplay.
Backlash – Measured in thousandths of an inch at a radius on the shaft, the backlash is the rotational movement of the output shaft while the input shaft is held in a stationary position.
Thermal Rating – The thermal rating of a speed reducer is based on its ability to dissipate heat (generated by friction) while allowing the maximum continuous power/torque to be transmitted.
Prime Mover – The prime mover is the power source. The most common prime movers are electric motors, hydraulic motors, internal combustion engines, and air motors
Input Horsepower – The amount of power applied to the input shaft of a speed reducer.
Output Horsepower – Based on losses causes by gearbox inefficiencies, output horsepower is the power available at the output shaft.
Overhung Load – An overhung load is induced by a force (or weight) applied to a shaft at a right angle, beyond the outermost bearing. Overhung loads generate undue forces on shafts, gear sets, and bearings, and should be minimized.
Service Factor – Service factor (also referred to as “safety factor”) is a manufacturer’s predefined multiplier used to calculate speed reducer load. Because applications differ in severity, service factors are added to the speed reducer’s load ratings to ensure proper sizing.
Face Width – The measurement across the face of the gear.
Outside Diameter – On an individual gear, the outside diameter is the measurement from the top of the gear’s tooth to the diametrically opposed tooth.
Center Distance – On a gear assembly (most commonly used in sizing worm gearboxes), the center distance refers to the measurement between the centerlines of mating gears.
Pitch Diameter – The point at which two- gears meet and power is transmitted. The diameter is the measurement from the pitch point to the diametrically opposed pitch point.
Diametral Pitch – Defined in terms of a ratio, the diametral pitch is the ratio between the pitch diameter and the number of gear teeth.
Types of Gearing
Spur Gears
Spur gears (with straight-cut teeth) are simple in design and can be accurately produced. They are commonly used in planetary gear speed reducers, as the axial forces generated by inaccuracies and deformations (twisting) are usually too low to be considered.
Helical Gears
Helical gears with angled teeth run smoothly and can carry heavier loads than those with straight-cut teeth. Due to the additional axial forces produced, the shaft bearings must be designed to compensate accordingly.
Double Helical
The double helical allows for large tooth widths and can carry particularly heavy loads. The axial forces produced on each individual helical set cancel each other out. Unfortunately, deviations in tooth helix angles can cause axial vibration.
Internal Gears
Internal gears (gears with teeth on the inside) have greater load-carrying capacity than external, due to the favorable tooth contact. However, Internal gears are more difficult to produce. The most common use of internal gears is in planetary (epicyclical) gear reducer applications. Planetary gear reducers alone are complex, and depending upon the application, the bearing arrangement becomes more complex.
Bevel Gears
The common characteristic of bevel-type gearing is that the shaft axes intersect each other. There are three basic designs categorized by tooth form.
Straight Bevel Gears
Straight bevel gears are cut with teeth parallel to the shaft’s axis. The gear mesh begins and ends across the total tooth width. The relatively high noise limits the use of these gears to low-speed drives at moderate power.
Bevel / Helical
Bevel gears with helical (angled) straight teeth are usually ground and the mesh is gradual. Even though the total overlap is bigger, and noise behavior is lower than straight bevel-cut teeth, bevel gears are not frequently used.
Spiral Bevel
Spiral bevel gears with spiral cut angled, curved teeth have clear advantages in load capacity. Those with ground teeth are quieter than other bevel gear types. For bevel gears that have to transmit high power, spiral bevel gears are the most frequently used.
Hypoid Gears
In a hypoid gear set the pinion shaft axis is displaced so that the shafts do not intersect, but instead, cross. Hypoid gears usually have spiral-cut teeth. Their advantages derive from the larger pinion and smaller circumferential force for the same torque. Moreover, the axis displacement, often allows the pinion to be supported at both sides, which creates a stiffer bearing arrangement. The sliding motion in the longitudinal direction of teeth also improves noise behavior. However, the additional sliding motion in the mesh increases friction, wear, and risk of smearing, thus requiring the use of hypoid oils with high additive content. Hypoid gears are most commonly used in vehicle drivelines (for example differential).
Worm Gears
The worm and wheel shaft axes cross each other at a distance, and usually a 90° angle. Worm gears are suitable for large single-stage speed reduction. Worm drives are popular due to their quiet operation and vibration-damping characteristics. Unfortunately, the efficiency of worm gear sets is lower than bevel / spur and planetary gears because of the higher proportion of sliding motion. To reduce the friction, synthetic lubricants are now common. The most commonly used design is the cylindrical worm paired with a concave gear. The cylindrical worm can be hardened and ground to improve load-carrying capacity. It is also freely adjustable in the axial direction to simplify the bearing arrangement and mounting. There are two other designs: a concave worm with a concave gear, and the more seldom used concave worm with a cylindrical gear.
Types of Gearboxes
Gearboxes are characterized by having at least three members: the power input, power output, and the housing. Obviously, the input and output assemblies transmit power from one shaft to the other. It is also important to note that the housing transmits gear reaction forces to the base, which is important when sizing the speed reducer to a severe application.
The shaft input and output are parallel to each other designating it as a parallel speed reducer. The input gear (on input shaft P1) is smaller than the output gear (on output shaft P2) designating it as a speed-down reducer. Because there are only two gears, we know the output shaft rotation is opposite of the input shaft.
Parallel Gearbox
Named after the fact the shafts are parallel to each other, parallel gearbox components include housing, spur, helical, herringbone gearing, shafts, and bearings. The gearing used in parallel speed reducers offers the advantages of both mechanical efficiency and very low ratios (as low as 1:1). More significant in the selection of a parallel shaft gearbox is its relatively low cost. The disadvantages of parallel speed reducers are noise, lower load-carrying capacity, and high-end ratio (anywhere from 6:1 to 10:1 max).
Right Angle Gearbox
A right-angle gear speed reducer allows for input and output shaft configuration at a ninety-degree angle. Gear sets used in a right angle reducer are hypoid, spiral bevel, and the worm. Worm gear sets are most common in right angle reducers due to their low noise, smooth operation, and high ratios (as high as 70:1). However, it is the economical use of space that makes the right angle gearbox so popular, as it allows for a myriad of equipment design and safety options. There are many drawbacks to using worm-geared speed reducers. The fact that they are relegated to lower reduction ratios (5:1) creates the problem of design utilization. More than anything else, worm-geared reducers are inefficient.
Double Reduction
Double reduction gearboxes take advantage of all the right angle reducer characteristics, but they also accommodate even higher ratios.
Parallel Gearboxes
Named after the fact the shafts are parallel to each other, parallel gearbox components include housing, spur, helical, herringbone gearing, shafts, and bearings. The gearing used in parallel speed reducers offers the advantages of both mechanical efficiency and very low ratios (as low as 1:1). More significant in the selection of a parallel shaft gearbox is its relatively low cost. The disadvantages of parallel speed reducers are noise, lower load-carrying capacity, and a high-end ratio (anywhere from 6:1 to 10:1 max).
Planetary Gearboxes
Planetary gear speed reducers achieve high ratios in comparatively smaller operating envelopes. Planetary gearbox components include housing, sun gear, planet gear, ring gear, shafts, planet carriers, and bearings. The planetary (epicyclical) gear speed reducer is commonly used in industry due to its simple design: the central sun gear drives the planet gears, which revolve within a stationary internal ring gear. In contrast to the spur gear units previously described (the shafts of which are supported in stationary housings), the planetary gear unit has planet gears that are supported by bearings on the “planet carrier assembly”\ (short shafts) connected to a “planet arm” (planetary carrier). The revolving planet gears drive the carrier through the planet bearings, which are attached to the output shaft. Planetary gear drives are smaller in volume, overall weight, and centrifugal mass. Because the rolling and sliding velocities in the mesh are lower, the planetary reducer is quieter. The advantages led to a continuous increase in the economic importance of planetary gear drives in spite of their disadvantages (difficult inspection, maintenance, and repairs).
Shaft Mounted Gearboxes
Shaft-mounted gearboxes differ from all others due to the fact that the housing is mounted on the shaft it drives. The above picture illustrates where a shaft-mounted speed reducer is located on a conveyor head pulley. As can be seen, the speed reducer does not need support from anything other than the shaft. In addition, the electric motor is easily mounted to the speed reducer, again eliminating the need for external mounting support.
Because the shaft-mounted speed reducer operates like the foot-mounted helical gearbox, the same performance benefits apply. Benefits to shaft mounted speed reducers are easy installation and removal, true concentric mount fit, capability of handling high shock loads, high range of ratios, and a broad range of accessories to support any application need.
—-End—-
Keywords: Gearbox, Types, Spur Gear, Helical Gear, Bevel Gear, Worm Gear, Hypoid Gear, Double Helical Gear, Herringbone Gear, Planetary Gearbox, Internal Gear, External Gear, Gear Terminology.