Gears are one common way to transmit power and change the output torque or speed of a mechanical system. Understanding these basic concepts is required to make optimized design decisions which consider the trade-off between torque and speed for a system with a given power.
Speed is the measure of how fast an object is moving. The speed of an object is how far it will travel in a given amount of time. For rotating parts like gears and wheels, speed is expressed in how many revolutions are made in a given amount of time. Under ideal conditions, the rotation of a wheel is converted into linear speed and can be calculated by multiplying the diameter of the wheel by the rotations for a given time. The SI unit for speed is meters per second (m/s), but speed is also commonly expressed in feet per second (ft/s).
Torque is roughly the measure of the turning force on an object like a gear or a wheel. Mathematically, torque is defined as the rate of change of the angular momentum of an object. This can also be stated as a force that acts normal (at 90 degrees) to a radial lever arm which causes the object to rotate. A common example of torque is the use of a wrench in order to tighten or loosen a bolt. In that example, using a longer wrench can produce more torque on the bolt than using a shorter wrench. Torque is commonly expressed in N⋅m or in⋅lbs.
When torque is turning an object like a spur gear, the gear will create a straight line (linear) force at the point where the teeth contact the other gear. The magnitude of the torque created is the product of the rotational force applied and the length of the lever arm ,which in the case of a gear, is half of the pitch diameter (the radius).
Power (P) is the rate of work over time. The concept of power includes both a physical change and a time period in which the change occurs. This is different from the concept of work which only measures a physical change. The difference in these two concepts is that it takes the same amount of work to carry a brick up a mountain whether you walk or run, but running takes more power because the work is done in a shorter amount of time. The SI unit for power is the Watt (W) which is equivalent to one joule per second (J/s).
In competitive robotics, the total amount of available power is determined by the motors and batteries allowed to be used. The maximum speed at which an arm can lift a certain load is dictated by the maximum system power.
Meshing two or more gears together is known as a gear train. Selecting the gears in the gear train as larger or smaller relative to the input gear can either increase the output speed or increase the output torque, but the total power is not affected.
When a larger gear drives a smaller one, for one rotation of the larger gear the small gear must complete more revolutions - so the output will be faster than the input. If the situation is reversed, and aa smaller gear drives a larger output gear, then for one rotation of the input the output will complete less than one revolution – so the output will be slower than the input. The ratio of the sizes of the two gears is proportional to the speed and torque changes between them.
The ratio in size from the input (driving) gear to the output (driven) gear determines if the output is faster (less torque) or has more torque (slower). To calculate exactly how the gear ratio effects the relationship from input to output, find the ratio for the number of teeth between the two gears. In the image below, the ratio of the number of teeth from the input gear to the output gear is 72T:15T which means the input needs to turn 4.8 rotations for the output to complete one rotation.
What happens when a 45 tooth idler gear is inserted into the gear example? An idler gear is any intermediate (between input and output) gear which does not drive any output (work) shaft. Idler gears are used to transmit torque over longer distances than would be practical by using just a single pair of gears. Idler gears are also used to reverse the direction of the rotation of the final gear.
Regardless of the number or size of idler gears in the chain, only the first and last gear determine the reduction. Since idler gears do not change the gear reduction, the reduction in the example remains 72:15, but the direction of the output stage is now reversed from the previous example.
Idler gears are a good way to transmit power across distances in your robot. A common example of this is an all gear drivetrain. In this example the gears on the end are linked to the drive wheels and one of the center gears would be driven by a motor (not shown). The orange arrows indicate the relative rotation of each of the gears showing that the two wheels are mechanically linked and will always rotate in the same direction.
Because idler gears reverse the direction of rotation, it is important to pay attention to the number of gears in the drivetrain. In the picture below there is an even number of gears, and because of this the wheels will always spin in the opposite direction.
Some designs may require more reduction than is practical in a single stage. The ratio from the smallest gear available to the largest in the REV 15mm Build System is 125:15, so if a greater reduction then 8.3 is required, multiple reduction stages can be used in the same mechanism, this is called a compound gear reduction. There are multiple gear pairs in a compound reduction with each pair of gears linked by a shared axle. Below is an example of a two-stage reduction. The driving gear (input) of each pair is highlighted in orange.
Reduction is the concept lowering input speed to reduce overall output speed.
To calculate the total reduction of a compound reduction, identify the reduction of each stage and then multiply each reduction together.
Where:
CR is the total Compound Reduction
Rn is the total reduction of each stage
Using the image above as an example the compound reduction is 12:1.
For any gear system there are a limited number of gear sizes available, so in addition to being able to create greater reductions using compound reductions it is also possible to create a wider range of reduction values, or the same reduction of a single stage, but with smaller diameter gears.
When REV Robotics Gears are used in conjunction with the slots on Extrusion or Channel, the center to center distance between axles is completely adjustable. Slide and re-tighten the shaft mounting plates anywhere along the slots to mesh gears together. This system allows any combination of compatible REV Robotics Gears to be used together, allowing for a high level of flexibility. When adjusting the reduction of a system, just a single gear can be replaced, reducing the amount of reassembly time.
Any combination of gear sizes can be used together. However, REV Plastic Gears and Metal Gears should NOT be used in conjunction with each other.
When using the pitch featured on the Extended Motion Pattern there will not be the same level of flexibility for gear spacing. Flexibility can be maintained by using a combination of the slots and the pattern, which allows more gear options. If just the pattern is being used then center to center distance calculations will be more prevalent.
In order for gears to work effectively, and not become damaged, it’s important that the center-to-center distance is correctly adjusted. The gears in DETAIL A ,of the figure below, may work under very light load, but they will certainly not work and will skip under any significant loading. The gears in that example are too far apart and so the teeth of each gear barely contact each other. The gears in DETAIL B are correctly spaced and will provide smooth and reliable operation.
To correctly space REV Robotics Gears along slots, use the following procedure:
Securely fix the axle of either the input or output gear. In the case of a gear train with multiple idlers or a compound reduction, consider which axle makes the most sense to fix, such as the very first input gear or the very last gear.
Starting with the fixed axle, identify all the driving and driven gears for any gears on that axle. One by one, loosen these axles and slide them until the teeth of both gears are fully engaged and parallel. Re-tighten the axle mounts.
Continue the procedure from Step 2 for each fixed axle until all the gears are tightly meshed and all the axles have been re-tightened.
When using a gear train in conjunction with the Extended Motion Pattern, only certain combinations of gears will work.
To learn more about pitch diameter return to the top level Gear page.
The gear spacing adjustment procedure above is sufficient in cases where the slot system is being used. To mathematically calculate the correct center to center distance use the Gear Measurements section to determine the pitch diameter (PD) of both gears.
Where:
CCD is the Center to Center Distance
PD1 is the pitch diameter for gear one
PD2 is the pitch diameter for gear two
Using the example in the figure with the 60 tooth and 72 tooth gears, the correct center to center distance calculation for a 72T:60T gear ratio is 49.5 mm.
Outside Diameter A
Pitch Diameter B
15 Tooth Gear
0.5 in
12.7 mm
0.44 in
11.25 mm
30 Tooth Gear
0.95 in
24.0 mm
0.89 in
22.5 mm
45 Tooth Gear
1.39 in
35.3 mm
1.33 in
33.75 mm
60 Tooth Gear
1.83 in
46.5 mm
1.77 in
45.0 mm
72 Tooth Gear
2.19 in
55.5 mm
2.13 in
54 mm
90 Tooth Gear
2.72 in
69.0 mm
2.66 in
67.5 mm
125 Tooth Gear
3.75 in
93.5 mm
3.69 in
93.75 mm
The Motion Interface Pattern is a circular M3 hole pattern on a 16mm diameter that interfaces with certain REV Brackets and the UltraPlanetary 5mm Hex Output (REV-41-1604).
The 15 tooth and 30 tooth plastic gears do not have a motion pattern due to size constraints. However, they share the same 0.75 module as the other acetal gears. The table below provides the outer diameter and pitch diameter of these gears.
Outside Diameter A
Pitch Diameter B
15 Tooth Gear
0.5 in
12.7 mm
0.35 in
9 mm
30 Tooth Gear
0.95 in
24.0 mm
0.89 in
22.5 mm
Outside Diameter A
Pitch Diameter B
42 Tooth Gear
1.39 in
35.2 mm
1.32 in
33.6 mm
56 Tooth Gear
1.83 in
46.4 mm
1.73 in
44.0 mm
70 Tooth Gear
2.27 in
57.6 mm
2.20 in
56.0 mm
86 Tooth Gear
2.77 in
70.3 mm
2.71 in
68.8 mm
The Motion Interface Pattern is a circular M3 hole pattern on a 16mm diameter that interfaces with certain REV Brackets and the UltraPlanetary 5mm Hex Output (REV-41-1604).
The 12 Tooth and 28 Tooth Metal Gears do not have a motion pattern due to size constraints. They are also made from sintered steel rather than aluminum. However, they share the same 0.8 module as the other metal gears. The table below provides the outer diameter and pitch diameter of these gears.
Outside Diameter A
Pitch Diameter B
12 Tooth Gear
0.44 in
11.2 mm
0.38 in
9.6 mm
28 Tooth Gear
0.95 in
24.0 mm
0.88 in
35.2 m
The 12 Tooth Pinion Gear (REV-41-1660) while sharing the 0.8 module differs from the other metal gears significantly. See the drawing for relevant information for the 12 Tooth Pinion Gear.
Gears have teeth that mesh with other gears in order to transmit torque. Gears can be used to change the speed, torque (turning force), or direction of a motor’s original output. For gears to be compatible with each other, the meshing teeth must have the same shape (size and pitch).
Gears offer more flexibility in transforming motion than sprockets and chain because there are a larger variety of gear sizes available.
There are many different types of gears; one of the simplest and most commonly used is a spur gear, and that is the gear type used in the REV DUO Build System. Spur gears consist of a disk with straight teeth projecting radially (outward from the center) and these gears will only mesh correctly with other gears if they are on parallel shafts.
Common and important features of a spur gear are highlighted in the image below.
Number of Teeth (N) is the total count of the number of teeth (projections) around the whole circumference of a gear. For gears with very few or very large teeth it is easy to simply count the number of teeth. However, for gears with a higher number of teeth, or when the teeth are smaller, attempting to count the teeth is not very practical or accurate.
Module (M) represents the amount of pitch diameter in mm per tooth. Gears with a higher module will have bigger teeth. Module (M) can be calculated using some combination of pitch diameter (PD), number of teeth(N), or outside diameter(OD).
Equations for calculating module:
To help with calculations: REV Plastic Gears have a 0.75 module, while REV Metal Gears have a 0.8 module.
Pitch Diameter (PD) is the imaginary circle that mates with any other gear’s pitch diameter when the gears are properly spaced. The pitch diameter will always be smaller than the outside diameter of a gear.
When creating simplified models for gears, first create a circle with the pitch diameter for each gear used in the system then constrain the circles tangent (just barely touching) to each other. Pitch Diameter (PD) can be calculated using some combination of module(M), number of teeth (N), or outside diameter (OD).
Equations for calculating pitch diameter:
Outside Diameter (OD) is the true outside diameter of a gear. The outside diameter will always be larger than the pitch diameter. The outside diameter should be used when checking for interference when placing gears very close to other structures. Outside Diameter (OD) can be calculated using the formula below.
Equations for outside diameter:
† The majority of Metal Gears in the REV DUO Build System are made using aluminum. There are three exceptions. The 12 Tooth and 28 Tooth Gears are made out of Sintered Steel. The 12 Tooth Pinion Gear is made out of Machined Brass.
REV Metal Gears are NOT compatible with REV Plastic Gears. Gears from other building systems may have a very similar tooth profile but are not an exact match. It may be possible to use the two gears systems together successfully in some situations, but it is not recommended.
The face of the REV DUO Metal Gears displays the gears tooth count to help with calculations!
Meshing two or more gears together is known as a gear train. Selecting the gears in the gear train as larger or smaller relative to the input gear can either increase the output speed, or increase the output torque but the total power is not affected.
Physics concepts, like speed and power, have a lot of applications in the REV 15mm Build System. Click here to learn more about them.
A gear ratio is the ratio of the sizes of two gears. For instance, in the image below, the input gear is a 15 tooth gear and the output gear is a 72 tooth gear. So, the gear ratio is 72T:15T. The ratio in size from the input (driving) gear to the output (driven) gear determines if the output is faster (less torque) or has more torque (slower). The gear ratio is proportional to the speed and torque changes between them.
In the image above, the 15 tooth input gear is rotating clockwise. As the input gear rotates, it pushes down on the output gear where the teeth are meshed. This action transmits the motion to the output gear, but forces the output gear to rotate in the opposite direction of the input gear.
In order for gears to work effectively, and not become damaged, it’s important that the center-to-center distance is correctly adjusted. The gears in DETAIL A, of the figure below, may work under very light load, but they will certainly not work and will skip under any significant loading. The gears in that example are too far apart and so the teeth of each gear barely contact each other. The gears in DETAIL B are correctly spaced and will provide smooth and reliable operation.
Sometimes in a design it may be desirable to stack together multiples of the same gear on a shaft to increase the load carrying capacity of the gears. In the case where the number of teeth on the gear is not divisible by six, because of how they are oriented when put onto the hex shaft, the teeth may not be aligned between the two gears. To ensure all of the gears are clocked the same way, use the alignment shaft notch to put all the gears on the shaft with the same orientation.
Being aware of the alignment mark will ensure all of the gear teeth are aligned on the shaft. The figure below is an example of a basic robot arm which may have to lift a heavy load. Using two gears to lift the arm doubles the material interfacing with the hex shaft and will allow the arm to perform with heavier loads.
The REV DUO Build System includes both and Gears. The table below covers some of the basic specifications for the different types of gears.
Both the Metal and Plastic Gears are compatible with with a 8mm pitch.
All REV DUO Plastic Gears have a M3 bolt hole mounting pattern that is on an 8mm pitch. This makes it easy to directly mount REV Robotics and to gears. The webbed design, combined with a wide face width and small tooth profile, increases the gear strength without adding significant weight. REV Robotics Plastic Gears are designed to fit a 5mm hex shaft which eliminates the need for special hubs and setscrews.
Metal Gears are designed to work with in high-load applications. REV DUO Metal Gears also have a M3 hole pattern, but this pattern focuses on allowing gears to be doubled up for additional strength. In general, the REV DUO Metal Gears have a higher yield strength than the REV DUO Plastic Gears, especially when the metal gears are doubled up. Thickness of the gear teeth, known as face width, is a core component of gear strength.
All REV DUO Metal Gears are narrower than plastic gears, allowing for compact drivetrains and gearboxes within the same 15mm width as the Extrusion and Punch Tube ().
To learn more about gear ratios and how they affect speed and torque check out thesection
One way to change change the direction of rotation in a gear train is to add idlers. Idlers sit between the input and output gear in a gear train and can help you manipulate the rotation of the output gear. To learn more about idlers check out the section
If you are using gears to transmit motion over long distances, like in a drivetrain, please check out the section to understand more about how to properly employ that sort of mechanism.
As mentioned in the section, DUO Gears work with the 5mm Hex Shaft to drive motion along a a gear train. To learn more about using Hex Shafts and proper motion support and constraint visit the pages linked below:
To learn more about calculating center-to-center distance for Gears visit the section
Plastic
Metal
Module
0.75
0.8
Pressure Angle
20°
20°
Material
Acetal (Delrin/POM)
7075-T6 Aluminum†
Thickness
15mm
5mm