31:1 Metal Gearmotor 20Dx41L mm 12V CB with Extended Motor Shaft
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Pololu item #:
3487
Brand:
Pololu
Status:
Active and Preferred
These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 12V brushed DC motor with long-life carbon brushes combined with a 31.25:1 metal spur gearbox. The gearmotor is cylindrical with a diameter of 20 mm, and the D-shaped output shaft is 4 mm in diameter and extends 18 mm from the face plate of the gearbox. This version also has a 6 × 2 mm extended motor shaft.
Key specifications:
| voltage |
no-load performance |
stall extrapolation |
| 12 V |
450 RPM, 80 mA |
2.4 kg⋅cm (33 oz⋅in), 1.6 A |
Alternatives available with variations in these parameter(s):
gear ratio motor type extended motor shaft?
Select variant…
Compare all products in 12V Carbon Brush (CB) 20D Gearmotors.
Pictures
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20D mm metal gearmotor with precious metal brushes (left) next to a 20D CB version with long-life carbon brushes. |
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20D mm metal gearmotor precious metal brushes (left) next to 20D 6V CB and 12V CB long-life carbon brushes. |
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20D mm metal gearmotor CB with long-life carbon brushes. |
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20D mm metal gearmotor CB with long-life carbon brushes. |
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20D mm metal gearmotor with precious metal brushes. |
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20D mm metal gearmotor with precious metal brushes. |
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20D mm metal gearmotor with precious metal brushes and extended motor shaft. |
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20D mm metal gearmotor with a univeral aluminum mounting hub and mounted on a 20D mm metal gearmotor bracket. |
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Pololu 70×8mm wheel mounted to a 20D mm metal gearmotor using a universal aluminum mounting hub for 4mm shafts. |
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Dagu Wild Thumper wheel 120×60mm (metallic red) with Pololu 20D mm metal gearmotor. |
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Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. |
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Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. |
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Dimensions of the Pololu 20D mm metal gearmotors. Units are mm over [inches]. This diagram only applies to the listed gear ratios. |
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An assortment of Pololu metal gearmotors. |
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Overview
These cylindrical brushed DC gearmotors are available in a wide range of gear ratios, from 25:1 up to 488:1, and with three different motors: 6 V and 12 V motors with long-life carbon brushes (CB), and a 6 V motor with shorter-life precious metal brushes. All three motors offer similar performances at their respective nominal voltages, just with the 12 V motor drawing approximately half the current of the 6 V motor and with the carbon brush versions having longer lifetimes than the one with precious metal brushes.
The CB versions (shown on the right in the picture below), can be differentiated from versions with precious metal brushes (shown on the left) by their copper-colored terminals.
These gearmotors are optionally available with an additional 2 mm-diameter output shaft that protrudes 6 mm from the rear of the motor (see the middle picture below). This rear shaft rotates at the same speed as the input to the gearbox and offers a way to add an encoder, such as our magnetic encoder for 20D mm metal gearmotors as shown in the right picture below, to provide motor speed or position feedback.
 | 20D mm metal gearmotor CB with long-life carbon brushes. |
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 | 20D mm metal gearmotor CB with long-life carbon brushes and an extended motor shaft. |
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 | Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. |
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The gearmotors all have 20 mm diameter gearboxes and 4 mm diameter gearbox output shafts, so it is generally easy to swap one version for another if your design requirements change (though the length of the gearbox tends to increase with the gear ratio).
Please see the 20D metal gearmotor datasheet (1MB pdf) for more information, including detailed performance graphs for each 6V CB and 12V CB version. You can also use our dynamically sortable 20D metal gearmotor comparison table to search for the version that offers the best combination of speed, torque, and current draw for your particular application. A more basic comparison table is available below:
Note: Stalling or overloading gearmotors can greatly decrease their lifetimes and even result in immediate damage. In order to avoid damaging the gearbox, we recommend keeping continuously applied loads under 5 kg-cm (75 oz-in) for the versions with carbon brushes (12V CB and 6V CB) and under 3.5 kg-cm (50 oz-in) for the versions with precious metal brushes. Stalls can also result in rapid (potentially on the order of seconds) thermal damage to the motor windings and brushes, especially for motors like this that can deliver a lot of power for their size; a general recommendation for brushed DC motors is to limit continuous current to approximately 25% of the stall current.
In general, these kinds of motors can run at voltages above and below their nominal voltages; lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor.
Details for item #3487
Exact gear ratio: ``(28×25×25×25) / (14×10×10×10) = bb(31.25:1)``
These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 12V brushed DC motor with long-life carbon brushes combined with a 31.25:1 metal spur gearbox. The gearmotor is cylindrical with a diameter of 20 mm, and the D-shaped output shaft is 4 mm in diameter and extends 18 mm from the face plate of the gearbox. This version also has a 6 × 2 mm extended motor shaft.
Key specifications:
| voltage |
no-load performance |
stall extrapolation |
| 12 V |
450 RPM, 80 mA |
2.4 kg⋅cm (33 oz⋅in), 1.6 A |
Dimensions
The diagram below shows the dimensions of the 20D mm line of metal gearmotors (units are mm over [inches]). This diagram is also available as a downloadable PDF (99k pdf).
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Dimensions of the Pololu 20D mm metal gearmotors. Units are mm over [inches]. This diagram only applies to the listed gear ratios. |
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Warning: Do not screw too far into the mounting holes as the screws can hit the gears. We recommend screwing no more than 3.5 mm (0.14″) into the M2.5 mounting holes.
Gearmotor accessories
The gearmotor face plate has two mounting holes threaded for M2.5 screws, and you can use our custom-designed 20D mm metal gearmotor bracket (shown in the left picture below) to mount the gearmotor to your project via these mounting holes and the screws that come with the bracket.
 | Pololu 20D mm metal gearmotor bracket pair with included hardware. |
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 | 20D mm metal gearmotor with a univeral aluminum mounting hub and mounted on a 20D mm metal gearmotor bracket. |
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The 4 mm diameter gearbox output shaft works with Pololu universal aluminum mounting hub for 4mm shafts, which can be used to mount our larger Pololu wheels (60mm-, 70mm-, 80mm-, and 90mm-diameter) or custom wheels and mechanisms to the gearmotor’s output shaft as shown in the left picture below. Alternatively, you could use our 4mm scooter wheel adapter to mount many common scooter, skateboard, and inline skate wheels to the gearmotor’s output shaft as shown in the right picture below.
 | Pololu 70×8mm wheel mounted to a 20D mm metal gearmotor using a universal aluminum mounting hub for 4mm shafts. |
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 | A 20D mm gearmotor connected to a scooter wheel by the 4 mm scooter wheel adapter. |
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These motors also work with the hex adapters included with the 120mm-diameter Wild Thumper wheels (the left picture below shows a 20D mm gearmotor while the right picture shows a larger 25D mm gearmotor):
 | Dagu Wild Thumper wheel 120×60mm (metallic red) with Pololu 20D mm metal gearmotor. |
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 | Dagu Wild Thumper wheel 120×60mm (chrome) with Pololu 25D mm metal gearmotor. |
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For a general-purpose hex adapter, consider our 12mm hex wheel adapter, which lets you use this motor with many common hobby RC wheels.
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12mm Hex Wheel Adapter for 4mm Shaft on a 20D mm Metal Gearmotor. |
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You can use our magnetic encoder kit for 20D mm metal gearmotors to provide motor speed or position feedback. Please note that this kit only works on gearmotor versions that have an extended motor shaft.
| Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. |
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 | Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. |
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We have a number of motor controllers and motor drivers that work with these 20D mm metal gearmotors. In particular, we recommend our MC33296-based motor drivers, for which we have basic single and dual carriers and a dual-channel shield for Arduino. Our VNH5019-based motor drivers (available as single and dual carriers) also work well with these motors, and because they can handle higher currents, they will run much cooler than the MC33926 carriers. If you are looking for higher-level control interfaces, such as USB, RC, analog voltages, or serial, consider our SMC 18v7, Jrk 21v3, or TReX Jr.
 | Pololu dual MC33926 motor driver shield for Arduino. |
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 | VNH5019 motor driver carrier. |
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 | Simple Motor Controller 18v7, fully assembled. |
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We have an assortment of Hall effect-based current sensors to choose from for those who need to monitor motor current:
 | ACS711EX current sensor carrier -15.5A to +15.5A. |
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 | ACS714 current sensor carrier -5A to +5A. |
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Selecting the Right Gearmotor
We offer a wide selection of metal gearmotors that offer different combinations of speed and torque. Our metal gearmotor comparison table can help you find the motor that best meets your project’s requirements.
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Dimensions
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Size:
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20D × 43.2L mm1
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Weight:
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44 g
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Shaft diameter:
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4 mm2
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General specifications
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Gear ratio:
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31.25:1
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No-load speed @ 12V:
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450 rpm
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No-load current @ 12V:
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0.08 A
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Stall current @ 12V:
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1.6 A3
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Stall torque @ 12V:
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2.4 kg·cm3
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Max output power @ 12V:
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2.8 W
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No-load speed @ 6V:
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225 rpm4
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Stall current @ 6V:
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0.8 A4
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Stall torque @ 6V:
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1.2 kg·cm4
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Extended motor shaft?:
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Y
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Long-life carbon brushes?:
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Y
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Motor type:
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1.6A stall @ 12V CB (carbon brush)
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Performance at maximum efficiency
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Max efficiency @ 12V:
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39 %
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Speed at max efficiency:
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360 rpm
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Torque at max efficiency:
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0.44 kg·cm
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Current at max efficiency:
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0.35 A
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Output power at max efficiency:
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1.7 W
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Notes:
- 1
- Not including output shafts; output shafts add 24 mm to the total length.
- 2
- D shaft.
- 3
- Stalling is likely to damage the gearmotor. Stall parameters come from a theoretical extrapolation of performance at loads far from stall. As the motor heats up, as happens as it approaches an actual stall, the stall torque and current decrease.
- 4
- This motor will run at 6 V but is intended for operation at 12 V.
File downloads
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This file contains 3D models (in the step file format) of the 20D mm gearmotors. Note: these models only accurately represent our newer versions with 18mm-long output shafts; they are not accurate representations of the older versions with shorter output shafts (i.e. gear ratios 29:1, 56:1, 73:1, and 154:1).
Frequently-asked questions
- Is it okay to stall this motor at its operating voltage?
No! Stalls can result in rapid (potentially on the order of seconds) thermal damage to the motor windings and brushes; a general recommendation for brushed DC motor operation is 25% or less of the stall current, which means keeping continuously applied loads around 25% or less of the stall torque.
Additionally, for many of our gearmotors with high gear ratios, the extrapolated stall torque is beyond what the gearboxes are designed to handle, and a stall could instantly damage the gears. Make sure to keep applied loads within the published limits for your gearmotor.
- What do the dotted lines mean in the motor performance curves from the datasheet?
- In our motor performance curves, the solid lines show the regions where we actually took measurements while the dotted lines show extrapolations beyond that. For many motors, we took measurements out to about 50% of the stall torque, since continued operation at loads past this point increasingly risk thermal damage to the windings. However, for higher gear ratios, going to 50% of the stall torque exceeds what is safe for the gearbox, so the measurements are further limited to not much past our recommendation for continuously applied loads.
- What material is the gearbox output shaft made of, and can I get the shaft customized?
- The shaft is composed of hardened stainless steel. Some customization is generally possible (e.g., different length or no D-shaft) for sufficiently high-volume orders. If this is something you are interested in, please email us.
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