This gearmotor consists of a medium-power, 12 V brushed DC motor combined with a 20.4:1 metal spur gearbox. The gearmotor is cylindrical, with a diameter just under 25 mm, and the D-shaped output shaft is 4 mm in diameter and extends 12.5 mm from the face plate of the gearbox.
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These cylindrical brushed DC gearmotors are available in a wide range of gear ratios and with five different motors (two power levels of 6V motors and three power levels of 12V motors). The gearmotors all have the same 25 mm diameter case and 4 mm diameter gearbox output shaft, 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). All versions are also available with an integrated 48 CPR quadrature encoder on the motor shaft. Please see the 25D metal gearmotor comparison table for detailed specifications of all our 25D metal gearmotors. This dynamically-sortable table can help you find the gearmotor that offers the best blend 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. For these gearboxes, the recommended upper limit for instantaneous torque is 200 oz-in (15 kg-cm); we strongly advise keeping applied loads well under this limit. Stalls can also result in rapid (potentially on the order of seconds) thermal damage to the motor windings and brushes, especially for the versions that use high-power (HP) motors; a general recommendation for brushed DC motor operation is 25% or less 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.
Exact gear ratio: ``(22×22×22×23) / (12×10×10×10) ~~ bb(20.4:1)``
The face plate has two mounting holes threaded for M3 screws. You can use our custom-designed 25D mm metal gearmotor bracket (shown in the picture below) to mount the gearmotor to your project via these mounting holes and the screws that come with the bracket.
Pololu 25D mm metal gearmotor bracket pair |
Pololu 25D mm gearmotor with bracket |
The 4 mm diameter gearbox output shaft works with Pololu universal aluminium 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 adaptor to mount many common scooter, skateboard, and inline skate wheels to the gearmotor’s output shaft as shown in the right picture below.
Pololu 60x8mm wheel on a Pololu 25D mm metal gearmotor |
A 25D mm gearmotor connected to a scooter wheel by the 4 mm scooter wheel adapter |
These are the same type of motors used in the Wild Thumper all-terrain chassis, so the gearbox’s output shaft also works directly with the hex adapters included with the 120mm-diameter Wild Thumper wheels (the left picture below shows a 25D mm gearmotor while the right picture shows the smaller 20D mm gearmotor):
Dagu Wild Thumper wheel 120x60mm (chrome) with Pololu 25D mm metal gearmotor |
Dagu Wild Thumper wheel 120x60mm (metallic red) with Pololu 20D mm metal gearmotor |
12mm Hex Wheel Adapter for 4mm Shaft on a 20D mm Metal Gearmotor
The diagram below shows the dimensions of the 25D mm line of gearmotors (units are mm over [inches]). This diagram is also available as a downloadable PDF (223k pdf).
![Dimensions of the Pololu 25D mm metal gearmotors. Units are mm over [inches]](/cache/files/productimageoriginals/22821_dimensions_of_the_pololu_25d_mm_metal_gearmotors_units_are_mm_over_inches_.jpg)
Dimensions of the Pololu 25D mm metal gearmotors. Units are mm over [inches]
25D mm metal gearmotor with 48 CPR encoder: close-up view of encoder
The versions of these gearmotors with encoders use a A two-channel Hall effect sensor to detect the rotation of a magnetic disk on a rear protrusion of the motor shaft. The quadrature encoder provides a resolution of 48 counts per revolution of the motor shaft when counting both edges of both channels. To compute the counts per revolution of the gearbox output, multiply the gear ratio by 48. The motor/encoder has six colour-coded, 11" (28 cm) leads terminated by a 1×6 female header with a 0.1″ pitch, as shown in the main product picture. This header works with standard 0.1″ male headers and our male jumper and precrimped wires. If this header is not convenient for your application, you can pull the crimped wires out of the header or cut the header off. The following table describes the wire functions:
| Colour | Function |
|---|---|
| Red | motor power (connects to one motor terminal) |
| Black | motor power (connects to the other motor terminal) |
| Green | encoder GND |
| Blue | encoder Vcc (3.5 – 20 V) |
| Yellow | encoder A output |
| White | encoder B output |
The Hall sensor requires an input voltage, Vcc, between 3.5 and 20 V and draws a maximum of 10 mA. The A and B outputs are square waves from 0 V to Vcc approximately 90° out of phase. The frequency of the transitions tells you the speed of the motor, and the order of the transitions tells you the direction. The following oscilloscope capture shows the A and B (yellow and white) encoder outputs using a motor voltage of 6 V and a Hall sensor Vcc of 5 V:
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| Encoder A and B outputs for 25D mm HP 6V metal gearmotor with 48 CPR encoder (motor running at 6 V). |
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By counting both the rising and falling edges of both the A and B outputs, it is possible to get 48 counts per revolution of the motor shaft. Using just a single edge of one channel results in 12 counts per revolution of the motor shaft, so the frequency of the A output in the above oscilloscope capture is 12 times the motor rotation frequency.
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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| Some of the Pololu metal gearmotors. |
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| Size: | 25D x 50L mm |
|---|---|
| Weight: | 85 g |
| Shaft diameter: | 4 mm |
| Gear ratio: | 20.4:1 |
|---|---|
| Free-run speed @ 6V: | 185 rpm1 |
| Stall current @ 6V: | 1050 mA1 |
| Stall torque @ 6V: | 21 oz·in1 |
| Free-run speed @ 12V: | 370 rpm |
| Free-run current @ 12V: | 200 mA2 |
| Stall current @ 12V: | 2100 mA |
| Stall torque @ 12V: | 42 oz·in |
| Motor type: | 2.1A stall @ 12V (MP 12V) |
| Encoders?: | N |
Customer Richard Nguyen has documented his work modifying the Wild Thumper chassis to use 25D motors with encoders in place of the chassis’s included motors.
Note: This is not an easy modification, and the chassis can be damaged if it is not done properly, so we generally recommend against it, and we can only provide very limited support for those who want to attempt it. The manufacturer did not intend for the chassis to be modified in this way, and we do not know how well such a modification will work out.
No; the information we have available for this motor can be found on its product page. However, you can approximate various additional motor parameters from the information found in the “Specs” tab.
The electrical resistance of the motor can be approximated by dividing the rated voltage by the stall current (at the rated voltage). The electromotive force constant (Ke) can be approximated by dividing the rated voltage by the free-run speed (at the rated voltage). To approximate the motor torque constant (Kt), you can divide the stall torque by the stall current.
For pretty much any DC motor, the current, speed, power, and efficiency curves as a function of torque will look like those in the graph below (assuming motor voltage and temperature are constant):
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The current and speed curves are approximately linear, and the product pages for our motors provide the approximate end points for these lines: (0 torque, no-load current) and (stall torque, stall current) for the red line, and (0 torque, no-load speed) and (stall torque, 0 speed) for the blue line.
The orange output power curve is the product of the speed and the torque, which results in an inverted parabola with its peak at 50% of the stall torque.
The green efficiency curve is the output power divided by the input power, where the input power is current times voltage. The voltage is constant, so you can divide the output power curve by the current line to get the general shape of the efficiency curve, which in turn lets you identify the torque, speed, and current that correspond to max efficiency.
There are many programs out there that you can use to generate these curves. For example, if you have access to MATLAB, you can use this customer-created MATLAB script to generate these motor plots for you from the specifications we provide for each gearmotor.
Note: A good general rule of thumb is to keep the continuous load on a DC motor from exceeding approximately 20% to 30% of the stall torque. Stalling gearmotors can greatly decrease their lifetimes, occasionally resulting in immediate damage to the gearbox or thermal damage to the motor windings or brushes. Do not expect to be able to safely operate a brushed DC gearmotor all the way to stall. The safe operating range will depend on the specifics of the gearmotor itself.
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![Dimensions of the Pololu 25D mm metal gearmotors. Units are mm over [inches]](/Cache/Files/ProductImageOriginals/22821_Dimensions_of_the_Pololu_25D_mm_metal_gearmotors_Units_are_mm_over_inches_.jpg?v636600100920371319)
