156:1 Metal Gearmotor 20Dx44L mm 6V with Extended Motor Shaft


Description

Overview

These cylindrical brushed DC gearmotors are available in a wide range of gear ratios, from 25:1 up to 488:1, and they are optionally available with an additional 2 mm-diameter output shaft that protrudes 6 mm from the rear of the motor. This optional 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, to provide motor speed or position feedback.

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 mm metal gearmotor comparison table for detailed specifications of all our 20D mm 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:

Rated
Voltage
Stall
Current
@ Rated Voltage
No-Load
Speed
@ Rated Voltage
Approximate
Stall Torque
@ Rated Voltage


Single-Shaft
(Gearbox Only)


Dual-Shaft
(Gearbox & Motor)
6 V 3.2 A 560 RPM 30 oz-in 25:1 6V 25:1 6V dual-shaft
450 RPM 35 oz-in 31:1 6V 31:1 6V dual-shaft
225 RPM 60 oz-in 63:1 6V 63:1 6V dual-shaft
180 RPM 75 oz-in 78:1 6V 78:1 6V dual-shaft
140 RPM 90 oz-in 100:1 6V 100:1 6V dual-shaft
110 RPM 100 oz-in 125:1 6V 125:1 6V dual-shaft
90 RPM 115 oz-in 156:1 6V 156:1 6V dual-shaft
70 RPM 125 oz-in 195:1 6V 195:1 6V dual-shaft
55 RPM 140 oz-in 250:1 6V 250:1 6V dual-shaft
45 RPM 150 oz-in 313:1 6V 313:1 6V dual-shaft
35 RPM 160 oz-in 391:1 6V 391:1 6V dual-shaft
29 RPM 170 oz-in 488:1 6V 488:1 6V dual-shaft

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 125 oz-in (9 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.

These motors are intended for operation at around 6 V. 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 #3468

Exact gear ratio: ``(28×28×25×25×25×25) / (14×14×10×10×10×10) = bb(156.25:1)``

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 (183k pdf).

Dimensions of the Pololu 20D mm metal gearmotors.  Units are mm over [inches].  This diagram only applies to the listed gear ratios.

Dimensions of the Pololu 20D mm metal gearmotors. Units are mm over [inches]. This diagram only applies to the listed gear ratios.

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.

20D mm metal gearmotor with a univeral aluminum mounting hub and mounted on a 20D mm metal gearmotor bracket.

20D mm metal gearmotor with a univeral aluminum mounting hub and mounted on a 20D mm metal gearmotor 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 70×8mm wheel mounted to a 20D mm metal gearmotor using a universal aluminum mounting hub for 4mm shafts.

Pololu 70×8mm wheel mounted to a 20D mm metal gearmotor using a universal aluminum mounting hub for 4mm shafts.

A 20D mm gearmotor connected to a scooter wheel by the 4 mm scooter wheel adaptor.

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.

Dagu Wild Thumper wheel 120×60mm (metallic red) with Pololu 20D mm metal gearmotor.

Dagu Wild Thumper wheel 120×60mm (chrome) with Pololu 25D mm metal gearmotor.

For a general-purpose hex adaptor, consider our 12mm hex wheel adaptor, which lets you use this motor with many common hobby RC wheels.

12mm Hex Wheel Adaptor for 4mm Shaft on a 20D mm Metal Gearmotor.

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.

Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft.

Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. (1)

Magnetic Encoder Kit for 20D mm Metal Gearmotors assembled on a 20D mm metal gearmotor with extended motor shaft. (1)

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, analogue voltages, or serial, consider our SMC 18v7, Jrk 21v3, or TReX Jr.

Pololu dual MC33926 motor driver shield for Arduino.

VNH5019 motor driver carrier.

Simple Motor Controller 18v7, fully assembled.

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.

ACS714 current sensor carrier -5A to +5A.

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.

Some of the Pololu metal gearmotors.


Specifications

Dimensions

Size: 20D × 44L mm
Weight: 45 g
Shaft diameter: 4 mm

General specifications

Gear ratio: 156.25:1
Free-run speed @ 6V: 90 rpm
Free-run current @ 6V: 200 mA
Stall current @ 6V: 3200 mA
Stall torque @ 6V: 110 oz·in
Extended motor shaft?: Y
Motor type: 3.2A stall @ 6V

Resources

File downloads

Dimension diagram for the Pololu 20D mm metal gearmotors (183k pdf)
3D models of 20D mm gearmotors (1MB zip)
This file contains 3D models (in the step file format) of the 20D mm gearmotors.

Recommended links

MATLAB script to plot motor performance curves for Pololu brushed DC gearmotors
This MATLAB script, written by Ali Asgher Mansoor Habiby, plots speed, power, current draw, and efficiency as they vary with torque when you input the gearmotor specifications. It also prints the resistance of the motor, and the current draw and torque at which maximum efficiency and maximum power occur.

FAQs

What material is the gearbox output shaft made of, and can I get the shaft customised?
The shaft is composed of hardened stainless steel. Some customisation 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.
I need additional information about this motor; do you have a datasheet?

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):

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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