# How much weight can a motor take before it can't run? [closed]

I bought these 50:1 gear motors for a project robot I'm building for this YouTube walkthrough series with a couple of Arduino's, two other servo motors, a motor controller and various hardware.

The photo below shows where the motors are placed on the sides near the bottom...

My question is, without testing and getting everything done, how do I determine the weight these motors can handle with these wheels? Is there some formula that will help me get a general idea of the max weight these diameter of wheels can hold on the motors mentioned above?

## closed as off-topic by Scott Seidman, PeterJ, Daniel Grillo, Ricardo, Adam HaunMay 29 '15 at 16:18

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• I'm voting to close this question as off-topic because this is mechanical engineering. – Scott Seidman May 29 '15 at 10:59

The data is all given on the page you linked.

The relevant quantity is the "Stall Torque", which is the maximum torque the motor is able to exert. Note - as the warning text, below the table, says, you should not plan to operate at this stall torque for more than a few seconds at a time.

To convert stall torque to "a weight", take the stall torque in ounce-inches and divide by the radius of your wheel, in inches. The result will be a number of ounces, representing the maximum weight the motor can hold against gravity.

However you are probably more interested in moving a robot around a horizontal surface on wheels. (Your question only mentions "a weight" which is vague.)

Let's suppose we have a robot with a single driving wheel, as shown above. It is driving around on a perfectly level surface. Since the ground is flat, gravity does not affect the robot (except to keep it pinned to the ground) so the robot does not require any torque to stay still.

Assuming frictionless bearings, any motor will be able to move the robot. The more powerful the motor, the faster the robot will accelerate.

The acceleration of the robot is given by combining:

$$Force (N) = Mass (kg) \times Acceleration (m/s²)$$

and

$$Torque (N.m) = Force (N) \times Wheel radius (m)$$

Suppose we have a car that weighs 500 kg, with a motor exerting 5,000 N.m of torque on a driving wheel with radius 0.5 m.

The force (N) exerted by the wheel is 5,000 N.m / 0.5m = 1,000 N. The acceleration (m/s²) is 1,000 N / 500 kg = 2 m/s².

Note that there is no single "maximum weight". These equations say that no matter how weak the motor, you will be able to move the car. However a small motor won't accelerate the car very fast, or be able to drive the car up a hill, or be able to overcome the friction of the bearings and air drag. You have to decide what your criteria are for these parameters, which will determine the maximum weight of your robot, for a given drive torque.

• Note: It is recommended to do all calculations in SI units (kilograms, metres, Newtons, N.m, m/s²) to avoid embarrassing unit conversion errors. Particularly, note that motor torques expressed in 'oz-in' or 'lb-ft' or 'kg-cm' are defined in terms of a weight hanging in earth gravity, so they include a surprising factor of 9.81 m/s² or 32 ft/s². – Li-aung Yip May 29 '15 at 6:51
• Very Cool answer +1 – Andy aka May 29 '15 at 8:05
• In the case of the wheeled robot, you may not be interested in the acceleration, but you may be interested in the question : can it climb a 10% slope, or more tricky, can it climb the step at the edge of a 15mm carpet. Some geometry with that wheel will convert that step into a slope; you want enough torque to roll the robot's weight up that slope. – Brian Drummond May 29 '15 at 9:07

The strength of the ABS plastic which the wheels are made of depends on the exact formulation including whether or not glass fibers are mixed in, etc. I was able to find some generic info on tensile strength of ABS plastic here: http://www.matweb.com/reference/tensilestrength.aspx

"In polymers, the tensile modulus and compressive modulus can be close or may vary widely."

The best way to find out the strength of the wheels without testing them yourself is to ask the manufacturer.

'The weight the motors can handle' is completely different from 'the maximum weight the wheels can hold'. What the motor can handle will depend on the terrain more-so than the weight.

The easiest way to find the answer is to ask the manufacturer. Alternatively, you can compare similar products on more informative sales sites: http://www.robotstorehk.com/motors/MOTOR-SET-001.html

The above site rates their similar product as being able to support 4 kg.