Need to Calculate Power and Torque for a Wheelbarrow? Here's How!

[RogerCh]

How can I calculate power and torque requirements for a device resembling a wheelbarrow using 12v battery and motor/gearbox drive?

 

[BvU]

Hello Roger,

As you can imagine, the requirements on performance come in: if your wheelbarrow has to carry a ton up a steep hill of loose sand, more is asked of it than if it only has to carry a doll on a smooth marble floor ...

 

[jack action]

To find the power you need (in watts), you need to multiply the desired speed (in meters per second) and the summation of the forces involved (in Newtons).

The forces involved are:

• Rolling resistance ($f_r mg$);
• Inertia ($ma = \frac{a}{g}mg$);
• Grade resistance ($Gmg$);
• Mechanical losses (10-20%).

There is also aerodynamic resistance, but with the low speed of a wheelbarrow, it can be ignored.

Since all of the previous are based on the weight of the wheelbarrow ($mg$), for typical use (avg acceleration of 0.15 g's, smooth & flat terrain, 15% losses) you will probably need a total force representing about 10% of the weight ($0.1mg$). But if you have severe conditions (steep grade and/or rough terrain), it can be easily 3 times as much.

So if your wheelbarrow weights 50 lb (= 222.4 N) and you want to go 3 mph (1.341 m/s), you would need between 30 W (= 0.1 * 222.4 * 1.341) and 90 W (= 0.3 * 222.4 * 1.341) of power.

If you know the power, knowing torque is irrelevant.

 

[CWatters]

One option is to tow the Barrow behind a car at a constant speed. While doing that measure the tension in the tow rope. The power in watts equals tension in Newton's times the velocity in meters per second.

You can work out the torque you need from the power, velocity and wheel radius.

Use representative terrain.