Mechanics and Construction > Mechanics and Construction

Motor torque selection

**mado**:

I have a carriage that moves on 4 wheels and weight 65kg I made design for the motor based on http://www.societyofrobots.com/mechanics_dynamics.shtml but the resulted torque was very high

Velocity = 3.14 ft/s

Acceleration = 1.5 ft/s

Mass = 143 lbs or 65kg

wheel diameter= .5ft

RMF = 143* 3.14*1.4 /2*pi = 107 .2 lbs ft rps

If I choosed rps to be 2 rps the torque will be 53 lbs ft and this is very high so where is the error here?

**jwatte**:

65 kg is a lot of payload. Accelerating at 1.5 ft/s/s is a fair bit of thrust (evel though it's "only" 1/20th of a g, if I can do unit conversions in my head.) You will need strong motors to achieve that level of acceleration.

Also note that typical motors run in the thousands of rpm, and don't have a lot of torque, but use gear boxes to trade rpm for torque. Thus, a 12V motor with 1,000 kV constant and 2 ozin of torque would run at 12,000 rpm at 12 V; use a 500:1 gearbox and it gets to 24 rpm and has 1,000 ozin of torque! (That motor would be too small for your design case, but that's a pretty small RC car motor I described...)

**mado**:

if i used two motors of 2 oz of torque , will it be enough to move the load ?

i need to know how to get enough torque .

how to calculate the required torque?

**jwatte**:

"2 oz" makes no sense. For torque, you need force times lever length. Do you mean 2 ozin?

No, that by itself is unlikely to overcome the losses in the system. However, a 2 ozin motor with a big enough gearbox (say, 1000:1) might be able to start moving the load. It will likely not have the degree of acceleration you need, though.

You already calculated the necessary parameters in post 1.

**mado**:

so referring to your talk i got that the my acceleration need to be bigger and the problem here is to get gear box with 1000:1 ratio , did i hit?

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