### Author Topic: Max load on rc cart depending on motor torque  (Read 4814 times)

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#### Stevic

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##### Max load on rc cart depending on motor torque
« on: January 13, 2012, 01:58:03 PM »
I'm making rc cart which is powered by two windscreen wiper motors on each front wheel. Torque of one motor is 3.5[Nm]. Because of low rotation speed of 70[rpm] I'll make transmision in 3:1 ratio to increase the speed. Wheels diamters are 5[cm].
My question is: What is the maximum load that the cart can carry?
Please help me if you know! Good with electronics but not so good with mechanics

#### newInRobotics

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##### Re: Max load on rc cart depending on motor torque
« Reply #1 on: January 14, 2012, 10:09:02 PM »
As it is, on flat ground, 2 motors would be able to carry 2 * (3.5N.m / 5cm) = 14.28kg. With gearbox of Your choice it will be able to carry 4.76kg. Power consumption will be vast as motors will be working near stall conditions.
« Last Edit: January 14, 2012, 10:10:25 PM by newInRobotics »
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#### Stevic

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##### Re: Max load on rc cart depending on motor torque
« Reply #2 on: January 15, 2012, 05:28:15 AM »
Thank you but I think you calculated it wrong. What you calculated is lifting power but not pulling. Take this for comparison: wheelchair (for grannies) has one motor with torque of 8 Nm and can carry those corpses weight 70-100kg.
« Last Edit: January 15, 2012, 07:25:07 AM by Stevic »

#### Soeren

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##### Re: Max load on rc cart depending on motor torque
« Reply #3 on: January 15, 2012, 01:12:08 PM »
Hi,

Thank you but I think you calculated it wrong. What you calculated is lifting power but not pulling. Take this for comparison: wheelchair (for grannies) has one motor with torque of 8 Nm and can carry those corpses weight 70-100kg.
2 x 3.5 Nm = 7 Nm
7 Nm = 71.38 kgf-cm
Wheel radius = 2.5 cm
71.38/2.5 = 28.552 kgf
Divided by 3 (your gear up ratio) it's ~9.5kgf

If the 3.5Nm is the stall torque, then the total weight ('bot, batteries, load und alles) should be quite a bit less than that.

The only error in nIR's calculation was that he used the wheel diameter rather than its radius.
1 Nm of force is 1 Nm of force, no matter the direction.

8 Nm = 81.5 kgf-cm will not move even the tiniest person without a large gear reduction. Without reduction and assuming a wheel radius of 10 cm, it will have 8.15 kgf on the wheels and the chair, motor and battery alone weigh way more.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Stevic

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##### Re: Max load on rc cart depending on motor torque
« Reply #4 on: January 15, 2012, 04:11:15 PM »
Actualy torque is 3.5-15Nm. At 15Nm it is stalled. It's a windscreen wiper motor. What do you say about this calculation:

I'm trying to find the torque required of an electric vehicle to pull a 1000kg trolley on four Nylon Wheels on smooth concrete. The trolley fully supports its own weight. The vehicle has 2 rubber drive wheels of Radius( r = .125m). Now I'm not sure if am on the right track because i cant find much information on Rolling resistance.

Using Fr = Crr * N to find the rolling resistive force.
Where Fr = Rolling resistance force
Crr = coefficient of rolling resistance
N = Normal force

Rolling resistance of trolley
Assuming Crr = .004
N = 1000kg * 9.81 m/s^2
Ft= (.004*9810) = 39.24

rolling resistance of vehicle
Fv = (45kg * 9.81 m/S^2 * .08)
Fv = 35.316
Ftotal = Fv +Ft
Ftotal = 35.316 + 39.24 = 74.6N

So if I use the equation for toque
T = F * r
T = 74.6 * .125
T = 9.32 N.m

This should give me the minimum required torque to get the trolley and vehicle rolling on a smooth horizontal surface. Yes/No?

I found this on physics forum. Only problem i found in this extremly small dinamical friction coefficient of wheel itself with axle of only 0.004. But if you look again it is added to friction force of wheel to ground.

#### newInRobotics

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##### Re: Max load on rc cart depending on motor torque
« Reply #5 on: January 15, 2012, 05:17:13 PM »
The only error in nIR's calculation was that he used the wheel diameter rather than its radius.
Oh yeah, just noticed that
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### Soeren

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##### Re: Max load on rc cart depending on motor torque
« Reply #6 on: January 15, 2012, 05:57:35 PM »
Actualy torque is 3.5-15Nm. At 15Nm it is stalled. It's a windscreen wiper motor. What do you say about this calculation:
OK, if 3.5 Nm is the rated working torque you get 9.5 kgf with the 5 cm wheels and 3 times upwards gearing.

I'm trying to find the torque required of an electric vehicle to pull a 1000kg trolley on four Nylon Wheels on smooth concrete. The trolley fully supports its own weight. The vehicle has 2 rubber drive wheels of Radius( r = .125m). Now I'm not sure if am on the right track because i cant find much information on Rolling resistance.
A good starting point would be to calculate the needed force assuming zero rolling resistance. It's easily factored in afterwards anyway.

But, you need to have the total weight (vehicle+trolley+loads), the wheel size and the needed minimum speed in the calculation.

Occasional sanity checks are a good way of assuring you're not going into long chains of calculations on bad data or formulas.
Like 1 ton + whatever - that's around the weight of a car.
1hp=~775W, what hp's do you estimate is needed to make a small car move at the speed you have in mind(?)

Using [...]
This should give me the minimum required torque to get the trolley and vehicle rolling on a smooth horizontal surface. Yes/No?[/b]
No (and please, a little less bold face).
In you calculation, the rolling resistance means you need less force?? (Sanity check time)
And why include gravitational acceleration?

I found this on physics forum. Only problem i found in this extremly small dinamical friction coefficient of wheel itself with axle of only 0.004. But if you look again it is added to friction force of wheel to ground.
Is it?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

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