What is the Best Method for Calculating Rolling Resistance in Train Carriage?

In summary, JR plans to use a gearbox that has a low coefficient of friction in order to reduce the time it will take to move a 10 ton cart. He needs to calculate the torque needed and size the gearbox accordingly.
  • #1
SevenToFive
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I would like to see if I am on the correct path here and didn't miss anything for a project at work. If I am trying to move a cart that weighs 10 tons, this includes the weight of the cart and its load, the wheels of the cart are 10 inches in diameter, and with a 40:1 ratio gearbox I need to figure out the torque needed to move this cart.

The cart is steel wheels on a steel rail so I figured 0.5 coefficient of friction.
The total tractive effort (TTE) = Rolling Resistance(RR) + Grade Force(GF) + Force for Accelerate(FA)

For RR=weight * surface friction = 20,000lbs * 0.5
RR=10000lbs

GF= Weight * Sin(Degrees) = 20000lbs * Sin(0)
GF=0

FA = Weight * max speed(ft/s) /(32.2ft/s^2 * Time in Seconds)
Max speed would be radius * rpm * 0.10472 = 5" * 44rpm * 0.10472
Max speed (ft/s) = 1.92ft/s
20000lbs * 1.92ft/s /(32.2ft/x^2 * 1sec)
Force Acceleration (FA) = 1193lbs

TTE=10000lbs+0+1193lbs
TTE=11193lbs

Wheel torque = TTE * Wheel Radius
Wheel torque = 11193lbs * 5inches
Wheel torque needed to move the 10 ton cart is 15,965in-lbs.

If I am close I can size the gearbox and then only have to worry about the operating temperature that the gearbox will be in since it is being used to move a cart in and out of a heat treating furnace.

Thanks to all who reply.
 
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  • #2
The way that I recall, the rolling resistance comes from the tire slightly deforming while in contact with the surface. I think the resistance that you want is from the wheel bearings, not the friction between the wheel and track. I think it will be much less than 0.5
I base this on the fact that I can put my 3000 pound vehicle in neutral on a level surface, and be able to push it. I'm guessing that I'm not exerting a lot more than 100 pounds force.

If the wheel starts sliding in relation to the track, then you are loosing power. The goal is to have a high enough static friction (between wheel and track) so that it does not slide.
 
  • #3
There is more than one coefficient of friction here. The 0.5 coefficient of friction relates to spinning or skidding the wheels. It does not apply to your situation. The coefficient of rolling friction is the force to make the cart move divided by the weight of the cart. That can be from 0.0003 to 0.005 (based on a quick internet search). It will vary widely depending on smoothness of track and wheels, bearings, alignment, slope, etc.

I realize this is physicsforum, where we try to calculate everything, but this is a case where you would be well advised to run a test. Get a spring scale, a rope, and a few people. Pull on the rope and measure the force. Do this in both directions in case there is a slight slope.

Walmart has a deer scale that measures up to 550 lbs for $25. https://www.walmart.com/ip/Allen-550-lb-Deer-Scale/17403736. You don't need a super accurate scale because you will put a safety factor of about 3 on to whatever you measure.
 
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  • #4
JRMichler said:
There is more than one coefficient of friction here. The 0.5 coefficient of friction relates to spinning or skidding the wheels. It does not apply to your situation. The coefficient of rolling friction is the force to make the cart move divided by the weight of the cart. That can be from 0.0003 to 0.005 (based on a quick internet search). It will vary widely depending on smoothness of track and wheels, bearings, alignment, slope, etc.

I realize this is physicsforum, where we try to calculate everything, but this is a case where you would be well advised to run a test. Get a spring scale, a rope, and a few people. Pull on the rope and measure the force. Do this in both directions in case there is a slight slope.

Walmart has a deer scale that measures up to 550 lbs for $25. https://www.walmart.com/ip/Allen-550-lb-Deer-Scale/17403736. You don't need a super accurate scale because you will put a safety factor of about 3 on to whatever you measure.

JR, I would like to do the simple way of just measuring rather than doing all of the calculations but this unit isn't even built yet and where it is being built is 2500 miles away from me.

Looking through my past work, I think my calculated torque value is way way to high. I am going have to run through the numbers again and as JR mentioned using a 0.005 rather than a 0.5 for friction.

Thanks
 
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  • #5
Unless I didn't see it, you seem to have missed the most important part: time required to accelerate to speed.

Websearch for Smart Motion Cheat Sheet, is a nice summary of equations of motion. Other nice sources are gear motor Engineering Guides from a variety of manufacturers.

You need to calculate peak torque, which is mostly the torque needed to accelerate that inertia (rotational & linear) up to your speed in a specified time. Add to that all the friction, gravity, etc., plus safety factor. Torque is cheap, be sure to use plenty of it.

T = J x α (Torque = mass moment of inertia X angular acceleration)
α ≈ Δvelocity / Δtime
 
  • #6

1. What is torque and how does it relate to moving a loaded cart?

Torque is a measure of the rotational force applied to an object, and in this case, the loaded cart. It is directly related to the amount of force applied and the distance from the center of rotation. In order to move a loaded cart, a certain amount of torque is needed to overcome the resistance or friction between the cart and the surface it is on.

2. How is torque calculated for a loaded cart?

The formula for torque is T = F x r, where T is torque, F is the applied force, and r is the distance from the center of rotation. So, to calculate the torque needed to move a loaded cart, you would need to know the amount of force applied and the distance from the center of the cart.

3. Can the weight of the load affect the amount of torque needed to move the cart?

Yes, the weight of the load on the cart can affect the amount of torque needed to move it. The heavier the load, the more torque will be required to overcome the resistance or friction between the cart and the surface it is on.

4. How does the surface affect the torque needed to move a loaded cart?

The type of surface the cart is on can greatly affect the amount of torque needed to move it. A smooth surface will require less torque compared to a rough surface, as there is less friction to overcome. Additionally, a flat surface will require less torque compared to an inclined surface, as the cart will need to be lifted up against gravity.

5. Are there any other factors that can affect the torque needed to move a loaded cart?

Yes, there are other factors that can affect the torque needed to move a loaded cart. These include the type and condition of the wheels on the cart, the presence of any obstacles or inclines in the path, and the overall weight distribution and balance of the load on the cart.

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