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A short class with Sarah and her students. The other ways are under More.
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Today we will start by discussing rolling resistance, which is expressed in kg per ton of vehicle weight. Who can explain how to calculate the rolling resistance for a machine weighing 50 tons and having a rolling resistance of 28 kg per ton?
Isn't it just multiplying the weight by the rolling resistance value? So, it would be 50 tons times 28 kg per ton?
Exactly! So, what do we get when we perform that multiplication?
That would be 1400 kg of rolling resistance.
Well done! Remember, the formula we use is: Total Rolling Resistance = Gross Weight × Rolling Resistance Value. Can anyone tell me why it's important to know the rolling resistance?
It helps us select the right machine for a project based on the strength needed to overcome that resistance!
Good connection! Knowing this helps in estimating the power requirement too.
Does that mean higher weight will always lead to higher rolling resistance?
Correct! Heavier machines will generally experience greater rolling resistance. Great observation!
Moving on, let's talk about grade resistance. Can anyone define what happens when a machine is climbing a slope?
It faces resistance because it’s moving against gravity!
Great! And how can we calculate the resistance based on grade percentage?
For every 1% grade, we need 10 kg of tractive effort per ton, right?
Exactly! So, if the slope is 5%, what would be the tractive effort needed for a vehicle weighing 10 tons?
That would be 5 times 10, which is 50 kg, multiplied by 10 tons, resulting in 500 kg.
Awesome! That’s how grade resistance builds up. Now, what about grade assistance? How does that play into our calculations when descending?
It reduces the power needed since gravity assists in moving downwards!
Correct! Understanding these concepts is essential for project planning. Let’s summarize: rolling resistance opposes movement, while grade assistance can ease the load when going downhill.
Now let's calculate total resistance. Remember, total resistance is the sum of rolling resistance and grade resistance. How do we calculate total resistance based on our previous examples?
We would add the rolling resistance and grade resistance values together!
Exactly. So if we have 1400 kg of rolling resistance and 600 kg of grade resistance, what is the total?
That would be 2000 kg!
Right! And why is it important to know this total resistance?
So we can ensure the machinery selected can handle that resistance correctly!
Well said! Next, we will dive into usable power. This will tie everything together.
Let's shift our focus to usable power. How do we determine the usable power from the total power available in our machinery?
We subtract the power used to overcome resistance from the total power, correct?
Exactly! If our total is 7000 kg and we have 1500 kg used for resistance, what remains for towing?
That would be 5500 kg available for towing!
Well done! Knowing the usable power directly influences the machinery we select for our tasks. What factors could affect usable power?
Altitude and temperature would be big factors as they affect efficiency!
Correct! Keep in mind that various conditions can change how we assess power capabilities.
The combined force opposing the motion of a vehicle due to rolling and grade resistance.
The power available for productive work after overcoming all resistances.
The difference in power requirements when going uphill (resistance) versus downhill (assistance).
When up the hill we go with might, grade resistance puts up a fight.
Imagine riding a bike uphill; the steeper the hill, the more effort you need. This is like how machines need more force to overcome grade resistance.
Remember 'R.G.U.' to recall:
G.R.R.A. helps to remember:
Rolling Resistance,
Required Power,
Grade Resistance,
Grade Resistance,
Usable Power.
Assistance while going down.
Term
Rolling Resistance
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Term
Grade Resistance
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Term
Usable Power
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The resistance that a vehicle experiences when rolling over a surface, typically expressed in kg per ton.
The additional resistance encountered by a vehicle when moving uphill against gravity.
The reduction in power needed when moving downhill, due to gravitational pull aiding the motion.
The sum of rolling resistance and grade resistance that a vehicle must overcome during operation.
The amount of power available for productive work after accounting for resistance forces.
The force required to move a vehicle, specifically in relation to overcoming resistance.
This section explains how to calculate rolling resistance and grade resistance in vehicles, along with the concept of grade assistance when moving downhill.
Through calculations for rolling and penetration resistance based on machine weight and slope percentage, the section highlights the importance of understanding resistance forces in selecting appropriate machinery for construction projects. It also introduces the concept of grade assistance while descending slopes.
This section elaborates on the different types of resistance encountered by vehicles during operation, focusing on rolling resistance and grade resistance, as well as the beneficial aspect of grade assistance when descending slopes.
Rolling Resistance Calculation: The section begins by explaining that rolling resistance is quantified in kg per ton of vehicle weight. The example provided highlights a machine weighing 50 tons having a rolling resistance of 28 kg per ton, leading to a total of 1400 kg of rolling resistance from detailed calculations.
Penetration Resistance: Further calculations for penetration resistance based on the tire's depth of penetration into the surface show that a depth of 6 centimeters translates into 1800 kg of resistance.
Total Resistance: By summing the rolling and penetration resistance, the total resistance of 3200 kg is established, highlighting the required tractive effort needed for machine operation.
Grade Resistance Explanation: The content explains how grade resistance occurs when a machine climbs slopes, requiring additional effort against gravity. The relation between slope percentage and the tractive effort needed to overcome this resistance is emphasized.
Grade Assistance Introduction: Conversely, the concept of grade assistance is introduced, indicating how gravitational pull can aid in movement on a downhill slope, thus reducing power requirements.
Calculating Grade Resistance: The formula for calculating grade resistance is introduced, outlining that for every 1% grade, a vehicle requires 10 kg of tractive effort per ton. This guideline is applicable for slopes less than 10%.
Equivalent Gradient Conversion: Instructions on converting rolling resistance into an equivalent gradient are also outlined, enhancing the understanding of how different resistances relate.
Usable Power Insights: Finally, the section transitions into the discussions regarding available and usable power for machinery, emphasizing the factors that affect these power calculations, including site conditions and manufacturer specifications.