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A short class with Sarah and her students. The other ways are under More.
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Today, we'll discuss rolling resistance. Can anyone tell me how we might calculate it from a vehicle's weight?
We can convert the weight from kilograms to tons.
Excellent! For instance, if the gross weight of a machine is 50,000 kg, how many tons is that?
That would be 50 tons.
Correct! Now, if the rolling resistance is 28 kg per ton, what would the total rolling resistance be?
It would be 1400 kg, right? Because 50 tons times 28 kg is 1400 kg.
Exactly! Rolling resistance is a critical factor in our calculations.
Remember the acronym ROLL: Resistance, Overcome, Load, Lift, to help remember this concept.
Now, let's calculate the total resistance. We already know our rolling resistance is 1400 kg. What about penetration resistance?
Isn't it based on how deep the tire sinks into the ground?
Exactly! Here, if the tire sinks 6 cm and it's 6 kg per ton per centimeter, how do we calculate that?
We'd multiply 6 kg by 6 cm and then by the 50 tons!
Perfect! That gives us a penetration resistance of 1800 kg. So, how do we find the total resistance?
We add the rolling resistance and penetration resistance together!
Correct! The total resistance is 3200 kg, which is crucial for understanding the power needed.
Next, let's discuss grade resistance, which occurs when machines climb slopes. Why do we need more power for this?
Because the machine is working against gravity!
Exactly! The steepness of the slope impacts how much tractive effort is required. Who remembers how that’s quantified?
Each percent of slope requires an additional 10 kg per ton of machine weight.
Right! If we have a slope of 5%, how much extra power do we need for a 50-ton machine?
That would be 5 times 10 kg per ton times 50 tons, which equals 2500 kg!
Great job! Remember that better routes can minimize this resistance and save cost.
Let's now discuss available power. What do we mean by this term?
It's the power the manufacturer states for a machine?
Exactly! However, this becomes usable power based on project conditions. Can anyone explain how these conditions affect usable power?
Factors like altitude and temperature can lower the effectiveness of that power.
Correct! With higher altitudes, engine performance drops due to lower atmospheric pressure. How can we calculate usable power?
By subtracting the power needed for resistances from the total available power!
Exactly! That’s the key to understanding how much power is really available for work.
The resistance experienced by vehicles due to weight and surface interaction.
Resistance due to tire sinking into the ground, based on depth.
Force required to overcome the effects of gravity on slopes.
The sum of both rolling and penetration resistance.
Power rated by manufacturers under defined standard conditions.
The effective power available after accounting for resistance forces.
To roll and climb, do calculate; resistance you must never hate.
Imagine a heavy truck named Tread. Tread had to roll over muddy ground and steep hills. Each time he tackled a hill, he learned to calculate his rolling and grade resistance to keep moving forward.
Remember the acronym RAMP: Resistance, Altitude, Machines, Power. It will help you recall the key aspects of determining usable power.
G.R.A.D.E.: Grade Resistance And Driving Effort helps understand the concept of how slope affects power.
Term
What is rolling resistance?
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Term
How is total resistance calculated?
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What affects usable power?
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What is the significance of slope percentage?
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The resistance a vehicle encounters when moving over a surface due to its weight and surface interaction.
Resistance caused by a tire sinking into the ground, requiring additional tractive effort.
The additional force required to move up or down a slope due to gravity.
The amount of power available for actual work after accounting for resistances.
The power rating provided by the manufacturer under standard conditions.
This section explains how to calculate the available power needed for heavy machinery to overcome various resistance forces during operation.
In this section, we discuss the calculation of rolling resistance, penetration resistance, and grade resistance that affect the tractive effort required from a machine. We also explore the distinction between available power and usable power, emphasizing the significance of these calculations in the context of selecting appropriate machinery for specific tasks.
In this section, we delve into the nuances of determining available power for machinery operating under varied conditions. We start by converting vehicle weights from kilograms to tons, allowing easier calculation of rolling resistance, typically measured in kilograms per ton. The gross weight of the machine (50 tons) is multiplied by the rolling resistance value (28 kg per ton) to yield a total rolling resistance of 1400 kg. Next, we calculate penetration resistance based on the depth of tire classification (6 cm), resulting in additional resistance of 1800 kg. When both resistances are summed, the total resistance is identified at 3200 kg, indicating the minimum tractive effort necessary to overcome these forces.
Continuing, we differentiate between grade resistance and grade assistance—grades influence how much power must be exerted to climb or descend slopes. By examining the slope’s percentage, the tractive effort needed is adjusted, illustrating the importance of considering terrain in machinery selection. Grade resistance is quantified using the manageable formula where each 1% of grade results in a requirement of 10 kg per ton of machine weight. This section concludes by examining the 'usable power,' defined as the power available for actual work after overcoming resistance, emphasizing project conditions like altitude and temperature as key indicators of how much of the available power a machine can actually utilize.