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A short class with Sarah and her students. The other ways are under More.
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Today, we're starting our discussion on calculating usable force, beginning with rolling resistance. Who can tell me why we need to convert weights from kilograms to tons?
Is it because rolling resistance is often expressed in kg per ton, making it easier for calculations?
Exactly! In our example, if the gross weight of a machine is 50,000 kg, how many tons is that?
That would be 50 tons, since you divide by 1000.
Correct! Now, if we have a rolling resistance of 28 kg per ton, who wants to calculate the total rolling resistance for 50 tons?
That would be 50 times 28, which equals 1400 kg.
Awesome! So, rolling resistance can be calculated using the formula: Roll Resistance = Gross Weight (in tons) × Rolling Resistance (kg per ton). Remember, 'Roll Resistance = GR × RR', where GR is gross weight and RR is rolling resistance.
That gives us a good starting point for calculating usable force!
Great! In summary, we’ve covered rolling resistance calculations and their significance in selecting machines.
Moving on, let’s talk about penetration resistance. Can someone explain what it is?
It’s the resistance a tire faces when it sinks into the ground, right?
Correct! For our scenario, if a tire sinks 6 cm into a surface, and resistance is specified as 6 kg per ton per centimeter, how do we calculate the total penetration resistance?
We would multiply the penetration of 6 cm by 6 and then times the gross weight of 50 tons.
So, that’s 6 × 6 × 50 = 1800 kg.
Excellent! Now we can find the total resistance by adding rolling resistance and penetration resistance.
That would be 1400 kg + 1800 kg = 3200 kg total resistance.
Precisely! Remember our formula: Total Resistance = Rolling Resistance + Penetration Resistance.
That’s really helpful!
Let’s summarize: We've learned how to calculate penetration resistance and total resistance.
Today, we’ll dive into grade resistance. Why do we need to consider slope in our calculations?
Because machines have to exert more effort when moving uphill, right?
Spot on! Referring to a 4% slope, can anyone calculate the grade resistance?
Using the guideline, it's 10 kg per ton for 1% gradient. So for 4%, it’s 10 times 4 times the gross weight, or 10 × 4 × 50 tons.
That gives us 600 kg for grade resistance.
Just right! So how does this grade resistance compare to rolling resistance when deciding on machinery?
Higher grade resistance means we need a machine that can provide more power to overcome it.
Exactly! To sum up, we've elaborated on grade resistance and its implications for equipment selection.
Now, let’s connect what we’ve learned to usable power. Why is determining usable power important?
It's needed to understand how much power is actually available for work after accounting for resistances!
Perfect! Given a maximum rimpull of 7000 kg and resistance of 1500 kg, what’s the usable pull available for towing?
That would be 7000 kg minus 1500 kg, resulting in 5500 kg for towing.
Correct again! And what nuances must we consider regarding usable power?
Conditions, like altitude and surface traction, influence how much power we can effectively use.
Exactly! So, to conclude, we explored how to calculate usable power and its importance in machinery operation.
Calculated by multiplying gross weight by rolling resistance value.
Resistance due to tire depth into the surface, calculated based on penetration depth.
Varies with slope percentage, impacting machinery power needs.
Determined by subtracting the total resistances from available power.
When the tire sinks in, resistance is a spin; 6 kg per ton makes the effort begin.
Imagine a truck on a slope trying to make way. Each kilogram of resistance is like a stone in the path, heavy yet stay. Rolling down seems easy with weight being light, but as it climbs higher, more effort feels right.
Remember 'RPG' for calculations: Rolling resistance, Penetration resistance, General resistance equals total needed!
R.U.F. - Remember Usable Force, as it guides us in power to tow!
Term
What is rolling resistance?
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Term
What does penetration resistance refer to?
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Term
Define grade resistance.
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Term
How do you define usable power?
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The force resisting the motion of a vehicle when rolling on a surface, typically expressed in kg per ton.
The resistance faced by tires when sinking into a surface, associated with the depth of penetration.
The additional effort required for machinery to move uphill against gravity, dependent on the slope's percentage.
The power available for performing work after accounting for resistances that oppose the machine's movement.
This section discusses calculating the usable force of machinery based on rolling and grade resistance, highlighting the importance of understanding these factors for selecting appropriate equipment.
In this section, the key calculations required for determining rolling resistance, grade resistance, and usable force in machinery operations are explained. The importance of these calculations in selecting the right machinery power for project sites is emphasized, along with the effects of different surface conditions.
This section covers how to calculate usable force, focusing on two primary resistances: rolling resistance and grade resistance. The gross weight of the machine is converted into tons to simplify calculations, with a gross weight of 50,000 kg translating to 50 tons. The rolling resistance is specified as 28 kg per ton, leading to a total rolling resistance of 1400 kg.
Next, the section explains penetration resistance when the tire sinks 6 cm into the surface, which adds an additional 1800 kg of resistance to the initially calculated total. The total resistance is thus calculated as the sum of rolling resistance and penetration resistance, culminating in a total of 3200 kg of resistance that equipment must overcome.
The section further discusses grade resistance associated with machines climbing slopes. The explanation details how tractive effort required varies with the percentage of slope, illustrating the contrast between grade resistance and grade assistance when moving downhill. Specific calculations relate to 4% slopes, establishing a guideline of 10 kg per ton for grade resistance under 1% gradient, allowing students to translate these resistances into usable force for equipment selection.
Issues related to available power versus usable power, and how project conditions affect each, are also delved into, with calculations illustrating the reduction of usable power due to resistance from elements such as gradation and traction. This culminates in understanding usable power as what remains after overcoming resistance. The section finishes by exploring power requirements through real-world examples, guiding students through practical calculations to ascertain usable force in machinery scenarios.