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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 start with rolling resistance, a critical factor in calculating usable power. Let's talk about how we convert weight to tons.
Why do we convert weight into tons specifically?
Great question! We express rolling resistance as kg per ton, simplifying our calculations. So, if we have a machine weighing 50,000 kg, it becomes 50 tons.
How do we calculate the rolling resistance using that?
Exactly! You’ll multiply the weight in tons by the rolling resistance rate, which is often given. If it's 28 kg per ton, we get a rolling resistance of 1400 kg from 50 tons.
What does that 1400 kg mean?
It tells us how much force is resisting the motion of the machine!
Does that mean we must always counteract that force?
Yes, and it is key to calculate total resistance now!
Next, let's discuss penetration resistance. This occurs when a tire sinks into a surface, correct?
Yes! How do we compute that?
We multiply the depth of penetration by a specific rate given per ton per centimeter. For example, a 6 cm depth at 6 kg per ton gives us a total of 1800 kg.
So we add that to our rolling resistance, right?
Exactly! That gives us the total resistance which we need to consider for power calculations.
What's the next step once we have total resistance?
We must ensure machines can overcome this resistance. Power is required based on these calculations.
Now, onto the concept of total and usable power! What is total power?
Isn’t it the sum of all resistances we just calculated?
Correct! Total power is what we need to overcome resistances, e.g., rolling and penetration. What's usable power then?
It's what's left after resistances are accounted for?
Yes! So, if we have a total power requirement of 3200 kg, for example, and our machine can offer 7000 kg of pull, what's available for the actual work?
That would be 3800 kg!
Close! After using 3200 kg, only 3800 kg is left for lifting or towing. This is how you determine usable power.
Let’s discuss grade resistance and assistance. What happens when a machine goes uphill?
It needs more power to overcome gravity!
Exactly! For every 1% grade, we generally need 10 kg per ton, multiplying with the machine's weight provides the effort needed.
And going downhill is the opposite?
Yes! That’s called grade assistance, where gravity helps reduce the power required.
How do slopes affect our total power calculations?
They can substantially increase the resistance needed, hence, it’s crucial to choose haul routes wisely.
Finally, let’s apply everything to a scenario! Let’s say we have a tractor weighing 15 tons. What’s the first step?
Calculate rolling resistance and grade resistance.
Indeed! For a rolling resistance of 60 kg per ton and a 4% slope, how do we get totals?
Rolling would be 900 kg and grade would be 600 kg!
Perfect! So, total resistance we’ll need to overcome?
1500 kg!
Exactly. If the machine offers a rimpull of 7000 kg, what’s our usable power?
5500 kg available for towing!
That’s right! Always remember, usable power is the power left for work after traffic resistances.
A critical factor in how machinery overcomes motion when calculating total resistance.
The force exerted while tires dig into surfaces affecting total machine resistance.
Increased force significantly required to move machines uphill depending on slope steepness.
The reduction of resistance power needed when descending slopes due to gravitational help.
The power left post-resistance computations that can be utilized for effective machinery function.
In the cab cannot slack, power's needed to track, resist or glide, we must abide, and calculate right, to pull with might.
Once, a tractor named Timmy had to cross a hill. With 15 tons and a slope, he figured his rolling and lifting efforts carefully, and with true calculation, he reached the other side effortlessly!
R-G-R-P: Remember – Rolling, Grade, Resistance, Penetration.
P.U.S.H: Power Under Stated Heavyweights.
Term
What is rolling resistance?
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Term
Define penetration resistance.
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Term
What is usable power?
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Term
Explain grade assistance.
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The force resisting the motion when a body rolls on a surface.
The resistance experienced by a tire as it sinks into a surface.
The resistance that opposes a machine's movement when climbing a slope.
The reduction in power needed when moving downhill due to gravitational assistance.
The power available for performing work after overcoming resistance.
This section outlines how to calculate usable power for machinery by analyzing resistances such as rolling and grade resistance.
The section covers the process of converting vehicle weight from kilograms to tons to compute rolling resistance, penetration resistance, and grade resistance. It emphasizes the distinction between total and usable power, and how conditions like slope and traction affect machine efficiency. The section culminates in practical examples illustrating calculations required for determining usable power.
In the section 'Usable Power Calculation,' key principles of calculating usable power in machinery are presented. The beginning discusses the conversion of gross weight from kilograms to tons, highlighting the importance of expressing rolling resistance in kg/ton. For example, a machine weighing 50,000 kg converts to 50 tons, leading to a rolling resistance calculation of 50 tons multiplied by 28 kg/ton, resulting in 1400 kg of rolling resistance. The penetration resistance is also determined based on the tire's sinking depth into the surface, which is multiplied by a specified kg/ton per centimeter figure.
The sum of rolling and penetration resistance yields the total resistance — in the example, 3200 kg — which indicates the tractive effort necessary to overcome that resistance. The section progresses to discuss grade resistance and grade assistance when a machine traverses slopes. The teacher explains that for each 1% of slope, 10 kg/ton of effort is needed to calculate grade resistance. For instance, a 4% gradient requires 600 kg of effort for a 15-ton machine.
Furthermore, available power, defined by SAE ratings, is also discussed. The divergence between available power and usable power is clarified — usable power is what remains after accounting for resistances, emphasizing the impact of project conditions on efficiency. Finally, the teacher details calculations for usable power, calculations based on traction, and underfoot conditions, reiterating how pulling forces exerted by wheel-mounted machines (rimpull) or tracked vehicles (drawbar pull) are crucial for efficient operation.