3.2. Calculating Maximum Usable Rimpull

Today, we're going to discuss rimpull. Can anyone tell me what rimpull means?

Is it the pulling force that a wheel can exert on the ground?

Exactly! Rimpull is the force available at the wheels to do work. Remember, it is primarily determined by two factors: the coefficient of traction and the weight on the powered wheels.

What’s the coefficient of traction?

Great question! The coefficient of traction indicates how well the wheels grip the surface. A higher coefficient means better grip!

How does weight factor into this?

The more weight on the driving wheels, the more rimpull we can generate. Hence, knowing how much of the total weight rests on the drive wheels is vital! It leads us to calculate usable rimpull more accurately.

To remember this, think of 'RIMPULL' - 'R' for Resistance, 'I' for Input weight, 'M' for Maximum force that can be generated. This acronym will help you relate weight and traction back to usable power.

So if we have more traction, we can do more work with the same vehicle?

Yes, that's correct! Let's move on to how we calculate this, including the influences of the environment and the specifics of the load.

Now, we will calculate maximum usable rimpull. Can anyone tell me the formula?

It's the coefficient of traction times the weight on the powered running gear, right?

Correct! Let's apply it now. For a scraper with a gross weight of 76,000 kg and a coefficient of traction of 0.7, what would the maximum usable rimpull be?

So, 0.7 times half of 76,000 kg would be what, 26,600 kg?

Exactly! Now this value represents the maximum rimpull available. But there's more; we need to consider rolling resistance, which is 2% of the gross weight. Can someone calculate that?

That would be 0.02 times 76,000 kg, which is 1,520 kg.

Great! Why is this figure important?

It shows how much pulling force we need to overcome just to keep the scraper moving!

Precisely! Remember to always subtract resistances from your rimpull to get your effective pulling capacity.

Let's talk about altitude effects now. How does altitude impact a machine's performance?

Doesn't it reduce air pressure, making combustion less efficient?

Absolutely! Particularly for combustion engines. At 600 meters, we need to adjust the rimpull further. Can anyone tell me how we would do this?

We would reduce it by 3% for every 300 meters above 300 meters.

Correct! So how would we calculate the deduction at 600 meters?

For the first 300 meters there's no deduction, but for the next 300, the deduction would be 3% of the maximum rimpull! So, that's 3% of 26,600 kg.

Great work! This accounts for the reduction of effective rimpull. Remember, lower efficiency from altitude means we must closely manage power outputs.

To help you remember: ALTITUDE - 'A' for Air pressure, 'L' for Lower performance, 'T' for Tolerate reductions, 'I' for Impact on rimpull, 'T' for Temperature, 'U' for Usage, 'D' for Deduction needed, 'E' for Efficiency decrease.

Now that we understand the theory, let’s see it in practice. How do we determine rimpull in different gears?

We use the horsepower formula, factoring in gear speed, right?

Yes! The formula is Rimpull = 273.6 x HP x Efficiency / Speed. Let’s calculate for the first gear at 6 km/h with 500 HP and 80% efficiency.

That gives us 18,240 kg as the supplied rimpull!

Exactly! And after altitude adjustment, what would our available rimpull be?

It would be 17,692.8 kg after subtracting 547.2 kg for altitude.

Right! And how does this help us in operational decisions?

We can determine if we have enough power to move up inclines based on our rimpull!

Exactly! Remember, always compare supplied rimpull against the required to avoid slippage.

Finally, let's reflect on how we select gears. What are the criteria we should consider?

We need to make sure the available rimpull meets the required rimpull for moving uphill.

Good! Can anyone illustrate this using the values we've calculated?

In the top gear, the available rimpull is less than required, so we cannot use it on inclines.

But first and third gears would work because their rimpull exceeds requirements.

Exactly right! Your choice of gear should always hinge on operational conditions. Remember: GEAR - 'G' for Grip, 'E' for Efficiency, 'A' for Adjustment to rimpull, 'R' for Resistance handling.

In summary, to optimize our rimpull calculations, always consider weight, traction, altitude impacts, and gear selection to enhance productivity.