Chen Hao's pen was still hovering over the paper, the crooked words "What is transmission ratio?" seeming to have just been squeezed out of his mind and hadn't been digested yet. He didn't move or speak, just stared at those words, as if looking at them a little longer would reveal the answer.

Nana didn't urge him. The progress bar on the terminal screen moved quietly. The teaching module had been loaded. The first animation was frozen in the initial frame: a handle connected to a wheel, a belt extending out, and at the end a high-speed rotating spindle.

"You just said that the smaller one leads the larger one, turning slowly but powerfully," Chen Hao finally spoke, his voice a little dry. "Then what I said before... was it backwards?"

"You use the large hand crank to drive the small wheel," Nana said, pulling up the structural diagram from the previous section. "It's like using a tractor to pull a bicycle."

"So it's not that it doesn't want to turn, but that it feels I'm insulting its physical dignity?"

"It simply operates according to the rules."

"The rules are truly unforgiving." He scratched the back of his head and slammed the marker on the table. "Let's do it again. This time I'll listen carefully."

Nana nodded, tapped the terminal, and the animation resumed. On screen, a handle turned, driving a small wheel, which in turn transmitted power to a larger wheel via a belt. The rotation speed slowed noticeably, but the torque value jumped upwards.

“It’s like—” she paused, “the irrigation canals we’ve built. The main canal has a large flow, but after branching into several tributaries, the flow of each tributary decreases, but the total pressure remains the same.”

"Oh!" Chen Hao's eyes lit up. "You mean, strength can also be divided into different parts?"

"That's one way to understand it."

"So if I want the spindle to fly, I have to make the smaller wheel on its side be carried by the larger wheel?"

"Correct. A strong input and slow rotation are necessary for a weak output and fast rotation."

He looked down and drew a large circle inside a smaller circle on the paper, connected them with an arrow, and wrote "Decelerate and increase torque". Then he drew another set on the other side and labeled it "Increase speed and decrease torque".

"So the ancients weren't clever, they just learned the tricks by trying them out?"

"Most of the progress of human civilization has been achieved by repeating mistakes."

"So, am I contributing to the history of textiles? Even if that brick hasn't been formed yet?"

"You're laying the foundation right now."

“It’s the kind that leaks cement.” He sighed and rubbed his sore eyes. “But these words… ‘torque,’ ‘slip ratio,’ ‘meshing angle’… sound like they’re designed to discourage lazy people.”

"These are just ways of describing things. You don't need the terminology, as long as you understand the phenomenon."

“The problem is I don’t understand the phenomenon.” He pointed to the spinning belt in the animation. “Why does it sometimes slip? Last time, that rubber belt was properly secured, but it bounced off as soon as I applied force.”

Nana paused the screen and switched to a force analysis diagram: the contact surface between the two wheels showed the pressure distribution, and the belt tension was represented by light and dark colors.

"When the friction required for power transmission exceeds the actual adhesion, relative sliding will occur."

Speak like a human.

"The belt is too loose, or the surface of the wheel is too smooth, so it's hard to grip when it's turning."

"It's like running in flip-flops; no matter how hard you step, they're likely to slip off."

The analogy holds true.

"So what should we do? Tighten it? Apply tape? Or grooving the wheels to increase friction?"

"Tighten the tensioner or raise the wrap angle."

What is "corner wrapping"?

"How long is the arc where the belt wraps around the wheel?" She gestured for him to look at the diagram. "The larger the corner, the stronger the contact."

"So you can't just put it on halfway; you have to go around it more than half a circle?"

"right."

Chen Hao stared at the diagram for a few seconds, then suddenly looked up and said, "Could we... shorten the spindle shaft a bit, add a secondary pulley next to it, and make the belt travel a longer distance?"

"It's technically feasible, but it would increase structural complexity."

"But I don't want to see rubber bands stuck to your face anymore."

Nana tilted her head slightly. "That was an accident."

“But I’ve dreamt about it several times.” He grinned, then frowned. “But I still don’t understand… why do we have to take such a roundabout way? Can’t we just connect directly?”

“A direct connection cannot change speed, nor can it buffer the impact.” She pulled up the windmill’s maintenance records. “Remember the last time the blade broke? It was because there was no intermediate transmission, and the force of the strong wind rushed directly into the bearing.”

"So these wheels and belts in the middle are actually 'buffer mediators'?"

"You could also call him an energy translator."

"Translators are quite expensive, and we haven't got enough of them yet."

He was silent for a moment, then picked up his pen and wrote three questions in the corner of his notebook:

1. How can a small wheel drive a large wheel without slipping?

2. What exactly does torque feel like?

3. Is it possible to use something other than a belt?

After finishing writing, he blew on it, as if to disperse his jumbled thoughts.

"I always thought mechanics was just about assembling parts," he said. "Who would have thought I'd have to learn to talk too?"

"If you don't learn, the parts won't listen to you."

"So I have to learn to reason with them first?"

"At least we need to know what they can understand."

He looked down at his sketch again, his finger slowly tracing the transmission path. Suddenly, he looked up: "You know... if I make the handle side very light, making it easier to turn, but then connect a large wheel that turns slowly, and then accelerates the transmission to the spindle through another small wheel—wouldn't that count as a two-stage transmission?"

"Theoretically, yes, it can be called multi-stage transmission."

"It sounds like an automatic car."

"The principles are similar."

"Can we do it?"

"Materials are limited, but a simplified version can be tried."

"In other words, I don't necessarily have to do it all at once. I can first set up a 'transmission training course' so that each part can learn its own movement?"

"Your analogy... is barely acceptable."

Chen Hao grinned and relaxed his shoulders. He reached for the water bottle, found it empty, and simply put it back.

“When I was in school, I would daydream when the teacher talked about gear ratios.” He stared at the ceiling. “I always felt that these things were a million miles away from me. But now, if I don’t study, I won’t have any clothes to wear.”

"Survival has changed the priority."

“You’re right.” He cracked his knuckles. “Shall we continue? I still have a question I don’t understand—what does torque matching mean? I always hear you use that term, it sounds like you’re chanting a spell.”

Nana brought up a new page, displaying a set of data comparisons: the input and output torque curves fluctuated with different combinations of wheel diameters.

“Matching means making the power and speed transmitted at each level suitable for the needs of the next stage,” she said. “For example, when you ride a bicycle, you need to start in a low gear, otherwise you won’t be able to pedal; when you get faster, you can switch to a higher gear, otherwise you’ll be exhausted and still not be able to catch up.”

"So the transmission system has to be able to shift gears?"

"Ideally, yes."

"But our spinning wheels can't be changed by themselves; we can only use one set."

"Then choose the set of parameters that best suits the commonly used operating conditions."

"In other words, I have to decide whether it's the 'muscle man type' or the 'endurance type'?"

"If you insist on classifying it this way."

“Then I’ll definitely choose the endurance type.” He stroked his chin. “Anyway, I’m not in a rush to meet deadlines, so I just don’t want it to break down too often.”

"It is recommended to set the gear ratio between 3:1 and 4:1, as manual operation requires the least effort."

"Three to four?" he wrote on the paper. "Then I need to calculate what size wheels would be appropriate."

He pulled out the data book from before, flipping through page by page until he found the diameter records of the spindle shaft and the handle shaft. He then took out the calculator and pressed a few keys, his brow furrowing more and more deeply.

"That's not right," he muttered. "At this ratio, the output wheel would be wider than the table."

"With limited space, multi-stage compression can be used to reduce dimensions."

"Another multi-level question?"

"Otherwise, it would be difficult to balance efficiency and size."

Chen Hao leaned back in his chair, looking up at the roof beams. "So, if you want to be lazy, you actually have to use your brain more?"

"Behind every labor-saving device lies a more complex set of considerations."

"What a ridiculous design logic."

He sat up straight, unfolded the paper again, and drew more carefully this time: the first stage is a handle driving a small wheel, which is connected to a large wheel via a belt; the large wheel is coaxial with another small wheel, and the second stage belt transmits to the final small spindle wheel.

"Like this?" He pointed to the sketch, "Two-stage growth rate?"

"The structure is reasonable."

"But the parts have to be precisely aligned, otherwise the belt will run wild."

"A positioning slot and adjusting screws are required."

"Adjusting screws... We have fine-tuning levers salvaged from old printers in the warehouse, can we use them?"

"The materials are suitable, and the precision is sufficient."

"That's good." He breathed a sigh of relief. "I was worried we'd have to make the threads from scratch again."

"Not needed for now."

Chen Hao stared at the blueprints for a long time, then suddenly smiled and said, "Do you know what I'm most afraid of right now?"

"What?"

"I'm afraid that just as I've figured it out, you'll shut down the power and restart, and forget all my notes."

"My storage system will not lose data."

"But what if one day you suddenly say, 'According to the new directive, the textile project is cancelled,' and then tear this page up?"

"No such instructions have been received at present."

"That's good." He lowered his head and continued writing. "I'm risking going bald to learn physics."

Nana glanced at him and slightly adjusted the focus of the optical lens.

"Your hair volume is currently stable."

"Thanks, the robot can even comfort people."

"I'm just stating the facts."

He smiled and shook his head, pushed the notebook forward, and pointed to the drawing he had just finished: "So, next, shouldn't we calculate the specific dimensions? Like the belt length and the wheel spacing?"

"We can begin modeling and calculations."

“Wait a minute.” He suddenly raised his hand. “Let me go over it again—I want to make the hand crank easier and the spindle spin faster, with two-stage belt drive in the middle, each stage about three to one, for a total of about nine to one?”

"A ratio closer to 10 to 1 would be even more ideal."

"Ten to one?" He widened his eyes. "Then I'll become a generator?"

"You turn it once, and the spindle turns ten times, significantly improving efficiency."

"But my arm isn't a motor."

"The required force can be reduced by extending the handle."

“Extended handle…” he stroked his chin, “It saves effort, but it takes up a lot of space.”

"It can be unfolded when in use and folded when stored."

"You can do that?"

“The structural design allows it.”

Chen Hao was stunned for a few seconds, then suddenly slapped his thigh: "Hey, isn't this just like a telescopic clothes rack pole!"

"The principle is similar."

"You should have said so earlier!" he laughed. "You made me worry for so long."

"You didn't ask."

"I never expected it could be mixed and matched like this." He grabbed his pen and eagerly drew a foldable handle next to the drawing. "This saves effort and doesn't take up much space, perfect!"

He finished drawing, took a breath, and felt as if a burden had been lifted from his shoulders. His eyes brightened, no longer filled with confusion, but with a hint of eager anticipation.

"So it wasn't that the machine was disobedient," he said softly. "It's just that I haven't found the right way to chat."

Nana looked at him; the terminal interface was ready to load the next lesson: "Selection Principles of Gears and Belts".

“Next,” she said calmly, “we’ll look at the specific calculation method for torque matching.”