The file moved back and forth along the edge of the iron bar, sending metal shavings falling onto the workbench like fine snowflakes. Chen Hao shifted his position, moving his left leg forward half a step to make himself more stable. He had just smoothed out a section of the cut when he heard a soft "click" from the bearing housing.

He paused for a moment, looked down, and found that the spindle was crooked.

"Again?" he muttered, reaching out to steady it. But as soon as he touched the shaft, the whole structure started to wobble, like a clothesline swaying in the wind.

Nana stood to the side without saying a word, only slightly tilting her head as the scanning light swept across the bracket connection point.

Chen Hao gritted his teeth and tightened the fixing bolts again, this time with extra force. He wiped his sweat and flicked it onto the rag on the corner of the table. "I refuse to believe it. Yesterday it could spin twice, and now it can't even last one spin?"

He gripped the handle and gave it a sharp turn.

"Crunch—Squeak—"

The sound was shrill, like someone had stomped on a tin can. After turning the handle twice, it suddenly jammed, and the spindle only moved slightly, twitching for half a beat as if it had a spasm.

"Damn it." He let go and stared at the unresponsive axle. "This isn't a spinning wheel, it's an elderly Parkinson's rehabilitation trainer."

Nana brought up the projector, and a 3D model floated in the air, with red markers lighting up one after another. "The torque at the power input end is 3.2 Nm, but the output end is only 0.47 Nm, resulting in an energy loss rate of 85%. The problem is not with the bearings or shaft straightness, but with the interruption in the transmission path."

Speak like a human.

“If you crank it by hand, the power won’t reach the spindle. There’s no intermediate mechanism.”

"So it's too lazy to put in the effort?"

"That's one way to understand it."

Chen Hao sighed, plopped down on the low stool next to him, and grabbed the water bottle on the table, taking a swig. The water was a bit lukewarm, and drinking it felt like swallowing a mouthful of old cotton wool.

He tapped the bracket with his hand. "I clearly assembled it according to the diagram. The handle connects to the shaft, and the shaft connects to the spindle. It's that straightforward. Isn't that how the ancients did it?"

"The ancients used belt pulleys or gear sets to increase speed and convert torque."

"Do they have these?"

"have."

"Then why didn't you say so sooner?"

"At the time, all you asked was 'if it could be transferred.'"

He rolled his eyes. "I thought 'as long as it can be converted' was a technical goal, but I didn't expect it to be a philosophical proposition."

Nana didn't reply, but simply switched the terminal screen to a breakdown animation: a hand crank drives a small gear, which meshes with a large gear, and then the rotation is transmitted to a high-speed spindle via a belt. The whole process is smooth and stable.

"So we need to insert two gears as intermediaries?" Chen Hao stared for two seconds. "But we don't have any gears ready."

"A sprocket can be used as a substitute, or a conical friction wheel structure can be designed."

"Just hearing about it gives me a headache."

"Alternatively, rubber bands can be used for flexible transmission, which is low-cost and easy to assemble."

"A rubber band?" He frowned. "The elastic band on the last pair of underwear in our base hasn't been removed yet. What are you going to use to make one?"

"There are 20 meters of discarded cable sheaths on the third row of shelves in the north section of the warehouse. The material meets the requirements for elastic transmission."

"Oh, you still remember the inventory?"

“I remember everything.”

He snorted, got up, and walked to the corner to rummage through the parts box. After searching for a while, he pulled out a few dark rubber strips, squeezed them, and found they were still somewhat flexible.

"Can this thing really be used as a belt?"

"Theoretically feasible."

“Here we go again.” He rolled his eyes. “Last time you said a plowshare was theoretically feasible, but welding it almost dislocated my wrist.”

"You loosened the clamp yourself that time."

"That's when I felt inspired and wanted to express myself freely!"

He returned to the front of the table and tried to cut the rubber strip into a long band, which he then looped between the handle shaft and the spindle shaft. As soon as he pulled, it snapped with a crack.

“Brittle aging,” Nana said calmly. “I recommend preheating to improve ductility.”

"How do we preheat it? With a hair dryer? We don't have an industrial drying oven."

"You can use a constant temperature soldering iron to heat it slowly, and control the temperature to within 60 degrees Celsius."

“Alright.” He picked up the soldering iron and stretched the rubber strip while heating it. “I’m not building a spinning wheel now, I’m giving a broken rubber band postpartum rehabilitation.”

Ten minutes later, the newly made drive belt finally managed to wrap around the two pulleys. He took a deep breath and slowly turned the handle.

This time, the voice was a little softer.

The spindle started spinning, and although it trembled a bit, it was indeed moving continuously.

"It's moving!" His eyes lit up. "Look, it's spinning! At a constant speed! Stably! The first ray of dawn for the future textile giant has been born!"

Before she could finish speaking, the rubber belt suddenly slipped, making a sharp "squeak" sound, and then bounced off the axle, slapping against Nana's face.

She gently peeled it off and placed it on the table without expression.

“Insufficient tension and too small a contact area lead to decreased adhesion,” she said. “The current structure cannot maintain continuous operation.”

Chen Hao slumped his shoulders. "So all my work was for nothing?"

“It wasn’t a waste of time.” She pointed to the data curve on the terminal. “You’ve verified that direct drive is not feasible. This is an effective process of elimination.”

“I thought you would say, ‘Congratulations on taking the first step.’”

That's not true.

"You robots are so cold and heartless."

"I just don't glorify failure."

He leaned back in his chair, looking up at the ceiling. "So what do you suggest? I can't just manually twist the thread every day, can I? I might as well go back and collect leaves to wear."

"There are two solutions: one is to optimize the existing structure by adding a tensioning wheel or adjustment device; the other is to redesign the transmission system and adopt a more efficient power transmission method."

"It all sounds so complicated."

"It is a bit troublesome."

"Is there anything simpler? Like... magic activation?"

"No."

"Remote control using alien technology?"

"No."

"Did God take pity on me and suddenly send down a gust of wind to turn it around?"

"The probability is less than three in a thousand."

He let out a long sigh. "So, the only option is to learn?"

"To solve a problem, you must first understand the principles."

He stared at the string of technical terms on the terminal: "Gear train, torque matching, slip ratio... these terms look like they're deliberately designed to keep ordinary people away from machinery."

"They are merely tools for describing phenomena."

“But I was most afraid of this kind of tool when I was in school.” He scratched his head. “Whenever the teacher talked about gear ratios, I would start counting the cracks in the ceiling.”

Nana paused for a moment, then said, "You can choose to leave now."

"And then what? Keep wearing rags? Until the fibers pile up into a mountain and still can't be turned into clothes?"

"I can accept the status quo."

“No.” He shook his head. “I can already smell progress—though it may just be the smell of burning rubber.”

He sat up straight, picked up a marker, and drew a crooked circle on a piece of scrap paper.

“Come on,” he said. “Teach me. Start with the basics. What is… gear ratio?”

Nana brought up the teaching module, the interface refreshed, and the title of the first course popped up: "Fundamentals of Mechanical Transmission - The Journey of Force".

A simple animation begins to play: a handle turns, driving a small wheel, which in turn drives a large wheel via a belt, with the speed changes clearly marked.

“If the input wheel diameter is small and the output wheel diameter is large, the rotational speed will decrease and the torque will increase.” Her voice was steady. “Conversely, it can achieve speed increase.”

Chen Hao stared at the screen, his brows slowly furrowing. "So if I want the spindle to spin faster, I have to make it smaller than the handle?"

"correct."

"Then why did I do it backwards just now?"

"You made the drive wheel bigger than the driven wheel."

"No wonder it's too lazy to move." He smiled wryly. "It's not a malfunction, it's just that it thinks I'm too weak."

"It simply follows the laws of physics."

"Routines are so annoying."

He picked up a pen and wrote a string of words on the paper, crooked and messy:

What is the transmission ratio?

After finishing writing, he looked up at Nana and said, "You know... how could people in ancient times, who hadn't studied physics, have invented a spinning wheel?"

As soon as he finished speaking, he remained seated, pen tip hovering above the paper.

Nana's gaze fell on the line he had written, and the terminal screen displayed the loading progress bar for the next lesson, which was slowly progressing.

Her voice rang out: "Because they tried three thousand times."