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Comment by dale_glass | Hacker News Reader

Plastic gears that break can be intentional, good design for better repairability.

If a mechanism jams, something has to give. This may be the motor, which may survive being jammed indefinitely, or it could burn up, causing a more expensive problem. In a mixer, the motor is probably decently powerful and doesn't tolerate being jammed.

If there's enough gear reduction, then a jam can develop enough force to bend shafts or break mountings and other components. That can make the product impossible to repair, if some injection molded bit that was supposed to hold stuff in place was broken.

A well placed plastic gear that breaks and saves the rest of the mechanism can make repair far easier and cheaper. And gears are very standard components that are far easier to find replacements for than some weird injection molded thing being made for one specific model of mixer.

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Here's a comment I posted a year ago about the sacrificial plastic gear in a Baratza coffee grinder:

https://news.ycombinator.com/item?id=27018629

One of the replies is a truly frightening story about people who broke the sacrificial gear in the tailgate window mechanism of their Toyota Landcruiser and machined a new one out of brass!

>Plastic gears that break can be intentional, good design for better repairability.

If it was a planned safety features then spare gears could be included or easy to find and buy for cheap(a device I bought had a safety fuse and it comed with 3 extra ones , also it was designed to be simple to replace it, no screws or "genius" needed. On the other hand a laptop of mine got destroyed by a high power voltage, but not destroyed at once, made me spent some more money on repairs until it finally died, all because some cheap safety feature was not included (even if hard to replace))

But then it isn’t planned obsolescence. It is planned obsolescence if the plastic gear wears and breaks down without a jam occurring. In your model it is a fail safe or circuit breaker. In the GP it a part designed to fail sooner (in wall clock time) to make sure you buy a new product.

But that's the crux of engineering: these two things are in tension – the part which is designed to fail safely if a jam occurs will, by its very nature, also be subject to wearing down sooner than if it were made of cast iron and caused the entire mixer to fail catastrophically during a jam.

In reality, nothing is so simple as waving your hands and declaring "no plastic 'planned obsolescence' parts!". Everything is a trade-off. A more effective way of pushing companies to position the trade-off in a way that rewards long-term durability would probably be to mandate longer warranty periods, rather than try to dictate what gears ought to be made out of in a vacuum divorced from the tradeoffs involved.

The key would seem to be if the part that fails is easily replaceable, then. If I make a design that has a part meant to fail-safe in a situation that would otherwise be dangerous to the other devices (I'm an EE rather than an ME, so we'll call it a "fuse"), the good-faith repairability practice is to put the part in an accessible location and make it a part that is readily available. If I wanted to encourage replacement of the whole device, maybe I make it a soldered-on part in the potted section of the PCB.

I was quite pleased with the design for the current measurement fuse on my HP 3458A meter: https://youtu.be/sdyDUl5xYpE

There are overload protection mechanisms without sacrificial parts, e.g. ball detents. They don't inherently have to wear down faster. Plastic gears are likely cheaper and lighter, though.

But taken to an extreme each car would look like a military truck (because each component could have been made stronger) - to prove planned obsolescence you'd have to show that the rest of the device will last longer than the part and there's a comparable part that would work better.

For example timing chains vs timing belts, apparently there are reasons they choose one over the other, but maintenance is part of keeping them operational.

On our blender, which has a fairly powerful motor, that’s the exact design. There’s a plastic drive coupling that does eventually wear our, but serves primarily as a one-way shear coupling if the blender gets jammed with ice or whatever mechanical stresses a blender is bound to experience.

A 5 pack of the replacement parts plus the tool to hold the shaft still while installing costs $10+tax delivered from Amazon (ASIN B07Y7S33RF).

The trick is, someone has to think to and then want to repair it. When that coupler first wore out, many would assume “oh, the mixer wore out; we have to buy a new one” rather than “a replaceable part wore out; we have to buy and install a new one”.

A few weeks ago, a friend came over with a cracked phone screen and mentioned they were going to buy a new phone the next day. “Why not replace the screen? I did mine last month.” “You can do that?!”

People are in a default buy-again mindset.

Frictions are not a new thing: for a simple mixer you just need a friction coupling with two sandpaper-like washers, Not only it's safer than a braking gear who can spill debris around blocking something else, but it demand essentially no repair...

Great example of why "planned obsolescence" isn't always bad. In this case the weak gear acts alike a kind of mechanical fuse. So long as it's within a context of easy replacement.

The cheap way to do this (and imo better because much better definition of the breaking strain) is a shear pin.

There are other (better) but more expensive ways to prevent damage to the motor. The plastic gear in a mixer is the cheapest way to solve the problem while also ensuring the device wears out fairly quick compared to a better built unit. It’s a win win for the manufacture while a total loss for the consumer (and environment)

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