I've had the same problem with mowers sold by Home Depot and Toro. The HD one had teeth on the inside of the "back" side of the rim. The Toro I'm currently using has a gear molded into the back side of the hub. The Toro is supposed to adjust its speed to match your walking pace. I've never done a teardown to figure out how it is doing that.
My dad had a mower when I was a teenager where a knurled or toothed wheel engaged the tread of two tires (front or rear, I can't remember). Pretty sure that the system was adjustable but eventually you need new tires because, again, the drive was more durable than the driven wheels.
Some of the comments seem to be talking about tractors or more industrial types of mowers. I read this as a typical residential, walk-behind mower with self-propulsion. Most of these don't have real speed adjustments (other than the Toro).
I have a Sears reciprocating saw that failed because the drive shaft wore excessivley because the guide was not a traditional bushing, it is an iron casting, AND the manual provided absolutely no information about having to open the case and oil it (there is no oil port, so I didn't even think about it, so I'm partly to blame). And you can't get any replacement parts for any tool more than a couple of years old, I've had to make my own parts.
So, one had better take a good look at a machine before buying as the manufacturer may have been more concerned with their initial profit than the customer's long term satisfaction. And, there may be the calculation done, like Ford did, that dissatisfaction, or death, is a cost that they are willing to bear if failure is below a certain level.
The drive system needs some kind of a slip clutch. The teeth become disengaged well before the drive belt is sufficiently loose to disconnect the engine power. This is how the wheel teeth get chewed up.
That's pretty graphic, Armorris. I agree, the partial engagement is a very natural way of managing the speed of the mower. As a kid a mowed a ton of lawns, and you can't manage a lawn cut with a fully engaged drive (except on long straight stretches). With a fully engaged drive, you can dig ruts into the lawn when you hold back the mower on turns. So you have to slow it down with partial engagement.
As a matter of fact, I had been partially engaging the drive in order to slow down the forward motion of the mower, but that is normal use in my opinion. The engine has a governor and no throttle.
About 45 years ago, in high school, I worked part time for Sears as a TV repairman. Back then, they repaired only Sears products, and at reasonal cost.
My boss told me that Sears made their money on sales and that the service department was the "butt-hole" of the sales department. But if you let the butt-hole get stopped up, you'll see how much it's worth.
@Jaybird2005 I'm always extending the benefit of the doubt to manufacturers, having had to make some tough choices and comprimizes of my own when it comes to development. I didn't own the specific lawn mower in this story, but I would have hoped that if the design was for safety that advertising would have touted it as a positive, advertised this safety "feature", and then provided a spare polymer gear or two with purchase and stocked them in the department store at just over cost...
It does look like 'planned obsolescence', but it might also have been a safety device. If the mower gets up against something solid, like a tree, and the operator (possibly inexperienced) does not release the handle a metal-to-metal gear system may cause damage to the metal gears or cause the mower to bend it's deck or jump in an unexpected way. The existing system would simply strip the plastic gears. It may be better to have the mower stop moving than to risk some other, more severe, damage.
There may have been problems with stones or tree bark getting between the gears and causing severe damage (if they were metal) where the plastic might flex and allow the stones to be ejected with some minor damage to the plastic part rather than a gear grinding halt to the entire system.
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