One of the two front wheels that propel my Sears Craftsman self-powered rotary lawnmower stopped working, while the other wheel started working intermittently. When I removed the plastic cover over the drive mechanism, I saw what appears to be a worm-drive transmission that was coupling power from the engine via a belt to a shaft driving the two front wheels.
While squeezing the control bar on the handle, I could see the drive shaft turning, but one of the wheels was not turning. Once I removed the front wheels, I noticed that at least half of the teeth had been almost ground away inside of one wheel, while a smaller section of the teeth had been ground away in the other wheel.
When the drive control bar on the handle is squeezed, the worm-drive transmission is pivoted forward, presumably on a bracket attached to the deck. When that happens, the pulley tightens up against the belt, drawing power from the engine. As the transmission pivots, the drive shaft is moved forward, engaging the gears on the ends of the drive shaft into the teeth inside the front wheels. The front wheels are made of plastic with gear teeth molded inside the rim.
The gears on the ends of the drive shaft are made of metal and are engaged into these plastic teeth when the mower’s self-propulsion system is engaged. If the operator doesn’t fully squeeze the control bar up against the handle bar, the metal drive gears are not fully engaged and literally chew away the plastic teeth inside the front wheels. Replacing the wheels is only a temporary fix, as the new wheels will suffer a similar fate.
This entry was submitted by Andy Morris and edited by Rob Spiegel.
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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.
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.
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.
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).
While General Motors wasn't the first to apply plastic to a gearing system, they sure made a bundle out of it. From about 1968 to 1976, Pontiac introduced their plastic coated cam timing chain gear. Ostensibly, it was to "quiet down" the noise the two cam timing gears and chain made during normal engine operation. Of course, the difference in noise could be measured in practically negative decibels.
Normal timing gear/chain lifetime used to typically exceed 100k miles - even in those days. With the plastic coated gear, one never exceed 65k miles - if you were that lucky. and it always happened on the road and required a tow. At least at 100k miles, you knew from experience what to expect, and premature timing gear failure was not one of them.
The replacement, whether from GM or aftermarket, was always all steel. Gee!
Planned obsolescence? Even back then the accountants and management were already working their magic.
By the time the NTSC got involved, it was too late. GM had already changed back to the all-steel gears, and the gov't chose not to pursue the problem any more.
1) Seems to me that there should have been a two-stage linkage for the motion drive mechanism. The one stage would force the proper engagement of the driving gears into the wheel gears, and the "feathering" for motion control should have been accomplished w/ the "pressure that the drive belt exerted on the driving shaft sheave. Ultimately, the V-belt would have been the sacrificial element in this drive, and since V-belts are considered consumer items & consumables, they're available at a wide range of outlets including automotive stores, etc.
2) Regarding the SEARS / K-MART alliance. I believe that was one of the worst corporate marriages to come down the pike in many a decade. In this Tampa Bay area, and more specifically in our immediate hometown area, we have FOUR WAL*MART outlets, within bicycling distance, and ONE SAM'S CLUB. We also have TWO K-MART stores & ONE SEARS store. Driving past these outlets is very revealing, and is NOT tied to time of day OR day of week. The K-MART parking lots have a handful of vehicles parked, while the WAL*MART stores show almost full parking lots (and I'm NOT just relating at Christmas time!). The Sears store is a general merchandise store, including an automotive service center. Interestingly, it is located at the end of a very large mall, also populated w/ BEST BUY, DILLARDS, J.C. PENNEY, MACY*S (formerly Burdines), etc. The parking lot area in front of the Sears store is very modestly populated. I believe Sears would have been far better off to remain independent, while at the same time trimming their product lines to include only their "core" businesses, which are tools, appliances, & automotive accessories & light repair.
I have a Toro with the variable walking speed drive.I too have not looked at how it works, but I understand that these wheels will have to be replaced from time to time when they start slipping.I was made aware of that fact after I bought the mower by a friend that used to work for the city and had to replace them all the time.My drive skips on steep parts of my yard and makes a nasty sounding cogging noise.I have not tried fixing it yet, but I am hopping they use the same wheels in the front as they do in the back.So when my gears or cogs become worn I can just "rotate" the tires.
This place in MN has some pretty good prices...I just checked my price $19 per wheel!!!
Dead on the money ChasChas! The gears should engage fully before the belt tightens, thus always being fully meshed before any power is applied.
The gear disengagement is necessary to allow the mower to be pushed manually.
With regard to other posts about controlling speed: I've always controlled the speed by applying downward pressure on the handle to allow the front wheels to slip. That's why they put power to the FRONT wheels!
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