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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I have to agree with you about the Sears / Kmart match-up, Old Curmudgeon. What you described about the parking lots is very much true where I live. I did some very late Christmas shopping mid-day on Christmas Eve at Kmart and the store wasn't even crowded. The lines at the check-out were one or two people deep.
My (Sears) mower does not have a differential drive like in a car. When one wheel slips, the other wheel still pulls. IMO, a differential drive would not be a good idea on a mower where the terrain is uneven.
Thanks to Jeff Martin for the link to the great mower transmission manual. Nice educational material, even though my mower is not much like the ones described in it.
According to the manual I mentioned earlier I have a Toro 20031 with a 22" steel deck, a single speed bevel gear system (new in 2002) a second generation Personal Pace drive system, and spring racket wheel pinions (Fig 112 page 3-26)
Front wheel drive mowers should have a non-differential direct drive.You do not need it because you lift the front wheels when you turn the mower.The problem with these mowers is that the wheels slip in the grass when mowing up steep hills.I remember as a kid having to lift the rear of the mower off the ground to get enough weight on the front wheel to give it traction to power up an embankment.Remember Fictional Force = Us x Cos(angle) x weight.Rear wheel drive naturally applies that pressure to move up hills and you don't have to do that awkward rear lift.
My rear wheel drive mower it needs a differential for me to turn.But it also needs a direct drive so I have 2 wheel traction and don't become a spinning one wheel wonder.So does it have a possi traction system?Well yes it does.....in a way.It uses a cleaver racketing pinion gear.The pinion key is spring loaded in the shaft and the pinion keyway is flat on one side and ramped on the other.So it will racket forward but not backward.Cool design.So most of the problems with my wheels slipping is not from engaging gears (because mine are always engaged) .The problem is caused by my deck and adjustable wheel height mechanism is that it is compliant.It is sheet metal and not perfectly rigid so it will bend slightly and the pinion gear will slip.The cool thing is my variable speed is controlled by belt slip.It only requires one hand ...and even one finger to push my mower forward.That pushing on the handle is transferred to the bevel gear housing and it will pivot the pulley loosing and tightening the belt.The speed is variable up to 4.8 mph. I don't like the slipping belt slipping in the design because I thought it would wear out fast, but I have not had to replace it in since I bought the mower in 2004.
I think what it comes down to, Old Curmudgeon, is a corporate culture that values short-term savings. In the long run, no serious executive can argue that poor quality makes the company stronger. But cutting corners makes them look like heroes -- for a short time, anyway. Sparsely-populated parking lots certainly aren't their goal, but the geniuses who place value on the short term fix usually aren't around to see the results of their decisions.
This is not uncommon. Most garage door openers use a steel worm gear on the motor shaft, mated to a plastic gear driving the lift mechanism. Guess what is the most common failure of the opener?
For years, GM used a power headlight lift motor on Firebirds, Corvettes, and the Fiero. The motor was a modified power seat motor with a steel worm gear. It mated to a plastic gear on the lift mechanism. The motor drive electronics sensed motor current, and shut off when the current increased after the lift mechanism drove hard against a stop. When you see a "one-eyed" Firebird or Fiero, you know the plastic gear has stripped. Most Corvette owners fixed this problem with a new lift motor. It had to be new, as the gears in junkyard parts also were stripped.
Metal on polymer is often a very good choice under certain load conditions. With the right polymer, the interface requires no lubrication. Polymer geared wheels are very easy to manufacture.
"Planned obsolescence" is the unsavory name for simple design compromise.
This lawn mower could have been designed with case-hardened steel gears and a built in auto-lubrication system for them, but then no one would purchase this extremely expensive lawn mower.
As stated in other comments, the weak "link" was the wheels, but the actual flaw in the design lay elsewhere.
I have used both self propelled and non self propelled mowers and I find that it takes far less effort to mow a lawn without the alleged work saving feature. At least that is when the lawns are a bit smaller and have anything other than wide open spaces. Constantly turning and pulling the mower back is a lot more work if I must be dragging the heavy drive mechanism as well as the mower weight.
And of course the mowers are mostly designed to only last a year or two, so as to sell more mowers. So what comes across as a design goof may actually be intended to make the machine last only a bit beyond the warranty period. Simpler is often better.
Several years ago several neighbors and I took turns cutting the grass for a gentleman who had a stroke.He was in the process of recovery but certainly in no condition to continue with his ward work.The lawn was quite small so he had a push mower and never bothered to purchase a motorized self-propelled device.One of those rotary types that are no longer made.Although very skeptical at first, I started the process and was amazed to find out how much quicker I completed the job and what a "better cut" I got from the old, but very sharp, blades.For this job, the more simple process was the best process.
Actually, I don't think is was the plastic on metal that was the problem. I think it was the partial merging of the gears that created the mess. Now it makes me worry about how my lawn mower works. I happen to like the auto-pulling feature. For the first time in years I have a lawn to mow, and I appreciate all the help I can get!
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