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Plastic on metal gears
LloydP   5/22/2012 10:25:48 PM
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.

Charles Murray
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Re: Planned Obsolescence
Charles Murray   5/22/2012 8:21:27 PM
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.

Jeff Martin
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Differentials, front wheel drive, and ratcheting oneway clutches.
Jeff Martin   5/22/2012 4:25:36 PM

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.

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Re: A little more info.
armorris   5/22/2012 3:38:27 PM
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.

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Re: A little more info.
armorris   5/22/2012 3:27:30 PM

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.

Rob Spiegel
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Re: Planned Obsolescence
Rob Spiegel   5/22/2012 2:29:13 PM
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.

User Rank
Re: linkage problem
OldGeek   5/22/2012 12:57:22 PM
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!


Jeff Martin
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A little more info.
Jeff Martin   5/22/2012 12:33:50 PM

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!!!





Here is a GREAT manual on Toro transmissions. 




There they list the benefits of the engaging pinion drive.


The wheel pinion clutch has a three-fold purpose. One,

when the transmission is engaged, it causes the pinions

to move with the output shaft and drives the wheels.

Two, it provides differential action to allow easy turning

and excellent maneuverability. Three it disengages the

wheel pinions from the output shaft when the operator

is not squeezing the handle. This allows the wheels

to turn backwards without turning the gears inside the

transmission and provides low pull back forces similar to

those on a hand-push version (Fig. 136).


User Rank
Re: Planned Obsolescence
OLD_CURMUDGEON   5/22/2012 12:20:12 PM
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.

User Rank
Even GM got in on the act years ago....
MaruaderMan   5/22/2012 11:15:47 AM
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.

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