Sunday, April 13, 2014

MTD 12B-443F500 Seized Front Wheels

I have an old MTD front wheel drive lawnmower that's been sitting outside idle for a few years. Its front wheels don't freewheel in either direction -- they're both quite seized. 'Time to see what's up with that.

I'll take off that 9/16" hex nut[1] in the wheel and see where that gets me.

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I had to hammer the wheel off using a piece of 1" x 2" as a punch. Revealed are a splined sleeve and the drive pinion gear. Everything appears to be seized. 'Time for some WD-40 and some coaxing.

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The gear and its pin came off fairly easily. The splined wheel sleeve won't budge.

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An 18" pipe wrench got the splined sleeve off -- in two pieces. That's ok. The sleeve may still be salvageable. All it is is a liner for the wheel's bore, so I may still be able to reuse it.

I'll get after the right side wheel now. With both wheels off, I can check that the transmission's output shaft is free-turning. If that's ok, and it probably will be, I'll be well on my way to having this mower working as it ought to again.

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The right side wheel had to be pulled with a three-jaw puller, but it came off with its splined sleeve still inside it. The right side gear also had to be pulled -- it wouldn't come off easily as the left side gear did.[2] Here's a view of the whole kit and caboodle that got the job done.

Now that the seized wheels are off, I can tell that the transmission's output drive shaft does turn, though not as freely as I'd like. The right thing to do here would be to dismantle the driveshaft's bearings and clean them properly, so I guess I'll do that.

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The wheel height adjustment levers' retention is also the driveshaft bearings' retention.  A single, big snap-ring has to come off.

To get at the driveshaft's bearings, you have to remove a corner wheel-support plate at one side or the other. Two 3/8"-16 bolts and nuts hold each plate in place. Here's the left side one off the mower along with the bearing.

At this point, I can maybe pull the entire transmission and driveshaft from the right side driveshaft bearing, without dismounting the bearing itself. If I get real ambitious here, I may open up the transmission, and clean it and repack it with fresh grease.

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As it turned out, the right side bearing didn't want to let go of the driveshaft easily. I took off the wheel height adjustment lever, and now I have the entire bearing/driveshaft/transmission affair off the mower.

Now I have a lot of parts cleaning to do, plus I have to deal with the broken sleeve for the left side wheel. While I'm at it, I'll take the bottom cover off the transmission, and see how its grease pack looks.

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Here's a view of the underside of the transmission. It's held on with eight T20 Torx-recess screws.

The screws came out easily, but the bottom cover doesn't want to budge; I think it's stuck in place with gasket compound.

The transmission's input and output shafts feel fine as they turn, so I really have no compelling reason to open up the gearbox. I'll just let discretion be the better part of valour here, and leave the thing be. I will wire brush the ends of the driveshaft, though, where they ride in their bearings.

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Reassembling A Wheel-Support Plate

This is not as easy as is getting the thing apart. The big wavy washer in between the driveshaft bearing and the plate, and the springy height adjustment lever want to prevent you from seating the snap-ring. Here's a way to go about getting the assembly back together. (It can be done either on or off the mower. I've already done the right side one on the mower. The following photos will be of the left side one off the mower.)

1) Get the assembly together with the snap-ring part-way onto the outboard hub of the bearing, like so.

2) With a big pair of Channellocks, squeeze the whole bearing/snap-ring affair together. You'll get a partial seating of the snap-ring.

3) With a pin punch, hammer the face of the snap-ring all around to fully seat it.

4) Carefully inspect the snap-ring's seating to make certain that it's correct all around -- the snap-ring's appearance can be deceptive to a casual look. Do whatever it takes with the Channellocks or the pin punch to get the snap-ring fully seated.

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Dealing With The Broken Splined Sleeve

The sleeve is broken in such a way that it can be put together, and it will not collapse under an inward radial load. That is to say that it can be pressed back into place and resume doing its job, despite the awful damage that I did to it. Follow along with me here.

Here's a view of the splined sleeve in pieces.

And here it is 'assembled', and just started into its bore by hand.

Here it is in my press, being gently pressed back into its bore.

And here it is fully pressed back into place, and ready to go.

That worked out beautifully. For all practical purposes, the wheel is completely back to normal.

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[1] A Bit Of Hex Nut Lore:

The 3/8"-16 flanged hex nuts that MTD used on this machine are self-locking nuts of the 'prevailing torque' type; i.e. the nuts have been purposely distorted in manufacture so they lock onto a thread.

There are many variations of the prevailing torque principle. The nuts used here have had two opposite sides squished slightly to produce their prevailing torque effect. Here's a close-up view of one of the nuts.

Note the appearance of the upper and lower hex flats in the image. There you see the squishing that was done to give the nut its prevailing torque characteristic.

A prevailing torque nut can only be finger-tightened until the nut's distortion comes into play; from there on it has to be wrenched.

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[2] Note that the two gears are different. Should you get them mixed up, it's not hard to figure out which one is which. Just study the two and consider how the driveshaft's pins must operate on them; it'll become obvious which is which.

Or, you can just look at the picture below.

The left side gear is at the left; the right side gear is at the right.

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