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Background: This is my Murray 38702X8A (38", dual blades, 12 hp B&S, bought new in early 1993). My regular repair/maintenance guy died a few years ago, so I've been handling everything myself since then. The mower has been generally rock solid since new, requiring only regular maintenance and a replaced front axle assembly 15 years ago -- caught the left front wheel on a low wall, bent up the axle pretty good, replaced the crappy original with an HD unit.

Doing PM a few weeks ago, I discovered worn spindle bearings. Bought all new parts including complete bearing assemblies, spindles, and bolts. Also bought fresh blades. I've swapped old for new, reattached the deck, and now she vibrates like a paint shaker when I engage the blades. This is not the normal vibration that occurs when blades are first engaged and settles out after a few seconds. This is pretty amazing -- I can see the deck vibrating through about 1/4 inch of travel. Vibration persists through all throttle settings. Mower is very smooth with engine running with blades disengaged. Engine crank and pulley spin true.

Here's what I've done so far to diagnose:

- Checked to be sure all the deck attachment points are properly pinned in place.

- Checked for tightness of the bearing assembly bolts.

- Removed the blades, checked balance (both good), and reinstalled, being sure they were centered properly, holes are proper size, blades not upside-down, and that all hardware was installed in proper orientation and tight.

- Spun the blades by hand to be sure they aren't bent or otherwise misaligned.

- Visually checked for runout of the spindles by spinning by hand. Also had the wife fire up the engine and engage the blades; watched for runout as they coasted down. Everything looks good.

- Checked spindle pulleys by the same methods. There is just enough of a wobble to be visible (less than 1/64"), but not enough to account for the violent shaking.

What am I missing? I'd hate to have to euthanize Ol' Red after 23 years of exemplary service (or suffer the shame of flat-bedding it to the local repair shop to expose my mistake).

Help, please. Thanks.

Burgess

Remove the blade belt and check it for cracks, missing lumps or stiff sections.
While it is off, spin each blade by hand and watch / listen for irregularities.
While the belt of off check all of the pulleys.
The V pulleys for a rock or bit of stick etc stuck in the bottom of the V.
Check the flat idler for flat spots.
Check the flat idler bearing for smoothness & quietness.

Finally have a really good look at the pivot points for the tensioning pulley.
If they are flogged out oval then can cause horrid vibrations.

bert is correct, deck idlers & belt are probable problem, use Murray OEM parts

Of all the potential sources mentioned, the belt is the only thing I haven't already checked, replaced, cleaned, or adjusted. It's probably due.

Thanks for the replies.

A while back I felt an "indexing" sensation after sharpening , installing and rotating the blades on my 2004 dyt 4000 48". The original belt had over 20 deep cracks, but there was no vibration when running (just info). I wonder if something appears tight at rest, but is actually loose under load, like the new spindle bearing assemblies or the spindle to deck attachment fasteners?

Unlikely the oem spindles are the cause, however they probably are assembled by hand with a press and mistakes can occur. The ball assemblies are fragile. Another thought. The attachment of the spindles with original hardware, and/or corrosion. Only an idea to again check the spindles for movement by using the blades to put load on the spindles and observe any excess looseness or gritty feel, and location of the spindle assemblies . Hope you find the problem soon.

Edit :Outside chance...could the blade pilot holes be larger than the orig and somehow you are locking down the balanced blades slightly off center?

motoman said:

I wonder if something appears tight at rest, but is actually loose under load, like the new spindle bearing assemblies or the spindle to deck attachment fasteners?

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I don't think so. First thing I did was to go back and check all the fasteners. Tight as a tick. The assemblies also seem to be tight and smooth. Attached the blades and spun by hand before putting the belt back on and if felt buttery.

Edit :Outside chance...could the blade pilot holes be larger than the orig and somehow you are locking down the balanced blades slightly off center?

Click to expand...

Pilot hole looks to be spot on. Blades seemed tight and centered when I cinched them down. Each end of each blade swings through the same arc relative to a reference point marked on the deck.

Amazingly, not a single shop in town had the correct belt in stock today, but I'll have a Murray part in hand tomorrow afternoon. Hope that does it.

Thanks for the reply.

So, the new Murray belt did not solve the vibration problem. Pretty sure now it's in the blades. I'll disassemble again this weekend, measure and rebalance everything, and see if i can find it.

Stay tuned.

longirons said:

So, the new Murray belt did not solve the vibration problem. Pretty sure now it's in the blades. I'll disassemble again this weekend, measure and rebalance everything, and see if i can find it.

Stay tuned.

Click to expand...

Have you tried removing the blades THEN running it ?? ..... If it still vibrates, I would suspect one of those new bearings you replaced..... even new bearings are sometimes defective !! Seems you tried all the other obvious things . Please keep us informed , good luck , Boobala

Boobala had an a good idea there, think about removing one blade at a time to see if there is any change. You might have one blade that for some reason is causing the vibrations.
Tom

Static (bubble type) balancing only works on some automotive wheel assembles because of the "dynamic" forces a tire sees. But seems blades are usually well balanced with a static balance unless something else is wrong. What happens when you put on the old blades? Do the new blades "nestle" perfectly when stacked together, and look identical ?

Although you eyeballed the runout of the spindles, maybe its time to remove the deck and use a dial indicator on the pulleys and spindle shafts. Without a magnetic stand and dial indicator use a wire pointer and feeler gauge. The runout required for good balance may be the problem. I believe you said 1/64" or .015" (check me). Seems like only a couple thousandths are expected at a bearing shaft. Others please chime in.

Boobala said:

Have you tried removing the blades THEN running it ??

Click to expand...

Have not. Good idea. Will do so the next chance I have to attack it again.

motoman said:

What happens when you put on the old blades? Do the new blades "nestle" perfectly when stacked together, and look identical ?

Click to expand...

New blades nest perfectly. Old blades are gone, I'm afraid, chucked in the trash (maybe prematurely).

The runout required for good balance may be the problem. I believe you said 1/64" or .015" (check me). Seems like only a couple thousandths are expected at a bearing shaft.

Click to expand...

Well, I said "less than 1/64," which it is. Maybe just the few thou expected. However, that was measured with a pair of uncalibrated eyes. I'll do a proper runout measurement the next chance I get.

Just more blabber on bearings...I once hit a brick with a front wheel drive car. Then a balance problem started. The wheels were rebalanced-still a problem. You could not feel any looseness jacked with weight off wheel , no runout. Finally replaced the hub and bearing-solved. Today I read a similar story in one of the auto mags. A typical new bearing assembly problem is the operator pushes the bearing in on the inner race and damages the balls which start working on the race. Or the ball separators become buggered and drop into the mix. Often cannot feel gritty sensation when bad. I bought a cheap HF stethoscope when I had a another front end problem. The thing uses batteries to amplify noise. The old way...put one end of a screw driver on the suspect and the other end on your ear while you rotate the shaft.

motoman said:

Today I read a similar story in one of the auto mags. A typical new bearing assembly problem is the operator pushes the bearing in on the inner race and damages the balls which start working on the race.

Click to expand...

The same thing happens on mower bearings to professional mechanics and DIY'ers alike when pushing on the outer races with a spacer between the two bearings unless one bearing is a hand push fit.

Well, it's been along, long time, but I thought I'd report back that the problem is solved, but I have no idea why.

I used the Murray several times this year, racking up about 10 hours over the warm months, with no change. Still shaking. A few weeks back, I was doing my last cut of the year. Everything was already brown, and I was mostly just smoothing down the top of the grass. There were a few drifts of leaves in the back yard that I didn't feel like raking, so I drove over them with the mower. On the second pile (pretty deep, maybe 8-10"), the mower bogged, shook even harder than normal, there was a bump/bang from the deck, and -- poof -- the shake disappeared.

I got off and under to see if I could see anything obviously changed, but, whatever it was, it was invisible to me. Cranked back up and ran it for another half hour or so. Smooth as silk.

:confused3:

longirons said:

Well, it's been along, long time, but I thought I'd report back that the problem is solved, but I have no idea why.

I used the Murray several times this year, racking up about 10 hours over the warm months, with no change. Still shaking. A few weeks back, I was doing my last cut of the year. Everything was already brown, and I was mostly just smoothing down the top of the grass. There were a few drifts of leaves in the back yard that I didn't feel like raking, so I drove over them with the mower. On the second pile (pretty deep, maybe 8-10"), the mower bogged, shook even harder than normal, there was a bump/bang from the deck, and -- poof -- the shake disappeared.

I got off and under to see if I could see anything obviously changed, but, whatever it was, it was invisible to me. Cranked back up and ran it for another half hour or so. Smooth as silk.

:confused3:

Click to expand...

Better bag up some leaves and keep em handy, in case there's "a next-time" ...:laughing:..:laughing:

Something hidden from your view dislodged removing the imbalance (chunk of limb)?