Has this ever happened to you?

[Brent_Baumer 5]

See the pics below. What you are looking at is the whole center drive pulley assembly from a 48" mower deck. The center shaft is broken in two. The wife was mowing at the time and claims she did not hit anything. The blades show no damage and the bearings still turn freely. This shaft and the bearings are only about 1 1/2 years old. What happened? Manufacturing defect? Brent http://ourworld.cs.com/Bcbaumer/arbor_1.jpg

[Brent_Baumer 5]

Here's another pic. The break is very clean. BTW, this happened late in this past mowing season. I just got around to taking the top belt guards off to disassemble today. I would have expected something else to go before the shaft.....Brent http://ourworld.cs.com/Bcbaumer/arbor_2.jpg

[Brent_Baumer 5]

That's unusual! It's hard to tell from a photo, but it doesn't look like the blade hit anything. If the bearings turn free, I'd say the shaft was defective (not unheard of). Others may say that the shaft was not installed properly, or there is no trace of grease. I think you'd know if you had to "drive" the assemble together when you rebuilt it, and the bearings are lubed and sealed when made. Finish disassembly and look at the shaft with a magnifing glass. See if there are any signs of it being "wound up". Contact Simplicity. They may give you another shaft, and I would think they'd want to know if the shaft has a dangerous defect.

[BenB 3]

Brent I also cannot see any damage to the blade. My experience with "hitting something" including some very large rocks and,a steel pipe has been that the blade suffers damage, the belt slips, or the engine stalls, but never a broken shaft. By the way... I wonder how many other wifes are mowing their lawns? I thought that my wife was the only one, or at least she is in my area. (Brent is a lucky man) For Dutch: Please tell me what "wound up" refers to? That is a very interesting term. Thanks Ben

[Brent_Baumer 5]

My wife usually only volunteers when the young'uns are being unusually bad. This year she mowed quite a bit at the end of the summer because my right arm was in a full cast for 8 weeks and a wrist cast for another 4. I couldn't work the hydro very well with my left hand and steer at the same time so she took over mowing duties for awhile. I'm gonna take this into the dealer and see what he says. Maybe I'll save about $50-$70 or whatever these things cost if they determine it to be defective. I don't remember now how much it cost but I know the assembly wasn't cheap. Brent

[StinKy 0]

When I have rebiult my spindles I make sure that there is neutral pressure on the upper and lower bearings as assembled. I measure the distance between the upper and lower bearing seats with a dial caliper and adjust the spacer that rides on the shaft between the bearings so when the whole assembly is drawn up tight the bearings are'nt being jammed into their seats but simply riding on them. I make the center spacer about .005 shorter than the distance between the upper and lower seats. This has worked well for me and was born out of necessity since I've never seen any bearing set-up instructions from Simplicity. Dont know if it would have anything to do with your breakage tho. "wound up"-------spiral marks on the face of the breaks indicating rotary motion was cause of break????? My 2-cents Dick

[sask 0]

I had a shaft on a power steering pump of my van sheer in half exactly like yours. It was fine one minute, and the next the pulley was spinning along by itself with the half of the shaft it was still attached to. No other problem with the pump. I would think there is a good chance you had a defective shaft, and the dealer should give you a new assembly. Pat

[sask 0]

Ben, By "wound up" I mean an actual twist in the shaft. You can usually see the twist by looking closely at the "grain" of the metal. To me, a twist would indicate that something stopped the shaft from turning while under power (like hitting a rock or stump).

[powermax_paul 1]

Brent, It's probably a fatigue fracture. This shaft is under a bending load from the main belt (I assume). Because the shaft is rotating, that bending load alternates back and forth from one side of the shaft to the other. We all know that you can break a nail or bolt by bending it back and forth. That's known in engineering terms as fatigue. Fatigue can be caused by smaller forces than what is required to actually "yield" or bend a member. That alternating stress state in the shaft can cause any tiny nicks in the shaft to grow into a hairline crack and the crack grows until it fails. The solution is to design a larger shaft to reduce the stress state or use a material less prone to fatigue failure. My guess is that some bean counter at Simplicity coerced the engr. dept. into cheapening the shaft by using a cheaper material. And no-one looked into the loading and it's consequences. Paul

[Roy 0]

I agree with Paul. It looks like classic fatique bending failure. Any chance the shaft was bent and not running straight? Roy

[Brent_Baumer 5]

Any alignment problems are the dealers fault in this case. I replaced the shaft, bearings and bearing housing on another deck a couple years ago and had a heckuva time doing it since I don't have a press. He only charged me something like $5 or $10 in labor to rebuild this setup last year so I let him do it. I just bolted the assembly back on the deck and started mowing. It worked fine for many hours. Just can't believe it wasn't made weak to start with. Thanks to all for the input. Brent

[powermax_paul 1]

Roy, The shaft could have been straight as an arrow right up till it failed. That's the funny thing about fatigue. It's a phenomenon that airframe manufacturers pay extreme attention to. Alternating stress states that may not actually deform a part can cause a failure at any moment. Commercial aircraft are closely inspected by maintenance personell for cracks in their frames. Paul

[Roy 0]

Paul, I understand fatique and how it works. I was just curious to know if perhaps the shaft was bent which would have contributed to shortening the life considerably. Roy

[jackcobb 11]

I rebuilt my deck, sandblasting, prime, slip plate, bearings, the works, 3 winters ago. Mid-way through the season my center shaft broke just like yours. Top pully assy flying across the yard. Visiting with the dealer, he felt I didn't have the top nut torqued properly. (Loosening of the nut set up a vibration that broke the shaft). He took it back in his shop and put a new shaft in. Used an air gun to torque the nut on, with locktight too (said it is not uncommon for the top nut to loosen up). So it has ran two 1/2 years with no further problems. I did notice that the top pulley (shown in your pic)that runs overtop of the decks main belt guard shields, was rubbing up agaist the deck shields, bottom side of the top pulley. I felt this also might be causing a harmonic vibration that caused the shaft to break in that area. I trimmed both left and right shields with a gas tourch, making the diameter of the hole about 1/2" greater. The pulley now turns completly free of the deck shields. If my wife had been mowing I would have blamed her too. But I was on the seat when the pully took flight.

[powermax_paul 1]

Sorry Roy, Didn't mean to diminish your response. You're right though. If the shaft were even slightly bent, it would add to the harmonic vibration causing early failure. Paul

[ReedS 6]

Iknow that this a wag but what about an unbalanced blade?

[powermax_paul 1]

Yes, an unbalanced blade could set up a harmonic vibration. If the degree of unbalance is enough it could cause the shaft/pulley assembly to vibrate at it's "natural frequency". This is a frequency that is determined by the mass of the pulleys and the spring constant of the shaft (flexibility of the shaft in bending). Typically this frequency is high because the shaft is very stiff. The greater the mass of the pulleys, however, the lower the natural frequency and also the greater the magnitude of the vibration. If one were to reduce the mass of the pulleys, say change to aluminum rather than cast iron, the magmitude of the vibration woud be reduced significantly. For instance, if you fastened a marble to a piece of coat hanger wire it would sway a couple of inches when provoked. If you put a golf ball on that same piece of wire, it would sway quite a bit more, and a lot slower. Paul

[ibellis 0]

Looks like the assembly has been very hot,(belt slippage). High heat could cause the high carbon shaft to Crystallize and break. I have seen this type of failure on farm equipment.