I acquired a used tractor built by a major Japanese manufacturer. It was a small diesel tractor with a three-point hitch and a mowing deck behind it. It worked fairly well, at first.
One day while I was cutting the grass, the mowing deck belt broke. This belt was very long. It extended past the three blade pulleys, two idler pulleys, and then to the big drive wheel attached to the shaft that goes to the power take-off from the rear of the tractor. I wrestled the mowing deck off of the tractor -- no small chore -- extracted the belt, and then marched off to the local dealer.
A new belt was $60. I brought it home and then wrestled with that belt to fit it on to the mowing deck. When I finally got it back together, I greased all the Zerk fittings for good measure, blew the grass clippings off of the deck, and then hopped back on the tractor.
I got the rest of my lawn mowed. A week or so later, I was out mowing again. The tractor isn't fast, but it has plenty of torque. While mowing through some deeper parts of the grass, the tractor makes a little bit more noise and throws off a bit more smoke. The clippings were flying. A couple hours later, toward the end of the effort, the belt broke again. I cursed and took the belt back to the dealer.
I showed him the broken belt and asked him what he thought might have caused it. He looked at it and asked, "You live close by?" "Yeah," I replied. "Go home, and bring back the idler pulleys." Puzzled, I drove home and unbolted the two idler pulleys. They both seemed to spin smoothly. I shrugged, and took them to the store.
The parts guy spun the first one and said, "This one is OK." He spun the second one and then said, "You've got galled bearings on that one."
I said that it was spinning just fine. He pointed to it and said, "Listen to the bearings as they move. Notice the play in it. When this thing gets hot, it will seize." He said he could replace the bad bearing for $25. I also had to buy another new belt.
I wrestled the thing back together, with a bit more finesse this time. I reattached the mowing deck to the tractor. The mowing deck ran fine for several more seasons before I had to replace the other bearing. I now check the bearings once a year.
This entry was submitted by Jacob Brodsky and edited by Rob Spiegel.
Jacob Brodsky is a registered professional engineer in the state of Maryland.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
I had the same thing happen to my 1985 Toyota Camry (albeit much more expensive). Luckily I was just pulling out from in front of my hour when the car stalled and refused to restart. I had the AAA tow to the local garage where they said the timing belt slipped. They replaced the belt and everything was find for a few months. Then the same thing happened again, this time I was two blocks from home. This time I had it towed to my brother-in-law's garage. The replaced the belt but also replaced both idlers. Never had a problem with the timing belt again. And I never went back to the other garage, either...
This story is a good example of the fact that often the part that breaks is not the real problem. Sometimes, it's just an innocent bystander. This is important to remember in failure analysis. The impulse is always to focus on the part which broke, and try to find something wrong with it. But it's important to look at the entire mechanical system, rather than just one component in isolation.
Very nice diagnosis, and it's true, as Dave says, you have to check all the parts connected with the problem. Often, like in medicine, the failure point is a symptom, not the cause. This reminds me of the stuff the guys in Gold Rush on Discovery have to deal with all the time with their second-hand back loaders.
Good point, Dave. That's really the lesson here. Fixing what broke doesn't mean you have addressed the problem. There are a number of medical conditions that are similar, where attending to the symptoms actually hides the real problem.
Indeed, I had some doubts when the first belt broke because it looked worn. I had no idea of how long it had been in service. There were no tell-tale squeals or squeaks so I assumed it had to be the belt. But when a brand new belt with only a couple hours on it broke, I knew something wasn't right.
These bearings are actually one of those press-fit things that slide in to the idler puly wheel. It's "sealed" so that you can't lubricate it easily. Given how much dust and grass clippings go flying around, I'm not surprised that the mowing deck manufacturer used them. And in general I get five or six years of service out of them before I get nervous and just replace them out of prophylactic care.
It's cheaper than a new belt (and I get about three seasons out of a belt).
That makes sense replacing the bearings now that you know what is causing the belts to break. Seems like a flaw in the design, though, that the bearings would wear out so frequently.
Actually, I don't know for a fact that the bearings are worn that far. The galled bearings may well have been original equipment from the 1970s (I discovered this problem in 1997). That's not too bad, really.
My caution is because it's cheaper to replace this stuff than to wonder if it's going to bite me again. I know what these bearings feel like when they're new and when I begin to detect some play, I replace them out of caution. I don't think they're bad yet. However, since I'm not certain how far I can go with the existing bearings, I replace them on the theory that it's cheaper to do that than to buy a new belt.
Keep in mind that I'm not working with a micrometer here. I'm servicing this thing in my barn with just hand tools (socket wrenches, a bottle jack, screwdrivers, etc.).
Jacob, good if the ball bearings are gone, ofcource there should be some play while rotating. This play can cause more wear and tear, which may end up in replacing the associated components/parts. So it's always better to have a periodic check take for lubrication and wear & tear for the moving parts. Otherwise it may end up in a mess.
Very good article for maintenance! I too have several tractors on the farm for chores from lawn care to mowing the pastures. Something I have noticed going missing from most operator manuals these days is the trouble shooting section, or at least any trouble shooting beyond if it doesn't run check the gas tank....
Having purchased a set of factory service manuals with every car/truck I own, they at least (Ford anyway) have a very in depth trouble shooting portion for each major assembly. Proper diagnostics can go a very long way in fixing the problem right the first time once and for all. A great example is the rear axle... if you look at the number of descriptors of sounds describing conditions discovered while driving you'll know what I mean. Clunk, howl, whine, whoop, are just a few of the many adjectives that describe subtle differences in what's really going on in that rust covered cast iron box between the rear tires.
Going full circle here, I guess my point was that in lesser cost products (but we'd like to think even low dollar items would have the same attention as expensive ones) the basics of maintenance are often neglected or given much lower priority. Maybe it's so you spend more in repair parts which helps their bottom line, maybe because they don't care, or maybe because they just never took the time to really research it, but we, as engineers, really owe it to ourselves to sometimes take our jobs home with us to really ferret out what's going on to save ourselves added expenses later.
Some products aren't too bad if they go bad catastrophically, sometimes the repair parts are more then the base unit, or it's just cheaper to buy a whole new one, like a weed eater for example, but a $5000 zero turn mower for example, that's a different story. And if that goes bad, say you loose a blade spindle, you don't want that to come flying out!
There was a tractor company in the mid 50s to maybe 70s that made light duty farm tractors. They were called ECONOMY tractors. It was a very simple design. Used a 1 cylinder, 11 hp WISCONSIN air-cooled engine coupled to a non-synchromesh 3-speed manual transmission with the power being delivered to the rear end via an enclosed driveshaft. It was rumored that the original tractors were designed w/ "left over" FORD MODEL "A" transmissions & rear ends, but I never confirmed that.
The engine was horizintal shaft w/ it aligned w/ the major axis of the tractor. The front end of the tractor featured a multi-shived pulley attached to the crankshaft. Between the front wheels & the rear wheels there was sufficient room to hang a 48"{ wide mower deck w/ 3 blades. The mower deck was fastened to the frame w/ clevis pins & locks and was relatively easy to attach & remove. To power this mower, a very long V-belt was connected to the front pulley and changed from the vertical orientation to horizontal via idler pulleys. The other end of the belt attached to the center 3-groove pulley connected to the center blade shaft. Two much shorter belts connected the power to the outer blades.
The two idler pulleys were DEFINITELY designed by the "bean-counting" dept. In profile, they were two steel "hat sections" spotwelded together with a center boss area so that a roller bearing could be pressed in. In theory, that would be sufficient, but given the closeness to the ground & the high dust (dirt, sand) content of the effluent from the blades, these idlers did not last very long. And, considering that they were sealed bearings, there was no provision for any lubrication.
The three main blade shafts were designed by a mechanical engineer with experience. They were housed in precision stancheons w/ a roller bearing at the top & a roller bearing at the bottom. Additionally, there were very aggressive shaft seals at both ends, even though the pulley had a "skirt" feature so that the top of the stancheon was buried unside the pulley. And, each one of these stancheons had a hefty ZERK fitting attached so that they could be greased periodically.
The idlers had to be replaced almost every mowing season which extended from early April into November. While I investigated alternative solutions for these idlers, I could find no catalog items which could be readily adapted to fulfill the requirements, so we learned to live with it, and every year we'd go to the tractor outlet to buy at least two sets.... one for immediate use, one as spares.
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