[handlewanker]

Hi all, typical example of cost cutting.

The manufacturer uses state of the art production technology purely to cut cost, not to produce a better product.

If the part fails, then provided the warranty is out, it's the customer who has the problem.

In all cases where part failure is the problem, redesign is the only option, otherwise you just go on repeating the same mistake.

I came up against this in cordless drill technology where the gears were all made by sintering, which as materials go is very much like cast iron in texture, but with probably steel like strength, but also prone to suddenly giving at the end of the tensile stress cycle like cast iron.

The easy way out is to fit a new part, provided you can get it from the dealer, but if it means sending to Hangdong Ona Gong in deepest China, then I'd get it investment cast in bronze, and with the order go into business supplying all those other people that are definately going to need a better part too.

Oh yeah, and make one for the other side too.
Ian.

[kb3crh]

Look at the discoloration from the heat treat on both sides and in the break. On one side the discoloration goes right to the hole. This is a defect that, combined with the sharp corner probably caused the failure.
I'm not sure how effective heat treating is on sintered parts.

[Geof]

Look at the discoloration from the heat treat on both sides and in the break......

The discoloration is just oil. I doubt very much that this was heat treated, sintered parts in my experience are quite hard.

[bajaranger]

When you get the replacement gear, which you said had a different P/N, will you post pictures to show us how they fixed the failure problem. Im curious now. New process altogether, better geometry?...

Thanks.

[Geof]

When you get the replacement gear, which you said had a different P/N, will you post pictures to show us how they fixed the failure problem. Im curious now. New process altogether, better geometry?...

Thanks.

Yes I plan on doing that, yesterday I got tied up with things and never even managed to phone the local Cub Cadet dealer. There is no panic on fixing this mower because we have two others.

[LeeWay]

What model Cub Cadet? This may hit closer to home than I had initially thought.

I have the Super LT 1550.

[Geof]

What model Cub Cadet? This may hit closer to home than I had initially thought.

I have the Super LT 1550.

Timesaver i 1046.

If yours has the zero radius steering where the front wheels turn a full 90 degrees and you take off the plastic covers over the steering linkage you may see the identical gear.

[One of Many]

Powdered metal parts can be identified typically by the edge periphery having a relief chamfer that reduces edge crumbling via inconsistent compression integrity into sharp corners of the die.

Sintered parts do go through more than one operation. There is the initial powder compression, another sizing die press and a heat curing tunnel to complete the powder bonding and ultimiate hardness. Most of which is going to be material/application dependant. Some parts can go through secondary machining processes then sizing and curing.

For the most part, the powder will be somewhat malible and often is coated with a bonding or flux agent that assists in the pre and post binding/curing process. Just the powdering process can get pretty involved.

DC

[Geof]

Powdered metal parts can be identified typically by the edge periphery having a relief chamfer that reduces edge crumbling via inconsistent compression integrity into sharp corners of the die...DC

Yes it is there, bit difficult to see in the pictures but quite obvious on the part.

And the rest of your post gone done turned me into a liar, obviously it wasn't made in one operation, hence the question mark.

Now I have a question for you:

I have found that the part number has been replaced by one with an A. My conclusion is that my one is not the first to fail; I would go so far as to speculate failure has been endemic.

How would you rework the design with the existing constraints of same hole size and shape and same overall gear shape?

Just a bigger boss, make the whole thing thicker, incorporate a steel insert for the hole (is this possible)?

[LeeWay]

Thanks, Geof.
Mines not zero radius turn, but I'll check it out anyway.

If I was going to rework this part, I would start with the shaft. It would have been a tapered shaft and a hardened shoulder bolt would have threaded into the gear to lock it in place on the shaft.
Much simpler to make all the way around I think. It could have been beefed up around the center hole too. I don't think those gears need to sink too far into it's mate.

On my truck, the steering shaft is splined on the box side and the shaft has a simple clamp to keep it in place, much like tie rod ends have.

[Geof]

The shaft is tapered with two flats at 180 degrees. The 'shoulder bolt' is a wimpy M6x1 with a thin flat washer.

When I get the repalcement part I will take pics of the assembly.

[handlewanker]

Geof, as far as the design goes, it's easier to mill two flats on the shaft than sink a keyway or mill splines.

So from the manufacturer's point of view, with cost saving foremost, location both radially and axially is accomplished by the taper and the two flats in the bore, mating with the taper shaft and the squared out cavity, all in one operation.

The very fact that the tapered shaft is exerting a force radially is creating a failure factor in the very unlinear grain disposition of the sintered material, much like cast iron that is strong in compression but fails in expansion.

It only wants an assembler to exert undue force when tightening the nut on the tapered end of the shaft, due to the shallowness of the taper angle, to make it fail, which by the stain at the break appears to have been partially cracked before.

I hate to think what would happen if this method and material were used on the tie rod ends on a cars steering mechanism.
Ian.

[One of Many]

No intent on my part of making you into a liar, just clarification.

Any minor change could result in a part number change.

I'd suspect the most simple change to the die would be removing material to enlarge or fillet around the boss. You will soon find out.....as long as it doesn't also involve further changes in the associated assembly.

From the darkening, it is apparent that the crack may have propagated from the large end of the internal taper at the boss/counter bore. The counter bore looks to be a shoulder stop for the taper fit? That locating diameter would need to be consistent with a tad bit of stretch interference fit. Any bit of slop would compound the problem.

A forged blank could punch the profile and internal shape I suppose.

As a similar part repair and on some woodruff key shaft repairs. I have heard that a copper key blank disk is inserted and a weld built up around the copper blank without actually welding into the copper. I have yet to try that, but heard is was common to avoid removing large shafts for machining etc. I ask more as requesting confirmation than any suggestion in your application.

DC

[Geof]

The boss only makes it thicker where it goes on the shaft and the counterbore doesn't seem to do anything; there is no shoulder on the shaft it just pulls tight on the taper.

The very dark spot is almost certainly where the crack started but even there you cannot see the typical fretting and polishing you get with movement and growth of a fatigue crack which propagates very slowly before final catastrophic failure; this part didn't spend much if any time fatiguing it just went bang.

Handlewanker is right about it being a simple low cost manufacturing method; simple I guess is a bit of a misnomer because the machines for poweder metallurgy are far from simple. However, once everything is setup parts like this pop out like crazy. It is very likely that had the shaft been made even 2mm larger in diameter this failure would never have occurred. I also have to say we do not baby our mowers and our lawn is a lonnnngggg way from being golf course smooth.

[One of Many]

While I was in Field Service for an OEM that made the presses, I picked up a few tidbits of information on the process over the years. I think there were something like 700+ PM parts in the average automobile.

Low cost on the input side equals low prices to the end user and an attempt to under cut the competitors. At these production numbers and cost/price competition, the parts are engineered to the bare minimum to fit the need, IF everything in the real world goes exactly as tested in the lab, vendor supply and factory floor. That is not to say yours is cheaply made, but economically efficient for afforability. That is not always a bad, all things considered.

Go buy an MTD riding mower and see how long it lasts. Proof there is a limit to minimal engineering…….to at least get it past the warranty period. The auto industry of the 80's was about as bad!

DC

[Geof]

.....Go buy an MTD riding mower and see how long it lasts. Proof there is a limit to minimal engineering…….to at least get it past the warranty period. ....DC

Funny you should say that; it is an MTD, now. I ordered the part and found MTD had bought Cub Cadet.

I like the way you phrased it; economically efficient for afforability, I am going to steal that one. :-)

[One of Many]

Funny you should say that; it is an MTD, now. I ordered the part and found MTD had bought Cub Cadet.

I like the way you phrased it; economically efficient for afforability, I am going to steal that one. :-)

Ha! That is funny. No slam against your cadet but, if they had to sell out the MTD influence is right around the cutting corner.

Most of the MTD problems I hear often are about the rear drive member and axle seals failing after the axles wear through the bushings. There are no bearings. Some within a 3 year life span mowing about a 1/4 acre 1/week 3-5 months/yr?

That was supposed to be affordability.

My JD 111h is pushing 28 years old and still going strong. I bought it used 15 years ago for $500. Yes I'm a cheap skate, although prefer the posh esoteric term of Frugalist.

DC

[Geof]

I finally got the replacment gear, actually I bought a left hand and a right hand just in case the right hand one breaks. They are the same dimensions as the broken one but they have a mottled appearance and look like they have been heated; the broken one has an even coloration.

So I did a bit of Googling and found some information about the physical properties of sintered items being improved by a heat treatment step following sintering so maybe this is the only change.

[One of Many]

I finally got the replacment gear, actually I bought a left hand and a right hand just in case the right hand one breaks. They are the same dimensions as the broken one but they have a mottled appearance and look like they have been heated; the broken one has an even coloration.

So I did a bit of Googling and found some information about the physical properties of sintered items being improved by a heat treatment step following sintering so maybe this is the only change.

That's interesting there were no dimensional changes.

I'd imagine the deeper heat treating intention was to improve ductility short of brittle. PM parts are considered green similar to ceramics in their raw state. My understanding was that basic post heat treat helped in molecular bonding. Seems proper the next logical phase would be deeper physical attributes of the alloy selected. Which the alloy itself could point to the other change to gain that added property.

As long as the shaft doesn't become the sacrificial component as a result?

DC