A little disappointed with Kubota

[crazyal]

You said in your first post that the center hole in the dish was too large for the axle hub. Is this true or not?

Not the center hole, the holes for the mounting bolts/ studs.

 

[crazyal]

No. I didnt miss it. I addressed it earlier. The fasteners were likely damaged by the wheel movement. Check recommended torque on Class 10.9 M16 fine thread. I find figures for coarse thread cluster around 250/260. Fine thread will typically accommodate about 10% more. So ~275/285

Torque to yield figures are up to 40% higher - - - ~ 380.

All the bolts say is "7" on them. If I had to guess they are not grade 10.9 (since there's no markings to indicate they are and the silver zinc color).

 

[crazyal]

as has been stated a couple of times, the lug nuts are supposed to be conical, like car and truck nuts, somebody rigged this for failure, maybe they were just too lazy to get the proper nuts, and used flat nuts, thinking it'll work.. who knows what the original owner was thinking before the OP bought this..

On post #49 I posted the picture of the bolt Kubota calls out to be used. It has a captured lock washer on the bolt. The center "disk" just has regular holes in it. Neither have a conical shape for a typical car style lug nut.

 

[rScotty]

All one has to do is look at Kubota's Illustrated parts to see the proper fasteners consist of lock washer and conventional nut for the studs and bolt with lock washer for either R4 or R1 rear wheels.

It would be really nice if people would verify info before they post information and state it as fact.

Good luck to the OP in resolving the issue.

You are right in that illustration is bizarre. Maybe it is because that illustration is an artist's concept. So the drawing itself might be correct or maybe not.

But there is absolutely no doubt that split washers are never used where bolt clamping pressure is critical. Since a split washer can expand radially when compressed or anytime that the load changes, there is no way that an engineer can count on using bolt torque to design an appropriate clamping force. That is why hardened flat washers are used.

A lot of our world is held together with clamping bolts. Think of big equipment and even bridges. Standard engineering design procedures for calculating what is necessary have been in place for over 100 years. It's a basic part of every engineering cirriculum. There's a reason why most applications call for a flat nut and hardened washer against a flat surface. Clamping pressure works because the torque on the nut stretches the bolt enough to turn it into a spring...but without taking the material up to the yield point. Everyone who designs anything mechanical knows how to do this quite accurately. Generations of designers have done it the same way.

If you put a lock washer into the stack, you cannot rely on bolt torque to give you clamping pressure or bolt stress because the split washer can expand radially. That's why lock washers are not used where clamping pressure has to be accurate. And if bolt torque isn't a reliable and repeatable measure of clamping then where are you??

There is something amiss with the way that Al's Kubota's wheels are clamped to the axle hub. I don't know what it is and have no idea how it got that way. In my opinion, I think he needs to contact a Kubota technical rep for his area and let them tell him what is wrong.

But any and every engineer is going to tell you that the way it is shown in the original photo is not the normal way to use bolt clamping forces.

My suspicion is that it is either a mistake or not stock.
rScotty

 

[ovrszd]

To reaffirm, the wheels are hub centric. The bolt holes are not tapered. The nuts are not tapered.

With that said an agreed with, your wheels were loose. It wasn't vertical load that caused the holes to become elongated. This was caused by rotational forces, allowed because they were loose.

 

[oldballs]

This has been an informative thread to follow, especially about bolts, nuts, washers and engineering...even though I've never had such a problem.

Thanks to all,
Mike

 

[SPYDERLK]

I think it is too late to fix this without replacement. - - Arent those cracks around the bolt hole? I have 4 Kubotas long term with that type of mounting and have no indication of any motion much less failure. Those are 16mm Class10.9 fine threads capable of sustaining well over 250ft-lb. - Your bolts/studs may have experienced additional stress due to the cracking. - The lock washers are not a plus although I have not experienced trouble. They present two problems; 1) scarring the disk, and 2) applying the clamp force on too small a footprint, thereby adding stress to the scarred hole periphery. It would be better to have a larger OD high strength flat washer against the disk - with or without the lockwasher.

Im not sure where your trouble originated, but when you renew the setup you will start with a built in advantage if you get some 5/8" Grade 8 flatwashers from TSC and incorporate them as mentioned.​

Any time I have ever seen wheels that looked like that they had been run loose. I do not expect this to be different, although it is possible the wheel or hardware are incorrect for the application. If it were not any of the aforementioned scenarios, this forum would be flooded with similar text and photos from other Kubota owners, as well as the dealership shop I run. BTW both that wheel and all the hardware is scrap metal. If the photo is of the better of the two, the other set is junk as well.

No. I didnt miss it. I addressed it earlier. The fasteners were likely damaged by the wheel movement. Check recommended torque on Class 10.9 M16 fine thread. I find figures for coarse thread cluster around 250/260. Fine thread will typically accommodate about 10% more. So ~275/285

Torque to yield figures are up to 40% higher - - - ~ 380.

Well if you didn't miss it, you certainly don't understand it. Quote from crazyal:


No matter what torque figures YOU came up with, crazyal said he broke one of the studs just by going a little past the torque for the bolts. And the original torque value stated was only 160 ft-lbs.

= = Fastener fatigue/damage due to whatever it was that damaged the dish mounting surfaces. = = We dont know the tractors history.

Actually I was wrong and edited. the 40% higher yield figure would be closer to 380 ft-lb. ... However bolts torqued to yield should not be reused in critical service - and these bolts/studs are meant for reuse. 300ftlb max use should allow enough safety margin since it is well below yield. :confused2: I cant see any reason for the 160 figure in that application since the bolts are much stronger and there are no affected components that would be sensitive to distortion, like is sometimes the case on automotive disk brakes.

Even Class 8.8 fine thread is rated to above 200 ftlb.

 

[crazyal]

Here's some background that some have asked for. I bought it new and it has almost 600 hours on it now. More than half of that has been using a front mount snow blower. The wheels are NOT filled with ballast but I always have anywhere from 800lbs (grading scraper) to a 1500lb ballast box on the back. Since discovering this I have closely checked the center disks for cracks and found none. My best guess is that they are powder coated vs painted and maybe the power coating was put on too thick. It took 9 years before I noticed this. That first picture I posted is blown up quite a bit. The actual amount the wheels are moving is probably somewhere between 1/16" to 1/8". If not for the bolt loosening up on the bad wheel I seriously doubt I would of realized they were moving. In fact the marks on the good wheel could easily be mistaken for damage by a socket installing them. The spot that's bare metal now from the rubbing is right next to the silver lock washer so even that could easily be missed.

I have the belleville washers but haven't ordered bolts yet. I don't think the original bolts were grade 10.9 (which is similar to an SAE grade 8 bolt) since they have no markings and are the standard silver color. Instead they look to be grade 8.8 (which I think is similar to a grade 5 SAE bolt). Also looking at bolt torque charts a M16x2.0 (I didn't find M16x1.5) is 137lb/pds for a zinc plated bolt. That should be about 160 (what the manual calls for) for a fine threaded bolt. That's very close to what a 5/8" grade 5 bolt (M16 is just slightly larger in diameter) would be torqued down to which also leads me to believe they aren't grade 10.9. Unless someone has a good reason not to I was going to replace them with grade 10.9 fasteners. The original bolts are 300mm long and with just the lock nut theends of the bolts are flush with the backside of the axle when torqued down. So if I go with 50mm length that will give me about 5mm for the extra washer I'm going to add and enough threads sticking out the backside of the axle to put a nut on it to prevent them from loosening up. The standard thickness for a M16 nut is 14mm so it should work just fine. There's miles of room on the backside of the axle and it's easy to reach.

 

[s219]

This is common and not specific to Kubota at all. In fact, Titan makes the wheel/tire combos for Kubota in the US, and a bunch of other brands. Many many tractors only use flat washers under the lug nuts or bolts.

I wonder if this is an upshot of having wheels that are intended to flip on some applications. Not normally with R4, but R1 tires commonly come on wheels that can be flipped to widen (or narrow) stance. That could be one reason they go with hub-centric wheels that use straight holes and flat washers. I don't think conical would work in that scenario, at least not on a steel wheel that doesn't have much thickness.

 

[crazyal]

To reaffirm, the wheels are hub centric. The bolt holes are not tapered. The nuts are not tapered.

With that said an agreed with, your wheels were loose. It wasn't vertical load that caused the holes to become elongated. This was caused by rotational forces, allowed because they were loose.

Define loose? Yes they did (and are moving) which means they are not tight enough, or loose. The bolts are torqued down to their correct specifications and have never loosened up. These two are in conflict with each other and in a perfect world it would never happen. But it is. My best guess is (after reading some of the posts) is that the wheels might have too much power coat on them and it's causing the problem. It could be acting as a lubricating agent reducing the clamping force of the bolts. That's all I've got.