Rtv 500 differential lock problems

[Steelauzzy]

I’m 2018 I bought 3 identical rtv500’s with sequential serial numbers for use on a golf resort project I was working on. Within the first 25-50 hours all three differential locks on all three units failed. Not catastrophic just stopped grabbing. As most of our use was on the resort and in our gravel yard it wasn’t a high priority.
recently I took the first unit in to the dealer before the warranty was up to get it sorted. Assuming this would be no biggie. After sitting in their yard for a couple months they called and said there was a chance it would not be covered if it was obvious that it was abused. And that it was expected to be 24 hours to just inspect or around 2000$.
Fast forward another month they called me with a bill for 6000$. Of course this is half the cost of a new unit so I was much less than impressed.
I requested to inspect the components to see why why had ruled it operator abuse. Upon very close inspection with a bright light there was a hairline crack in the casting at the corner of both locker contacts. The surface of the contact was also polished and probably had a coupler thou wear. Yes you would litterally need a microscope or micrometer and a good part to compare any measurable wear.
The manager said this is common in diffs that have been worked hard. And in his experience he has seen this tons. He also said that if it was a design issue there would be tons of other units with the same problem. And since they had never heard of it that it must be our fault. Specifically they said we must have had a wheel in the air revved to top speed then engaged the locker. This was a change from their original story that we must have locked the diff while moving.

my first question was how such a small change in the angle of the contact and a hairline crack would render the locker non functional. Also given that the back side contact wasn’t worn it didn’t offer a theory as to why it didn’t work driving backwards.

so I have requested the old parts and a set of new parts to take to a lab to get testing. I’m also wondering if anyone else has had any issues with their diffs. You pretty much need to get stuck and have someone watching to see if it’s bad.
I plan on getting to the bottom of this and it will be interesting to open up the other two machines to see how they failed last kubota wouldn’t let me see the other side of the coupling or the mechanism that pushes it together.

 

[Steelauzzy]

This is the current update on the warranty. The first unit is fixed. I was charged about 2400$ for the "reduced labor only" Kubota covered the parts. The second unit went in and I was told Kubota wouldn't be covering anything and that i was now on the hook for over 9000$ in repairs to the second unit. They advised me to bolt it back up as is and sell it or let it die. In response i came up with the following summary of the problem.




1) the machines. We purchased 3 identical RTV 500 units for use at the golf course project. we noticed within the first couple months around 50 hours on each machine that the differential locks did not function at all. finding it odd we figured it was a clutch or spring of sorts that needed adjusting. As it did not impede operation for the most part we did not rush them in for service but keeping in communications with the dealer we made sure to bring them in before the warranty expired.
All 3 units have non functioning diff locks. So far we have opened two units. the first had two hairline cracks and some slight wear on the forward face of the contact carrier side. The second unit the contact lugs were ripped clean off.
We requested that the old and new parts be provided to me for outside testing and inspection. This request was approved but then before I could take them they were reinstalled back in the machine without my authorization and then I was told that the old parts were now kubotas property and that i could not take them for inspection without approval. I expect this approval to be granted without hesitation.
The second unit is at kubota and I will be taking the old part as well as a new replacement from the 500 model as well as one from the new 520 model in for testing. the hypothesis I want to explore is the hardness difference between the contactor and the carrier. I want to compare these as well as the hardness of several other common oem and aftermarket locker components.
A question that came up on the first unit is that given the fact that the carrier was still in tact and have zero noticeable wear on the back face of the lugs then why did it not function even a bit in forward or reverse? This has not been answered. Again to be clear this must be answered in writing.
the second unit also has evidence that the front differential needs replacing. It was noted that a battery was installed in the front of the unit that ran the hour clock for a while before being noticed. Either way the unit is within a few hours of the first scheduled front differential oil change. Somehow this has also been passed over by warranty.

The third unit we expect to be similar to the first two once inspected.

So now this gets interesting.
This is a photo of my first unit. notice the the lugs are intact and there is some wear on the forward face and no wear on the reverse face. In this example the only conclusion that can be drawn is that the locker would have slightly reduced function in forward and little to zero reduced function in reverse. That is unless this is a fundamentally flawed design. Now notice the size of the lugs they cover approximately 30% of the face. The other coupling covers the other 30% this leaves 40% free space to slide back and forth when starting stopping turning left right etc. So as you drive with this engaged its going to constantly be hammering back and forth. Then if the hit is hard the spring will depress and the hammer will skip over and slam in on the next 1/2 revolution.

This photo demonstrates a fundamentally identical mechanism.
this is a link to the patent design of yet another similar design. US2637426A - Impact type clutch - Google Patents
As you can see by the patent this design is known as an "impact clutch" they are most commonly found in impact drills like the above pictured dewalt 18v impact screw gun. The mechanism rotates and after every impact the spring depresses allowing the hammer to skip over to the next lug. Note the spring used on the dewalt is several times stronger than the kubota spring. It takes two strong hands full power to depress it enough to get the hammer to slide over. Now notice in the patents one of the most critical components of making this design last for millions of stikes is the metallurgy and case hardening of the parts. This usually involves machining or forging the parts rather than casting and then blasting them with tiny ball bearings to harden the surface. Notice the tiny orange peel texture on the opposite cam of the kubota aswell as the two cams for the 4wd clutch. This is why taking the parts in for a rockwell hardness test.

now this picture is the industry standard for face contact differential lockers. notice the contact lobes or teeth do not allow any play. if the force overcomes the holding capacity of the locker the springs allow the coupler to slip then grab again once the two axles come back to a similar speed. These lockers as pictured usually come with lifetime warranty. Note if the wheel does break free it will accelerate then there is potential of this locking at high speed. The manufacturer anticipates this and ensures the parts are strong enough to hold up under any possible situation.




Now we move back to the Kubota 500 series rtv. Ive just noticed that kubota changed the model from the rtv500 to rtv520 the new model comes with an amended manual. On page 36 there is a new box added that instructs the operator to disengaged the locker immediately after use prior to turning. This is because the fundamental design of this locker will experience failure as witnessed if it remains engaged under normal operation. The wear and tear experienced is not from engaging while the wheels were spinning but rather from leaving it engaged when not needed. Note many off road vehicles have fixed rear axles. The diff lockers or spool diffs are only problematic when driving on pavement or on soft turf. In any other real world scenario it is acceptable to leave it locked.

In summary the failures experienced are without a doubt the result of a design flaw, a casting flaw, poor material choice, poor material treatment choice and lack of warnings to make operators aware of the flaw and potential for damage.