Differential Lock Question

[Henro]

I have a simple question about the differential lock on both my Kubota tractors. But mainly on the BX

I used to use my differential lock on my BX 2200 more frequently than I have over the last probably 10 years. One reason being that generally I don’t need it. But today I did need it for about three seconds and I decided to use the differential lockafter struggling without using it. And it worked fine of course

But I’ve hesitated to use it, mainly because I’ve read horror stories here, and probably over a TBN, where people have ended up breaking the pin off that causes the lock to occur. And at least one time, worst case, the broken pin fell down, and something in the transmission or rear differential caused the pin to poke hole in the case bottom.

What I’d like to really understand is: I’ve read many times that you have to activate the differential lock when the wheels are not turning. And I’ve heard the description that the way the differential lock works is that the pin slides into a hole in a disk. Probably multiple holes in the disk and it goes into one of them.

What doesn’t add up to me is that I know from experience, and from what has been reported here, if the lock pedal does not go down easily don’t force it. So I don’t. But with the wheel(s) stopped often it will not engage, in other words the pin won’t go in the hole, probably due to misalignment between the pin and the hole.

What works for me is to just cause the rear wheel(s) to turn ever so slowly, while gently pushing on the differential lock pedal. It seems like doing this causes the disc with the holes in it to spin slowly, and the differential lock pin drops in a hole when one comes by.

The only other option that I see is to constantly have the wheel stopped. Try the differential lock. If it doesn’t work spin the wheel a little bit more. Stop. Try the differential lock. And continue doing this until you get lucky.

In my imagination, I see a disc with holes in it turning slowly if the wheels are turning slowly( one RPM, or probably less), which allows the pin to drop in the hole, when the hole comes to the pin. This is all while putting a slight light pressure on the differential rock pedal.

That’s the two scenarios I see. Is there one I’m missing? AND why is doing what I do a no-no?

I get why trying to apply the differential lock when the wheels are turning fast is very risky. Not something I will ever do…

 

[85Hokie]

You nailed it - I as a former teacher, I could not have explained it better!

 

[NCL4701]

The slow and careful engagement you describe is the way I was taught to use a mechanical diff lock on a tractor, for the exact reasons you very competently articulated. I have yet to bust one, but my tractoring days aren’t done so I guess I still have a chance.

IME, the horror stories of which you speak, in reality, go something like this… Tractor gets stuck. Doesn’t really matter if 2WD or 4WD. One rear wheel spinning, one not. Whether due to inexperience or panic or frustration or whatever, the operator increases throttle and gets the one wheel spinning at a pretty good clip. Not going anywhere, so engage the diff lock while the one wheel is spinning quickly and the other is at a dead stop and maybe has good traction or maybe doesn’t. The pin engages, shears (or something else in the rear end breaks) and mayhem ensues.

Thinking about the physics involved, I have to use my tractor as an example as I don’t know much of the specifics of the BX. The rotating mass of just one rear wheel assembly on my L consists of a wheel and tire (about a couple hundred pounds), liquid ballast (about 500lb), axle shaft and some gears (maybe another 50lb) = very roughly 750lb of rotating mass. With one of them spinning at even moderate speed, that’s a lot of inertia. On the other side, the stationary one, which weighs the same, has a lot of inertia as well, plus if it has good traction it has the inertia of the whole tractor to get moving. The numbers would be smaller for a BX but the lock pin is likely smaller as well.

Bottom line, engaging a mechanical diff lock while one wheel is spun up at any speed is possible, but it doesn’t require a mechanical engineer to figure out it slams the absolute $hit out of the lock pin. Pretty much the rotating version of snatching a chain. (Yeah, that has been known to break a variety of things, too.)

Certainly possible a diff lock has failed without any operator error being involved. Much more likely those horror stories are a result of operator error or abuse, even if it’s stress fractures caused by prior abuse that fail when being used properly.

 

[Bee-Positive]

Spot on Brother! Nice write up.
Like slamming your car into park doing 30 mph, somethings gotta give.

 

[D2Cat]

Kind of the same response as putting the auto transmission in park while parked on an incline, then when backing up the parking pawls release with a bang.

 

[Hoserman]

I don't use mind hardly at all, but what you described is exactly how I use mine. If the rear wheel starts to break traction, I stop and gently press the peddle while slowly depressing the forward pedal and it works great. good info thread. Well done.

 

[Bee-Positive]

Here's the list of bullets from my users manual. I've seen identical verbiage in other manuals.

• Do not operate the tractor at high speed with differential lock engaged.
• Do not attempt to turn with the differential lock engaged.
• Be sure to release the differential lock before making a turn in field conditions.
• When using the differential lock, always slow the engine down.
• To prevent damage to power train, do not engage differential lock when one wheel is spinning and the other is completely stopped.

The no turning part but makes perfect sense. I imagine those breaking parts are heading the warnings.