Difference between 4WD and MFWD

[ovrszd]

John Deere uses a live axle set up. At least on the ones I saw at the dealership a couple of days ago including a 990 (similar to my L4400) and several Ag/utility sized machines. They must think its an okay design.

I would also have to say that looking at that .pdf schematic I personally don't see the bevel gear (Kubota's terms) differential set up as a weak link. As I mentioned before, based on the size of the bevel and pinion gears it looks to be about a 1:1 gear ratio. I see nothing weak in that. The size of the bevel gears (both the 'ring' and the 'pinion') don't really indicate that they are weak to me. They might be smaller than what you see in the rear because the final drives by each wheel may have a reduction ratio but it is also (and probably mostly) because the front tires and wheels are 1/2 the size of the fronts.

I don't know what the mechanism in the bevel gear assemblies looks like. That might indeed represent a 'weak spot' but I don't see any inherent weakness or anything failure prone in the diff. And again, nothing in my manual suggests there is anything in the front driveline that requires any special care or attention compared to the rest of the tractor.

If the front tires on the tractor are tall enough to raise the center of the axle high enough to not be a hinderance then a live axle setup works great.

The weakness in this design is not the pinion or ring gear, it's the size of the spyder gear and spyder gear pin. Look at the bottom exploded diagram. Part number (2) and pin number (8). The size of these two parts are critical to the strength of this assembly. This diff will fit in your hand. Inside it are four gears and a pin. splined on one end is the ring gear. A smaller gear diameter or size means less teeth. Less teeth means more repititve wear to accomplish the same mission.

The assembly at the wheels consists of a bevel gear on the end of the horizontal axle shaft coming from the diff, a bevel gear running against it and fastened to the top end of the verticle pinion shaft, a bevel gear at the bottom end of the pinion shaft that runs against a ring gear that is splined onto the end of the wheel hub shaft that the wheel runs on.

Every time you add a gear you add wear points and weakness. That is why in another discussion I stated that the front drive assembly is more wear and subsequently failure prone than the rear drive assembly. Simply because of complexity. The front axle and wheel assembly of my Kubota has 14 gears and 14 bearings. The rear axle has six gears and six bearings.

 

[N80]

Okay then, back to my original question. Why does Kubota use this complex system with tiny spyder gears that, according to what you are telling me, introduces weakness. Why would they do this? More complexity, weaker, harder to manufactur, more expensive. There must be _some_ advantage and it isn't, as you suggest, so that they can use small tires/wheels on the front. The John Deeres with typical live axles and typical pumpkins don't use any larger tires than my comparable L4400. So that's not it. So again, what advantage is gained by this complex, inherently weak system that Kubota uses even on its largest Ag tractors (with comparably tiny spyder gears in their comparably tiny front diffs).

Improved turning radius in a 4wd is all that Kubota mentions. Is that it?

I'm just trying to make sense of this. Anyone else have any insight?

 

[ovrszd]

Okay then, back to my original question. Why does Kubota use this complex system with tiny spyder gears that, according to what you are telling me, introduces weakness. Why would they do this? More complexity, weaker, harder to manufactur, more expensive. There must be _some_ advantage and it isn't, as you suggest, so that they can use small tires/wheels on the front. The John Deeres with typical live axles and typical pumpkins don't use any larger tires than my comparable L4400. So that's not it. So again, what advantage is gained by this complex, inherently weak system that Kubota uses even on its largest Ag tractors (with comparably tiny spyder gears in their comparably tiny front diffs).

Improved turning radius in a 4wd is all that Kubota mentions. Is that it?

I'm just trying to make sense of this. Anyone else have any insight?

As always in our discussions N80, it will have to be someone else that reasons with you. I have explained this in detail. You are intent on picking apart what I say and taking things out of context to question. That's your choice. I have nothing more to offer. I only had this discussion to offer explanations to others that might be reading. If you still don't understand, I can't help you. Anyone else want to jump in, feel free.

 

[Superduper]

Ok, there's lots of guesses here so one more won't hurt. On a true 4wd system, you have a front differential, you have a rear differential, and you have a transfer case that acts like a differential between the front and rear axles. On the MFWD (mechanical front wheel drive, I believe the front wheels get driven mechanically without the differential action that normally takes place in a normal 4WD system.

During a turn, the inner wheel spins at a slower speed than the outer tire. Additionally, the front axle probably spins at a different speed than the rear axle during the turn. These speed differences (in a truck, for example) would be adjusted for in the differential, and allows even tire wear in all tires. As you can see, because the MFWD system is actually locked without any differential (or speed compensating function), operating the MFWD system on firm ground for extended periods is not recommended. Many tractors with MFWD also have a lever to "lock" axle for better traction. This suggest to me that the rear axle is the only differential in the tractor.

 

[ovrszd]

Ok, there's lots of guesses here so one more won't hurt. On a true 4wd system, you have a front differential, you have a rear differential, and you have a transfer case that acts like a differential between the front and rear axles. On the MFWD (mechanical front wheel drive, I believe the front wheels get driven mechanically without the differential action that normally takes place in a normal 4WD system.

During a turn, the inner wheel spins at a slower speed than the outer tire. Additionally, the front axle probably spins at a different speed than the rear axle during the turn. These speed differences (in a truck, for example) would be adjusted for in the differential, and allows even tire wear in all tires. As you can see, because the MFWD system is actually locked without any differential (or speed compensating function), operating the MFWD system on firm ground for extended periods is not recommended. Many tractors with MFWD also have a lever to "lock" axle for better traction. This suggest to me that the rear axle is the only differential in the tractor.

Hi Superduty, your description of what you called a true 4WD system is right on the mark for a MFWD system like Kubota uses. The difference between what Kubota uses and a 4wd Jeep or truck uses is that Kubota obtains the specific gear ratio in the front axle using a ring/pinion gear setup at each front wheel. The differential in the center of the axle only allows different speeds of each front wheel and splits the driveshaft power two directions to each front wheel using a ring/pinion and a carrier with spyder gears.

During a turn each front wheel is allowed to rotate at a different speed because of the spyder gears in the front axle center differential. The front driveshaft and rear driveshaft are locked together by the transfer case and rotate at the same speed regardless of a turning situation or straight line situation.

If there is a fixed variation of ground speed or ground travel between the front wheels and rear wheels, it is calculated into the variation in final drive ratios of each axle. This calculation is based on ring gear/pinion gear ratio and circumference of the front and rear tires. Some tractors are set to operate at exactly the same ground speed front and rear. Some tractors are designed to run a few percent faster in the front than the rear. The goal of a faster front is to help prevent scrubbing when turning and to make the front tires more effective in directing the tractor when turned since they are pulling slightly faster than the rear.

Depending on how your particular tractor is set up, it will either bind when going straight or bind when it is in a turn. That is unavoidable in a MFWD setup. If the ratio of the front/rear is equal that tractor will bind when turning. If the ratio of the front/rear has the front running a few percent faster that tractor will bind when going straight.

Because of that one fact it is stressful and will cause faster wear on the driveline and tires if you operate with the MFWD engaged on a firm surface.

That fact has been demonstrated and proven in several discussions on this website. Some will argue that their tractor does not bind going straight and/or while turning with the MFWD engaged. To those operators I say your tractor is special.

 

[SPYDERLK]

That's the point I was trying to make in our previous discussion about the strengths/weaknesses of various drive systems, especially in the front drive system. The power and traction of the Kubota is magnified thru the ring/pinion at each front tire. This magnified torque is transmitted thru a tiny set of spyder gears because of the small ring gear in the center of the axle housing. Spyder gears probably half the size of those in the rear diff. This setup creates a weak spot for potential trouble. So the operator should take precautions to lessen the load on that component, i.e., not using MFWD unless needed. Binding is inevitable in the Kubota system, either when driving straight or when turning, depending on the variation of ratios between the front axle and rear axle. It's a mechanical system and has to bind during one or the other.

ovrszd, This is a very elucidating statement. Allow me to manipulate your conclusion a little tho. -- The torque multiplication is happeniing at each wheel drive and is directed toward each wheel. In other words the shafts from the front spiders are turning very fast and are geared down AT the wheel. The front spiders are delivering low torque at high speed. This way they dont have to be real strong. They just never have to put up with much force because the high forces are generated only on the wheelward side and are isolated there by the wheel bevel drive.
N80, Im betting that the parts are appropriately robust to satisfy needs at each point in the system. Even the little ones are ample to do what they do.
ovrszd, I want to use another quote from you: "Depending on how your particular tractor is set up, it will either bind when going straight or bind when it is in a turn. That is unavoidable in a MFWD setup. If the ratio of the front/rear is equal that tractor will bind when turning. If the ratio of the front/rear has the front running a few percent faster that tractor will bind when going straight.

Because of that one fact it is stressful and will cause faster wear on the driveline and tires if you operate with the MFWD engaged on a firm surface.

That fact has been demonstrated and proven in several discussions on this website. Some will argue that their tractor does not bind going straight and/or while turning with the MFWD engaged. To those operators I say your tractor is special."

This is absolutely true. I would, however, have used the term "fictitious".
Larry

 

[ovrszd]

ovrszd, This is a very elucidating statement. Allow me to manipulate your conclusion a little tho. -- The torque multiplication is happeniing at each wheel drive and is directed toward each wheel. In other words the shafts from the front spiders are turning very fast and are geared down AT the wheel. The front spiders are delivering low torque at high speed. This way they dont have to be real strong. They just never have to put up with much force because the high forces are generated only on the wheelward side and are isolated there by the wheel bevel drive.
N80, Im betting that the parts are appropriately robust to satisfy needs at each point in the system. Even the little ones are ample to do what they do.
ovrszd, I want to use another quote from you: "Depending on how your particular tractor is set up, it will either bind when going straight or bind when it is in a turn. That is unavoidable in a MFWD setup. If the ratio of the front/rear is equal that tractor will bind when turning. If the ratio of the front/rear has the front running a few percent faster that tractor will bind when going straight.

Because of that one fact it is stressful and will cause faster wear on the driveline and tires if you operate with the MFWD engaged on a firm surface.

That fact has been demonstrated and proven in several discussions on this website. Some will argue that their tractor does not bind going straight and/or while turning with the MFWD engaged. To those operators I say your tractor is special."

This is absolutely true. I would, however, have used the term "fictitious".
Larry

Hahahaha!!! Okay, change that to "fictitious"!!!!

Good point about the torque multiplication. You are very correct. Same basic concept applies when looking at a "sun gear" setup. That's how those little bitty gears can stand all that stress!!! Good point Larry.

 

[Soundguy]

Hey now.. don't be bringing any fact into this conversation... conjecture and shear lack of knowledge were doing -just fine-. (grin)

soundguy

 

[ovrszd]

Hey now.. don't be bringing any fact into this conversation... conjecture and shear lack of knowledge were doing -just fine-. (grin)

soundguy

I don't know if we should pay any attention to you Soundguy!!! I saw your list in the Roll Call thread. You definitely have an addiction problem!!!! Hahahaha!!!

 

[N80]

N80, Im betting that the parts are appropriately robust to satisfy needs at each point in the system. Even the little ones are ample to do what they do.

Thank you. That has been my sole contention in all of these 4wd posts.

That fact has been demonstrated and proven in several discussions on this website. Some will argue that their tractor does not bind going straight and/or while turning with the MFWD engaged. To those operators I say your tractor is special.

I agree. And to clarify, when I discussed how my tractor operated my only point in that matter was that you cannot _feel_ the binding in my L4400 even in a tight turn. It was someone else who suggested that my tractor was 'special'. I was quick to point out that the L4400 might be the least 'special' tractor that Kubota makes.