Final drive assemblies

[froggy]

I have looked on the web, but cannot seem to find a site that discusses the various final drive assemblies (spur, bull gear and planetary) for tractors and their pros and cons for each. Can anyone point me in the right direction?

 

[fishpick]

I HATE to use this as a reference - because - it's not a reference... but you CAN get to the cited articles from here... which make it a place to find a good reference...
Gear - Wikipedia, the free encyclopedia

And if that makes no sense - here's the explanation of why it's an evil URL...
YouTube - Michael Scott (Steve Carrell), NBC's The Office on Wikipedia

 

[froggy]

You are right on both counts. That should be the last place anyone should go to for information.

 

[N80]

I disagree about Wikipedia. They are perfectly up front with you about their sources and the nature of their info. It only becomes a problem if you aprroach Wikipedia as if it is a definitive source. It isn't and they don't claim to be. If you understand that, then it is a great source of great information as long as you approach the info critically and with common sense and confirm information that you are either unsure of or that is imporatant to you in some way.

The real problem is when people approach any source as definitive. Always approach all information critically.Always confirm all sources if the information you are seeking is important to you.

Edit: And I'm with the OP, I'd love to see a discussion about these different final drive assemblies also. The final drives on my little Kubota are pretty weird. I don't even know what they are called. I've got the parts manual but it just calls it the 'final drive assembly' if I remember correctly. Seems unusually complex.

 

[AKfish]

I've had this question for some time... too lazy to get out there and do some digging --- Wickypeepee --- or otherwise.

The differences between Spur and planetary final drives; that is. (I have a basic understanding of planetary - ring gear and pinion - configuration.)

I have noticed that the 3x20 series are spur gear and the 4x20 are planetary drives.

Just assumed that the spur gears are not as heavy duty as the planetary gear tractors. Have never read a discussion on TBN that detailed the pros and cons.

AKfish

 

[Farmerford]

Spur gears (also known as bull and pinion gears) work fine until the gear ratio gets high, say 5 or 10 to 1, or higher. Depending on the design, at certain ratios and above only one tooth on the pinion and bull gear are engaged at the time. That single tooth must carry the entire torque being transmitted by the gear set, resulting in a high load on the tooth in terms of both bending moment and pressure on the tooth face.

Spur gears are also fairly large and result in an offset in the driveline since the centerline of the pinion gear is removed from the centerline of the bull gear by one half the sum of the pitch diameters.

Planetary gearsets instead typically have three (or more) planet gears, each of which is engaged with (usually) more than one tooth on the pinion (sun gear) and more than one tooth on the ring (outer, internal toothed) gear. Therefore the planetary gearset is physically smaller and lighter than the bull gearset. The planetary gearset also allows (requires?) that the input and output shafts be in line.

I suspect (but do not know) that planetary gearsets are more expensive to manufacture than bull and pinion gearsets. If so, they probably are used on the larger tractors because they handle the torque better and allow a more compact transmission case.

Virtually all automotive automatic transmissions use planetary gearsets, as do the "powershift" transmissions on construction equipment like crawler tractors and loaders.

Since a tractor manufactor is presumably trying to make its tractor as cheaply as possible, I am not sure the type of final drives are an indication of quality. After all, a planetary gearset can be undersized for the load just as easily as a bull and pinion gear set can be. It is possible to build bull gear and pinion gearsets that will handle enormous torque. Two examples are the bull gear and pinion final drives in every Caterpillar dozer through D-9 size until the Hi-drive was introduced 20 or so years ago, and the turbine drive reduction gear in steamships like the WW II battleships.

In wheel and track driven equipment, the planetary gearsets are often incorporated into the wheel hub. If the last gear reduction is at the "end" of the drive train, then the drive train components ahead of it can turn faster and, since a shaft rotating at a higher speed transmits the same horsepower with less torque than a slower turning shaft, the shafts can be smaller, lighter, and cheaper. The Caterpillar Hi-Drive has a planetary gearset in the elevated drive sprocket.

 

[N80]

Thank you very much for that explanation. My 45hp Kubota has a spur gear set up. I'm not sure, but I think all Kubotas do. Would be interested in hearing if the larger Kubotas, like the M7040 or M105 actually have them.

All of my farmer friend's John Deeres from the big ones to the small ones have straight rear axles, which are presumably the planetary type. (The newest one is 30 years old.) Same with his much older Cases.

So, as best you know the only disadvantage to planetary gears is the cost?

 

[froggy]

Farmerford,

Thanks for the explanation. The reason I brought this up was that I was comparing tractors on the CaseIH web site and noticed the that the larger CUTs varied from spur (DX55 and Massey 1552) to bull gear (Kubota GL5240) to planetary (JD4720).

Then as I compared the larger tractors such as the JD5325 & 5425, the Case JXC series and the Kubota "M" series, planetary was the final drive assembly for each.

In the past I remember some less than sterling comments on bull gears and also outboard planetary systems.

Now the question is, how much of a difference in there in planetary systems: "outboard" and "in-board".

Thanks,

froggy

 

[Farmerford]

The only general disadvantages I know of are cost and the need for good lubrication for the planet shafts. In bull gear and pinion sets, the gear shafts are typically in antifriction (ball, roller, etc.) bearings set in a fixed foundation (the transmission case, rear end housing, etc.), and are easily lubricated by the splashing of the bull gear in the sump. Large units, like the bigger Caterpillar final drives, have a lube pump driven by the pinion to pump oil up onto the top of the bull gear.

The input and output shafts in the planetary gearset are also typically held in antifriction bearings in a fixed foundation. But in the planetary gearset there are also the planet gears not found in the bull gear and pinion. The planets rotate on shafts held by the planet carrier that, in the typical case, is itself rotating. And because of the need to keep costs and size down the planets are usually mounted on plain bearings (ie, bronze bushings). Splashing oil from the sump is not very effective at lubricating these plain bearings because they are shrouded by the ring gear and planet carrier, and also are themselves rotating, so the sump is usually designed so that the planet carrier is partially immersed in oil during operation (the overdrive gear set on old cars) or the oil is also an operating fluid that flows over the planets (the power shift transmission in my John Deere crawler loader). So if the lubricating fluid gets low or is somehow blocked, the planet bushings quickly begin to wipe and seize.

But I suppose that is one disadvantage that regular maintenance can offset.

 

[Farmerford]

Froggy:

In my experience outboard planetaries are troublesome only when the manufacturer cuts too many corners.

The bull gear and pinion by its nature requires a large final shaft to the wheel/track sprocket because the gear reduction has already taken place and the torque on the shaft is high. In addition, the size of the bull gear almost insures that it will be mounted near the center of the machine. Therefore, the axle from the bull gear to the wheel/sprocket is relatively large and long, and it is supported by bearings near each end that are relatively far apart. Thus the axles and bearings, if adequately sized, are capable of carrying great torque and weight, as well as large side loads from the wheel (as on a hillside, when turning, etc.). Think of the long axle housings on the old Farmall H's and M's and John Deere A's and B's. I know of some that ran daily on our farm from the 40's to the 60's (and later) and never was there a problem with the rear axles or axle bearings.

But the drive axle in an outboard planetary gearset is really a very short shaft that runs from either the outer side of the ring gear or the planet carrier (depending on the design) to the wheel. This short shaft puts much greater stress on its bearings than the comparable bull gear axle because the bearings are so much closer together, producing much greater multiplication of the wheel forces. And if the shaft flexes it can alter the geometry of the gears in the planetary gearset. The only problems I know of are where the manufacturer skimps on the size of the stub axle and bearings.

So I suppose the best approach is to select a reputable manufacturer who may value long term reputation over short term profits enough that it will not cut too many corners.

Of course, as I say that I am cussing Kubota for putting a tool box the size of a teacup on the L4330 I bought last year. Who can you trust!