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  1. #1

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    Jun 2004
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    Default 2 questions on tractors in general, thanks.

    Hi guys.

    I'm an engineering student and I've recently devised a device that can potentially improve the efficiency and the performance of tractors. It's already been considered by trucking companies. But to help me perfect it for tractors, I'd appreciate it if you guys can answer a few questions since I've never operated a tractor before. Thanks a lot.


    2. Anyone here familar with mechanical geared tractors? How many
    different gear ratios does a "typical" one have? And what are the
    ratios generally (from the engine to the PTO of wheel)? I heard they
    have huge gear reductions.

    3. If anyone have ever driven or seen a tractor plow field (where the
    load is very high), and if the operator let go of the gas, how fast
    does the tractor come to a stop? (in seconds)

    That's all! Any comments would be welcomed.

  2. #2
    jmc
    jmc is offline
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    Ford 1920 4x4 (traded in on Kubota). Case 480F TLB w/4 in 1 bucket, 4x4. Gehl CTL60 tracked loader, Kubota L4330 GST

    Default Re: 2 questions on tractors in general, thanks.

    Here are the forward speeds, in MPH, for one version of the kubota L4330 at 2600 RPM. Tire diameter is 48.9 inches. You can calculate the gear ratios.

    1.0
    1.4
    1.9
    2.2
    2.7
    3.3
    4.0
    4.8
    5.6
    6.7
    11.0
    16.2

    Don't know about large tractors but mine stops immediately under high loads.


  3. #3
    Epic Contributor Bird's Avatar
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    Default Re: 2 questions on tractors in general, thanks.

    </font><font color="blue" class="small">( How many different gear ratios does a "typical" one have? )</font>

    I'm not sure what a "typical" tractor would be. Are you talking about large ag tractors, utility tractors, or compact tractors. Personally, I've operated geared tractors with from 3 to 18 speeds.

    </font><font color="blue" class="small">( if the operator let go of the gas, how fast does the tractor come to a stop? )</font>

    Since most tractors have a hand throttle, and some also have a foot accelerator pedal, I guess you're talking about shoving the throttle to an idle, and if you have a plow in the ground, you're probably talking about stopping in a fraction of a second.

  4. #4

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    Default Re: 2 questions on tractors in general, thanks.

    thanks to you guys that replied.

    Now, I noticed that there are plenty of ratios. (above 10 easily). Why do tractors have so much different ratios, when their top speed is usually much less than road vehicles. There must be a reason for the amount of gear ratios. Is it because that nature of the tasks that tractors do requires more precise gear ratios?

    And as a follow up to my second question, you guys mentioned that the tractor would stop immediately if it's under high load such as plowing. This is expected. That means there usually isn't much gear shifting when you guys are plowing. Because the speed is the same for the whole task, and changing gears would mean the tractor would come to a stop (mechanical gear box). Right?




  5. #5
    Veteran Member
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    N.E. KY
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    Century 3035

    Default Re: 2 questions on tractors in general, thanks.

    </font><font color="blue" class="small">( ...That means there usually isn't much gear shifting when you guys are plowing. Because the speed is the same for the whole task, and changing gears would mean the tractor would come to a stop (mechanical gear box). Right?
    )</font>

    That is correct. Typically you select your ground speed by selecting a gear, for PTO tasks ecspecially, as the engine will be throttled up to PTO speed. On some tractors, usually older models, you can NOT shift gears on the move! On these machines you have to clutch, stop, shift.

  6. #6
    Silver Member
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    Chenango County, NY
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    JD 216 Lawn Tractor

    Default Re: 2 questions on tractors in general, thanks.

    I've driven both manual trucks and manual tractors. There is a vast difference the the applications between them.

    On a truck (or any other vehicle for that matter), you shift on the go. This means that you let off the gas, engage the clutch, shift gears, give a little on the gas and let out on the clutch. Therefore, in most instances, you vary the throttle and gear range for the speed.

    On a tractor with PTO implements, most often, you run at a constant throttle. This means that the PTO will be at the proper RPM to operate the impelment. When you shift, there is usually little fiddling with the throttle. As Bird mentioned above, often gear tractors have a throttle lever on or near the steering column which you set to the proper RPM for the task. You then pick the gear ratio for the speed / power you want.

    To answer your question regarding the high number of ratios for a vehicle with limited speeds, they are necessary! If you are forced to run the engine at 2600 RPMs so that the brush hog behind you keeps cutting well, the only way to vary speed is by shifting to a lower ratio. You can't slow down by letting off the throttle or you risk stalling out the mower.

    Also, you mention letting off the gas. I doubt many farmers will do this on a regular basis! (Very bad for PTO implents engaged in a task). Most often, if you want to stop, you engage the clutch and leave the throttle where it is.

    In the end, you really can't compare vehicles to tractors as far as how / why one shifts...

    On another note, there are synchronized trannys for tractors. Usually this means you can pick a range and then shift on-the-go between the gears. To change a range, one usually must stop. (But this is a whole 'nother topic.)

  7. #7

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    Default Re: 2 questions on tractors in general, thanks.

    "To answer your question regarding the high number of ratios for a vehicle with limited speeds, they are necessary! If you are forced to run the engine at 2600 RPMs so that the brush hog behind you keeps cutting well, the only way to vary speed is by shifting to a lower ratio. You can't slow down by letting off the throttle or you risk stalling out the mower."

    Does this mean the PTO implement and the wheels of the tractor are separate? So one can spin independent of another? There isn't a fixed gear ratio between them?

    In the case you mentioned, if you are forced to run the engine at 2600 rpm so that the brush hog behind cuts well, do you mean the brush hog has to maintain that SPEED (2600 rpm from the engine) , or that POWER (power output at 2600 rpm)?

    Because if there is a fixed ratio between the PTO implement and the wheel, then when you downshift, your wheel speed will slowdown, and also the cutter speed will slow down accordingly, while keeping the engine at 2600 rpm. But the cutter will get more torque, at a slower speed, but same power.

    If there isn't a fixed gear ratio between the cutter and the wheel, then you can simply downshift on the wheel's gear ratio while the cutter stay at the same speed.


  8. #8
    Platinum Member
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    New England...Central MA
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    TC35D/16LA

    Default Re: 2 questions on tractors in general, thanks.

    <font color="blue"> Does this mean the PTO implement and the wheels of the tractor are separate? </font> Yup!!

    <font color="blue"> So one can spin independent of another? </font> Yup!!

    <font color="blue"> There isn't a fixed gear ratio between them?</font> Nope!!

    Now if someone wants to go through the live / independent PTO discussion......

  9. #9
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    JD4320 with TNT, electric diverter, cruise control and air suspension seat.

    Default Re: 2 questions on tractors in general, thanks.

    Here is a good overview from TractorSmart:

    "Transmission Driven PTO. This is the simplest form of PTO design, in that it is driven (as the name implies) directly off a gear in the transmission. It is generally found in smaller, straight-shift tractors, which use a single clutch disc to transmit engine power to the drive train. The PTO is either fully OFF, or fully ON, as determined by the position of the PTO control lever. In order to select the ON position it is necessary to completely depress the clutch pedal, bringing all rotation of the transmission to a stop, before engaging the ON position. Re-engaging the clutch couples the engine back to the transmission, causing the PTO shaft to rotate, and also causing the tractor to move; if you have selected a gear other than neutral. This everything-happens-all-at-once type of PTO can be a nuisance, especially if you are working in close quarters with a rear mower or tiller. In the early days, this type PTO was the only game in town. As implements evolved, there arose a problem, in that the rotating mass of certain implements such as rotary cutters, with their heavy blades and blade carriers, had a tendency to push the tractor, even though the clutch was released. This was due to the momentum that had built up in the blades/blade carrier assembly. That momentum simply transferred its energy, via the PTO shaft, into the transmission, and back out of the transmission to the drive wheels of the tractor. That made for some exciting rides! (Many times through a fence, or over an embankment.) On older vintage tractors, this is a dangerous set-up. There is, however, a safety device, which is built-in on newer tractors, and available as an accessory for older tractors. This is known as the "over-running" clutch, or one-way clutch. Imagine that your PTO shaft is actually composed of a front, engine side (input) shaft, and a rear, implement side (output) shaft. The output shaft can only transmit power as long as the input shaft is driving it. As soon as you cease to impart a driving force to the output shaft, i.e. release the tractor's clutch, the output shaft is allowed to freely over-run the input shaft, thanks to the one-way clutch. In other words, this one-way transmission of PTO power makes it impossible for the implement to push the tractor. The engine/transmission can rotate the implement, but not vice-versa. If you want to know whether your tractor has this feature, place the PTO control lever in the ON position, stop the engine (put the key in your pocket) and with your hand, reach down and try to turn the PTO shaft in a clockwise direction. If you have a one-way clutch, you will be able to rotate the shaft, encountering only a slight resistance from the one-way clutch. Conversely, you should not be able to turn the shaft counter-clockwise (without also turning the engine).

    Advantages of this type of PTO are simple design, low initial cost and less expense to maintain.

    Disadvantages are the inability to fully control tractor direction and speed without disrupting implement power. In the case of a straight shift tractor with a mid mount mower deck that is driven by a mid PTO, you would be well advised to investigate thoroughly, the operational characteristics. You DO NOT want the mower to stop every time you change directions!



    Live PTO. While still maintaining a relatively simple design, the usability of Live PTO is far ahead of the transmission driven variety. Nowadays, Live PTO refers more to operational characteristics than it does a configuration of specific parts. Huh? There was a time, not so long ago, that if you had a live PTO tractor, it meant that you had a dual clutch system. Dual clutch simply means that there are two clutch discs involved; one to engage/disengage the tractor's transmission, and one to engage/disengage the PTO. Depressing the clutch pedal approximately half way down would disengage the transmission clutch, while depressing it all the way down would also disengage the PTO clutch. This allows you to control the direction and speed of the tractor, without disturbing power to the implement. There is also the ability to feather the engagement of the PTO shaft, allowing a smooth start up of the implement before pulling out with the transmission. It only takes a few minutes in a tight field, with a mower mounted, to convince yourself that this is a much better set-up than the transmission driven PTO. This dual clutch arrangement is the most common, and best known style of live PTO. The advent of the hydrostatic transmission in recent years has given us another style of live PTO. While the parts list would more resemble that of transmission driven PTO, the functionality is the same as that of a dual clutch live PTO, in that tractor speed and direction can be variable, while PTO output remains constant.

    Advantages are: relatively simple design, operator friendly in function, and safety. Safe, because, with the tractor movement and implement power dependent upon the position of the clutch pedal, you have a built-in panic button (the clutch pedal) that you can instantly depress to bring everything to a halt should things go awry.

    Disadvantages are: more expensive to repair, extra routine adjustments (on dual clutches) to maintain proper operation, and a little more leg muscle is required to depress the clutch through both stages of the clutch.

    Independent PTO. Available in two "flavors"; mechanical independent, and hydraulic independent.

    Mechanical independent is essentially identical to a dual clutch live PTO, except that there is a separate hand lever to control the engage/release of the PTO clutch disc. Really, there are two controls for the PTO. In addition to the hand clutch lever, there is another lever that actually couples/uncouples the PTO shaft in the drive train. To start the PTO turning requires that you first move the hand clutch lever to disengage the PTO clutch. Then, you select the ON position with the PTO control lever. Next, using the hand clutch lever, you re-engage the PTO clutch, which causes the PTO shaft to begin rotating. One nice advantage of this system is that you can slowly, in a controlled manner, engage the PTO clutch, while the tractor is on the go. Disadvantages include having to fiddle with two control levers. Also there looms the potential for severe engine damage should you use this system incorrectly. Incorrectly means running around all day with the PTO hand clutch lever in the disengaged position, in order to stop rotation of the PTO shaft. Doing that puts a constant thrust load on the engine crankshaft, leading to accelerated wear of the crankshaft's thrust washers. The eventual result of this abuse is a ruined, non-rebuildable cylinder block. If you acquire a tractor with this type of independent PTO, please be sure that you are familiar with the correct operating procedures.

    Hydraulic independent PTO is the most expensive, complex, and, in most cases, user friendly type available. Expensive and complex because operation depends upon various pumps, valves, filters, etc., all functioning correctly to drive the PTO shaft. User friendly, because one need only move a single lever, or push a button, to start/stop the PTO shaft. You are able to select ON and OFF, independently of tractor travel and speed. The heart of hydraulic independent PTO is the clutch pack. The clutch pack generally consist of several drive discs, and several driven discs. The drive disc stack is coupled to the engine side, so that any time the engine is running, these discs are rotating. The driven discs are coupled to the PTO shaft and will rotate only when the PTO is selected to the ON position. Rotation of the driven discs is achieved by directing pressurized oil behind a piston in the clutch pack, causing the piston to move into a position which squeezes all of the discs together, making them behave as a single unit. The drive discs were already turning, now the driven discs are turning as well, imparting rotation to the PTO shaft. Some form of PTO brake is incorporated into most hydraulic independent PTO systems. The function of the brake is to overcome the tendency of the PTO shaft to continue rotating, even when it is in the OFF position. This undesirable rotation is caused by oil drag between the drive and driven discs in the clutch pack. Without a brake, light load implements, such as hay rakes, might continue to slowly rotate, even with the PTO OFF. At best, this is a nuisance; at worst it could present a dangerous situation. PTO brakes are getting more reliable, however they are still subject to accelerated wear and damage, if continually called upon to stop heavy implements. A good example is the heavy duty rotary cutters that are popular today. There is a lot of potential energy built up in those heavy blades and stump jumpers when they are rotating at operating speed. When you select the PTO to the OFF position, your are disengaging the clutch pack, and redirecting the pressurized oil to the PTO brake system. Imagine trying to bring your truck to a stop from 50 MPH, using only half of the brake pads on one wheel, and you'll get the general idea. Some of these systems have an intermediate coast position between ON and OFF, which allows natural deceleration of the implement, saving wear and tear on the brake components. In the absence of that, you may wish to consider using an over-running coupler on the tractor PTO shaft, in order to avoid costly damage when using heavy implements.

    Advantages are: ease of operation and maximum flexibility in determining when you want the PTO to be ON and OFF.

    Disadvantages are: higher acquisition cost, more complex and difficult to troubleshoot/repair, higher maintenance expense and not quite as safe as live PTO. (If you needed to stop the PTO in a panic situation, can you find that little lever or button as quickly as you can "stomp" the clutch pedal all the way down?)"

  10. #10
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    Default Re: 2 questions on tractors in general, thanks.

    The PTO and ground gearing are two separate things entirely. U.S. domestic tractors have either a 540rpm or 1000rpm pto or they have both. The only bearing on pto speed is throttle position, usually the max rated speed of the engine.

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