Hydrostatic vs belt and pulley automatic

jfw432July 22, 2010

I'm trying to figure out the difference between these two transmissions. I understand both operate in a similar fashion as far as the operator is concerned. While one takes a belt form the engine to control a hydraulic pump to drive the wheels and the other takes a belt to drive a varying wheel to drive the wheel like a CVT transmission in a car.

I understand the belt and pulley automatic is far less complicated. However, what is the real difference between the two? Is one more sturdy than the other? How much maintenance is involved with them? Can both withstand the stress of going up and down hills all the time?

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jfw, your inquest almost answers itself in the points you bring up:^)
I'll give my slant on the subjects at hand, and add a bit of complexity to your description of the works.
Hydrostatic drive power transmission: Uses an engine driven (usually by belt) hydraulic pump to power a hydraulic motor. The hydraulic oil flow is managed by a control valve that adjusts the volume of oil flow through the hydraulic motor to vary the ground speed of the tractor. Increasing the volume of hydraulic oil flow through the motor increases the ground speed. The control valve is a "double acting" (aka directional) valve in that the direction the control valve is moved from the neutral center position determines which direction the hydraulic oil flows through the hydraulic motor. Reversing the direction of hyd oil flow through the hyd motor reverses the direction of travel of the tractor.
Advantages of hydrostatic drive are: Very smooth when adjusting ground speed or changing direction of travel.
Because there are no gears involved in changes of ground speed or direction of travel, you will never have to "hunt for" another gear shift notch, or tease the transmission into letting you engage that next gear (sometimes accompanied by gnashing or grinding of the gears inside a manual gearbox tranny).
Drawbacks to hydrostatic drive are: Actual transmission of power from engine to to where the drive axle shafts turn is less efficient than CVT due to internal fluid leakage in the pump, valve, and motor. This loss of efficiency tends to get worse as the three major components accumulate wear. The efficiency is also affected by the temperature/viscosity/ of the oil, and the state of degradation the oil is in at any given moment.
Another drawback is that hydrostatic processes generate a lot of heat which must be shed (hence the fan on the input pulley) or the build up of heat will lead to foaming and burning (carbonizing) of the oil inside the transmission. When the oil foams, hydrostatic processes diminish or stop, and so does the tractor.
Maintenance to the typical modern hydrostatic drive transmission is an adventure in creativity, and a challenge for the owner because marketing trends by the OEMs have made this genre "throwaways".
Oil changes on many of these hydro units require the transaxle assembly be removed from the chassis and literally "turned upside down" over a catch pan in order to drain the oil. Few of them have an external filter for easy replacement, many have an internal filter which require opening the transaxle case.............not exactly what I call "routine maintenance".

The variable belt drive, vari-drive, or CVT transmission is "a transmission" in and of itself, sort of.
The CVT portion of the power train allows for a fixed range of RPM reduction between the engine crankshaft pulley (CVT input pulley), and the CVT output pulley (belt to the transaxle housing input pulley).
The ratio of input RPM versus output RPM is contolled by the operator through a linkage system.
The lower the output pulley speed, the slower the ground speed....but with more TORQUE applied to the axle shafts.
The higher the output pulley speed, the faster the tractor moves across the ground......but with less torque applied to the axle shafts.
Because the CVT pulleys adjust the ratio within a limited range, typically the transaxle has a manually operated gearbox to give a wider choice of ground speed and wheel torque combinations. The manual gearbox also provides for the change in direction of travel (forward and reverse).
Advantages of CVT are: More efficient in actual power transmission (when belts are in good order).
Heat build up in the transaxle during use does not decrease the efficiency or performance of the transmission system.
Drawbacks of CVT: Expect to replace both belts at about 2-3 year intervals.
Belt replacement can be in the neighborhood of $80 to $100 for the pair because you need to use OEM belts (which are naturally more expensive than generics).
Because there is some "shifting of gears" involved in the manual gearbox when going between forward and reverse, there will be times when the transaxle gears don't "line up" exactly and you will have to hunt for the engagement sweet spot, sometimes a frustrating event.

As I stated, those are my opinions.

    Bookmark   July 22, 2010 at 12:00PM
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There are some operation similarities between a hydrostatic transmission and CVT (Transmatic). I have bought and sold a lot of gear and transmatic lawn tractors, and fewer hydrostatics, but have had enough of each to form some generalized opinions. A similar application for CVT are snowmobiles, although they differ by using integral centrifugal clutches, a special type of belt, and both pulleys are moveable to provide variable ratios.

The Transmatic is almost unique to MTD (and the hundreds of private labels they manufacture for). Deere used to use a similar transmission (Variator), but other than a rear engine rider, don't use much any more to my understanding. In operation, there is a clutch between the engine and CVT pulleys which slackens or tightens the first belt. There is a speed range control which sets the maximum speed ration, and there is a forward-neutral-reverse control on the rear axle. The rear axle is separate from the CVT and connected by a second belt.

The hydrostatic is either directly coupled to the engine (higher end equipment) or belted to the engine using a clutching pulley (similar to the transmatic). The hydrostatic consists of a hydraulic pump closely coupled to a hydraulic motor which then drives through a differential to the rear wheels. The main operational control is either a hand lever or footpedal which controls both forward-reverse direction and ground speed. This is accomplished by controlling both the direction of flow and the amount of flow internally to the hydrostatic transmission. The clutch on a belted unit is used for starting, but the engine can continue to run with the transmission in neutral.

I find the hydro easier to use, particularly if it is direct connected and has foot pedal control. No brake is usually needed (other than parking) because the hydro functions as a retarder when in the center (neutral) position. In fact, it's almost impossible to roll a hydro around not running, and most mfgs put a bypass on the hydro for this reason. With the transmatic you use the clutch to get underway, the machine then starts in the lowest gear (pulley ratio) and then advances to the tallest ratio limited by the speed control. To come to a stop, you depress the clutch which also actuates the brake, and then start process over. If you want to slow down to a lower "gear", you need to clutch and allow machine to slow and then select a lower "gear".

Both transmission types are pretty well proven. The hydro is popular but not good if overloaded or used for ground engaging equipment. Excessive heat will wear out a hydro with expensive results. Also, hydros do not tend to hold up well in hilly operation. Hydros fitted to true, heavy duty garden tractors and CUT's are meant for this service and perform much better. The Transmatic is generally pretty simple and robust, although they tend to go through belts. Also, the rear axle on an MTD Transmatic is made by Peerless and can be the weak link.

    Bookmark   July 22, 2010 at 12:17PM
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Two excellent responses. Thank you both. I like how the hydrostatic trannys pretty much stop themselves when you let off the control lever but I think I will most likely be going with the transmatic. During the mowing season, it's often around 100F when I mow my yard. Throw in a lot of hills and hauling a trailer or aerator and I can only imagine that an entry level hydrostatic tranny will not hold up to the abuse. I can also live with replacing belts every couple years but replacing or servicing a hydrostatic is most likely beyond what I want to deal with.

Thanks again.

    Bookmark   July 22, 2010 at 2:18PM
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There are plenty of hydros that will handle ground engaging equipment. Wheel Horse tractors from the 1960's prove this.

It is true, however, that under abuse the hydro will be expensive to fix (even those units designed for repair and maintenance) where as the CVT will usually only need new belts (though the wear and tear may affect other portions of the tractor).

Incidentally, the continuously variable transmission concept is making its way into more and more automobiles!

    Bookmark   July 22, 2010 at 2:48PM
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Hi, I am hoping that one of you knowledgable posters might be able to give me a bit more insignt into how transmatic works: my old MTD Lawnflite now only goes slow or slightly slower and I can't see what's wrong.
The two relevant belts look in good condition (though I would see if one were stretched) and the linkages seem to do the right thing: the "variable speed dual pulley" (as MTD call it) sits in a different position front-back depending on the gear lever position. The tensioner keeps the rear belt tight. But this makes we wonder what actually changes the gear ratio: should the dual pulley centre section be moving around? I am not neccesarily looking for a remote fault diagnosis: hopefully if I can understand better how transmatic works I will be able to see what is wrong.
Thanks, Angus

    Bookmark   August 12, 2011 at 7:22AM
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The ratio change is affected by the centre section of the dual pulley.
The centre section must be free to to move up & down when the machine operator moves the ground speed control to speed up, or to slow down the machine's rate of travel across the terrain.
The movement of the centre section of the dual pulley assembly alters the "apparent diameter" of each vee groove.
The 2 belts are: Drive belt..........carries power from engine pulley to input groove of dual pulley.
Driven belt........carries power from output groove of dual pulley to the gearbox input input pulley.
Which way the operator moves the ground speed control will determine whether the dual pulley drive belt groove becomes smaller diameter, or larger diameter. When the drive belt groove "apparent diameter" becomes smaller.......the driven belt groove diameter becomes larger (and vice versa).
When the ground speed control moves the centre section of the dual pulley to make the drive belt groove a small diameter, the ground speed of the machine will be slow, but the torque output to the axle shafts of the gearbox will be great.
If the speed control moves the centre section of the dual pulley to make the drive belt groove a larger diameter, the ground speed of the machine will be great, but the torque output to the gearbox axle shafts will be low.
By design, the 2 belts are a "matched set" to the specific application and when renewing belts, both belts should be replaced as a set.
The centre section of dual pulley literally "forces" the belts to assume the changing diameter, so every bit of wear to the vee grooves, and to the sides of the belt vee will have an effect on the ground speed and torque output parameters of the machine.

    Bookmark   August 12, 2011 at 11:40AM
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OK, I know little about CVT transmissions, although I've driven a few snowmobiles. The snowmobiles and autos that use CVT do not have gears, except for F/R in the case of the auto. Both types of vehicles operate in a much greater range of power and speed than a typical lawn tractor. Therefore, why does a lawn tractor need more than one gear (other than F/R) in a CVT?

I can't imagine it is a power issue, since snowmobiles have a great range of speed with not much more power. Since lawn tractors are pretty much limited to a top speed around 5-6mph, that should free up the CVT designer to give it a lot more torque to the axle for the given engine HP. Any CVT experts care to respond?

I, too, mow in hilly areas, and it frustrates me that I have to spend twice as much for a lawn tractor (over the base models) just to get a "reliable-enough" hydro tranny that can handle the hills. You'd think there'd be a better mousetrap out there...

    Bookmark   August 15, 2011 at 10:40AM
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earthworm(6 Pennsylvania)

So, I'm thinking that the bottom line is this..
The hydrostatic is great, but the good ones must have a way of changing the hydraulic fluid and the filter....And it
will be less efficient that the CVT..
The greater efficiency will come at a prive...every 2-3 years $100 to $200 for two new belts.
Here, the DIY is worth $100.
So, when buying, ask this
1 hydrostatic, is the oil and filter changeable ?
2 CVT, the the belts available right now ?And are they changeable and not a federal case)...
And the responses were great and very informative.

    Bookmark   August 16, 2011 at 1:22PM
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It should be noted that millions and millions of hydrostatic L&G riders are sold that are trouble free. The misconception that hydrostatics in entry level riders often fail is based on a very small (representative) number of posters complaining. People who are out mowing on their riders seldom post that they are working well.

My experience is that hydros in entry level riders fail most often because they are intentional or unknowingly abused by the operators by asking them to mow too large an area, too hilly an area, pull more weight then they are designed to pull, pulling accessories that they are not designed to pull, and then there's the way oversized operator.

John Deere warrants their X300 lawn tractor for 4 years or 300 hours and it has a sealed, non-servicable Tuff Torq K46 hydrostatic transmission. If those transmissions were so unreliable think JD would warranty it that long?

CVTs look great on paper and work well in snow mobiles but the dust and dirt mowing environment is a bad environment for CVTs.

There are a great many utility and sub cut tractors around here with most owned and operated by old timers who know tractors. Talking to them over the years they were belly laughing at the though of hydrostatics on real tractors till they got to try one and now there are very few who don't own hydro sub cuts and utility tractors with the most common comment... should have done this years ago.

    Bookmark   August 16, 2011 at 5:47PM
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earthworm(6 Pennsylvania)

Excellent discussion.
I wonder, does the Craftsman "tight turn" tractor's transmission cut the power to one wheel when the steering is turned full crank ?? I suspect that the Sears salesman may not know this...even Sears may not know this....
I visit their site and the info is vague and missing...
Kerry's on the Trindle Rd, Mechanicsburg is the answer ??

    Bookmark   August 17, 2011 at 11:01AM
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