[AT] OT: hydrostat mowers( Hydraulic drive)

[Bill Bruer]

Thanks to all who have replied & participated.  I don't know the 
technicalities or differences in the systems.  I can't readily observe the 
unit in the Cub Cadet, but today I was able to look at the IZT's in my 
zero-turn mower.  They have cooling fans on top of the pulleys for the belt 
drive so the fan speed is in direct relation to the engine speed.  Also, 
they are specifically designed for mowers so I am assuming they work most 
efficiently at high engine & pump speed.  Other than the heat factor, 
though, I'm still not sure which way of controlling ground speed is least 
likely to cause damage when pulling an implement.

The IZT's are Hydro-Gear model 310-2600 units.  The General Description in 
the service manual says: "The 310-2600 is a self contained unit designed for 
the transfer and control of power.  It provides an infinitely variable speed 
range between zero and maximum in both forward and reverse modes of 
operation.  This transaxle uses a variable displacement pump with a maximum 
displacement of 10cc per revolution, and motor with a fixed displacement of 
21cc per revolution. The variable displacement pump features a cradle 
mounted swashplate with a direct-proportional displacement control. 
Reversing the direction of the swashplate reverses the flow of oil from the 
pump and thus reverses the direction of the motor output rotation."

Thanks again,
Bill


----- Original Message ----- 
From: "Steve W." <falcon at telenet.net>
To: "Antique tractor email discussion group" <at at lists.antique-tractor.com>
Sent: Friday, June 18, 2010 10:27 PM
Subject: Re: [AT] OT: hydrostat mowers( Hydraulic drive)


Gene Dotson wrote:
>     Mowers do not use hydrostatic drive systems. They are hydraulic 
> drives.
> Big difference in their design. Hydraulic drives use a constant 
> displacement
> pump that always delivers the same volume of oil each revolution. The 
> motor
> is also constant displacement and requires a set amount of oil during each
> revolution. Speed is controlled by the hydraulic drive system by 
> controlling
> the amount of oil bypassed. More oil bypassed, the slower the speed and 
> more
> heat from the bypassed oil, just like a stuck relief valve. in a hydraulic
> system. You will get maximum torque with the high speed setting with no
> bypassed oil.
>
>     True hydrostatig like used in combines and large tractors use a 
> variable
> output  pump and a variable displacement motor. They have maximum torque 
> at
> the low speed setting as the pump output is reduced by shortening the 
> stroke
> and the pump displacement per revolution is increased by increasing the
> length of piston stroke.
>
>     There are certainly other systems that work on the same princilal.
>
>     Link below may shed some light on the systems.
>
> http://www.tpub.com/content/construction/14273/css/14273_221.htm
>
>             Gene

Gene the Cub Cadets from the first hydro up to the current models are
true hydrostatic drive. They use a swash plate which not only controls
speed but direction by changing the pump displacement as the operator
moves the control lever. The motor is a unique unit that uses pistons
against a fixed swash plate to power the tractor.

The older Case garden tractors on the other hand do use hydraulic drive
with a bypass controlling the system.

-- 
Steve W.
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