The Birth of the Hydrostatic Transmission

IHC engineers had been laboring to create a dependable hydraulic hydrostatic (hydrostat) transmission since the mid-1950s. And for a short time, they even tried out a General Motors heavy-duty, 4-speed Hydramatic transmission that combined the hydraulic operation of a planetary gearbox (allowing much of the shifting to be automated) with a fluid coupling instead of a friction clutch. But those experiments proved fruitless.

IHC’s devotion to the concept began when it was employed in the space-age experimental model HT-340, a gas-turbine powered tractor with a hydrostatic drive. While that tractor dazzled crowds at farm shows, it never went into production.

The Answer Was a Piston-Type Pump

However, within a short time, IHC introduced a hydrostatic transmission that employed a piston-type hydraulic pump (which was driven by the tractor’s engine) to pressurize and pump oil to a piston-type hydraulic motor coupled to a 2-speed transmission. That gearbox provided for two operating speed ranges of zero to 8 mph (low range) and zero to 20 mph (high range).

The transmission was operated using two levers. located on either side of the tractor’s dashboard. A lever on the left side of the dash regulated the amount of oil being pumped which, in turn, determined tractor speed. Moving that lever forward (from its locked neutral position) increased forward speed, while pulling the lever back increased rear motion.

The lever on the right side of the dash was used to change speed ranges.

Foot Clutch Addition Made Farmers Feel at Home

What made the hydrostat appealing to farmers was its unique foot clutch. The IHC hydrostat didn’t need a clutch, as its hydraulic pump was fastened directly to the flywheel. A foot pedal clutch was added to allow a farmer to stop the tractor at any time. Engaging the clutch diverted oil from the pump side of the transmission, stopping the tractor. Releasing the clutch gradually pressurized the pump, mimicking the operation of a mechanical clutch, which farmers were familiar with using.

IHC was so convinced in their design, they introduced a payloader with a hydrostat, marking it as the first use of this technology in that industry. Starting in 1965, their innovation was used to propel IHC model 403 and 503 combines, marking another industry first. Soon, all harvesters in agriculture ran exclusively with hydrostats.

But the hydrostat did not see use in tractors until the Model 656. The transmission technology in this case was “right sized” for the power range of tractors that handled lighter tillage chores (tillage generates high draft loads on tractors) than larger models.

The hydrostat’s huge advantage was seen in lighter-duty jobs, such as loader work or baling hay. The transmission excelled at readily changing directions for loader work and allowed farmers to match the forward speed of a tractor with the operation of PTO-powered implements such as a baler.