The Right Ratios

March 14, 2009 02:50 PM

Sometimes we just have to accept that somebody else is watching out for us. Thank goodness the tractor engineers are doing just that because we can be guilty of our own demise when it comes to the way we treat the equipment we operate.

Modern-day tractors feature smart transmissions that use computer modules to control almost everything. When the tractor operator chooses a job that puts considerable strain on the transmission, the computer takes over and makes a decision beyond the operator's control.

What do I mean by considerable strain? The amount of pull is limited by traction, which is directly related to weight. When the load is high and the gear selection is low, the load
becomes an anchor (essentially increasing the weight) and the available horsepower overstresses the transmission before the traction breaks loose. The result is excessive wear and tear on the transmission.

To preserve transmission life, tractor manufacturers have decided to offer equipment that meets 95% of agricultural needs. When the task is beyond the designed limitations, the computer derates the tractor horsepower to avoid any catastrophic failures.

Rev it up. Each manufacturer derates a little differently, which may make a difference in your tractor selection.

Bill Manke and Tony McClelland, Case IH tractor product specialists, say the company establishes parameters including the tractor model, the gear selected, the configuration of the transmission/engine and engine revolutions per minute (rpm). The transmission controller then derates (to as much as 70% of maximum torque on some models) in the lower gears on the four-wheel-drive STX models.

The operator can still maintain maximum horsepower as long as rpm is kept high. Added protection to the drivetrain automatically puts the tractor in "kick to neutral" mode when rpm drops below 1,000.

Jerry Griffith, John Deere product line marketing manager for four-wheel-drives, says the company handles derate on higher horsepower models with a Power Management system, which limits the amount of available horsepower in the lower gear selections, based on the available traction and proper ballast at those speeds. The lower the gear (typically below the 6th gear), the greater the percentage of derate.

AGCO tractors that include derating use more than just gear selection. Using a machine software protec-tion strategy, the control modules read the situ-ation and limit horsepower to match normally available traction until the load exceeds acceptable levels, at which point derating takes affect. AGCO does allow for a momentary load surge before full derate occurs.
The Versatile four-wheel-drive tractors, by Buhler Industries, do not use derating to limit horsepower as torque increases. They do limit warranty coverage to agricultural operations.

Ask your dealer.
If you are buying a tractor for primary use outside of normal agricultural operations, you should have a discussion with your dealer about model selection. Some manufacturers offer a "scraper tractor" option that will cost you more but is built for the rugged use your operation will require.

"It is in the customer's best interest to use derating so the tractor isn't overbuilt and carrying a higher purchase cost," explains Jason Hoult, AGCO product marketing manager. "The customer can know that what he is buying in a used model has not been mistreated, and the hours are a good measure of the life of the machine, regardless of operating conditions."

If you thought the tractor you purchased uses all the horsepower all the time, know that someone is making sure your buy will still be a good buy in the used market down the road.

Doc Cottingham farms full-time near Pine Village, Ind., and writes this column for Farm Journal. To offer feedback or pose questions, e-mail him at  

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