This is Stan's original post:
 Stan IL&TN wrote:
Just wondering if you run faster engine RPM than the indicated 540 PTO speed when bush hogging or do you run her faster? I ask because I was told to run her wide open for the best looking cut and it's easier on the engine not to lug it. Just wondering what others do? |
All comments regarding combines, silage blowers, lawn mowers, recutters, bailers... they're all irrelevant to Stan's posting. His was about a 540rpm PTO-driven rotary mower. As was posted earlier, comparing engine speeds of for-purpose engine-driven machinery (combines, lawn-mowers, etc) is irrelevant to the 540rpm implement. The answer to the question of WHY is found in history, and engineering design and economics.
References to 'wide open' are improper terminology. Throttles and injection pumps have 'wide open' positions of 'full fueling'. These are GOVERNED engines, which by nature, are intended not only to limit engine speed, but hold it constant at a given speed. Governors modulate throttle position to maintain governed speed. A goverened engine will run at the desired speed with practically no throttle, when there is no load. As load is applied, engine speed will drop off slightly, and the governor will BEGIN to increase throttle opening (technically referred by governor and engine-management engineers as 'sag')... the engine will only see 'full throttle' from a governor when the load is substantial enough to require all that the engine is capable of providing. Once full throttle is reached, any additional load applied will slow the motor down BELOW the goverened speed, which is usually referred to by engineers as 'overload fall-off'.
Running an engines of for-purpose machines is irrelevant, because the machine is designed only for that purpose, and the engine governance, and all driven loads, are appropriately ratioed to their optimal speeds by drive mechanisms therin. Engineers determine proper speed of all those pieces, assemble the drivetrain ratios, and then calculate the proper final drive ratio by comparing the prime mover's fuel and power curves, calculate performance lifespan and durability, and then apply the appropriate drive ratio, and set (now program) the engine speed governor accordingly.
540rpm PTO output systems on tractors have been well-standardized since the late 1950's. Prior to that time, manufacturers made implements intended to be driven by their OWN tractors. They Generally worked within the realm of ASAE standards of the mid '30's, wherin the common 540RPM PTO shaft operated at 540rpm +/- 10rpm, in a clockwise direction (when viewed at the output of the driving machine). Through the late '50's, the expiration of Harry Ferguson's 3-point patents (there were many) gave the ASAE the ability to publish a 'standard' of hitching geometry, and they 'categorized' hitching sizes to tractor size. At the same time, they published the PTO standards.
Once that occured, agricultural manufacturers both on the tractor end, and in the implement business, went to manufacturing implements to allow compatibility with other manufacturers.
Prior to the 60's, belt-driven implements were very common. Belt-driven implements fell in their own neighborhood based on belt
speed (in feet-per-minute), and the operator had to calculate a
reasonable speed based on knowledge of the prior drive machine
(oftentimes a steamer). It isn't unusual, particularly after the late '30's for full-goverened speed to be bumped up... manufacturers did this as part of the 'horsepower wars' to improve package performance. When doing so, the 540rpm PTO's final gearing either had to be altered (to accomodate the resulting higher speed), or the engine had to be slowed-down, and the PTO output tested (think Nebraska Tests) at less than full-goverened speed. Manufacturers knew this would be a loss of performance-point (if the engine didn't create higher torque without the governed speed increase), however, it was a small price to pay vs. cost of tooling for lower PTO gear ratio. Manufacturers also had to contend with belt hp performance, but towards the middle '60's. the importance of belt HP to the agricultural market fell off- most belt-driven implements were designed around lower-power tractors, hence, high belt-hp wasn't an incredible selling point. That being the case, manufacturers didn't invest the engineering or production resources to alter belt-drive ratios, instead, they just left it as-was, and entrusted the operator to use due caution. As time went on, and power demands increased, and PTO implements got big and mean, full-governed-speeds increased, manufacturers finally started revisiting rearend designs, PTO drive concepts (Cockshutt's live-PTO!), and PTO drive ratios to optimize PTO output power, especially on the Category II and III machines. Agricultural engineers were constantly working to provide the highest PTO output numbers at the lowest fuel consumption and longest lifespan, it was not possible to make a drive ratio that yielded the desired output combination. After the '60's, ROAD SPEED was also a consideration, and tractors were made with gearing and maximum governed speed to provide faster transit times between fields.
Amidst all this, was the implement manufacturer, who used the ASAE and ASME standards guidelines for selecting coupling equipment and calculating load applicability. Once the design was done, proper implement manufacturers documented their products so that the operator would follow guidelines on operation. Allis-Chalmers, for example, never made ANY implement, that didn't get passed to end user, without also being handed a book.
This is why, when you look at a tachometer, that SOME have the 540rpm 'window' in the same vicinity as full governed speed... while others, it's not.
How a person goes about using their equipment, is totally up to them, and totally determines how the driving force, and driven implement survive, and all the casual and catastrophic results that may occur. The performance suggestions and safety mandates of the engineers of the machine-in-use should be abided by if you desire to get best performance, lifespan, and safety from the combination. There is nothing wrong with telling another guy what YOU do, however, it is neither a wise nor honorable practice to do so without indicating that you're operating outside of the manufacturer's suggested realm, and it is an extreme disservice and liability to recomend to another a practice which occurs outside the envelope of the original design guidelines. It is one thing to have equipment damage or injury due to one's own actions, it is totally another to suffer damage, injury, or death as a result of ill advise. The damage to equipment and people can frequently be repaired, but the dead cannot be brought back to life. The loss of friendship is irreplaceable.
I have nobody to blame for my injuries except myself, and I plan on keeping it that way. I would rather be the one injured, than to advise someone into peril.
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