Shaft speed and torque must go hand-in-hand to create horsepower

Sunday, June 8, 2014

Methods for calculating power vary but transmissions, PTOs and engines are all evolving towards efficiency

by RALPH WINFIELD

Contrary to popular belief, a horsepower is still a horsepower and always will be, even when metric units are used.

When mechanization started to take over and replace the horse, it was necessary to create a replacement value. Thus the value of 33,000 lb-ft per minute was established as the amount of power that a good horse could produce. Note that "power" must always have a time (e.g, minutes) associated with it. Work does not.

If you want horsepower in metric, simply multiply the horsepower value by 0.7457 to give you kiloWatts (kW).

The most common formula for rotating horsepower is HP=2ϖ NT/33,000. Or it can also be reduced to HP=NT/5255. In both cases, N is the shaft speed in revolutions per minute (rpm) – the time element – and T is torque in lb-ft.

When you have your tractor tested on a shop dynamometer to check the horsepower, the value quoted will in fact be a function of the PTO shaft speed and the torque created at the standard pto speed of 540 or 1,000 rpm.

PTO speeds. I trust that you can see why the older standard PTO speed of 540 had to be increased to 1,000 rpm when we started to acquire tractors approaching or exceeding 100 horsepower. As torque increased at the higher horsepower, the pto shaft and bearings and gearboxes would have had to be upsized significantly if 540 rpm had been maintained as the only standard speed.

Do you now know why John Deere had to leave the old reliable two-cylinder concept behind in 1960, when it wanted to compete with higher horsepower engines and tractors? Their model 830 tractor produced over 80 horsepower and had a very heavy crankshaft, as well as other driveline components. Their newer model 4010 tractor, which developed the same horsepower, weighed 1,000 pounds (454 kilograms) less.

New concepts. For certain applications, PTO speed is critical as the equipment is designed to perform at the standard PTO speed of 540 or 1,000 rpm. A few examples: balers, fertilizer spreaders and standby generators.

For field operations, most tractor manufacturers developed some form of shift-on-the-go capability to allow standard PTO shaft speed to be maintained while varying forward speed in order to optimize equipment performance.

Some tractor manufacturers have also incorporated an "economy PTO" feature. This permits the standard PTO speed to be maintained at a "restricted" engine speed to improve fuel efficiency for lighter loads. Two applications come to mind here.

The first application involves the PTO-driven standby generator. PTO speed is very critical to maintain the correct generator speed, usually 1,800 rpm, in order to maintain the correct frequency of 60 cycles per second (Hz).

The second application is in spreading fertilizer. If the correct PTO speed is not maintained, the spread pattern will be adversely affected. This will affect yield. A yield monitor on the combine can and will show yield variation in wheat fields if nitrogen fertilizer is not spread uniformly.

An interesting concept. In a recent article, I discussed variable speed transmissions to allow for a range of drawbar load variations while maintaining optimum engine performance and fuel economy.

While doing some research online, I came upon a manufacturer who was promoting a variable speed transmission to connect an engine to an electrical generator. This transmission was able to maintain a constant output shaft speed of 1,800 rpm to the generator while allowing the engine speed to even go below that speed when electrical demand decreased in order to improve engine efficiency, i.e, fuel usage.

We have seen many interesting technical developments in recent years, primarily to reduce energy consumption as well as environmental pollution. Unfortunately, my crystal ball is cloudy, but I am willing to bet that we will see additional "engineering achievements" come to fruition as soon as the economic push justifies implementation.

An addendum. I realize that some Nebraska tractor test results that I talked about in an earlier article may be somewhat confusing as they are now given on the Internet sites in both imperial and metric equivalents. In an attempt to clear the confusion, I trust that I will not add to it. Here goes.

The same "Nebraska Tractor Tests" are now being carried out in Britain and other locations in Europe, primarily for the benefit of manufacturers building tractors in those areas for sale in Europe and North America. Some examples are: McCormick, Fendt and Massey-Ferguson.

As a result, all Nebraska tractor test results are now given in imperial as well as metric units. For example, the efficiency in "Horsepower Hours Per U.S. Gallon of Fuel" is also given as: kWh/L as well as kg/kWh.

Does this sound confusing? It can be, but since both the metric and imperial are given, you can use your units of choice. Because the values are all intended to compare one tractor with others, all the numbers are relative. In most instances, the higher the value, the more efficient the tractor.

Please remember that, when I talk about imperial units, the gallons are those smaller U.S. ones. U.S. customary units, as they are sometimes called. They are entirely different from the British imperial system, which Canadians follow. Therefore, six U.S. gallons equal five imperial gallons. Please do not use fluid ounces for comparison. The U.S. fluid ounce is larger than the imperial one!

Let us not forget that drawbar pounds pull is given in metric as kiloNewtons (kN).

Here's an example. A larger 4-WD tractor being tested for drawbar performance might provide the following set of figures for maximum power. (See Figure 1).

Another European influence. As I determined many times when visiting Europe and writing operator/service manuals, hydraulic and air pressures are not given as kilopascals, which is what we use as metric values in Canada. They use "bars" which is much more realistic. One bar is the standard barometric pressure, which is the equivalent of 14.5 lb/in2 (psi) in imperial.

If you stop to check car tire pressure in Europe and are pondering the pressure scale, the person in line behind you will soon tell you to set the meter at two bars, which is 29 psi. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.