What you need to know about planetary gear sets

Thursday, May 1, 2014

Planetary gear sets are not new – Henry Ford used them in the Model T – but the technology has greatly improved and they are now found in a wide range of machinery, cars and trucks. Here's a basic primer

by RALPH WINFIELD


My guess is that you have at least two – and probably as many as 20 – planetary gear sets on or in your tractors, swathers, combines, cars and trucks.

They are very versatile and permit power to be transmitted at higher shaft speeds until that power reaches its final point of use. Almost every farm has at least one tractor with mechanical front wheel drive (MFWD).

If you look at the front wheel hub, you will see a relatively large, round gear case on the outside of the wheel. That gear case will have a planetary gear set in it. I'll also bet that you will see a recessed plug with arrows showing you how to check the gear oil level or change it.

Many planetary gear sets are operated individually at the end of an axle. If they are, it is absolutely essential that you change the gear oil regularly, as suggested by the equipment manufacturer.

When used at the end of an axle to reduce shaft speed and increase the torque available to the driven wheel, there is no provision to filter the relatively small quantity of gear oil that is so critical to the performance and life of the gear set.

Especially during cold weather operation, the volume of oil and air will be reduced. Moist and often dirty air will be drawn in around the shaft seals to eliminate the vacuum that was created. When reheated by mechanical heat, air will escape and the process will be repeated on the next cold startup. This scenario would apply especially on the front wheels of a loader tractor that is used regularly to move manure or snow.

If that condensed moisture remains in the gear case, the gear oil will turn a milky colour and lose its ability to lubricate the gears. The end result will be an abrasion-damaged gear set that has to be replaced. Trust me, those gear sets are not cheap. The gear oil can be changed many times for the price of changing a gear set!

As noted, the oil level should be checked at least once a year, and probably be changed if there is any sign of metal cuttings in the gear oil. As you can appreciate, it is just not practical to filter the gear oil in an individual planetary gear set at the end of an axle.

Please do not misunderstand the choice or performance of planetary gear sets. They can and do perform very well in many installations.

If you are not familiar with them, you may recall the "stick steering systems" on swathers and bulldozers or, if you are old enough, the International Harvester No. 91 combines. All of those units achieved direction reversal by activating one of the two planetary gear sets that drive each wheel or track.

Be sure not to confuse those mechanical drives with the newer hydrostatic drive systems used on the large four-wheel-drive, high-clearance sprayers that many of us have seen in recent years.

Planetary gear sets have proven their worth in many simple applications, such as the ones I have already mentioned. However, as we demand higher technology, compound planetary gear sets are becoming more common when we move to Continuously Variable Transmissions (CVT) in tractors and combines, as well as in many new cars and truck transmissions.

In the past, we tried to use hydrostatic transmissions in tractors and combines. They worked well in giving us continuously variable travel speeds, as well as quick and easy direction reversal. Unfortunately, they were not very energy efficient when we had a high pull (drawbar) load. Slippage and oil heating were the indicators of power/energy losses.

Hydrostatic transmissions are still acceptable for combines. Most of the power and energy is directed mechanically to the threshing and cleaning functions. When machine loading is kept at maximum capacity, the driveline losses are relatively small and thus acceptable.

Planetary gear sets. These come in many shapes and sizes, but in all cases they have three basic components. There is a sun gear in the centre with gear teeth on the outside surface only. Next come the planetary gears, usually three, connected together by a planetary carrier that keeps them spaced equally at 120-degree spacing around the sun gear. On the outside will be a ring gear that has teeth on the inside surface only so that it can engage with the teeth on the planetary gears.

The relative sizes of these three components can vary to permit a specified gear (speed) reduction.The tooth pitch of all the gears must be the same so that they can mesh with each other.

Now for the clincher. We can also find compound planetary gear sets. What this term means is that two planetary gear sets are connected together on a common planetary gear carrier. However, the size of the planetaries can be larger, if and only if the sun gear or the associated ring gear is larger or smaller. Does this sound confusing? It is, but this is how the designer accomplishes different overall gear ratios.

With planetary gear sets, one component is the input, a second is the output, while the third component is held (prevented from turning.) By changing the input, output and locked component, we can get a wide range of output shaft speeds at constant speed input. However, in tractors and cars this can also be used to keep the input (engine) speed down to increase engine efficiency!

Does this sound confusing? It is. If you really want to try to understand how complex planetary systems work, you have to go on the Internet and see a number of videos that have been prepared to show the varying speed versus load characteristics.

Back to the basic system. If you look at the wheel hub shown in the accompanying photograph, you will see first the drain plug and fluid level marking. As indicated, you open the plug to check the fluid level when the oil level line is horizontal. By rotating the wheel a quarter turn, the fluid can be drained and then the wheel turned back to refill the gear set compartment with the specified lubricant.

Constant Variable Transmissions (CVTs). Tractor manufacturers use a variety of simple and compound planetary gear sets to provide CVTs. One even incorporates a closed centre hydraulic system to provide the control mechanism to vary the mechanical speed ratios. This unique Fendt Vario system is also demonstrated on an Internet video.

To summarize, the concept of planetary gear systems is not new. Henry Ford used them in those "three-foot pedal" transmissions in all of the Model T Fords. What has happened in the last 20 years is that old technology has been greatly improved with the inclusion of very sophisticated electronic control systems.

Some of us predicted in the 1960s that computers would never be practical in the field, home or office when we had to program the massive IBM computer that took up an entire room at the University of Toronto. It had to be fed punched cards to tell it how to calculate on a step-by-step basis using the Fortran language. We now have more computing power on our desktop computers!

I also remember when my first combine-mounted GPS had to see a ground-based beacon in the United States to accurately establish my combine's position in order to draw yield maps. Now we have auto-steering on combines, sprayers and tractors following GPS signals! BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.