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techdirections September 2013 : Page 22

Make Simple Planetary Gear Sets Truly Simple By Timothy W. Dell and Robert S. Norman tdell@pittstate.edu; rnorman@pittstate.edu P LANETARY gear sets are used in almost all fields of engineering, such as automotive, aerospace, agricultural, and construc-tion. Examples of planetary gear set applications include automotive automatic transmissions, agricultural power take off (PTO) drives, con-struction equipment torque dividers, and turbo-prop gear boxes. Instruc-tors themselves sometimes struggle to learn how they work—so, need-less to say, they find it challenging to effectively teach their students how the gear sets work. After visiting with other educa-tors and students, it’s apparent to us that sometimes teachers teach and students learn planetary operation by simply memorizing a matrix. (See Table 1.) However, when students simply memorize a matrix, they typically will not retain the informa-tion over the long term. Using the approach described in this article will greatly improve student learning regarding planetary gear sets. Example of a simple planetary gear set Simple planetary gear set with ring gear removed Table 1—Simple Planetary Gear Set Operation Held Slow Reverse Fast Reverse Slow OD Fast OD Slow reduction Fast reduction Direct drive Neutral P/C P/C Sun Ring Ring Sun Input Sun Ring P/C P/C Sun Ring Two inputs One input Output Ring Sun Ring Sun P/C P/C Start with the Basics A simple planetary gear set has the following components: a sun gear, surrounded by a planetary carrier that contains planetary pinions that orbit around the sun, and a ring gear. (See Fig. 1.) The ring gear is sometimes called the internal gear or annulus gear . The ring gear surrounds the planetary pinions. For a planetary gear set to be classified as a “simple” planetary gear set, it must have only one ring, one sun, and one planetary carrier. Compound planetary gear sets are, of course, Timothy W. Dell and Robert S. Norman are associ-ate professors, Automotive Technology Department, Pittsburg (KS) State University. 22 tech directions X SEPTEMBER 2013

Make Simple Planetary Gear Sets Truly Simple

Timothy W. Dell & Robert S. Norman


PLANETARY gear sets are used in almost all fields of engineering, such as automotive, aerospace, agricultural, and construction. Examples of planetary gear set applications include automotive automatic transmissions, agricultural power take off (PTO) drives, construction equipment torque dividers, and turbo-prop gear boxes. Instructors themselves sometimes struggle to learn how they work—so, needless to say, they find it challenging to effectively teach their students how the gear sets work.

After visiting with other educators and students, it’s apparent to us that sometimes teachers teach and students learn planetary operation by simply memorizing a matrix. (See Table 1.) However, when students simply memorize a matrix, they typically will not retain the information over the long term. Using the approach described in this article will greatly improve student learning regarding planetary gear sets.

Start with the Basics
A simple planetary gear set has the following components: a sun gear, surrounded by a planetary carrier that contains planetary pinions that orbit around the sun, and a ring gear. (See Fig. 1.) The ring gear is sometimes called the internal gear or annulus gear. The ring gear surrounds the planetary pinions. For a planetary gear set to be classified as a “simple” planetary gear set, it must have only one ring, one sun, and one planetary carrier.

Compound planetary gear sets are, of course, also frequently used in engineering fields. A compound planetary gear set consists of at least two simple planetary gear sets that work together and also share a common component. In the case of a Simpson compound planetary gear set, the sun gear is shared. The shared sun gear is what compounds together the two simple planetary gear sets.

In a Ravigneaux compound gear set, the two planetary carriers are compounded together to form a common carrier with two sets of pinions. An 8-speed transmission may have four simple planetary gear sets compounded together to allow power flow from one simple gear set to the next.

One simple planetary gear set can be designed to transmit eight different power flows:
-slow reverse,
-fast reverse,
-slow speed reduction,
-fast speed reduction,
-slow overdrive,
-fast overdrive,
-direct drive, and
-neutral.

Each of the three planetary members (the sun, planetary carrier, or ring) can be designed as the (1) input member, (2) holding member, or (3) output member (the output shaft).

Sometimes a simple planetary gear set is designed to be used for just one purpose. An example is the inboard planetary found on the rear axle of an agricultural tractor (Photo 1). In this application, the planetary gear set serves as a final drive assembly that provides a slow speed reduction that also multiplies the torque coming from the differential side gear. In this configuration, the gear set is designed for only one power flow and always provides the same gear reduction ratio.

For the simple planetary gear set to transmit power, the gear set must incorporate the use of planetary controls, which commonly consist of a band, one-way clutch, and multiple disc clutch or multiple disc brake. The planetary controls serve the purpose of holding a planetary gear, coupling two planetary gears together, or driving a planetary gear (input).

Figure 2 shows a multiple disc clutch that is used to transmit a slow-speed reduction power flow. Whenever hydraulic fluid pressure is applied to the piston, it compresses the clutch pack that connects the input shaft to the sun gear. The ring gear is fixed to the housing, and as a result the planetary carrier delivers power to the output shaft and multiplies torque through the planetary gear set.

Students often struggle with learning what it takes to produce each of the eight different power flows that are possible within one simple planetary gear set.

What Is the Planetary Carrier Doing?
The first step for teaching students simple planetary gear set power flows is to have them learn what the planetary carrier is doing in (1) reverse, (2) overdrive, and (3) speed reduction. We consider this step the most important.

Reverse: planetary carrier = held Speed reduction (torque multiplication): planetary carrier = output
Overdrive (speed increase): planetary carrier = input

To help students remember this process, you’ll need to have a working simple planetary gear set in the classroom that you can use to demonstrate the power flows. Our faculty have a variety of mock-up gear set trainers, some of which can be projected onto a large overhead screen. Some instructors even use software to demonstrate the eight different power flows. One of our trainers was built by removing a simple planetary gear set from a traditional four-speed overdrive automatic transmission, then mounting it in a fixture to enable the instructor to hold the sun gear, the planetary carrier, or the ring gear. (See Photo 2.)

Big Gear vs. Small Gear
The second important step in teaching planetary gear sets is to reinforce what it takes to increase speed and decrease speed in terms of the size of the input gear and output gear. Most students already have a grasp on this concept or can quickly relearn this important point.

Small input and large output = speed reduction and torque multiplication

You can use whatever analogy works for you, such as a 10-speed bicycle or a ring and pinion gear that are used in the final drive of a conventional automotive differential. Students must learn that any time a small-sized input gear is used with a large output gear, the power flow will result in a reduced output speed and an increase in torque multiplication. (See Fig. 3.)

Large input and small output = speed increase and reduction in torque

Equally important is for students to learn that any time a gear set— whether it be a spur gear, helical gear, bevel gear, or a planetary gear set—has a large input gear and a small output gear, it will result in an increase in speed and a reduction in torque. (See Fig. 4.)

Planetary Gear Size Relationships
The last important thing for your students to learn is the relationship of gear sizes for each of the three planetary gears:

Sun gear = smallest gear
Planetary carrier = middle sized gear
Ring gear = largest gear

Note that the individual planetary pinions (located inside of the planetary carrier) are not mentioned here and are not important for understanding simple planetary power flows. If you want, you can explain to your students that more planetary pinions for a given tooth width and gear tooth radius will provide more torque-handling capacity.

Let the Learning Begin!
After illustrating the three laws listed above, you can now act as a facilitator and let the students explain to you how to configure a simple planetary gear set to achieve:

-slow reverse,
-fast reverse,
-slow reduction,
-fast reduction,
-slow overdrive, and
-fast overdrive.

We recommend writing two laws on the board:
1. What the carrier is doing in:
a. Reverse,
b. Speed reduction, and
c. Overdrive.
2. Little gear and big gear:
a. Small input and large output = speed reduction (slow)
b. Large input and small output = speed increase (fast)

Reverse
Reverse is often the most easily understood by students and should be taught first. Provide students with the chart in Fig. 5 and have them fill in the information in the sequence specified below. Figure 6 shows the filled-in chart.

This is a good time to demonstrate both slow and fast reverse power flows to your students with your planetary trainer.

Speed Reduction
When progressing to the next power flow, you’ll most likely need to slow your presentation. This is where students tend to get lost. The first question is to ask is, “Any time the gear set is in a speed reduction, what is the planetary carrier doing?” You hope they will remember that the planetary carrier is the output member. You can use the example of the agricultural tractor axle to help reinforce that the output shaft is physically coupled to the planetary carrier and show students Photo 1.

Once students can comprehend that the planetary carrier is the output (anytime the gear set is in a speed reduction), then slow your presentation and tell the students that now is the time that they need to be able to generate the right question. It is important to emphasize that they already know what gear is being used as the output, which is the planetary carrier.

Consequently, when attempting to distinguish between slow or fast reduction, the key question the students must ask is: “What size of input is required for slow and fast?” If they do not get this part, they will struggle if you attempt to move forward.

Have students then answer the input questions for speed reduction. (See Fig. 7.) And finally, have the students fill in the remaining answers for what is held. (See Fig. 8.)

Overdrive
Follow the same steps and sequence for overdrive. Ask the students to explain what the planetary carrier is doing in “overdrive,” which is the input. (Provide Fig. 9.)

The next step is to determine difference between slow and fast overdrive. Ask your students to specify the next important question. Since they already know that the planetary carrier is the input, they must ask themselves “What is the output for slow and fast overdrive?” (See Fig. 10.)

Then, students can fill in the remaining answers for what is held.

Neutral
The planetary gear set will not produce any output power so as long as there is:

1. No input, or
2. Just one input but nothing is coupled or held.

Direct Drive
Some of your students may need a refresher on what the concept “direct drive” actually means, such as the gear set will transmit the same output torque and the same output speed that was delivered by the input shaft. There are two methods for achieving direct drive in a simple planetary gear set:

1. Deliver two inputs at the same speed and in the same direction, or
2. Deliver one input and couple the two remaining planetary gears together.

Either of those methods will result in the gear set assembly locking together as one complete unit and revolving as a complete assembly. (Note that in a true direct drive power flow, the individual gears are not spinning internally. This tip is used in troubleshooting automatic transmission noises. If a noise is eliminated in direct drive, then the gear set might have a gear with a chipped planetary tooth, or a bad planetary pinion bearing or bushing.)

Keep in mind that instructors sometimes loosely state that two inputs automatically equals direct drive. Technically that is not the case. The two inputs must be driven at the same speed and in the same direction—if the planetary gear set is receiving two inputs that are not at the same speed or are not in the same direction, then the gear set will result in one of six possible configurations which nets a change of speed and or a change of direction:

-slow reverse,
-fast reverse,
-slow reduction,
-fast reduction,
-slow overdrive, or
-fast overdrive.

The other method for achieving one of the six configurations listed above (a change of speed or a change of direction) is to simply have one input (driving member) and one holding member.

Timothy W. Dell and Robert S. Norman are associate professors, Automotive Technology Department, Pittsburg (KS) State University.

Read the full article at http://www.omagdigital.com/article/Make+Simple+Planetary+Gear+Sets+Truly+Simple/1480475/171541/article.html.

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