Bp-fullguide - Planetary Gears

7/29/2019 Bp-fullguide - Planetary Gears

1/10

Basic Power Train - 4007 KT801096-R2Page 2-8 May 2005

Planetary Gears

Many modern automatic transmissions use

planetary gears to obtain different speeds andchange of direction. The first planetary

transmissions used simple sets of planetarygears which consist of three rotating

members, the internal gear (or ring gear), thesun gear, and the planetary pinion set,

consisting of the planet pinions and planetpinion carrier, or cage. In a simple (single) set

of planetary gears, its members are not

connected to members of other planetary gear

sets. Later we will study compound planetarygears, which are two or more planetary setsthat are interconnected.

Figure 2.17 A simple set of planetary gears

The planetary gear set has two main advantages. First, they are compact and take up less space than

other gear combinations with the same output. Secondly, they are stronger due to the fact that multiplegears are in mesh at all times.

The reason the system is called a planetary

gear system is that the planet gears rotate andat the same time revolve around the sun gear,

just as the planets in the solar system revolvearound the sun. In the planetary gear system

the planet gears are assembled on shafts in aplanet carrier or cage. Arrangements can be

made to put power into any of the threerotating members and, at the same time, hold

one of the other members so that the gearratio through the system can be increased or

decreased. In addition, by the properarrangement of turning and holding, the

system can reverse rotation.

In Figure 2.18, one can see that the carrier canbe held stationary by applying the clutch.When the carrier is held, the direction of

rotation reverses.

Figure 2.18 Carrier held by a clutch


2/10

KT801096-R2 Basic Power Train - 4007

May 2005 Page 2-9

Notice that the input shaft with its attached sun gear is rotating clockwise. The carrier and pinion gear

shafts are held by the clutch pack (when engaged). You learned previously in the chapter that when twoexternal tooth gears (such as the sun gear and

pinions above) are in mesh there is a changeof direction. So, the pinions drive the ring

gear in the opposite direction of sun gearrotation.

In Figure 2.19, a simple set of planetary gears

is used in the final drive system of a largetruck. This gearing is enclosed in the wheel

and the ring gear is spline-fitted to the fixedshaft to hold it stationary. The small sun gear

brings input power to the set and transfers thepower to the planet carrier. Whenever the

carrier is the output member, there is a

reduction of speed. And whenever there is a

reduction of speed, there is an increase of

torque. The wheel hub is bolted to the carrier

so the tire turns with reduced speeds.

Figure 2.19 Ring gear is permanently held

In the planetary set shown in Figure 2.20, thering gear is held stationary when the clutch is

applied.

Figure 2.20 Ring gear held by a clutch


3/10


The chart in Figure 2.1 shows the six conditions that can result in the planetary gear system from turning

or holding various members. Not all conditions shown in the chart will be used in constructionequipment power trains, but they should be studied for understanding of the action of the planetary gear

system.

Figure 2.1 - Six Actions (Results) Possible From One Single Set of Planetary Gears

The six reactions, shown in the chart in Figure 2.21, are sometimes taught as the carrier rules. If oneknows the carrier rules, they can easily see the end reaction a particular planetary set will have. Learn to

look at a planetary set and identify through which member power is being supplied to the unit. Next,look for one of the members to be held (either fixed or by clutch application). Finally, identify which

member takes the power out of the unit (output member).

The following terms are used when studying gear train actions:

Input member - The member through which power enters the planetary set.

Output member - The member through which power leaves the planetary set.

Reactionary member - The held member or when there are two inputs, the slower moving inputcauses a change in speed or torque and thus becomes reactionary.

Overdrive - A type of reaction that is the result of a larger gear with more gear teeth driving asmaller gear. Because of this the output member turns faster than the input member. Anotherway to look at this condition is that there is an increase of speed and a proportional decrease of

torque.Reduction - A type of reaction, which is the result of a small gear driving a larger gear. Because

of this, the output member turns slower than the input member. In a reduction action, there is a

decrease of speed and an increase of torque.

Direct Drive - When two members are locked together or if there are two inputs at the samespeed, the results will be that the entire planetary unit will turn as one. The input and output

speed and torque will be the same.


4/10


May 2005 Page 2-11

Neutral- When there is no reactionary or held member, power comes into the unit but will notleave it.

For practice, study each of the following three drawings to find out which conditions cause certain

actions (results).

In Figure 2.22 the sun gear is held. If the carrier is theinput, the carrier pinions are forced to walk around the

stationary sun gear. This causes the ring gear (output) toturn faster than the input. The result is an overdrivecondition. Notice also that because the external tooth

(input) drive gear is in mesh with, and driving an internal

tooth (output) ring gear, the power enters and leaves the

planetary set in the same direction.

Still looking at Figure 2.22, switch the input and output

members. Now, with the ring gear as the input and carrieras the output, the result is a reduction.

Figure 2.22 Sun gear held

As shown in Figure 2.23, if the ring gear is the held member of the planetary gear set, and if the sun gear

is the input, the pinions, which are in mesh with both, would be forced to turn opposite the sun gearrotation. The pinions must walk around the internal tooth ring gear, moving the carrier (to which theirmounting shafts are attached) in the same direction as the

sun gear. Because the input sun gear has fewer teeththan the ring gear, it will turn faster than the carrier. The

result is a reduction.

Look again at Figure 2.23 but reverse the input and

output members. Now the carrier and its pinions are theinput and the sun gear is the output. As the carrier is

turned, its pinions must walk around inside the ring gear,which has more teeth than the smaller sun gear. Because

of this difference in gear teeth, the sun gear is drivenfaster than the speed of the input carrier. This is an

overdrive condition.

Figure 2.23 Ring gear held


5/10


If no member were held, all power would spin out within the planetary. This results in a neutralcondition.

When the carrier is held and the sun gear is the inputmember, the sun gear forces the carrier mounted pinions

to rotate on their axis. The carrier pinions turn theopposite direction from the input sun gear. The pinions

transfer the power to the output ring gear in the reversedirection. Again, because the sun gear has fewer teeththan the ring gear, the output ring gear would turn slower

than the input sun gear. The result is a reduction in thereverse direction.

On the other hand, if the carrier was held but the ringgear is made to be the input, and the sun gear is the

output, the result would be a reversing overdrive. As the

ring gear is turned it forces the pinions, which are inmesh with it, to turn faster because they have fewer teeth.The fast turning pinions drive the output sun gear in the

opposite direction.

Figure 2.24 Carrier held

If a clutch were attached to the planetary in such away that it would lock any two planetary

members together, the entire planetary set wouldrotate as one. This is a direct drive.

The easiest member of a planetary gear set to

hold is the outer-most member or ring gear.Clutches or bands can simply be installed to fit

around the ring gear. Komatsu found a way to

hold the ring gear and obtain a reverse in

direction. They made a carrier having coupledpinions as shown in Figure 2.25.

Figure 2.25 Carrier with three sets of coupled pinions


6/10


May 2005 Page 2-13

In Figure 2.26 the sun gear is the input and is in mesh with the first

pinion of each coupled pinion set. Because both gears are externalthe first pinions turn the opposite direction of the sun gear. Next, the

first pinions drive the second pinion of each coupled pinion set. Thefirst and second pinions are both external tooth gears so they must

turn opposite each other. Therefore, the second pinion turns in thesame direction as the input sun gear. When the ring gear is held, the

second pinions are forced to walk around inside the internal toothring gear. This causes the output carrier to rotate in the direction

opposite the sun gear.

Figure 2.26 Carrier rotates opposite

direction of sun gear

Compound Planetary Gears

Over time, engineers discovered that they could make a smaller transmission with compound sets of

planetary gears. Instead of having one simple set for each speed range and direction, with compounding,

they can reduce the number of moving parts, cut manufacturing costs, and sometimes get more than one

reaction out of a planetary set.

Figure 2.27 illustrates a forward - reversecompound planetary set. Notice that the

forward clutch holds the rear sun gear whenapplied. Also, the reverse clutch holds both

forward and reverse ring gears when it isactuated. Finally, you need to see that thepinions of both sets are mounted on shafts

connected to the same carrier and that thecarrier is always the output member. Power

always enters this unit from the input shaftthat drives the forward sun gear.

Figure 2.27 Compound forward and reverse planetary set


7/10


If the reverse clutch were applied, the ring gear is held stationary. Did

you notice that the front carrier had coupled pinions? The power istransferred from the front sun gear to the first pinion, then to the second

pinion, which is forced to walk around the internal ring gear. Thiscauses the carrier to rotate opposite the input shaft and at reduced speed.

Figure 2.28 Reverse clutch

applied

When the forward clutch is actuated, it holds the rear sun gear stationary. Power from the input shaft

drives the right-hand sun gear, which in turn drives the first pinions of the right-hand planetary set.

Every time the first pinions turn, the pinions of the leftside planetary set are also driven because they are splinedto the same shaft. The pinions of the left side planetary

are forced to walk around the immobile left sun gear.

Because both the left planetary set pinions and first

pinions of the right planetary set are connected to theoutput carrier by common shafts, the rear (left) pinions

force the output carrier to turn in the same direction asthe input sun gear, and at a reduced speed.

Figure 2.29 Forward clutch applied

The drawing in Figure 2.30 shows a typical 1st

and 2nd speed shift mechanism. Notice that the

power from the input shaft drives the left-hand carrier. Also see that both sun gears are

attached to the output shaft. The left-hand (2nd

speed) clutch would hold both the left ring

gear and the right carrier because they are

interconnected. The right-hand (1st

speed)

clutch only holds the right-hand ring gearstationary when applied.

Figure 2.30 1st and 2nd speed compound planetary set


8/10


May 2005 Page 2-15

When the 2nd speed clutch (left-hand) is applied, the pinions of the input

carrier are forced to walk around the inside of the held ring gear.Because these pinions are also in mesh with the left sun gear, they also

transfer driving power to the output shaft. The result is an overdrive.

igure 2.31 2nd speed

If the 1st

speed clutch is applied instead of the 2nd

speed clutch,the right-hand ring gear is held. Now incoming power from theleft carrier is transferred to its pinions. These pinions use the

left-hand sun gear as their reactionary member even though it is

turning. These pinions are also in mesh with the left ring gear

and transfer driving power to it. Because the left clutch in notengaged, the left ring gear turns and transfers power to the right-

hand carrier as they are connected together.

Figure 2.32 1st

speed

This is a planetary gear system used as a high-low speed shift mechanism. It has two sun gears fitted onthe common input shaft that rotate continuously with it. The right-hand carrier is the input to the right-

hand planetary set. In 1st speed, the 1st speed clutch holds the right-hand ring gear stationary;

consequently, power is transferred from the pinions to the right-hand sun gear. The result is an

overdrive.

Comparing the size of the two sun gears and the pinions driving them can see the major difference

between 1st and 2nd. The small 1

st

speed pinions driving a larger sun gear causes the output shaft to turnslower in 1st

speed.


9/10


Figure 2.33 shows a planetary gear system

used as a high-low speed shift mechanism. Ithas two sun gears fitted on the common input

shaft and they all rotate continuously together.The power from the sun gears is transmitted

through a couple of carriers to the bluecolored ring gear that is the output member.

When either of the two clutches is actuated,all the members in the output-side (right)

planetary gearing are turning.

Figure 2.33 High and low speed planetary set

When the low speed clutch is applied, the input-side (left) carrier isheld. The power entering the left planetary (see Figure 2.33) is

ignored in the drawing in Figure 2.34. This is because all the powerhere is spinning out until the high-speed clutch is applied. Under the

conditions shown here, power from the right side sun gear transfersto the right side planet pinions. The pinions are rotating on their axis

driving the output ring gear at a reduction and in the oppositedirection from the input sun gear.

Figure 2.34 Low speed


10/10


May 2005 Page 2-17

When the system is set in high speed, the high-speed clutch

will be activated. This locks the left side ring gear stationary.In this case, power now enters from the first sun gear and is

transmitted to the first and then second pinions of the leftside coupled pinion sets. The second pinions are also in mesh

with the stationary ring gear and are force to walk aroundinside of it. In this way, the power is sent to the carrier to

drive the output side ring gear in the reverse direction of theinput sun gear. The increase in speed is due to the difference

in gear ratio.

Figure 3.5 High speed

Documents

Bp-fullguide - Planetary Gears