950G Series II Wheel Loader Caterpillar

Introduction to Planetary Transmission

Illustration 1	g01085433

Planetary transmissions use planetary gears in order to transmit power. Planetary transmissions use planetary gears to make speed and directional changes. Hydraulic clutches control the rotation of the planetary gear components. This allows the planetary gears to serve as a direct coupler, a reduction gear, or a reversing gear.

Planetary gear sets are compact units. The countershaft has been eliminated and the input shaft and the output shaft rotate on the same axis. A planetary gear set permits gear ratios to be changed without actually engaging or disengaging gears. This allows little interruption of the power flow.

In planetary gear sets, the load is spread over several gears. This decreases the load on each tooth. The planetary system spreads the load evenly around the circumference of the system. This eliminates sideways stress on the shafts.

Illustration 2	g01085435

A planetary gear set consists of the following components:

• A sun gear is the center of the planetary set.

• Three or more intermediate gears are called planetary gears.

• A planetary carrier holds the planetary gears.

• A ring gear is the outer boundary of the planetary set.

The planetary transmission controls the power through the planetary gears with clutch packs that consists of discs and plates. Each clutch pack is encased in a separate housing.

In some planetary transmissions, the clutch packs are mounted on the perimeter of the planetary set. The inside teeth of the discs are engaged with the outside teeth of the ring gear. Notches on the outside diameter of the plates are engaged with pins in the clutch housing. The pins keep the plates from rotating. The following examples will represent this type of transmission.

Planetary Transmission Clutch

Illustration 3	g01085436

Illustration 3 shows the components in a clutch. Springs are located between the clutch housing and the piston. The springs keep the clutches disengaged by keeping the clutch piston from pushing against the plates. The clutches engage when oil is sent into the area behind the piston. When the pressure of the oil in the area behind the piston increases, the piston moves to the right against the force of the spring. This pushes the discs and the plates together. The clutch is now engaged and the ring gear is held stationary. When the pressure of the oil that holds the piston decreases, the spring forces the piston back into the housing. This releases the discs and the plates. The ring gear is no longer held and the ring gear rotates freely.

Planetary Transmission Clutch Plates

Illustration 4	g01085438

The clutch plates (Illustration 4) are mounted inside the clutch housing. Notches (arrows) on the outside diameter of the plates are engaged with pins in the clutch housing which keep the plates from rotating.

Planetary Transmission Clutch Disc

Illustration 5	g01085439

The clutch discs (Illustration 5) are connected to the ring gear and the clutch discs rotate with the gear. The inside teeth of the discs are engaged with the outside teeth of the ring gear. The discs are made of friction type materials that are based on the requirements of the application.

Planetary Transmission Clutch Housing

Illustration 6	g01085441

Each clutch in the transmission is encased in a housing (Illustration 6). The housing holds the clutch piston and the plates in place. Pins are used to prevent the plates from turning.

Planetary Gear Set

Illustration 7	g01085442

Understanding the basic concepts of the planetary gear set will help understand the functions of a planetary transmission.

The components of a planetary gear set are shown in Illustration 7. The planetary gears (1) are encased in a planetary carrier (2). The outer gear is called the ring gear (3). The gear in the center is called the sun gear (4) .

Less space will be required in a transmission if planetary gear sets are used instead of external tooth gears, because all the gears can be inside the ring gear.

Another advantage of the ring gear is they have twice the tooth contact as external tooth gears. Internal tooth gears are stronger and have a longer wear life than the external tooth gears.

When an external tooth gear is driven by another external tooth gear, the two gears rotate in opposite directions. When an external tooth gear and an internal tooth gear are meshed, the gears will rotate in the same direction.

Planetary gears turn freely on their own bearings. The number of teeth does not affect the ratio of the other two gears. In planetary gear sets there are usually three or four planetary gears that turn on bearings.

Planetary Gear Ratios

S + R = C

S - Sun Gear Teeth

R - Ring Gear Teeth

C - Carrier Teeth

Gear Ratio = Driven Gear Teeth/Drive Gear Teeth

In a simple planetary gear set with a sun gear that has 30 teeth and a ring gear that has 90 teeth, the effective number for the carrier is 120 teeth. This is calculated by adding the number of teeth on the sun gear to the number of teeth on the ring gear:

• 30 + 90 = 120 (S + R = C)

To calculate the gear ratio through this gear set, divide the number of teeth of the drive member into the number of teeth on the driven member. For example, if the sun gear is the drive member, the ring gear is the driven member, and the carrier is being held, the ratio would be:

• 90/30 or 3:1

If the carrier is the drive member and the ring gear is the driven member with the sun gear being held, the ratio would be:

• 90/120 or 0.75:1

There are eight possible conditions that can be used with a simple planetary gear set. Six of the conditions are shown in Illustration 8 through Illustration 13. The other two conditions are direct drive, when two members are locked to each other; and neutral, when there is no drive member or held member.

All planetary gear sets follow basic rules. The rules listed below will help to understand the planetary power shift transmission operation.

• Two external gears in mesh (sun gear and planetary gears) will rotate in opposite directions.

• An internal (ring) and external (planetary) gear that is in mesh will rotate in the same direction.

• There must be an input member and a held member in order to get output from a planetary gear set (except direct drive).

• When any two components of a planetary gear set are driven in the same direction at the same speed, direct drive (1:1) ratio will result.

• A carrier will always follow the input.

• If a carrier is the driving member, overdrive will result.

• If a carrier is the output member, reduction will result.

• If a carrier is held, reverse will result.

Planetary Gear Set in Forward Reduction

Illustration 8	g01085443

The changes of speed, direction, and torque are accomplished by restraining or by driving various components of the planetary gear set. There are many different combinations that are possible. In order to transmit power through a planetary set, one member is held, one member is driving, and one member is driven.

With the ring gear as the input and the carrier as the output (Illustration 8), the planetary gears walk around the stationary sun gear and the gear set is in forward reduction.

Planetary Gear Set in Max Forward Reduction

Illustration 9	g01085446

With the sun gear as the input and the carrier as the output (Illustration 9), the planetary gears walk around the inside of the stationary ring gear and the gear set is in maximum forward reduction.

Planetary Gear Set in Forward Overdrive

Illustration 10	g01085450

With the carrier as the input and the ring gear as the output, (Illustration 10), the planetary gears walk around the stationary sun gear and the gear set is in forward overdrive.

Planetary Gear Set in Max Forward Overdrive

Illustration 11	g01085451

With the carrier as the input and the sun gear as the output (Illustration 11), the planetary gears walk around the inside of the stationary ring gear and the gear set is in maximum forward overdrive.

Planetary Gear Set in Reverse Reduction

Illustration 12	g01085453

With the sun gear as the input, the ring gear as the output, and the carrier stationary (Illustration 12), the planetary gears act as idler gears. The gear set operates in reverse reduction.

Planetary Gear Set in Reverse Overdrive

Illustration 13	g01085508

With the ring gear as the input, the sun gear as the output, and the carrier stationary (Illustration 13), the planetary gears act as idler gears. The gear set operates in reverse overdrive.

Two-piece Shaft

Illustration 14	g01085509

A two-piece shaft is shown in Illustration 14. The shaft on the left is the input shaft. The sun gears of the reverse and forward planetary gear sets are mounted on the input shaft.

The shaft on the right is the output shaft. The sun gears for the second speed and first speed planetary sets are mounted on the output shaft.

Note: Illustration 15 through 19 shows that planetary gear sets are added to the shafts in order to represent a basic planetary transmission.

Illustration 15	g01085511

Illustration 15 shows that planetary gears have been added to each sun gear. Planetary gear sets are usually referred to by the numbers that start from the input (left) end.

Illustration 16	g01085512

The front carrier of the reverse planetary gear set has been added to the illustration in Illustration 16. Half of the planetary carrier is omitted to show how it is mounted and how it holds the planet gears.

Illustration 17	g01085513

In Illustration 17, a center carrier has been added to the transmission assembly. The center carrier connects the input shaft to the output shaft. The center carrier contains the planetary gears for forward speed and for second speed.

Illustration 18	g01085514

All three planetary carriers are mounted on the shafts in Illustration 18. From left to right, the planetary carriers are the front carrier, the center carrier, and the rear carrier.

Four Planetary Gear Sets

Illustration 19	g01085515

Illustration 19 shows four planetary gear sets. From the input end (left) the planetary gears are: no. (1) (reverse), no. (2) (forward), no. (3) (second) and no. (4) (first).

Ring gears and clutches must be added in order to make a complete transmission, then the entire assembly must be put into a protective housing.

Illustration 20	g01085517

In some planetary power shift transmissions, there is a planetary gear set for each transmission speed. There is a set for forward and a set for reverse.

Illustration 20 shows four planetary gear sets that are assembled into a compact group.

Two-Speed, Two-Directional Planetary Transmission

Illustration 21	g01085518

Illustration 21 is a schematic representation of a two-speed, two-direction planetary power shift transmission. Illustration 21 is an exploded view of the assembled planetary gear set that is shown in Illustration 20.

Engine power is transmitted to the input shaft (1) through a torque converter or a torque divider. The sun gears for both the forward and reverse directions are mounted on the input shaft. The sun gears always rotate when the input shaft is driven. The center carrier (2) is the carrier for the planetary gears of the reverse set and the second speed set. The output shaft (3) and the sun gear for the second speed are mounted on the center carrier. The sun gear for first speed is mounted to the output shaft (4) .

Note: The arrangement of the planetary gear sets from the engine to the output shaft are listed from left to right: reverse, forward, second and first.

Directional Planetary Gear Set in Forward

Illustration 22	g01085520

Illustration 22 shows the planetary gear sets for forward and reverse, which are the directional half of the transmission. Power is transmitted from the engine to the input shaft (left end of planetary gear set). The ring gear of the planetary gear set for forward is stopped. This portion of the transmission is now engaged in forward gear.

The input shaft is driven. Since the sun gears are mounted on the input shaft, the sun gears are also driven. The reverse sun gear (on the left) is rotating the planetary gears. However, no power is transmitted through the reverse planetary because no member of the planetary is held.

The sun gear of the forward planetary rotates with the input shaft. Therefore, the planetary gears rotate in the opposite direction. Because the ring gear is stopped, the planetary gears must revolve in the same direction as the sun gear is rotating. This causes the planetary carrier to rotate in the same direction. This is the power flow for the forward direction.

Directional Planetary Gear Set in Reverse

Illustration 23	g01085521

Illustration 23 shows the power flow when the planetary carrier for the reverse planetary gear set is stopped. The input shaft (at left) drives the sun gear of the reverse planetary set. The sun gear drives the planetary gears. Because the planetary carrier is stopped, the planetary gears must rotate in place and drive the ring gear. Ring gear rotation is now opposite sun gear rotation.

The ring gear of the reverse planetary set is fastened to the planetary carrier gears of the forward planetary set. Therefore, the planetary carrier of the forward planetary set also rotates in reverse or rotates opposite to input gear rotation.

Speed Planetary Gear Set in Second

Illustration 24	g01085524

Illustration 24 shows the speed part of the transmission in Second gear. The planetary carrier on the left is part of the planetary carrier of the forward planetary set. The planetary carrier is driven either clockwise or either counterclockwise, depending on which planetary gear set (forward or reverse) is transmitting power.

The ring gear of the planetary set for second gear is stopped. Because the planetary carrier is rotating and the ring gear is stopped, the sun gear of the second planetary gear set is driven. The sun gear and the output shaft rotate in the same direction as the planetary carrier.

No member of the first planetary gear set is held. Therefore, all components are free to rotate and no power is transmitted through the first gear planetary gear set.

Speed Planetary Gear Set in First

Illustration 25	g01085525

For first gear operation (Illustration 25), the ring gear of the second gear planetary set is released and the ring gear of the first gear planetary set is stopped. The planetary carrier at the left is still driven through the directional half of the transmission. The load on the output shaft provides resistance to sun gear rotation. Therefore, the ring gear of the second gear planetary set must rotate. This ring gear is fastened to the planetary carrier of the first gear planetary set.

The ring gear of the first gear planetary set is stopped. This causes the planetary gears to walk around the inside of the held ring gear and drive the first speed sun gear and the output shaft. The output shaft rotation is in the same direction as the rotation of the planetary carrier on the left.

Planetary Gear Set in First Gear Forward

Illustration 26	g01085526

In first gear forward (Illustration 26), the ring gears of the forward (F) and first speed (1) planetary sets are stopped. No power is transmitted through the reverse (R) planetary set because no member is held. When the ring gear of the forward planetary set is stopped, the rotating sun gear causes the planetary gears to revolve around the sun gear. The forward planetary gears are mounted on the center carrier, and the center carrier must rotate.

The rotating center carrier drives the ring gear of the second (2) gear planetary set. The sun gear of the second gear planetary set is the held member because the rotation is restricted by the load on the output shaft. The planetary gears will cause the ring gear to rotate. The ring gear of the second speed planetary set is connected to the planetary carrier of the first speed planetary set. Because the first speed ring gear is held, the planetary gears drive the first speed sun gear and deliver power to the output shaft. The machine moves forward in first gear.

Planetary Gear Set in First Gear Reverse

Illustration 27	g01085527

In first gear reverse (Illustration 27), the planetary carrier of the reverse (R) planetary set and the ring gear of the first (1) gear planetary set are held. When the planetary carrier of the reverse planetary set is held, the planetary gears rotate. The planetary gears drive the reverse ring gear in the opposite direction from the input shaft. The reverse ring gear causes the center carrier to rotate. The load on the output shaft holds the sun gear of the second (2) gear planetary set. The center carrier will cause the planetary gears to drive the second speed ring gear. The second speed ring gear connects to the planetary carrier of the first speed planetary set. The first speed ring gear is held. The planetary gears revolve around the inside of the first speed ring gear. The planetary gears drive the first speed sun gear and the output shaft.

Planetary Gear Set in Second Gear Forward

Illustration 28	g01085529

In second gear forward, the ring gears of the forward (F) and second (2) speed planetary sets are stopped. No power is transmitted through the reverse (R) planetary set because no member is held. When the ring gear of the forward planetary set is stopped, the rotating sun gear causes the planetary gears to revolve around the sun gear. The forward planetary gears are mounted on the center carrier, and the center carrier must rotate.

The second speed ring gear is held. The center carrier causes the planetary gears to revolve around the inside of the second speed ring gear. The planetary gears drive the second speed sun gear and the output shaft.

Planetary Gear Set in Second Gear Reverse

Illustration 29	g01085530

In second gear reverse (Illustration 29), the planetary carrier in the reverse (R) planetary set and the ring gear of the second (2) speed planetary set are held. When the planetary carrier of the reverse planetary set is held, the planetary gears rotate. The planetary gears drive the reverse ring gear in the opposite direction from the input shaft. The reverse ring gear causes the center carrier to rotate. The second speed ring gear is held. The center carrier causes the planetary gears to revolve around the inside of the second speed ring gear. The planetary gears drive the second speed sun gear and the output shaft.

Transmission Clutch Engagement Chart

Table 1

Transmission Speed	Direction Clutch Engaged	Speed Clutch Engaged
First Speed Forward	2	5
Second Speed Forward	2	4
Third Speed Forward	2	3
Neutral	-	3
First Speed Reverse	1	5
Second Speed Reverse	1	4
Third Speed Reverse	1	3

The chart in Table 1 lists the clutches that are engaged for operation in each gear range. This chart applies to D9R Tractor-type Tractor models.

A chart of this type can be a useful reference when troubleshooting a transmission performance problem. For example, if the machine operator says that the transmission slips in first speed forward and first speed reverse, the problem is probably in clutch No. 5 since clutch no. 5 is common to both gear ranges. If the transmission slips in first gear forward but not in first gear reverse, the problem is more likely in clutch no. 2.