950G Series II Wheel Loader Caterpillar

Fluid Coupling

Illustration 1	g01084710

The torque converter is a form of fluid coupling that is used to transmit power from the engine to the input shaft of the transmission. Torque converters use fluid (oil) to hydraulically connect the flywheel of the engine to the input shaft of the transmission. Unless the machine is equipped with a lockup clutch, there is no direct connection between the engine and the transmission.

There are three types of hydraulic mechanisms that are used to transmit power, the fluid coupling (Illustration 1), the torque converter, and the torque divider. All three hydraulic mechanisms are fluid drive devices that use the energy of fluid in motion in order to transmit power.

Illustration 2	g01084712

Operation of a fluid coupling can be compared to the action of two electric fans (Illustration 2) that are placed face to face and close together. If one fan is running the energy of the moving air will cause the other fan to turn.

In a fluid coupling the fluid acts like the air between the two fans. The output fluid power of the driving component acts as the input power for the driven component. Liquid has more mass than air so liquid transmits more energy.

Mechanical power from the engine is converted to fluid power and fluid power is converted back to mechanical power to drive the output shaft.

Impeller and Turbine

Illustration 3	g01084713

Illustration 3 shows the two halves of the fluid coupling. A number of straight, radial blades extend from the inside to the outside edge. The blades in the part on the right side are a part of the housing. This part is called the impeller or the pump. The blades in the part on the left side are part of the turbine.

The impeller changes mechanical power from the engine to fluid power. The turbine changes fluid power back to mechanical power in order to drive the transmission. The impeller and turbine are mounted very close together for efficiency.

Illustration 4	g01084714

The turbine and the impeller have a rounded profile (Illustration 4). When the turbine on the left is cut along the axis, the cross section will look like the illustration on the right.

This rounded profile is shown in the following schematic cross sections of the fluid coupling.

Illustration 5	g01084716

The schematic in Illustration 5 represents the fluid coupling. The pump impeller shaft (1) connects to the engine flywheel. The turbine output shaft (2) connects to the driven unit.

The impeller and turbine both turn in the housing (3). The impeller and turbine are not directly connected together in anyway. The housing is filled with oil.

When the engine is started, the impeller starts turning. As the impeller turns, the impeller throws oil from the center toward the outside edge. The shape of the impeller and centrifugal force move the oil outward and across into the turbine. The oil strikes the turbine blades. The energy of the moving oil is absorbed by the turbine and starts turning the turbine. As the oil strikes the turbine, the oil slows down and flows inward, toward the center in order to re-enter the impeller.

When the oil leaves the turbine, the oil is flowing in a direction opposite the oil flow in the impeller and tends to oppose the impeller.

The heavier arrows inside the impeller represent oil increasing speed and energy as the oil moves through the impeller. The smaller arrows inside the impeller represent oil slowing down and losing the energy to the turbine.

Rotary Oil Flow

Illustration 6	g01084721

Illustration 6 shows the two basic kinds of oil flow in a fluid coupling: Rotary flow (large arrows) and vortex flow (small arrows). Rotary flow occurs when the oil is traveling with the impeller and the turbine in the direction of rotation. This happens when the impeller and the turbine are traveling at nearly the same speed, for example when the machine is coasting or when the machine is being roaded with little or no load. The oil is thrown outward by centrifugal force in both the impeller and the turbine (small arrows). The oil simply follows the impeller and the turbine. (large arrows).

With rotary oil flow, there is minimum slip or difference in rotational speed between the impeller and the turbine. The turbine output torque is zero.

Vortex Oil Flow

Illustration 7	g01084723

Vortex oil flow, shown in Illustration 7, occurs when the oil travels outward through the impeller, across to the turbine and inward through the turbine, then back to the impeller. The impeller is turning with the engine. The turbine is stalled or held stationary by a load. The oil that is traveling across and strikes the turbine blades limits oil movement in the direction of rotation with the impeller. The oil flow path looks like a spiral.

With vortex flow, there is maximum slip between the impeller and the turbine. The output torque is greatest when the turbine is stalled.

Under normal operating conditions, the oil flow in a fluid coupling will combine both rotary and vortex flow. The imaginary oil flow path will be like a coil of wire, which loosens or becomes tighter depending on upon the amount or degree of slip between the impeller and the turbine.

Note: In a fluid coupling, the input torque equals the output torque. The fluid coupling transmits power but will not multiply torque. As the oil flows from the impeller to the turbine in a fluid coupling, the oil is not moving in the same direction as the turbine. This produces an unnecessary load on the engine. A stator is required to multiply torque.

Torque Converter

Illustration 8	g01084724

A torque converter is a fluid coupling with the addition of a stator. Like the fluid coupling, the torque converter couples the engine to the transmission and transmits the power that is required to move the machine. Illustration 8 shows an instructional cutaway. The housing is cut away to see the working parts inside.

Unlike the fluid coupling, the torque converter can also multiply torque from the engine, which increases torque to the transmission. The torque converter uses a stator, which redirects fluid back into the impeller in the direction of rotation. The force of oil from the stator increases the amount of torque that is transferred from the impeller to the turbine creating torque multiplication.

The basic components for the torque converter are listed below:

• Rotating housing

• Impeller

• Turbine

• Stator

• Output shaft

Illustration 9	g01084725

The rotating housing and impeller (1) turn with the engine. The turbine (2) turns the output shaft. The stator (3) is fixed and held stationary by the torque converter housing.

The oil flows upward from the rotating impeller, around the inside of the housing and downward past the turbine. From the turbine, oil is redirected back to the impeller by the stator.

The rotating housing is connected to the flywheel and surrounds the entire torque converter. An inlet relief valve and an outlet relief valve control the amount of oil pressure in the torque converter.

Illustration 10	g01084726

The impeller (1) is the driving member of the torque converter. The impeller is splined to the flywheel and turns at engine RPM. The impeller contains blades, which throws the oil against the blades of the turbine (Illustration 10). As the impeller spins, the impeller slings the oil outward toward the inside of the rotating housing. Oil is moving in the direction of rotation when the oil comes off the impeller blades.

The turbine (2) is the driven member of the torque converter with vanes that receive the oil flow from the impeller. The impact of the oil from the impeller on the turbine vanes causes the turbine to rotate. The turbine causes the output shaft to turn. The oil is moving in the opposite direction of engine/flywheel rotation when the oil comes off the turbine fins.

Stator

Illustration 11	g01084727

The stator is the stationary reaction member with vanes that multiply force by redirecting fluid flow from the turbine back to the impeller. The purpose of the stator is to change the direction of the flow of oil between the turbine and the impeller. Illustration 11 shows this direction change, which increases the momentum of the fluid, thereby increasing the torque capacity of the converter. The stator is connected to the torque converter housing. Momentum of the oil is in the same direction as the impeller. Oil hits the backside of the impeller blades causing the impeller to rotate. This is known as the reaction.

Torque Converter Oil Flow

Illustration 12	g01084729

The arrows in Illustration 12, show the oil flow pushing outward from the impeller and around the housing into the turbine. Oil drives the turbine and torque is transmitted to the output shaft. As the oil leaves the turbine blades, the oil strikes the stator which redirects the oil toward the direction of impeller rotation. The oil flow is guided upward in order to re-enter the impeller. The oil flows continuously between the torque converter components.

The output shaft, which is splined to the turbine, sends torque to the input shaft of the transmission. The output shaft is connected to the transmission through a yoke and drive shaft or directly to the transmission input gear.

Illustration 13	g01084730

Illustration 13 shows a cross section of the torque converter. Arrows indicate the oil flow in the torque converter. The oil inlet port is just above the output shaft and the outlet port is in the converter support below the output shaft.

Oil from the pump flows through the torque converter inlet relief valve (not shown). The torque converter inlet relief valve controls the maximum pressure of the oil in the torque converter.

Oil flows through the hub to the impeller and lubricates the bearing in the hub. Oil then flows through the torque converter. Oil exits the torque converter and flows through the outlet relief valve. The outlet relief valve controls the minimum pressure inside the torque converter. Oil must be maintained under pressure in the torque converter in order to reduce or minimize cavitation, which reduces converter efficiency. Cavitation is the formation of oil vapor bubbles around the blades.

The torque converter absorbs impact loads. The viscosity of the torque converter oil is a good medium for transmitting power. Oil is necessary to reduce cavitation, to carry away heat, and to lubricate torque converter components.

The torque converter adjusts to the machine load. Under a high load, the impeller spins faster than the turbine in order to increase torque and reduce speed. With a small load on the machine, the impeller and the turbine rotate at nearly the same speed. Speed increases and torque decreases. Under a stall condition, the turbine is stopped, the impeller is rotating, and maximum torque is produced.

Benefits of a Torque Converter

The torque converter multiplies torque when torque is needed for the load. The torque converter helps keep the engine from stalling during high load applications. The torque converter also allows the hydraulics of the machine to continue to work. The torque converter also permits the use of a power shift transmission.

Illustration 14	g01084732

A torque divider (Illustration 14) provides the combined benefits of a torque converter and a planetary gear drive. The torque divider consists of a conventional torque converter with an integrated planetary gear set that is in front of it. This arrangement allows for a variable split of engine torque between the converter and the planetary gear set. This split can be as high as 70/30 depending on the machine load. Both the converter and the planetary gear set output are connected to the torque divider output shaft.

Torque Converter and Planetary Gear Set

Illustration 15	g01084733

The torque divider is attached to the engine flywheel. During operation, the torque converter and planetary gear set work together to provide the most efficient split of engine torque.

The torque converter (Illustration 15, left diagram) provides torque multiplication for heavy loads. The planetary gear set (Illustration 15, right diagram) provides nearly 30% direct drive during light load situations.

Torque Divider Components

Illustration 16	g01084735

Torque dividers combine a fluid drive with a mechanical drive that adjust to load conditions. Like the torque converter, the torque divider (Illustration 16) consists of four components contained in a housing that is filled with oil by a pump.

The following components operate the same as a torque converter:

• The impeller (driving member)

• The turbine (driven member)

• The stator (reaction member)

• The output shaft

The torque divider also contains a planetary gear set.

The planetary gear set differentiates the torque divider from the torque converter. The planetary gear set provides direct drive when the machine is under light load. When the torque divider is under heavy load, the torque divider acts as a conventional torque converter in order to increase output torque.

The planetary gear set consists of the following components:

• Sun gear

• Ring gear

• Planetary gears

• Planetary carrier

The ring gear is splined to the turbine. The planetary carrier is splined to the output shaft. The sun gear is connected to the engine flywheel by splines and turns at engine rpm.

With a light load on the machine, the planetary carrier has low resistance to rotation. The sun gear, the planetary gears, the planetary carrier, and the ring gear turn at the same speed. The torque from the converter and planetary gear set is transmitted through the planetary carrier to the output shaft and to the transmission. Neither the torque converter nor the planetary gear set multiplies the torque from the engine when they turn at the same speed.

When the machine is under heavy load, the planetary carrier has a resistance to rotation. Since the sun gear is turning at the speed of the engine, this resistance to rotation causes the planet gears to turn on their shafts. The planet gears rotation is opposite the rotation of the ring gear. This causes a decrease in the speed of the ring gear. Since the turbine is connected to the ring gear, a decrease in speed will cause the torque converter to increase output torque. This torque is sent to the planetary carrier and the output shaft through the ring gear.

When the speed of the ring gear decreases, the torque of the engine through the sun gear and the planetary gear set also multiplies. This torque is also sent to the planetary carrier and the output shaft.

If resistance to the rotation of the planetary carrier becomes high enough, the ring gear will stop. During some very high load conditions, the rotation of the planetary carrier and the output shaft will stop. This is referred to as converter stall. This will cause the ring gear to turn slowly in the opposite direction. At this time torque multiplication of the torque converter and sun gear is at its maximum.

The following statements are the benefits of a Torque Divider:

• More continuous application of power

• Increase torque output

• Absorb shock

• Permit direct drive operation.

Torque dividers provide a more continuous application of power and increased torque output that is suitable for high loads. Torque dividers absorb shock, which provides a longer life for the power train. Torque dividers permits direct drive operation of the machine. This increases efficiency and provides better fuel economy.

Illustration 17	g01084736

Torque dividers are used in track-type tractors to push through hard dirt without surging.

Torque converters in track-type tractors are at stall more than any other machine that Caterpillar produces. Illustration 17 shows a track-type tractor that is equipped with a torque divider.

Lockup Clutch Torque Converter

Illustration 18	g01084740

Some machines need torque converter drive under certain conditions and direct drive during other conditions. The lockup clutch torque converter (Illustration 18) provides a direct connection between the transmission and the engine. The lockup clutch torque converter also operates the same way as a conventional torque converter when the converter is not in lockup mode.

The lockup clutch is located inside the torque converter housing. When the lockup clutch is engaged, the clutch connects the rotating housing directly to the output shaft and the turbine. The output shaft will turn at engine speed. Direct drive provides the highest drive train efficiency at high speeds. The lockup clutch connects the turbine to the rotating housing. The rotating housing rotates at the same speed as the impeller. When operating conditions demand direct drive, the lockup clutch is automatically engaged.

Illustration 19	g01084742

Illustration19 shows the lockup clutch components. The lockup clutch consists of the following components.

• Clutch

• Piston

• Plates

• Discs

A lockup clutch control valve that is located on the outer cover controls oil flow in order to engage the lockup clutch. In some machine applications the lockup clutch is controlled by a solenoid, which is activated by an Electronic Control Module (ECM).

When lockup clutch activation is demanded, oil flows through an oil passage in the output shaft to the lockup clutch piston. The lockup clutch piston and plates are connected to the converter housing with splines. The converter housing rotates at engine speed. The discs are connected to an adapter with splines and the adapter is fastened to the turbine with bolts. The oil pressure on the piston forces the piston against the lockup clutch plates and the discs. The plate and discs rotate together causing the turbine and the output shaft to rotate at the same speed as the converter housing. The turbine and the impeller are now turning at the same speed and there is no multiplication of torque from the torque converter.

When the lockup clutch is released the torque converter multiplies torque like the conventional torque converter.

The benefits for a lockup clutch torque converter are listed below:

• Allow flexibility in the machine application.

• Provides torque multiplication for high loads.

• Provides direct drive for high speeds.

The lockup clutch torque converter allows flexibility in the machine application. When a machine is under a high load, the lockup clutch torque converter operates like a conventional torque converter providing torque multiplication. When the machine is traveling at a high speed, the lockup clutch torque converter provides direct drive for higher speeds and improved fuel economy.

Illustration 20	g01084792

Several types of machines feature lockup clutch torque converters, such as the large wheel loaders and the tractor-scraper shown in Illustration 20.

One-way Clutch

Illustration 21	g01084794

The one-way clutch torque converter operates similarly to the conventional torque converter. The impeller uses fluid to drive the turbine and the output shaft. However, the stator is mounted on a one-way clutch instead of a stationary housing. This one-way clutch permits the stator to turn freely when torque multiplication is not required.

The cam connects the one-way clutch to the stator. The cam is splined to the stator. The rollers provide the mechanical connection between the cam and the hub. The springs hold the rollers in the cam opening. The hub connects the one-way clutch to the carrier. The hub is splined to the carrier.

When the load is heavy and torque multiplication is necessary, the force of the oil on the front of the stator vanes will try to turn the cam ring clockwise. This action will cause the rollers to be clamped between the cam and the hub. This locks the stator in place. The stator will then redirect oil back to the impeller in order to multiply torque.

As the speed of the impeller and turbine increase, the force of the oil begins to strike the back of the stator vanes turning the stator counterclockwise. When the stator is rotating in this direction, there is no clamping action and the rollers are allowed to roll on the hub and the stator freewheels. The stator does not send oil back to the impeller, causing the torque converter to act more like a fluid coupling.

A one-way clutch is also used with lockup clutch torque converters. In lockup clutch torque converters, the one-way clutch permits the stator to turn freely when the machine is in direct drive.

The benefits of One-way Torque converters are listed below:

• Torque Multiplication under heavy loads.

• Less heat buildup.

• Reduced converter drag.

Torque multiplication occurs under heavy loads.

The stator freewheels during light loads, resulting in less heat buildup and reduced converter drag.

Illustration 22	g01084796

Wheel tractor-scrapers, backhoe loaders, off-highway trucks, and articulated trucks are equipped with one-way clutches.

Impeller Clutch Torque Converter

Illustration 23	g01084797

The impeller clutch torque converter (Illustration 23) makes it possible to vary the output torque of the converter over a broad range. The impeller clutch is similar to the conventional torque converter except the impeller is driven by the rotating housing through an impeller clutch. The rotating housing turns at engine speed. The impeller clutch is a multi-disc clutch pack. The impeller clutch is hydraulically activated and controlled by the impeller clutch solenoid valve. The impeller clutch solenoid valve is controlled by an ECM.

Illustration 24	g01084801

Illustration 24 shows the impeller clutch components. The impeller clutch couples the impeller to the converter housing and consists of an impeller clutch piston, plates, and discs. When the ECM increases the current to the impeller clutch solenoid, impeller clutch pressure is reduced. When current from the ECM is at zero, the impeller clutch pressure is at the maximum and the converter operates like a conventional converter.

Illustration 25	g01084802

When the impeller clutch solenoid valve is not energized by the ECM there is no current flow to the solenoid. Oil flows to the impeller clutch oil passage from the carrier and forces the impeller clutch piston (1) against the plates (2) and discs (3). The piston and plates are connected to the impeller clutch housing with splines. The adapter is fastened to the impeller (4) with bolts. The friction between the discs and the plates locks the impeller to the converter housing. This causes the impeller to rotate at the same speed as the converter housing. The impeller displaces the full amount of oil and the torque converter is at maximum torque output.

As the current flow to the solenoid is increased, oil pressure to the piston is decreased. The friction between the plates and discs decreases and the impeller slips (turns slower). This forces less oil to the turbine. With less force on the turbine, there is less torque at the output shaft.

The impeller displacement depends on the speed of the impeller. A slower speed means less displacement and less power transfer. The clutch slips in order to prevent the wheels from slipping. The machine operator can set the amount of slip to suit the work that is being done, by varying the amount of current that is sent to the solenoid. This varies the pressure behind the clutch piston.

The benefits for an Impeller Clutch Torque Converter are listed below:

• Decrease wheel slippage

• Reduces tire wear.

• Increases available engine power for the hydraulic system

The most important benefit of the impeller clutch is the ability to keep the wheels from slipping. A wheel loader's wheels are particularly likely to slip during heavy bucket loading operations. The tires wear more rapidly when slippage occurs and replacement tires are the largest expense of operating a wheel loader. The impeller clutch also increases available engine power.

Illustration 26	g01084804

Illustration 26 shows a 992G Wheel Loader that is equipped with an impeller clutch torque converter.

Variable Capacity Torque Converter

Illustration 27	g01084807

The variable capacity torque converter (Illustration 27) allows the operator to limit the amount of torque increase in the torque converter. This reduces wheel spin and diverts power to the hydraulic system.

The main components of the torque converter are listed below:

• Inner impeller

• Outer impeller

• Impeller clutch

• Turbine

• Stator

The inner impeller, the turbine, and the stator all function essentially the same as in the conventional torque converter previously covered. The principal difference is that the impeller is split so there is an additional impeller for greater torque management flexibility.

Outer Impeller

Illustration 28	g01084808

The outer impeller (arrow) is the second impeller within the torque converter. The outer impeller rotates with the converter housing when oil pressure acts on the clutch piston to engage the clutch pack. When maximum oil pressure fully engages the clutch, the outer impeller turns with the inner impeller. When there is a reduction of oil pressure, there is clutch slippage. This results in the outer impeller turning slower and a reduction of torque converter capacity.

Impeller Clutch

Illustration 29	g01084811

The impeller clutch (arrow) is hydraulically activated and controlled by the transmission hydraulic system. The clutch connects the outer impeller to the rotating housing allowing the inner and outer impellers to turn together.

Illustration 30	g01084813

In the full power mode (Illustration 30, right diagram), oil pressure acts on the clutch piston which engages the impeller clutch and causes the outer impeller to turn with the inner impeller. With both impellers rotating at the speed of the housing, the impellers displace the full amount of oil and the torque converter produces maximum torque. When the clutch is completely engaged there is no clutch slippage. This allows the torque converter to operate as a conventional torque converter.

In the reduced power mode (Illustration 30, left diagram), oil pressure decreases at the clutch piston. This allows the clutch to slip. The clutch transfers some of the power from the rotating housing to one impeller. One impeller rotates at the same speed as the rotating housing, and one impeller rotates slower. The impellers do not displace the full amount of oil and torque converter output is reduced. At minimum capacity, the operation of the variable capacity torque converter is similar to a conventional torque converter, except that the effective size of the impeller has been reduced due to impeller clutch slippage.

Impeller displacement depends on the speed of the impeller. Slower speed means less displacement and less displacement means less power transfer. The clutch slips in order to keep the wheels from slipping. The machine operator sets the amount of slip by varying the pressure behind the clutch piston.

The following lists the benefits of a variable capacity torque converter.

• Decreases wheel slippage

• Reduces tire wear.

• Increases available engine power for the hydraulic system.

Similar to the impeller clutch torque converter, the variable capacity torque converter keeps the wheels from slipping during bucket loading operations. The variable capacity torque converter also increases available engine power.

Torque Converter Tests

• Stall Tests

• Inlet Relief Valve Test

• Outlet Relief Valve Test

The stall test is performed when a problem with the torque converter is suspected.

Torque converter stall occurs when the output shaft speed is zero. The converter stall test is performed while the engine is operated at full throttle. This test will give an indication of engine and drive train performance based on engine speed. A lower or higher speed than specified is an indication of either engine or drive train problems. A low converter stall speed is generally an indication of an engine performance problem. A high converter stall speed is generally an indication of a drive train problem.

The converter relief valve tests include the inlet relief valve test and the outlet relief valve test.

The inlet relief valve for the torque converter controls the maximum pressure to the converter. The inlet relief valves primary purpose is to prevent damage to the converter components when the engine is started with cold oil.

The outlet relief valve maintains pressure in the torque converter. Pressure must be maintained in the torque converter in order to prevent cavitation and ensure efficient converter operation. Low pressure could be an indication of converter leakage, poor pump flow, or a malfunctioning relief valve. High pressure could be an indication of a malfunctioning relief valve or a blockage in the system. Perform this test by checking the converter outlet relief valve pressure at the proper pressure port.