Hydrokinetic Drives

Although all types of fluid couplings are used in starting and controlling pumps, the one most commonly used is the hydrokinetic machine (Figure 1).

Basic Principle In the hydrokinetic drive, commonly known as a fluid drive or hydraulic coupling, oil fluid particles are accelerated in the impeller (driving member) and then decelerated as they impinge on the blades of the runner (driven member). Thus, power is delivered in accordance with the basic law of kinetic energy:

FIGURE 1 Power-transmitting elements of a hydrokinetic coupling (American Davidson)
FIGURE 2 Hydrokinetic coupling, scoop-trimming type (American Davidson)

where F represents energy, M is the mass of the working fluid, V is the velocity of the oil particles before impingement, and V2 is the velocity after impingement on the runner blades.

This principle is used in traction units and, with modification, in torque converters. Neither of these offers controlled variable speed.

In variable-speed units, the mass of the working fluid can be changed while the machine is operating and infinitely variable output speed is achieved. Variation of oil quantity can be accomplished in four ways: scoop-trimming couplings, leakoff couplings, scoop-control couplings, and put-and-take couplings (Figures 2 to 5).

Components The following components are common to all the above types with few exceptions.

The housing of the fluid drive serves four purposes—as a reservoir for the nonworking oil, as a support for the bearings and scoop tube, as a guard to surround the moving parts, and as a container for oil particles and vapors that prevents their escape to the atmos-

FIGURE 3 Hydrokinetic coupling, leakoff type (American Davidson)

phere. It also supports the oil pump when an internal pump is used. On small units, the housing is of end-bell construction; all others are split on the horizontal centerline to facilitate inspection and maintenance.

Bearings are used to support the shafts radially and axially. In the case of smaller industrial units, ball or roller bearings are usually used; larger machinery utilizes babbitt radial bearings and Kingsbury thrust bearings. Input sleeve bearing pillow blocks often support the internal oil pump driving and driven gears. In most cases, the thrust bearings are designed to handle only the internal thrust of the fluid drive. Thrust developed by sleeve bearing driving motors can be accepted by the hydraulic couplings, but driven machines must usually have provisions to absorb their own thrust.

Shafts support the rotors and transmit driving torque to and from them. In some cases, shafts are hollow and are used to supply oil to the bearings and to the working circuit (Figures 2 to 5).

Rotors are often compared to halves of grapefruit after the meat has been removed and may be fabricated in three ways. The lightest-duty units are equipped with die-cast rotors of SAE 356 aluminum. Heavier-duty units have rotors that are machined out of 4130 or 4340 aircraft-quality steel forgings.

Inner and outer casings are bowl-shaped members that bolt to the front of the impeller to contain the oil in two connected areas known as the working circuit (Figure 2). One chamber is formed by the impeller and inner casings. The other is formed between the inner and outer casings and can be called the scoop-tube chamber. Ports in the inner casing permit oil to flow from one chamber to the other.

The scoop tube (Figure 2) can be moved radially or rotated inside the scoop tube chamber and is supported by sleeve or antifriction bearings. The pickup end of the tube is between the two casings facing the direction of oil rotation. Linkages permit the tube to be moved from outside the housing, and seals prevent the leakage of oil or vapors at this penetration.

An oil pump is provided that may be an internally mounted gear pump driven from the input shaft or an externally mounted positive displacement motor-driven pump. In cases where extreme reliability is required, emergency standby ac- or dc-driven pumps may be furnished. These pumps furnish light turbine oil to lubricate, transmit power, and remove heat from the fluid drive. In many cases, they supply lubrication to the driver, the intermediate gear boxes, and the driven machine.

Oil coolers are required on all drives rated above 3 hp (2.2 kw). These coolers remove heat dissipated by the fluid drive and other machines for which they furnish lubrication. Shell-and-tube water-to-oil exchangers are normally supplied, although finned-tube air-to-oil exchangers are utilized where water is not available or economical. On pipeline work, it is common to use in-line coolers. The product of the pipeline is put through one side of the cooler to remove heat from the fluid-drive oil system.

FIGURE 4 Hydrokinetic coupling, scoop-control type

Manifolds are usually used on scoop-controlled couplings in lieu of housings. They provide passages to permit oil flow to and from the working circuit and support the scoop tube and, sometimes, one bearing on the output shaft.

Operation The flow of oil in the scoop-trimming fluid drive is begun by the circulating pump, which is driven at constant speed by the input shaft, or external motor driver. The circulating pump moves the oil from the reservoir at the bottom of the housing to an external oil cooler (if used) and then to the rotating elements. Oil entering the rotating casing is acted upon by centrifugal force caused by the casings rotating at the input speed. This

FIGURE 5 Hydrokinetic coupling, put-and-take type (American Davidson)

centrifugal force throws the oil outward against the side of the casing and into the impeller and runner, or working circuit, where it takes the form of an annular ring. Communication ports in the inner casing permit the oil level to equalize in the two chambers.

The amount of oil in the working circuit is regulated by the scoop tube acting as a sliding weir. The scoop tube removes the oil from the casing and empties it into the oil reservoir at the bottom of the housing, where it is ready to begin the cycle once more.

By either manual or automatic control, the scoop tube is moved in the casing. This, in turn, sets the level of the oil is the working circuit because the oil tends to seek the same level in the entire assembly. The scoop tube is designed to give fast response for both increase and decrease of output speed as required. In the leakoff type of fluid drive (Figure 3), the scoop tube and outer casings are not used. Oil flow is initiated by a pump, usually of the viscous or centrifugal type, driven by the input shaft, through a heat exchanger (if required) and to a two-way control valve. This control valve modulates between the two extreme positions: all oil to the working circuit and all oil dumped back to the reservoir. Oil in the working circuit is thrown out through orifices called leakoff ports. Flow is created by centrifugal head, which varies with the depth of oil in the coupling.

If oil is added to the coupling faster than it is thrown out of the orifices, the quantity of oil in the unit and the output speed increase. Obviously the converse is true as well, and if oil is put into the working circuit at exactly the same rate that it is "leaked off," the unit runs at constant speed. This type of unit lends itself well to closed-loop automatic control, which compensates for the differential flow through the leakoff ports. Manual control is questionable because oil must be added at exactly the rate it is discharged or output speed will drift.

In the scoop-control fluid drive, the communication ports in the inner casing are closed to form orifices and the scoop tube casing is sealed at the shaft. This breaks the unit into two separate chambers, the working circuit between the impeller and inner casing and the rotating reservoir between the inner and outer casings. The two are connected only by the orifices, or leakoff ports. Usually the housing, three bearings, and input shaft are omitted. In this configuration, the input rotor and casings are supported by the driving motor. In some cases, the mounting is accomplished through a solid hub as shown in Figure 4, and in others through a disk capable of flexing to absorb slight misalignment. The runner and output shaft are supported either by a pilot bearing and an outboard bearing or by a pilot bearing and the driven machine through a piloted flexible coupling.

Oil flow is initiated by the scoop in the reservoir acting as a pump. This flow is directed out through the manifold to the oil cooler, back to the manifold, and into the working circuit.

A portion of the oil constantly flows through calibrated nozzles in the inner casing to the outer casing, where it is held in an annular ring against the outer casing by centrifugal force. The fluid drive is initially charged with just enough oil to fill the impeller and runner and the cooler circuit so the idle oil in the outer casing is a subtraction from the working circuit. The movable scoop tube adjusts the oil quantity in the outer casing and thus regulates the oil quantity in the working circuit. The scoop tube can be fully engaged, where it skims off all the oil in the casing and thus fills the working circuit. Otherwise, it can be retracted completely so all the oil lies idle in the outer casing and the unit is "declutched." Intermediate positions regulate torque and speed of the drive.

Put-and-take couplings (Figure 5) have not been manufactured in recent years. There was, in the design of such couplings, a variation of the scoop control coupling wherein the position of the scoop tube was fixed; thus, the tube provided circulation only between working circuit and cooler. The amount of oil in the coupling itself was regulated by a gear-type pump that was operated in one direction to pump oil from a reservoir into a unit, stopped to maintain constant coupling speed, or reversed to remove oil from the drive and pump it into the reservoir. This created very unwieldy control systems having very poor response characteristics with some bunting, and the design became obsolete.

Reversibility can be obtained by reversing the driving motor, provided that the unit incorporates oil pumps that are not affected by input shaft rotations. In addition, units utilizing scoop tubes must have dual tips that can accept the flow of oil from either side.

In all fluid drives, the same fluid is utilized to transmit power, to remove absorbed heat, and to lubricate. Thus there is no requirement for internal seals, or slingers, and positive lubrication is assured. Because the power-transmitting medium is the heat-absorbing medium, there are no problems of heat transfer encountered in units utilizing oil pumps. This type of unit can be selected with the capability of dissipating 100% or more of the driving-motor rated power.

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