AN INTRODUCTION TO FLUID COUPLING AN INTRODUCTION TO FLUID COUPLING
INTRODUCTION
The term fluid coupling can be loosely used to describe any device utilizing a fluid to transmit power. The fluid is invariably a natural or synthetic oil because oil is capable of transmitting power, is a lubricant, and is able to absorb and dissipate heat. Manufacturers have tried water as the fluid in fluid couplings, but sealing problems (keeping water out of the bearings and oil out of the water) and corrosion have prevented its use in any standard catalog drive.
TYPES OF FLUID COUPLINGS
All fluid couplings may be broken down in four categories:
1. Hydrokinetic
2. Hydrodynamic
3. Hydroviscous
4. Hydrostatic
1- HYDROKINETIC DRIVES
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:
F represents energy,
M is the mass of the working fluid,
V is the velocity of the oil particles before impingement,
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 FIG 1
leakoff couplings, FIG 2
scoop-control couplings, FIG 3
put-and-take couplings FIG 4
2- HYDRODYNAMIC DRIVES
This type of fluid coupling is occasionally used to drive pumping equipment, usually in the portable pump field
Basic Principle In the most common forms of hydrodynamic drives, planetary gear trains utilize some components as oil pumps. Throttling the discharge of these pumps creates back pressure and increases drive torque.
3- HYDROVISCOUS DRIVES
Hydroviscous drives are relatively new in commercial use. There are several manufacturers are marketing this type of drive for a wide range of pump applications
Basic Principle Hydroviscous drives operate on the basic principle that oil has viscosity and energy is required to shear it. More energy is required to shear a thin film than a thick one. The hydroviscous drive varies its torque capability by varying the film thickness between driving and driven members.
4- HYDROSTATIC DRIVES
Basic Principle There are many variations of hydrostatic variable-speed drives, but in one form or another they invariably use positive displacement hydraulic pumps in conjunction with positive displacement hydraulic motors.
In some cases, varying amounts of fluid are bypassed from the pump discharge back to the pump suction. This provides a controllable variable flow to the positive displacement motor and therefore a variable output speed. This system has no particular advantages over the more common variable-speed drives. The higher-than-average first costs and above-average maintenance required explain why this type of hydrostatic system is seldom used. other cases, the hydrostatic drive system uses variable-flow positive displacement pumps that may be of the sliding vane type or axial piston type (Figure7). Reducing the discharge flow on the hydraulic pump reduces output speed; increasing pump flow increases output speed.
This type of variable-speed drive is offered in package form with pump, piping, and motor mounted in a common housing. It offers the capability of torque multiplication, maintains a relatively constant efficiency regardless of speed, has excellent control characteristics, and is widely used in the machine tool and other industries. The output shaft can be reversed by valving (without changing motor rotation). This design has inherently high first cost and maintenance requirements, precluding significant use as a pump driver.
Response
Hydrokinetic Drive In the scoop-trimming fluid drive, response speed is affected by many factors. The speed with which oil can be added to the working circuit (a factor of the size of the oil pumps) or removed from it (a factor of the size of the scoop tube) influences response capability.
In the leakoff unit, the size of the leakoff ports determines how quickly the unit will empty. However, the oil pumps must be sized to replace this oil and have additional capacity to fill the coupling in a reasonably short time.
Scoop-control units are limited by the ability of the scoop tube to pump oil from the reservoir into the working circuit and by the ability of the leakoff ports to return it to the reservoir. Some special marine couplings utilize quick-dumping valves, but these are seldom, if ever, used with pump drives.
Obviously the speed at which the scoop tube is moved is also significant. Large polar moments of inertia (WK² values) of the driven equipment will increase response time.
The scoop-trimming coupling offers the best overall response characteristics of the hydrokinetic drives, and standard catalog machines have normal fill times ranging from 10 to 15 s. They will accomplish 90% of a 10% step speed change in the 40 to 100% speed
Hydrodynamic and Hydroviscous Drive Both hydrodynamic and hydroviscous couplings respond very quickly to a change in demand for torque output. Both require a mechanical motion (change in valve position or change in spacing between disks) followed immediately by a change in pressure or in film thickness.
In most cases, the torque available for speed change and the WK² values involved are of such a magnitude that the major portion of the response time is caused by inertial effects rather than by the time required to change torque. This is particularly true in the deceleration of centrifugal pumps. Unless auxiliary brakes are built-in, none of the hydrokinetic,
hydrodynamic, or hydroviscous drives can provide dynamic braking.On a demand to decrease speed, they can at best reduce driving torque to zero. Under these circumstances, the only retarding force to slow the inertia of the driven machine is the load it developed. In the case of centrifugal pumps on fixed systems, this load would fall off as the cube of speed, and below 40% of full speed, such pumps have an almost insignificant braking effect.
SELECTION
The basic information required by the manufacturer for selection is as follows:
1. Speed and type of driver
2. Power required by the driven machine at, or at least, one operating point
3. Character of driven machine-smooth or pulsating load; how torque requirements change with speed
4. Cooling medium available and temperature of medium
5. Control type
6. Accessories
7. Special specification requirements
Standard catalog variable-speed fluid couplings are available from one or more manufacturers in the speeds and powers shown in Table 1. Special designs are available for higher power ratings.
Fluid couplings are utilized to drive pumps in virtually all pump applications requiring variable flow or pressure. They are used primarily to improve efficiency and controll ability, to permit no-load starting, and to reduce pump and system wear. They are standardized to the degree that units are available to handle most pumping applications. Most manufacturers stand ready to develop new designs as the requirements of the marketplace change.
AN INTRODUCTION TO FLUID COUPLING
INTRODUCTION
The term fluid coupling can be loosely used to describe any device utilizing a fluid to transmit power. The fluid is invariably a natural or synthetic oil because oil is capable of transmitting power, is a lubricant, and is able to absorb and dissipate heat. Manufacturers have tried water as the fluid in fluid couplings, but sealing problems (keeping water out of the bearings and oil out of the water) and corrosion have prevented its use in any standard catalog drive.
TYPES OF FLUID COUPLINGS
All fluid couplings may be broken down in four categories:
1. Hydrokinetic
2. Hydrodynamic
3. Hydroviscous
4. Hydrostatic
1- HYDROKINETIC DRIVES
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:
 |
| FIG 1 |
E= 1/2 M (V₁²-V₂²)
whereF represents energy,
M is the mass of the working fluid,
V is the velocity of the oil particles before impingement,
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 FIG 1
leakoff couplings, FIG 2
 |
| FIG 2 |
scoop-control couplings, FIG 3
 |
| FIG 3 |
put-and-take couplings FIG 4
 |
| FIG 4 |
This type of fluid coupling is occasionally used to drive pumping equipment, usually in the portable pump field
Basic Principle In the most common forms of hydrodynamic drives, planetary gear trains utilize some components as oil pumps. Throttling the discharge of these pumps creates back pressure and increases drive torque.
 |
| FIG 5 |
Hydroviscous drives are relatively new in commercial use. There are several manufacturers are marketing this type of drive for a wide range of pump applications
Basic Principle Hydroviscous drives operate on the basic principle that oil has viscosity and energy is required to shear it. More energy is required to shear a thin film than a thick one. The hydroviscous drive varies its torque capability by varying the film thickness between driving and driven members.
 |
| FIG 6 |
Basic Principle There are many variations of hydrostatic variable-speed drives, but in one form or another they invariably use positive displacement hydraulic pumps in conjunction with positive displacement hydraulic motors.
In some cases, varying amounts of fluid are bypassed from the pump discharge back to the pump suction. This provides a controllable variable flow to the positive displacement motor and therefore a variable output speed. This system has no particular advantages over the more common variable-speed drives. The higher-than-average first costs and above-average maintenance required explain why this type of hydrostatic system is seldom used. other cases, the hydrostatic drive system uses variable-flow positive displacement pumps that may be of the sliding vane type or axial piston type (Figure7). Reducing the discharge flow on the hydraulic pump reduces output speed; increasing pump flow increases output speed.
This type of variable-speed drive is offered in package form with pump, piping, and motor mounted in a common housing. It offers the capability of torque multiplication, maintains a relatively constant efficiency regardless of speed, has excellent control characteristics, and is widely used in the machine tool and other industries. The output shaft can be reversed by valving (without changing motor rotation). This design has inherently high first cost and maintenance requirements, precluding significant use as a pump driver.
 |
| FIG 7 |
Hydrokinetic Drive In the scoop-trimming fluid drive, response speed is affected by many factors. The speed with which oil can be added to the working circuit (a factor of the size of the oil pumps) or removed from it (a factor of the size of the scoop tube) influences response capability.
In the leakoff unit, the size of the leakoff ports determines how quickly the unit will empty. However, the oil pumps must be sized to replace this oil and have additional capacity to fill the coupling in a reasonably short time.
Scoop-control units are limited by the ability of the scoop tube to pump oil from the reservoir into the working circuit and by the ability of the leakoff ports to return it to the reservoir. Some special marine couplings utilize quick-dumping valves, but these are seldom, if ever, used with pump drives.
Obviously the speed at which the scoop tube is moved is also significant. Large polar moments of inertia (WK² values) of the driven equipment will increase response time.
The scoop-trimming coupling offers the best overall response characteristics of the hydrokinetic drives, and standard catalog machines have normal fill times ranging from 10 to 15 s. They will accomplish 90% of a 10% step speed change in the 40 to 100% speed
Hydrodynamic and Hydroviscous Drive Both hydrodynamic and hydroviscous couplings respond very quickly to a change in demand for torque output. Both require a mechanical motion (change in valve position or change in spacing between disks) followed immediately by a change in pressure or in film thickness.
In most cases, the torque available for speed change and the WK² values involved are of such a magnitude that the major portion of the response time is caused by inertial effects rather than by the time required to change torque. This is particularly true in the deceleration of centrifugal pumps. Unless auxiliary brakes are built-in, none of the hydrokinetic,
hydrodynamic, or hydroviscous drives can provide dynamic braking.On a demand to decrease speed, they can at best reduce driving torque to zero. Under these circumstances, the only retarding force to slow the inertia of the driven machine is the load it developed. In the case of centrifugal pumps on fixed systems, this load would fall off as the cube of speed, and below 40% of full speed, such pumps have an almost insignificant braking effect.
SELECTION
The basic information required by the manufacturer for selection is as follows:
1. Speed and type of driver
2. Power required by the driven machine at, or at least, one operating point
3. Character of driven machine-smooth or pulsating load; how torque requirements change with speed
4. Cooling medium available and temperature of medium
5. Control type
6. Accessories
7. Special specification requirements
Standard catalog variable-speed fluid couplings are available from one or more manufacturers in the speeds and powers shown in Table 1. Special designs are available for higher power ratings.
Fluid couplings are utilized to drive pumps in virtually all pump applications requiring variable flow or pressure. They are used primarily to improve efficiency and controll ability, to permit no-load starting, and to reduce pump and system wear. They are standardized to the degree that units are available to handle most pumping applications. Most manufacturers stand ready to develop new designs as the requirements of the marketplace change.
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