A coupling is used whenever there is a need to connect the shafts of a prime mover to a piece of driven machinery. The function of a coupling is to axially transmit shaft-to-shaft power from one component of a machinery train to another component of the machinery train. Depending on the coupling design, a coupling may also accommodate misalignment between shafts and transmit axial thrust loads from one machine to another.
In accordance with 31-SAMSS-004, lubricated couplings are not acceptable. Lubricated couplings, such as the gear or spring grid coupling, require periodic lubrication to prevent wear of coupling parts. Nonlubricated couplings do not require grease or oil lubricant.
Flexible couplings must be capable of accepting parallel misalignment of not less than 50 mils (1270 micrometers) between shaft ends, and they must be capable of absorbing twice the maximum axial growth plus twice the axial magnetic centering force of the electric motor.
Rigid Adjustable Spacer Type
In accordance with 31-SAMSS-004, rigid adjustable-type
couplings are required for vertical lineshaft pumps not equipped with thrust bearings. Spacer type couplings must be furnished for all horizontal, flexibly coupled, vertical in-line and vertical suspended pumps. Couplings for high-speed integral gear pumps may be of the nonspacer type. Figure 71 shows an example of a rigid adjustable-type coupling.
Flexible Disk Pack
The flexible metal disc coupling, which is shown in Figure 72, uses two hubs that are keyed to the shafts with a series of thin steel discs that are connected between the hubs. The hubs may be keyed to the shafts or hydraulically pressed onto the shafts.
The flexing element is the series of thin steel discs. The series of thin steel discs is called a disc-pack. Disc-pack couplings can be configured as either single or double disc pack. A double disc-pack coupling uses two packs of thin steel discs that are attached to the hubs through a center connector ring. Each disc-pack is alternately bolted to a coupling hub and the center connector ring. High strength connecting bolts are inserted through specially designed spacer inserts that are securely held in place with lock nuts. Removal or installation of the disc-packs can be performed without removing the coupling hubs or
disturbing the machinery. Disc-pack couplings do not require lubrication.
If the initial alignment of the shafts is not accurate, the assembly of the disc-packs to the coupling hubs is difficult because the closely fitted boltholes offer little clearance for bolt placement.
Excessive flexing that is caused by misalignment will eventually cause the metal discs to fatigue and break.
Disc-pack couplings have the following characteristics:
• They provide high torsional rigidity.
• They can accommodate some axial shaft movement (end float).
• They can accommodate minor shaft misalignment.
• They can transmit power without any backlash or mechanical looseness between the coupling halves.
Figure 72. Double Disc-Pack Coupling
Diaphragm
In accordance with 31-SAMSS-004, flexible disc-pack or
The diaphragm-type coupling, which is shown in Figure 73, consists of two assemblies that are bolted together with a spacer. Each assembly consists of a rigid hub, a diaphragm pack, and a splined adapter. The rigid hubs are either keyed, tapered, or splined to the shafts. The outside diameter (OD) of the diaphragm pack is bolted between the hub and a diaphragm pack guard. The inside diameter (ID) of the diaphragm pack is splined to accept the splines of the splined adapter. The spline transmits the torque from the driving unit through the coupling to the driven unit. The splined adapter is held in place with a clamp ring. The clamp ring is bolted to the splined adapter with bolts that pass through the diaphragm pack. Both of the splined adapters are bolted to a spacer to complete the coupling assembly.
The flexing element is the diaphragm pack. The diaphragm pack consists of several thin convoluted diaphragms. The convolution and its unrolling action result in the diaphragm's large axial capacity with low stresses. Disc and contour diaphragms accommodate for axial motion by pure deformation of material.
Removal or installation of the adapter and diaphragm packs can be performed without removing the coupling hubs or disturbing the machinery.
The diaphragm pack coupling compensates for misalignment.
The diaphragm pack makes the coupling axially soft so that the coupling does not impose excessive axial loads on the bearings or the shafts of the connected equipment. The diaphragm coupling is maintenance free and it requires no lubrication.
Shims may be used between the diaphragm pack and the rigid hub to adjust the effective coupling length.
Diaphragm couplings have the following characteristics:
• High and low ambient temperature capability.
• High-speed capability.
• High-torque capability.
• High-misalignment capability.
• Corrosion-resistant flex elements.
Elastomeric
In accordance with 31-SAMSS-004, elastomeric couplings or couplings that have a comparable torsional stiffness are
required for pumps driven by reciprocating engines and may be considered for pumps driven by synchronous electric motors.
Elastomeric couplings consist of two hubs and a flexible
elastomeric element. The hubs are fitted onto the shaft, and the elastomeric element is attached to the hubs. Figure 74 shows two of the many different styles of elastomeric couplings that are available: a splined rubber-type coupling, which is shown in Figure 74(A), and a rubber tire-type coupling, which is shown in Figure 74(B). Each style uses a different method for the
attachment of the element to the hubs.
The metal hubs of the splined rubber-type coupling are either cast or are machined with a cavity that contains internal and external splines. The rubber insert also has internal and external splines. These splines engage with the hub splines. The rubber insert is a one-piece construction on small diameter couplings.
On larger coupling sizes, the insert is a two-piece construction with the insert split horizontally. A steel lock ring fits into a
groove, which is located on the outside diameter of the insert, to hold the insert halves together.
The rubber tire-type coupling uses two metal hubs that have grooves machined into the hubs. A flexible rubber tire element fits into the grooves and is held in place with threaded fasteners and flanges. Some rubber tire elements are molded directly to the flanges. The flanges are threaded to receive bolts. The flexible rubber element is molded into one piece, and it is then split at one point to facilitate installation.
The rubber tire coupling can be assembled and disassembled without the removal of the hubs from the equipment. The side flange bolts are removed to allow the rubber tire element to be pulled out of the flange and to allow a new rubber element to be installed.
• The elastomeric couplings have the following characteristics:
• They provide high torsional flexibility.
• They can tolerate shock loads and reversing situations.
• They act as a vibration dampener.
• They can accommodate axial shaft movements (end float).
• They can accommodate angular shaft misalignment.
• They offer excellent resistance to electrical conductivity.
• They can be used in corrosive atmospheres (provided that there is careful material selection).
Elastomeric couplings are temperature-limited, they generally have a high overhung weight, and they are difficult to balance.
As the elastomers age, the assumed stiffness and damping values of the elastomer can change and reduce the flexibility of the coupling. Such reduction in the flexibility of the coupling will allow the train components to become overstressed.
Figure 74. Elastomeric Couplings
GLOSSARY
balanced seal A mechanical seal arrangement in which the effect of the hydraulic pressure in the seal chamber, on the seal face closing forces, has been modified through seal design to have a seal balance ration of less than 1.
barrier fluid A fluid that is introduced between dual mechanical seals to completely isolate the pump process liquid from the environment. Pressure of the barrier fluid is always higher than the process pressure being sealed.
bellows seal A type of mechanical seal that uses a flexible bellows to allow a static secondary seal and to provide spring-type loading on the primary ring.
buffer fluid A fluid used as a lubricant or buffer between dual mechanical seals. The fluid is always at a pressure lower than the pump process pressure.
cartridge seal A completely self-contained mechanical seal unit that contains the seal, gland, sleeve, and mating ring. A cartridge seal is pre-assembled and preset before installation.
dual mechanical seal A seal arrangement that uses more than one seal in the same seal chamber in any orientation which can utilize either a pressurized barrier fluid or nonpressurized buffer fluid.
flashing hydrocarbon service
Any service that requires vapor suppression by cooling or pressurization to prevent flashing. This category includes all hydrocarbon services where the fluid has a vapor pressure greater than 14.5 psia (1 bar) at
pumping temperature.
flexible coupling A coupling that permits minor amounts of flexibility by allowing the coupling components to slide or move relative to each other.
primary seal when in proximity to the face of an axially adjustable face seal assembly.
neutral fluid A fluid that does not react with the process fluid. Water and steam are typically used as neutral fluids.
nonflashing hydrocarbon service
A category that includes all hydrocarbon services that are predominately all hydrogen and carbon atoms;
however, other non-hydrocarbon constituents may be entrained in the stream. A product in this category does not require vapor suppression to prevent transformation from a liquid phase to a vapor phase. The vapor
pressure for non-flashing hydrocarbon service is less than 14.5 psia (1 bar).
nonhydrocarbon service A category that includes all services that cannot be defined as containing all hydrogen and carbon molecules; however, some hydrocarbons may be entrained in the fluids.
non-pusher type seal A mechanical seal in which the secondary static seal is fixed to the shaft.
packing A seal that prevents leakage around the plunger or rod of a reciprocating pump.
pusher-type seal A mechanical seal in which the secondary seal is mechanically pushed along the shaft or sleeve
(dynamic secondary seal) to compensate for face wear.
quench A neutral fluid (usually water or steam) that is
introduced on the atmospheric side of a mechanical seal to retard the formation of solids that may interfere with seal movement.
rigid coupling A coupling that is designed to draw the two shafts together tightly so that no relative motion can occur between the shafts.
seal balance ratio Sometimes expressed as a percentage, the ratio of seal face area that is exposed to closing force by hydraulic pressure in the seal chamber, to the total sealing face area.
seal chamber A component that is either integral with or separate from the pump case (housing) and that forms, between the shaft and casing, the region into which the shaft seal or packing is installed.
specific gravity The density of a liquid divided by the density of water at 60°F.