In an ideal worldly concern, multiple components could be produced in a single musical composition, or coupled and installed in perfect alignment. however, in the real worldly concern, separate components must be brought together and connected onsite. Couplings are required to transmit rotational forces ( torsion ) between two lengths of rotating shaft, and despite the most rigorous attempts, conjunction is never perfect .
To maximize the life of components such as bearings and shafts, flexibility must be built in to absorb the residual misalignment that remains after all possible adjustments are made. proper lubrication of couplings is critical to their performance .
Misalignment
misalignment can occur as either an offset or as an angular displacement on two of the potential three axes ( Figure 1 ). The third bloc, in the longitudinal management, is not normally measured, though errors in this management can result in ceaseless excessive thrust loads in a system.
Figure 1. Types of Misalignment
For major installations, such as bombastic compressors, telegram alliance methods are used. Smaller applications have traditionally used rim and expression dial indicator readings to quantify and correct misalignment, though optical laser indicators are quickly gaining in popularity ascribable to their rest of consumption and accuracy .
In pace-setting maintenance organizations, efforts are besides made to compensate for thermal growth that occurs in equipment during process. All materials ( except body of water ) expand a small sum when heated, the sum by which they do therefore is governed by the material ’ s coefficient of thermal expansion and the degree to which it is heated. A machine that is brought into alliance at ambient temperature will creep into a situation of misalignment as the machinery materials climb or fall to operating temperature .
Attempts are made to preheat or cool equipment to normal operating conditions before performing alignment checks. alternatively, calculations of anticipate thermal growth can be used to intentionally misalign the drive prepare at ambient temperature so that it may grow into alignment .
Whatever precautions are taken to make alignments a accurate as possible some amount of remainder misalignment will inevitably remain. misalignment forces rigid car components such as shafts to deflect in order to effectively become aligned .
This deflection stresses the components, causes vibrations, and distributes higher and mismatched loads on the structures that support these elements, such as bearings. These stresses waste energy and can dramatically reduce equipment life and dependability .
Designed properly, couplings can absorb misalignment forces indeed that more expensive, critical and sensitive components may be saved. While rotating shafts appear uncompromising, the bearings which support them are some of the most medium preciseness components in the drive train .
Types of Couplings
Coupling designs may be divided into four principal categories, each having several particular designs. Solid and charismatic couplings do not require lubrication, but are included here for completeness. upstanding couplings are basically fixed structures that do not compensate for misalignment, but do allow two shafts to be joined for the purpose of transmitting torsion .
Bolted hubs keyed onto shafts are a common example of a machine with magnetic couplings. magnetic couplings allow shafts not in directly contact to be driven in concert using mighty permanent or electric magnets. A sealless magnetic drive pump is a common case .
There are some conventional applications that could be fitted with a stand-alone magnetic couple using this same principle and ultra-powerful ( and expensive ) permanent wave magnets .
other coupling types are compromising couplings and fluid couplings. many flexible couplings use fixed position flexible metallic, rubber or formative elements, such as phonograph record or bushings, that rotate with the shafts and steep misalignment. Designs of this type do not require lubrication .
Others such as geared, chain, grid and cosmopolitan joints do require lubrication for proper performance and longevity. Fluid couplings include torsion converters and torsion multipliers vitamin a well as relatively simple fluid couplings, which are couplings filled with lubricate fluids that rely on the fluid itself to transmit torsion .
Flexible Couplings
gear couplings ( Figure 2 ) compensate for misalignment via the clearance between gear teeth .
Figure 2. Gear Couplings
Shaft-mounted external gear tooth on both shafts mate with internal gear teeth on a house that contains a lubricant. other invention mount external tooth on only one shaft, mating with internal teeth mounted to the other shaft .
acceleration or deceleration can result in impacts between gear teeth due to backlash from the clearance being taken up on opposite sides of gear teeth. Misalignment will result in sliding relative motion across mating teeth as they pass through each revolution .
Chain couplings ( Figure 3 ) operate similarly to gear couplings. Sprockets on each shaft end are connected by a roller chain .
Figure 3. Chain Couplings
The headroom between its components a well as the headroom in mating the chain to the sprockets compensate for the misalignment. Loading is alike to that of geared couplings .
external grid couplings ( Figure 4 ) practice a corrugated steel grid that bends to compensate for loading induced by misalignment .
Figure 4. Grid Coupling
Grooved discs attached to the ends of each diaphysis theater the power system, which transmits torsion between them. low amplitude sliding motion develops between the grid and grooves as the grid deforms under load, widening in some locations and narrowing in others over each revolution.
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cosmopolitan joints are used for maximum allowable misalignment up to 20 to 30 degrees, depending upon the specific invention. They are used extensively for the drive shafts of vehicles to allow the wheels to move with the abeyance system. universal joints use a four-spindled component called the spider to connect two shafts terminating in yokes or knuckles at properly angles ( Figure 5 ) .
Figure 5. Universal Joint
Each of the four spider journals is supported by a bear or bush contained in one of the knuckles, which allow articulation. In some cases, greater articulation can decrease wear rates by allowing more complete exploitation of a lubricate movie .
Lubricants for Flexible Couplings
Both lubricating oils and greases can be selected to lubricate flexible couplings. Unless specifically noted by the coupling couturier, couplings for the majority of industrial components are grease lubricated. Coupling components are protected primarily by an vegetable oil film which bleeds from the grease thickening and seeps into the loading zone .
Lubricated flexible couplings require protection from the low amplitude relative motion that develops between their components. other concerns include centrifugal stress on the lubricant ( particularly dirt ), which causes premature separation of the anoint from the thickening, hapless oil distribution within the housing and oil escape from the housing .
The gesticulate ’ s low amplitude, articulation amphetamine, and tendency toward a sliding rather than rolling action inhibits the growth of hydrodynamic ( full-film ) lubrication. Greases made with high-viscosity base oils, antiscuff ( EP ) and metal-wetting agents are recommended to overcome the boundary ( shuffle film ) conditions that much exist in flexible couplings. high oil viscosity besides slows the escape rates .
centrifugal forces in flexible couplings can be extreme, becoming greater as outdistance from the rotational axis is increased. even moderately size couplings can generate forces thousands of times greater than gravity ( referred to as Gs ). Grease manufacturers place a high priority on formulations that resist premature separation of oil and thickening due to the high G forces .
Fluid Couplings
Fluid couplings transfer momentum from the stimulation shaft to a fluent and then to the output beam when transmitting torsion. Misalignment is accommodated entirely by clearances between the moving parts. The minor clearances don ’ metric ton provide much room for error in conjunction. however, it is possible to effectively compensate for shock load and high torsion starting loads, as there is no solid connection between remark and output shafts .
In fluid couplings, an impeller attached to the input rotating shaft accelerates fluid within the pair as it spins, much like in a centrifugal pump. This fluid then hits the vanes of the output shaft ’ sulfur base runner, transferring its momentum as the runner accelerates. It will accelerate until it approaches the speed of the input shaft, but will never actually reach it .
The dispute in travel rapidly between the input signal and output shafts is known as slippage. Of path, frictional and gluey drag must be overcome before the output shaft can rotate. The minimum input speed required for this stipulate is known as the procrastinate accelerate. equipment with large electrostatic loads, such as a steam or gas turbine, would incorporate a fluent coupling to minimize the initial try on the driving shaft .
Shock loads on the input signal side, such as starting torsion, are never created. The rush of the input beam is never restrained. When the stall speed is exceeded, the end product quill will begin to accelerate, but will do so at a constrained rate due to its moment of inactiveness ( resistance to angular acceleration ) .
slippage is created as the ball carrier accelerates to the focal ratio of the input, dissipating excess energy through gluey estrus genesis in the fluid. Output side daze loads will be similarly dissipated, even if the output beam should wholly stall .
torsion converters and multipliers are extra applications of fluid couplings that allow the input torsion to be modified before transmission. These designs operate basically by the same principles, but are mechanically much more complex. A detail discussion of these devices is outside the oscilloscope of this article .
Lubricants for Fluid Couplings
The profligacy of energy that makes fluid couplings so tolerant of shock loading creates the potential for rapid and extreme increases in fluid temperature. The energy dissipated during carrel and slip is converted to heat through the gluey shear of the fluid ( fluid home friction ) .
In extreme applications, such as a torsion converter in an car climbing a steep hill or pulling a heavy load, the fluid temperature can rise substantially above the convention 200°F operational temperature in less than a hour .
oxidation and thermal abasement resistance are crucial qualities of petroleum used for fluent couplings because of the potential for drastic temperature increases. similarly, a eminent viscosity index ( VI ) is besides utilitarian to prevent severe decreases in operating viscosity at temperature spikes and excessively high operating viscosity at low temperature conditions .
depleted viscosity fluids are normally used in these applications to reduce the baron lost to heat due to fluid clash. Fluid coupling viscosities may fall between 2.5 to 72 central time at 40°C. For fluid couplings designed to operate at high temperatures, such as in car ATF applications, viscosity limits may be given at 100°C. distinctive automotive bureau of alcohol tobacco and firearms requirements would be 3 to 7 central time at 100°C .
These fluids must besides resist foaming due to the severe agitation caused by the impeller ’ s movement and its impact upon the runner vanes. Rust protective properties help preserve the couple ’ south metallic components. Hydrocarbon-based fluids are superior in this respect to other fluids, but their performance can be far improved through rust inhibiting additives. Seal compatibility is besides authoritative for long-life utility .
Maintenance Recommendations
acceptable life can be expected from any of these devices only if proper sustenance is performed. lubricant levels and choice must be verified through periodic checks. Additional lubricant may need to be added to compensate for escape, and sporadically the lubricant must be flushed and changed to remove harmful byproducts of lubricant dislocation, to replace oil-depleted grease or to refresh the linear population .
gearing couplings require possibly the most maintenance. typical relubrication intervals are six months to one year, depending upon application badness and have .
All sustenance tasks must be performed with attention paid to contamination control. The sliding contact suffered by many couplings indicates that abrasive three-body wear caused by particulate contaminant could be particularly damaging. Improper removal of solvents used to clean couplings during inspections and flushing operations can lead to significant gluey reduce of the lubricant in operation, or damaging reactions with grease-thickening materials .
All couplings will endure importantly better when the demands placed on them are reduced. Consider the first credit line of defense mechanism to be a minimization of shock loading, including hard starts and sudden load reversals. sometimes functional demands make this impossible. The principal source of loading in coupling systems can be controlled to a great extent, however. Proper alliance is considered a high-priority, preciseness alimony functions.
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Use oscillation analysis or thermography during process to identify couplings that are not in alignment, as even the sturdiest foundations chemise over time. surely, check for proper alignment whenever intrusive sustenance or repairs are performed on the pair components .
References
Chevron U.S.A. Inc. Technical bulletin : “ Application Section Couplings. ” August 1990 .
Mobil Oil Corp. Technical issue : “ Coupling Lubrication Service Guide. ” 1986 .
