Shaft couplings are used extensively throughout industry to connect rotating equipment. This article will discuss the purposes of a coupling, identify different types of couplings, various effects of couplings, and define a few of the basic terms used when discussing couplings.
A '''coupling '''is used to connect the drive shaft of a power source, such as an electric motor or a diesel engine, to a shaft on a piece of driven machinery, such as a pump or an electrical generator as shown in''' Figure 1'''.
The primary purpose of a coupling is to transfer rotational motion from one shaft to another shaft at maximum efficiency. Couplings can also perform several additional functions, such as providing a means for disconnecting the driver motor or engine from the driven equipment, such as a pump. Couplings can also allow some degree of relative axial motion, reduce or eliminate vibration transfer from one component to another, and permit small amounts of misalignment.
The correct alignment of couplings and shafts helps to ensure that the equipment functions properly. If shafts are not aligned properly, damage to the equipment may result, and the life of the equipment and its availability will be reduced.
There are two major categories or types of couplings: rigid and ''flexible.''
Rigid couplings normally come in two types:
Flexible couplings normally come in six types:
- Spring grid
- Metal disk
Rigid couplings are used to connect machines where it is necessary to maintain shafts in precise alignment. They are used where the shaft of one machine supports and positions the shaft of another machine. Their advantage is that they provide longer shaft seal life by keeping the shaft alignment more precise, which reduces shaft wear on packing or mechanical seals.
Flexible couplings accomplish the purpose of any coupling, but their advantage over rigid couplings is they accommodate the unavoidable misalignment between shafts in some machinery. Flexible couplings also allow for a degree of axial movement between the coupled shafts as may occur due to thermal expansion. Flexible couplings are used extensively in industry, particularly in electrical machinery. When a generator is started, its shaft will seek an electrical center, causing possible shaft misalignment. Two basic designs for flexible couplings use mechanical flexibility or material flexibility.
Mechanical flexibility uses design clearances within the coupling to allow for slight movement. Material flexible couplings use materials that have sufficient resistance to failure due to flexing, allowing a long service life.
'''Figure 2 '''shows two common rigid couplings: the flanged coupling and the ribbed coupling. The flanged coupling is usually made with flat faces; the ribbed coupling is usually used where two or more long shafts are connected, e.g., a line shaft. If the shaft were small in diameter, the coupling would be solid instead of split and bored. Rigid couplings of this type are generally called '''''sleeve '''''couplings.
Flexible couplings compensate for temperature changes and permit end movement of the shafts without placing unwanted stresses on bearings and seals. Flexible couplings should not be used to compensate for misalignment at the time of installation. Types of flexible couplings include gear, spring grid, chain, metal disk, and elastomeric.
All flexible couplings consist of two hubs that are attached, one each to the driver and driven shafts. The hubs may be attached to the shaft by a keyway, taper fit, or other means. The hubs are connected in various ways, several of which are discussed later.
For all flexible couplings, the hubs must be aligned to within the manufacturer’s specifications for the driven equipment, driver, and coupling. In addition, some equipment may require that the coupling permit end float of the shaft, but limit the end float to a certain amount. Many equipment manufacturers specify alignment and end float requirements; others do not. In all cases, the manufacturer’s literature and drawings should be consulted.
A split sleeve connects the hubs of a gear coupling, as shown in''' Figure 3'''. The hubs are aligned on the shafts with the sleeve halves in place behind them. The sleeve halves then slide into place over the hubs; they are bolted together and aligned. The manufacturer recommends that the hub and sleeve teeth are packed with lubricant before the hubs and sleeve halves are placed on the shafts.
The hubs are geared to the sleeve. The gear teeth are designed with a small amount of backlash to permit minor parallel and angular misalignment. Gear couplings permit end float and may be designed to limit end float to a certain extent.
High-speed gear couplings are usually dynamically balanced. The manufacturer's instructions must be followed during assembly and disassembly to prevent the balance from being lost.
A high-strength metal grid connects the hubs of a spring grid coupling, as shown in '''Figure 4'''. The spring grid interlaces between grooves in the hubs and transmits rotational motion from the driver shaft to the driven shaft. The spring grid compensates for minor parallel and angular misalignment. In addition, '''Figure 5 '''shows how the equipment can be positioned so that the spring grid allows either unrestrained or limited end float.
To limit end float with a spring grid coupling, the equipment should be positioned with a wide space between the hubs so that the hub grooves are near the outside of the spring grid. Bringing the hub faces closer together toward the center of the spring grid will provide more room for end float.
The spring grid coupling can be used to connect the hubs while the alignment is made and should be packed with lubricant when it is installed to minimize air pockets. After the alignment, the cover end seals may be put in place and additional lubricant added through the grease fittings on the cover. Lubricants and lubricant change schedules should be in accordance with coupling manufacturers' specifications.
A metal disk coupling, shown in''' Figure 6''', uses several layers of thin steel, called a ''disk pack'', to compensate for misalignment. Generally, the disk pack is bolted alternately to the hub and a center spool. In some couplings, the center ring is left out, and the disk packs are bolted directly to each other.
The metal disk coupling requires no lubrication and transmits motion smoothly without backlash. Installation and removal of the disk packs are easy and can be done without removing any of the coupling hubs or machinery. Manufacturer’s instructions must be followed exactly during the assembly and disassembly of metal disk couplings.
Chain (or roller chain) couplings, shown in''' Figure 7''', consist of two hubs and a roller chain. The roller chain wraps around the outside of the coupling hubs. Teeth are cut into the flange of each hub to receive the chain. Variations of the basic chain coupling include the use of a double-length chain and a single-strand (tooth or timing) chain instead of a double strand of roller chain. Although most chain couplings will operate satisfactorily with minimum lubrication, the life of the coupling can be greatly extended with proper lubrication. To ensure proper lubrication, most chain couplings are equipped with covers to retain lubricant.
Elastomeric couplings consist of two hubs and a flexible elastomeric element. One type of commonly used elastomeric coupling has an elastomeric element with teeth that lock into teeth on the hubs, as shown in''' Figure 8a'''. The hubs are fitted onto the shaft. The elastomeric element can flex to compensate for minor angular and parallel misalignment, can absorb shock load, and can compensate for end float. The elastomeric element may be one piece, split or solid, or two pieces. A retaining ring is used to join the halves of two-piece elements.
In other types of elastomeric couplings, the elastomeric element is held in place by clamp rings on each hub. The elastomeric element is usually one piece, split or solid, and resembles a tire in appearance. When the element is in place, clamp ringbolts are used to clamp the element between the flange and clamp ring on each hub, as shown in''' Figure 8b'''.
Elastomeric couplings do not require lubrication. Many designs use a split elastomeric element that enables installation and removal of the element without the removal of any coupling hubs or machinery. '''Figure 8c '''shows an exploded view.
Three-jaw couplings, shown in''' Figure 9,''' consist of two hubs, each with three equally spaced jaws on their faces. The two hubs are aligned to each other. Fitted between the hubs before they are placed together is a flexible insert that can be made of various materials such as leather, rubber, polyurethane, or other materials, depending on the type of load. The three-jaw coupling is commonly used on small equipment with low-power inputs.
The terms defined below are used in association with couplings.
A cylindrical object of various lengths and diameters that transmits power and motion by rotation, as shown in''' Figure 10'''.
The part of a coupling, which actually transmits power and rotation from the driving shaft to the driven shaft, as in''' Figure 10'''.
A small metal piece inserted between a shaft and hub to prevent relative motion, as shown in''' Figure 10.'''
- Cylinder Bore
A hole with a uniform diameter, as shown in''' Figure 10'''.
- Tapered Bore
A hole with a gradual and uniform change in diameter, as shown in''' Figure 11'''.
The space between two hub faces, as shown in''' Figure 12'''.
- Match Marks
An impression or scratch made on the circumference of each hub to mark the rotational position for the alignment process, as shown in '''Figure 13'''. The individual can ensure both hubs rotate together by keeping the match marks even during rotation. This provides more accurate alignment results.
- Total Indicator Reading
The actual amount a shaft is misaligned divided by two.