coupling selection criteria

Usually couplings are supplied as part of any new equipment. Instead of having to select a new coupling, one is faced only with the need to replace an old one, or part of an old one. Assuming that the equipment manufacturer selected the right coupling type and size, couplings generate few problems. There are cases, however, when wither the coupling does not live up to expectations or when a new piece of equipment is purchased without a driver and a coupling must be selected. The process is not simple because there is no application for which only one type of coupling would work. The best approach is to let an application engineer from a coupling manufacturer make the selection. Today most manufacturers make more than one type of coupling and can objectively recommend the best one for the application.

Choosing a coupling of the correct size is very important. To do this one must know not only the required power and speed, but also the severity of service the coupling must accommodate. A correction factor, or service factor, must be applied.

Coupling manufacturers rate their couplings in horsepower per 100 r/min. for instance, if a pump required 50 hp (37.3kW) at 1750 r/min, it needs a coupling that can handle 2.86 hp (2.13k W) at 100 r/min. This is correct only if the pump is centrifugal and is driven by an electric motor. In this case the service factor is 1. If we have a double-acting reciprocating pump driven by an internal combustion engine, we have to use a service factor of 2.0 + 1.0 = 3.0 for a gear coupling and 2.0 + 0.5= 2.5 for an elastomer coupling, according to one manufacturer. As a result, we must choose a gear coupling that can handle 8.58 hp (6.4 kW) at 100 r/min or an elastomer coupling that can handle 7.15 hp (5.3 kW) at 100 r/min. it seems that we can choose a smaller coupling if we choose the elastomer type.

However, the elastomer coupling will be about 8 ¾ in (222 mm) in diameter, while the gear coupling will be only half that size! If size is not important, price can be the next selection criterion. But the price of the coupling alone is not a good guide; one should consider the total cost including maintenance, replacement parts, lost production, etc.

Although couplings represent a small percentage of the total cost of a piece of machinery, they can cause as much, if not more, trouble than the rest of the equipment if they are not properly selected. Buying an inadequate size or type of coupling will never be economical in the long run. Maintenance personals are frequently faced with the problem of replacing a worn-out or broken coupling. After the cause of the failure has been determined, careful consideration should be given to the type size, and style of the coupling that will used as a replacement. Whenever possible, it should satisfy all the needs of the drive.

Proper selection as to type of coupling is the first step of good maintenance. A well chosen coupling will operate with low cross loading of the connected shafts, have lower power absorption, induce no harmful vibrations or resonance into the system, and have negligible maintenance costs. The primary consideration in selecting the correct type of coupling, as well as its size and style are:

1. Type of driving and driven equipment
2. Torsion characteristics
3. Minimum and maximum torque
4. Normal and maximum rotating speeds
5. Shaft sizes
6. Span or distance between shaft ends
7. Changes in span due to thermal growth, racking of the bases, or axial movements of
the connected shafts during operation.
8. Equipment position (horizontal, inclined or vertical)
9. Ambient conditions (dry, wet, corrosive, dust, or grit)
10. Bearing locations
11. Cost (initial coupling price, installation, maintenance, and replacement)

The coupling should be conservatively selected for torque involved. Consideration must be given to all peak loads and shock loads encountered in normal service. If the coupling is to operate at high speeds, it should be dynamically balanced. Special coupling modifications dictated by the connected equipment should be made. If any doubt exists as to proper type or size of coupling to use, it is recommended that the manufacturer be consulted. Most manufacturers are usually qualified to make recommendations and assist in the coupling procurement.


GENERAL SELECTION CRITERIA:

TORQUE

The vendor shall furnish couplings with the maximum practical service factor, considering torque transmitted by the coupling, overhung movement, speed, and lubrication. Unless otherwise specified, the coupling, coupling-to-shaft junctures, and machinery shafting shall be capable of continuos operation at a torque determined by following equation (I) or (II), using an experience factor of 1.75. The purchaser will specify the expected magnitude, nature, and number of occurrences of transients to which the coupling will be subjected in service. The coupling, coupling-to-shaft juncture and shafting shall be capable of transmitting 115 percent of the purchaser-specified maximum transient torque without damage. The vendor shall state the coupling design rating (in horsepower per 100 revolutions per minute). The coupling size selection shall be submitted to the purchaser of approval.

Ts=
63,025 X Pnormal X SF
-------------------------- - Eq. (I)
Nnormal





In SI Units,


Ts=
9549 X Pnormal X SF
------------------------- - Eq. (II)
Nnormal


Where:

Ts = torque used to make the coupling selection, in inch-pounds (joules)


Pnormal = input power required by the driven machine at the specified normal operating point, in horsepower (kilowatts)

Nnormal = speed corresponding to the normal power, in revolution per minute

SF = experience factor derived from various modes of off-design operation that may result from such factors as a change in the density of the pumped fluid (molecular weight, temperature or pressure variation), unequal load sharing, fouling, and driver output at maximum conditions.


Note: Should reasonable attempts to achieve the specified experience factor fail to result in a coupling weight and subsequent overhung moment commensurate with the requirement for rotor dynamics of the connected machines, a lower factor may be selected by mutual agreement of the purchaser and the vendor. The selected value shall not be less than 1.25.