6.4 Shaft coupling
Shaft coupling
A shaft coupling is intended to transfer torque between two in-line, or nearly in-line, rotating shafts. The magnitude of the torque in the shafts is equal, although slipping and disengagement can cause speed variations. In its simplest, and perhaps oldest form, the coupling acts as a means of joining shafts. Another function is to join two shafts which are not necessarily in perfect alignment with each other. The coupling in this case must be capable of absorbing such misalignment. Shaft couplings can perform many different functions and have varying characteristics. Shaft couplings are thus usually divided into three main groups with sub-divisions, namely:
- Non-disengaging couplings
- solid
- torsionally rigid
- torsionally flexible
- Disengaging couplings
- clutch with maneuver mechanism
- free-wheeling clutches
- Limited torque couplings
- non-controlled
- controlled and variable
Some of the requirements for flexible couplings, including definitions, performance and operating conditions, dimensions of bores, reference to necessary components together with an appendix on alignment is to be found in ISO 14091 and ISO 10441 and API 671.
For pump applications it is usual to utilize a coupling from the first group above, although special installations make use of disengaging clutches and limited torque couplings. Examples being centrifugal clutches to reduce starting loads when using a direct started induction motor. Hydro dynamic clutches can be used for reducing starting loads and speed regulation. Combinations of brakes and reverse locks can be used in order to prevent reverse pump rotation for example.
Various types of shaft coupling
Non-disengaging couplings maintain, after assembly, a more or less flexible although continuous transmission of the rotational movement. The connection is only broken for disassembly, repair, etc. Flexible couplings of one form or another, which are capable of absorbing residual misalignment, are most common, although solid couplings do have their areas of use, see figure 6.4a . One example is the split muff coupling, the main advantage being its ease of assembly.
Another example is in the case of long-shaft pumps and stirrers, figure 6.4b where, because of space requirements when disassembling, it is necessary to join the shaft. Production technicalities also necessitate the joining of long shafts. The most usual couplings in these cases are disc or flange couplings.
Figure 6.4a Examples of solid shaft couplings.
Figure 6.4b Long-shaft pump.
Torsionally-rigid flexible shaft couplings consist of various types of diaphragm and gear couplings, see figure 6.4c. Their common feature is their ability to take up angular misalignment and axial deviations. Radial misalignment can only be absorbed by means of double coupling sections, whereby the size of the radial misalignment is determined by the angular misalignment times the distance between the coupling sections.
Figure 6.4c Examples of torsionally-rigid flexible shaft couplings.
Torsional/y flexible shaft couplings usually consist of flexible rubber, plastic or even steel elements, figure 6.4d. The first mentioned coupling elements require somewhat larger coupling diameters because of their lower load carrying capacity.
In order to simplify disassembly and service of machines, so-called spacer couplings are preferred, figure 8.4e. Removal of the coupling enables service of the respective machines without necessitating their removal.
Misalignment
The types of movement or deviation which can occur between two shafts are of three types, see figure 8.4f, namely:
Radial deviation, parallel misalignment, where the shafts are parallel although not lying on a common center line.
Axial deviation, end float, where the shaft center lines are in alignment although axial movement is possible.
Angular deviation, angular misalignment, where the center lines of the respective shafts form an angle, the intersection point of which lies between the ends of the shafts.
Figure 6.4g Types of misalignment.
These deviations can occur separately or in combinations. Suppliers of couplings provide information relating to the maximum permissible size of these deviations, usually stated for each individual type of deviation. It is important to know the maximum permissible values of combined misalignment deviations, see figure 6.4h and how the maximum permitted deviations are influenced by speed and the torque transmitted.
Figure 6.4h Permissible angular misalignment as function of axial deviation and radial misalignment for a particular size of double-diaphragm coupling.
The service life of both couplings and machines are influenced by oblique settings. Just how much the life of the machine is affected can only be judged when information regarding the precise magnitude of the torque and moments transmitted due to oblique setting are known. It is usual to refer only to the amount of obliquity permitted for a given coupling type, when it is the amount of obliquity tolerated by the machine for a given grade of misalignment, (figure 6.4i) which should really be investigated.
Figure 6.4i Relationship deviation/force
Shaft coupling forces and moments
A shaft coupling is normally constructed so as to only be subjected to torsional moments. In certain cases a coupling can be subjected to bending moments as well as axial and radial forces. For solid couplings this does not normally present a problem. For other types of coupling, however, these forces should be avoided, or at least be kept to a minimum, exceptions to this, however, are diaphragm couplings, which can absorb radial forces and carry, for example, long intermediate shafts. Special arrangements can sometimes be made for other types of coupling. It is always advisable to consult the supplier in each individual case.
Service factors
When determining the size of flexible, and even for certain other types of coupling, it is usual to evaluate a certain service factor or safety factor, which primarily takes account of the type of driving and driven machine and seldom gives consideration to the operating conditions. The difficulty of using such service factors lies in the subjective evaluation of the particular machines which are to be connected together. Another problem is that the manufacturer does not always supply basic data for the coupling, having instead only stated the maximum permissible load. Any concealed factors or margins, etc., are therefore unknown to the consumer.
In order to compare different couplings objectively and, above all, to facilitate more realistic calculations, a new method has been developed which takes into consideration the frequency of starting, temperature, the moments of inertia of the driving and driven machine, normal torque and maximum torque.
This method has been presented in the German coupling standard DIN 740, which, apart from the method of calculation, also contains dimensional standards. There are, however, two additional service factors which should be considered.
The first is the effect which oblique setting of the shafts can have on the coupling. A factor based on the extent of allowable misalignment expressed as a percentage of the maximum permissible deviation, should also be given.
The second factor should take into consideration the level of vibration of both machines, at least vibration velocities above 1.5-2 mm/sec. Note that for pumps, vibrations of up to 4.5 m/sec. can be permissible. For accurate values and guidelines regarding permissible levels of vibration and unbalance we refer to ISO 2041 and ISO 21940. The size of the various factors and their influence on coupling speed varies with different types, which is why the calculations and values given in DIN 740 must be used with a certain amount of caution and always with due regard to the suppliers instructions, which must apply.
A very important point in this context, to which too little consideration is given, is the magnitude of the starting torque, in the case of direct starting of a squirrel-cage induction motor. Measurements have shown that almost immediately after connection, approx. 0.04 sec., a maximum torque is reached which is between 6-10 times the rated torque and even higher in some cases. This is a result of the electrical sequence in the actual motor and the fact that connection of the three phases does not occur absolutely simultaneously. The actual maximum torque thus being much greater than the starting torque or overload torque quoted in motor catalogs.
An important factor for shaft coupling calculations is the relationship between the moments of inertia of the driving and driven machine. This quotient determines the percentage of torsional moment which is to be used for the acceleration of the motor and pump rotors. When starting, the torque passing through the shaft coupling is:
Where:
Mk = coupling torque at start (Nm)
Mi = internal motor torque (air-gap torque) at start (Nm)
Jmo = moment of inertia of motor (kgm²)
Jma = moment of inertia of driven machine (kgm²)
to = motor starting time without load (s)
t = motor starting time with load (s)
The moment of inertia of a centrifugal pump can be given in relation to that of the motor, the following figures can be used as a guide:
2-pole motor Jma = 0.04 • Jmo to 0.2 • Jmo
4-pole motor Jma 0.25 • Jmo to 0.6 • Jmo
6-pole motor Jma 0.6 • Jmo to 3.2 • Jmo
By inserting these figures in equation 6.4a and assuming that Mi is 6 to 10 times the rated torque, values for coupling torques at start of up to 3.8 times the rated torque for 4-pole motors and 7.6 times for 6-pole motors are obtained. Thus it can be concluded that care must be taken when sizing couplings which are subjected to direct start, especially when the driven machine has a large flywheel capacity.
Coupling Speed
Centrifugal forces increase with speed. The material of the coupling and the permissible peripheral velocities must be calculated. The maximum peripheral velocity for grey iron, for example, is 35 m/sec. To avoid vibration damage it is necessary, for couplings which are not fully machined, to carry out both static and dynamic balancing at much lower speeds than is necessary for those which are fully machined.
The mass of the coupling is often quite small in relation to the rotating masses in the driving and driven machines. For a pump unit the relationship coupling vs. total rotor weight is approx. 0.02-0.08. It thus follows that out-of-balance in the coupling normally, has less effect on bearings and vibration than out-of-balance of the actual main components. The following relationship applies:
F = m * e * ω² *10‾³
where
F = out-of-balance force (N)
m = mass (kg)
e = distance from center of rotation to center of gravity (mm)
ω = angular velocity (rad/s)
For highly resilient rubber element couplings with intermediate shaft, the out-of-balance can be further increased by whirling. It is also important that balancing is carried out using whole keys, half keys or without keys, depending upon the method of balancing the opposing component.
Example:
A fully machined coupling can be assumed to have a degree of balancing, without dynamic balancing i.e. approx. 0.08 mm permissible center line deviation at 3000 r/min. If the concentric tolerance for the shaft bore in the hub is 0.05 mm, the maximum center line deviation can thus be 0.13 mm. This is in no way abnormal. In many cases the tolerance alone reaches this value. This center line deviation generates an out-of-balance force of approx. 12 N per kg coupling weight at 3000 r/min. A coupling for 50 kW can weigh 10-15 kg, which thus generates a rotational out-of-balance force of 120-180 N.
Size and weight
The importance of small size and low weight so as to achieve as small a moment of inertia as possible, as well as reducing the out-out-balance forces, has been mentioned previously. In certain extreme cases light-alloy metal bearings are used to reduce weight. Apart from the necessity of maintaining a small size/transmitted torque ratio, it is also important, from the point of view of cost and standardisation that the coupling should be able to accommodate large variations in shaft diameter.
Environment
Corrosive and abrasive environments can affect the service life of the coupling by causing abnormal wear to the component elements. Extremes of heat and cold can affect the strength and elasticity of the component materials. Oils, chemicals and ozone can completely destroy a rubber element. A coupling made entirely of metal, diaphragm couplings, for example, are usually the only solution in such cases. The process industries offer a very poor environment from the point of view of coupling life. In certain types of industry, the petrochemical industry for instance, in refineries as well as oil and gas tankers, for example, it is necessary to use spark-free (flameproof) couplings. A spark-free diaphragm coupling can be manufactured by making the diaphragm of monel and the remaining components of bronze. Spark-free types are usually used in conjunction with fully enclosed flameproof electric motors in environments where there is risk of explosion, either continuously or normally during operation. Statutory regulations must be observed.
Figure 6.4i Spark-free coupling.
Another method of overcoming explosion risks, especially on board ship, is by means of gas-tight bulkheads and deck through-fittings consisting of two mechanical seals with barrier fluid between them, together with bellows which absorb obliques. This type of gas-tight through-fit-ting must be equipped with spark-free shaft couplings.
Installation and disassembly
To maintain maximum operational reliability and to simplify assembly and service it is important that the machines connected are securely mounted, preferably on a common foundation. Safety shields (coupling shields) must be fitted to rotating parts according to safety requirements. The manufacturer is primarily responsible, although in the case of foreign manufacture it is the responsibility of the importer. In certain cases the coupling must be installed behind an a protective shield, which must be capable of withstanding the impact of the coupling in the event of breakage. An alternative is to select a “burst” proof coupling, which is sufficiently guided, see figure 6.4j.
Figure 6.4j Burst proof coupling with guide flanges.
Alignment of couplings, or, more correctly, alignment of the shafts which the coupling is to connect, should be carried out as accurately as possible. A perfect alignment should be considered as an economic possibility, since alignment can considerably affect both service and maintenance costs. See Section 6.4k with regard to methods of shaft alignment. In the case of cardan couplings the angular deviation should be equally distributed between the two joints so as to avoid unequal rotational velocities. Furthermore, a universal coupling should always rotate with a slight amount of obliquity.
The attachment of a coupling half to a shaft usually presents a dilemma. The hub should be securely attached and preferably absorb part of the torque, so as to reduce the load on the key attachment, as well as being easy to detach. The practice of hub attachment is similar to that for motor shafts where the fit is usually H7/k6, light push fit, up to 48 mm diameter. A push fit H7/m6 is preferred for diameters above 55 mm.
The tighter fit is necessitated by the fact that the height of the key is reduced from 12.5% of the diameter at 24 mm diameter to only 6% at 100 mm shaft diameter. This reduction should also be compensated for by increasing the length of the hub. In the case of electric motors the key does not normally extend right to the end of the shaft, which also increases the strain on the key. This must also be compensated for by increased hub length.
Assembly and disassembly of the coupling halves must be carried out carefully so as to avoid damage to the bearings. This operation could be simplified considerably if the motor, pump and coupling suppliers fitted their equipment with suitable lugs, etc., to facilitate the attachment of pullers. For electric motors a drilled and tapped hole in the end of the shaft, as shown in figure 6.4k can be supplied at extra cost.
Figure 6.4k Tapped assembly hole in electric motor shaft.
Other methods of attaching the coupling halves are shrink fits, bolted joints or some form of clamping sleeve. Clamping sleeves, figure 6,4l, are used primarily for chain wheels and rope pulleys, but can be a useful alternative for shaft couplings where space permits.
Figure 6.4l Examples of clamping sleeves
The resilient elements in the shaft coupling must be easy to purchase, replace or repair. That it must be possible to replace without disturbing the machines or coupling hub, goes without saying.
Shaft coupling service life
The life of the coupling is influenced by many different factors, which vary according to the type of construction. One factor, which above all affects couplings with rubber elements, is the surrounding environment. The service life of a gear coupling is largely dependent upon regular lubrication using the correct type of lubricant according to the ambient temperature, etc. Alignment affects the service life of all couplings irrespective of type or manufacturer. For certain types of installations it can be suitable to construct the coupling so that a certain amount of emergency co-rotation occurs even in the event of failure of the flexible element. For other installations it may be necessary to use a limited torque coupling with “overload” function.
It is important to carry out regular service and alignment checks according to the manufacturer’s instructions, and equally important that these instructions are placed in the hands of the personnel concerned. Unfortunately methods or regulations for assessing the degree of wear are often lacking.