Primary factors that will affect the type and size of coupling used for an application include, but are not limited to: horsepower, torque, speed RPM , shaft sizes, environment conditions, type of prime mover, load characteristics of the driven equipment, space limitations and maintenance and installation requirements. Because all couplings have a broad band of speed, torque, and shaft size capabilities, those criteria are not the best place to start. First, determine what attributes beyond those basic criteria will be required for your application. If none stand out then simply choose the lowest cost that fits those basics. Almost always, though, there will be other considerations that will narrow your alternatives down to certain types of couplings.
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The goal of this handbook is to assist you with the process of sorting out the myriad of coupling styles that exist to select the one best suited to your application. This handbook is not a textbook. There are several of those in print which do a great job and are very useful for coupling designers. What we are attempting to do is to provide down-to-earth useable knowledge.
We want to arm you with information that you need to utilize the variety of styles that exist in flexible couplings to your best advantage and solve real world problems. Lovejoy has been manufacturing couplings since More importantly, we have the greatest breadth of coupling types offered by any single manufacturer in the world. Since couplings are our strategic focus, we feel you will find this handbook to be a valuable resource. A flexible coupling connects two shafts, end-to-end in the same line, for two main purposes.
The first is to transmit power torque from one shaft to the other, causing both to rotate in unison, at the same RPM. The second is to compensate for minor amounts of misalignment and random movement between the two shafts.
Belt, chain, gear and clutch drives also transmit power from one shaft to another, but not necessarily at the same RPM and not with the shafts in approximately the same line. Such compensation is vital because perfect alignment of two shafts is extremely difficult and rarely attained. The coupling will, to varying degrees, minimize the effect of misaligned shafts.
If not properly compensated, minor shaft misalignment can result in unnecessary wear and premature replacement of other system components. In certain cases, flexible couplings are selected for other protective functions as well. One is to provide a break point between driving and driven shafts that will act as a fuse if a severe torque overload occurs. This assures that the coupling will fail before something more costly breaks elsewhere along the drive train.
Each type of coupling has some advantage over another type. There is a trade-off associated with each, not the least of which can be purchase costs. Each design has strengths and weaknesses that must be taken into consideration because they can dramatically impact how well the coupling performs in the application. This handbook will be a guide to assessing the features and limitations of the many standard types of couplings on the market. Coupling catalogs will show the maximum angular misalignment tolerable in each coupling.
A coupling should not be operated with both angular and parallel misalignment at their maximum values. Example: Sliding the hub in either direction may change the position of a coupling hub, on its shaft. Thus affecting its axial position on the shaft. It is the change in axial position of the shaft and part of the coupling in a direction parallel to the axial centerline.
Can be caused by thermal growth or a floating rotor. Some couplings limit this displacement and are called limited end float couplings. Because of the inherent construction of some couplings, forces may be generated in the axial direction when operating at high speeds or under misalignment. Such forces can place additional loads on the support bearings. If one half of a coupling is held rigid and the other half can be rotated a slight amount with very little force , you have some amount of backlash.
The freedom of movement, or looseness, is the backlash and may be expressed in degrees. Backlash is not the same as torsional stiffness. BORE: The central hole that becomes the mounting surface for the coupling on the shaft.
Close tolerances are required. Other bore types can include hex, square, d-shaped, tapered, and spline. A spline bore is one with a series of parallel keyways formed internally in the hub and matching corresponding grooves cut in the shaft.
DAMPING: Some couplings greatly reduce the amount of vibration transmitted between driver and driven shafts because of the damping capacity of an elastomer in the coupling. It is a hysteresis effect that will generate heat.
The coupling must dissipate this heat or risk losing its strength by melting down. The stiffness of the elastomer affects the rate at which vibration is damped. All-metal couplings, for the most part have poor damping capacity.
The factors can compensate for temperature, material variations, fatigue strength, dimensional variations, tolerances, and potential stress risers to name a few. This is characteristic of couplings in which some portion of both halves operate in the same plane, allowing direct contact between those portions. An example of this is the jaw coupling, in which driving jaw faces push the driven jaw faces through an elastomer in compression between them; if the elastomer breaks away, the driving faces simply advance to push the driven faces directly.
Finite-life couplings are those that wear in normal operation, because of using sliding or rubbing parts to transmit torque and compensate for misalignment. These types usually have lower purchase costs than infinite-life couplings. Periodic maintenance is required. The distortion results in fatigue stresses rather than wear, and the couplings are designed and rated to operate within the fatigue capabilities of the coupling material.
This group includes tire, disc, diaphragm, some donut types, wrapped-spring, flex-link, and most motion-control types. An overload will fail an infinite-life coupling but may only reduce the life of a finite-life coupling. Infinite-life designs are most often used on maintenance-free systems where maximum torque requirements — including transient, cyclic and start-up torque — are known. It is the time rate of doing work. For power transmission it is the torque applied and rotational distance per unit of time.
Applied torque causes a shaft and its connected components to rotate at a certain RPM revolutions per minute. Horsepower HP is converted to torque as follows:. KEYWAY: A rectangular opening formed by matching rectangular slots cut axially lengthwise along both the coupling bore and shaft. A square or rectangular metal key is then inserted into the opening to lock the coupling and shaft in position.
Torque is transmitted from shaft to coupling through the keyway and key. Coupling catalogs will show the maximum parallel misalignment tolerable in each coupling. A coupling should not be operated with both parallel and angular misalignment at their maximum values. RADIAL: Any projection outward from the center of a shaft or cylindrically shaped object, or any motion along that line. The centerline of the projection or motion normally passes through the axial centerline of the object.
These are used for the purpose of guiding coupling size selection to a torque rating that will allow for unforeseen demands those characteristics might make on the coupling. Such characteristics can include peak torque, start-up torque, transients or cyclic torque, or any other empirical factor. Among couplings that use wear parts to transmit torque, service factors are intended to prevent premature failure of those parts due to accelerated wear or degradation.
Caution: Resist the temptation to specify in excess of the published service factors. An oversized coupling will not perform better or last longer, but will be unnecessarily expensive and force the system to waste energy. Always base coupling size and service factor on the actual torque requirements at the point of installation within the drive system.
SET SCREW: A headless screw, with hexagon shaped socket, used over a keyway to keep the key stock in place and prevent the coupling from moving axially along the shaft.
It can also be used for torque transmission on low torque applications. Even seemingly stiff all-metal couplings can have some degree of torsional twist. Values greater than 30 are hard very stiff. Values between 10 and 30 are torsionally flexible. Values less than 10 are considered very soft. The dynamic stiffness will be greater than the static. The dynamic torsional stiffness can be linear, a constant value, or non-linear, an increasing value.
It is equal to the difference between maximum and minimum limits of any specified dimensions. TORQUE: In rotary motion it is the force multiplied by the radius, to the axis of rotation, at which the force is applied. In English units F is in pounds and r is in inches, expressed as in. In metrics F is in Newtons and r is in meters, expressed as Newton-meters Nm. Some causes of torsional variation are the geometry of the rotating parts of internal combustion engines, cyclic and irregular torque demands of the driven equipment, and variations in the output of certain types of electric motors at startup.
These are attributes that affect the type of coupling best suited for an application. This is a long list of evaluation factors. For any one application there may be only three or four attributes which are extremely important.
In fact it would be difficult to satisfy more than a half dozen attributes with any one coupling. It is important to narrow the requirements for an application down to only the most critical attributes that come into play. In the next chapter we summarize the major coupling types discussed in the materials and provide some ratings of each coupling type against these factors. Adaptability of Design — Some couplings are available in a variety of configurations e. These alternatives can be important to users who want to standardize on a particular type of coupling design, but need to adapt it to suit different application requirements.
Alignment Capabilities — Different couplings have different limitations as to the amount of angular misalignment, parallel misalignment or axial displacement each can accommodate. First, determine the amount of misalignment that can reasonably be expected between the two pieces of equipment to be coupled and let that guide or influence coupling selection.
This is important in two situations. The first is when the BE dimension is very small and coupling hubs need to be installed further back from the shaft ends. The other is when axial float in the shafts is characteristic of system operation. This can include requirements for slider-type couplings or limited end float couplings. Backlash — Also defined in the basic terminology section.
Backlash is usually not desired in applications where precise positioning of the shafts is important.
The Coupling Handbook - II - Couplings
The basic jaw coupling known as an L-line, and pictured above next to "Compression Loaded" title , has many variations for specific applications. In Europe, a curved jaw coupling variation has become the standard industrial coupling, and Lovejoy sells a large number of curved jaw couplings into Europe through Lovejoy's German affiliate. Lovejoy also replaces a significant number of curved jaw coupling components on equipment in the United States that has been imported from overseas. Note: Just like Lovejoy's L line, curved jaw couplings are manufactured, finished, and readily available from Lovejoy's Downers Grove, IL manufacturing facility. While curved jaw couplings cannot be turned into jaw in-shear like straight jaw due to their tooth profile , one nice feature of curved jaw couplings is that, by tightening their tolerance and using a very stiff elastomer, the curved jaw couplings can be turned into a very affordable backlash free coupling. Backlash refers to the looseness of fit of a coupling, which is generally undesirable in very precise motion control applications. Shear Loaded : In addition to the Jaw In-Shear couplings, there are several other popular elastomer in-shear designs.
The Lovejoy Coupling Handbook