Couplings are mechanical elements that ‘couples’ two drive elements which enables motion to be transferred from one element to another. The drive elements are normally shafts. We tend to see lot of applications of the couplings mainly in the automobiles, for example the drive shaft which connects the engine and the rear axle in a bus or any automobile is connected by means of a universal joint. In order to transmit torque between two shafts that either tend to lie in the same line or slightly misaligned, a coupling is used. Based on the area of applications there are various types of coupling available. But they are generally categorised in the following varieties
1. Rigid Couplings
2. Flexible or Compensating Couplings
3. Clutches to are a type of Coupling – We will the dealing on them separately.
Rigid Couplings are mainly used in areas where the two shafts are coaxial to each other. There are many types of couplings that fall under the rigid couplings category. They are · Rigid Sleeve or Muff Couplings- This is the basic type of coupling. This consists of a pipe whose bore is finished to the required tolerance based on the shaft size. Based on the usage of the coupling a keyway in made in the bore in order to transmit the torque by means of the key. Two threaded holes are provided in order to lock the coupling in position. The photo shows a type of the rigid sleeve or muff coupling.
· Flanged Coupling – The coupling basically consists of two flanged end pieces as shown in the figure. A spigot and recess is provided in the flanges to provide location between them. The flanges are connected firmly by means of fitted bolts which are tightened accordingly to the torque to be transmitted.
FLEXIBLE OR COMPENSATING COUPLINGS
Flexible couplings are normally used in areas where the coaxiallity between the connecting shafts is not always assured and in areas where there is a possibility of occurrence of shocks in the transmission is applicable. They are also called as Elastic Couplings. By construction these couplings tend
to have a elastic member in between the two connecting entities. The different types of flexible couplings are
1. Flanged Pin Bush Couplings
2. Bibbly Coupling
3. Gear Tooth Coupling
4. Tyre couplings
5. Elastomeric Couplings – This consists of jaw type and S-flex couplings.
6. Oldhams Coupling
7. Universal Coupling or Hooke’s Coupling
8. Bellows Coupling
Each of the above couplings are quite unique in their construction and we can deal in detail on their constructional and working aspects in separate articles.
This article deals with Bibby Resilient type couplings. The article describes constructional aspects of the coupling and the selection procedures for the coupling. * Named after its Inventor Dr. James Bibby in 1917, this coupling is still one of the most sought after flexible couplings for heavy shock applications. Let us now look at the constructional features of the coupling.
1. The Coupling consists of two flanges or hubs specifically mounted on the drive and the driven shafts respectively. These hubs contain axial grooves cut on their circumference. 2. The two couplings are joined or held together by means of a specially designed grid spring. 3. The total assembly is enclosed in a casing or shell filled with grease for low speed applications or, in high speed application with high viscosity oil. Now let’s look at the working aspect of the coupling and how it tends to reduce vibration and prevents the build up of resonance in the system.
a.) The specially designed spring is wound up through the grooves forming a series of resilient bridges throughout the periphery of the coupling. b.) The grooves are tapered up at the edges (see photo) in order to provide extra flexible spans to the spring at normal loads and tends to support the spring at the sides whenever overloading occurs. c.) The stiffness of the spring depends on its unsupported length of each of its flexible span. The unsupported length tends to vary with the loads producing a varying stiffness for the coupling based on the loading. d.) This action tends to produce a detuning action altering the torsional vibration frequency of the system that prevents the build-up of resonance. Some applications of Bibby Couplings are
1. Cement Mills – in grinding and crushing machinery.
5. Marine Auxiliaries
6. Paper Mills
Bibby couplings can be used for applications with powers varying from as low as 1 h.p to about 100,000 h.p. per 100 rpm of the coupling. Correspondingly the couplings are available in varying sizes ranging from 100mm diameter to 4000mm diameter. Bibby couplings are capable of taking axial and radial misalignments to a very small extent only. It is necessary for the user to ensure that both axial and radial misalignments are kept to the minimum possible.
Selection procedure for Bibby Couplings:
The selection of the Bibby coupling is mainly based on the torque being transmitted by the coupling. Each coupling has a characteristic value for the torque transmitted by it to the revolutions per minute of the coupling.
The rating of the coupling is given by
Rating = Maximum power to be transmitted in h.p/ r.p.m of coupling In order to account for contingencies like shocks, sudden stops, stalling, etc. a factor of safety is to be considered during selection of the coupling. Rating of coupling to be Selected = Max. power of coupling* Factor/ r.p.m. of coupling The chart containing the factors to be considered is given below based on the application and driving machinery.
See the selection chart below for the selection of corresponding coupling based on the power. Note: This chart is an indicative for the selection of the Bibby coupling. Kindly refer to an actual manufacturer’s catalog for selection as ratings may vary from manufacturer to manufacturer.
This article deals with Gear Couplings. The article describes constructional aspects and selection procedures for flexible gear couplings. * Couplings are used as connecting elements between two shafts. The complexity in the type of connectivity, the power to be transmitted and the area of application tend to play a vital role in the selection of the type of the coupling. Gear Couplings belong to the category of flexible couplings that are capable of transmitting very high torques. Constructionally the gear coupling utilizes the advantages of gear engineering; practically the coupling is a complete gear assembly. Let us now look into the constructional aspects of the coupling.
1. The gear couplings consists of a forged sleeve with internal teeth cut on its inside. The forged sleeves are normally two halves possessing internal gears in both of them. In certain cases the forged sleeve tends to remain a single piece. 2. Two hubs with external teeth containing the same number of teeth as in the internal gear is present with each one being mounted on the driven and driver shaft respectively. The tooth profile of the external teeth is normally crowned in order to take up more loads. 3. The entire assembly in normally enclosed and is provided with gaskets at the joints and O-rings at vantage points in order to prevent the leakage of the lubricant filled inside the coupling, 4. Lubrication plugs are provided at vantage points in order to lubricate the couplings during predetermined intervals of time.
Tooth forms in the Coupling:
Three types of external teeth are used in gear couplings. The only difference is the manufacturing methodology of the same. a.) Straight Teeth: The external teeth in the hub are straight. During the condition of misalignment in the coupling the contact pattern between the internal gear and the external gear tends to be line type of contact as shown in the figure.
b.) Crowning with constant radius: The external teeth are barrel shaped with a constant radius in order to increase the area of contact and move the area of contact to a near middle portion of the teeth.
c.) Crowning with variable radius: The external teeth are barrel shaped with a variable radius instead of constant radius. This increases the area of the contact significantly compared to that of the constant radius crowning.
The maximum degree of misalignment permissible in gear couplings is from 0.5 Degrees to 1.5 Degrees. This varies with the size of the coupling being used. Selection procedure for gear coupling: The gear couplings are selected based on the torque ratings. 1. 1. The torque transmitted by the coupling is to be determined. 2. 2. Based on the application the appropriate service factor needs to be determined. A reference chart on the service factors is shown below.
3. 3. The equivalent torque is to be determined by multiplying the service factor with the torque transmitted. 4. 4. Select the coupling based on the equivalent torque value calculated. A reference selection chart is given below for example.
JAW TYPE ELASTOMERIC COUPLING:
This article deals with Jaw type Elastomeric coupling. The article describes the constructional aspects of the coupling and selection procedures. * Couplings are indispensable mechanical power transmission elements used for connecting shafts. Flexible couplings in particular are used in areas where misalignment needs to be accommodated (the flexibility is highly restricted based on the type of Elastomeric material used) and also to take up shock loads at the driven end, thus protecting the driving element like a motor or such. Jaw Type Couplings tend to have a flexible member or Elastomeric member between the two metal components. The flexible member is also called the “Spider.” The Spider material can be made from different types of rubber or plastic possessing varying degrees of hardness in order to suit the load carrying capacity of the system torsional characteristics. The constructional features of the Jaw type coupling are shown below. The dimensional drawing as well as the nomenclature of the coupling is also enclosed.
Here as some of the key aspects on the constructional features of the Jaw type coupling. These points are to be considered for the proper selection of a coupling. – The metal hub is constructed of sintered iron, cast iron, or aluminium or stainless steel. The type of metal hub is mainly based on the area of operation of the coupling and its inertia aspects. – The spider is normally made up of a nonmetallic element like rubber, urethane, hytrel because of their Elastomeric properties and in very rare cases bronze is used.
A beam coupling
A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque between two shafts while allowing for angular misalignment, parallel offset and even axial motion, of one shaft relative to the other. This design utilizes a single piece of material and becomes flexible by removal of material along a spiral path resulting in a curved flexible beam of helical shape. Since it is made from a single piece of material, the Beam Style coupling does not exhibit the backlash found in some multi-piece couplings. Another advantage of being an all machined coupling is the possibility to incorporate features into the final product while still keep the single piece integrity.
Changes to the lead of the helical beam provide changes to misalignment capabilities as well as other performance characteristics such as torque capacity and torsional stiffness. It is even possible to have multiple starts within the same helix. The material used to manufacture the beam coupling also affects its performance and suitability for specific applications such as food, medical and aerospace. Materials are typically aluminum alloy and stainless steel, but they can also be made in acetal, maraging steel and titanium. The most common applications are attaching encoders to shafts and motion control for robotics.
Animated Oldham coupler
An Oldham coupling has three discs, one coupled to the input, one coupled to the output, and a middle disc that is joined to the first two by tongue and groove. The tongue and groove on one side is perpendicular to the tongue and groove on the other. The middle disc rotates around its center at the same speed as the input and output shafts. Its center traces a circular orbit, twice per rotation, around the midpoint between input and output shafts. Often springs are used to reduce backlash of the mechanism. An advantage to this type of coupling, as compared to two universal joints, is its compact size. The coupler is named for John Oldham who invented it in Ireland, in 1821, to solve a paddle placement problem in a paddle steamer design. *
* it is recommended to use manufacturer’s alignment target values to set up the machine train to a defined non-zero alignment, due to the fact that later when the machine is at operation temperature the alignment condition is perfect Coupling maintenance and failure
Coupling maintenance is generally a simple matter, requiring a regularly scheduled inspection of each coupling. It consists of: * Performing visual inspections, checking for signs of wear or fatigue, and cleaning couplings regularly. * Checking and changing lubricant regularly if the coupling is lubricated. This maintenance is required annually for most couplings and more frequently for couplings in adverse environments or in demanding operating conditions. * Documenting the maintenance performed on each coupling, along with the date. Even with proper maintenance, however, couplings can fail. Underlying reasons for failure, other than maintenance, include: * Improper installation
* Poor coupling selection
* Operation beyond design capabilities.
The only way to improve coupling life is to understand what caused the failure and to correct it prior to installing a new coupling. Some external signs that indicate potential coupling failure include: * Abnormal noise, such as screeching, squealing or chattering * Excessive vibration or wobble
* Failed seals indicated by lubricant leakage or contamination. Checking the coupling balance
Couplings are normally balanced at the factory prior to being shipped, but they occasionally go out of balance in operation. Balancing can be difficult and expensive, and is normally done only when operating tolerances are such that the effort and the expense are justified. The amount of coupling unbalance that can be tolerated by any system is dictated by the characteristics of the specific connected machines and can be determined by detailed analysis or experience.
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