The Perfect Pair: Couplings

Coupling: the pairing of two items.  Couplings are a product that is designed to connect shafts that are manufactured separately such as motor and connecting mechanism. The main purpose is to transfer rotary motion or torque from one shaft to another without changing rotational speed.  At the same time, they need to compensate for any misalignment in random movement between two shafts. They need to reduce transmission or shock load vibrations and to protect the connected mechanism from overloads. Couplings, in general, do not disconnect from the shafts during operation unless torque limiting couplings are used. By careful selection, installation, and maintenance, couplings protect expensive machine elements from sudden excessive and unexpected loads by breaking or disconnecting. When connected shafts are misaligned, great stresses can be induced on the rotating and stationary components. Using a coupling between these components, while not completely isolating all misalignments, couplings can still achieve optimum alignment conditions in machinery.

Couplings compensate for three types of misalignment. One is angular misalignment.  When shafts are not on the same plane and there is a parallelism error between the two shaft centers measurable by an angle.  Lateral misalignment is when the shafts are not centered to each other or the angular error between two shaft centers. When shafts shift in the axial direction this is end play or axial misalignment.

Angular Misalignment

 

Lateral Misalignment

 

Axial Misalignment (End Play)

There are several ways to connect couplings to the shaft. One is using a set screw. For MISUMI couplings, usually one or two set screws are placed at 90 degrees apart. This type of connection is best used when the shaft is softer than the set screw. Unfortunately, the set screw causes damage to the shaft which makes the coupling harder to adjust or remove.  It is common to machine small flats onto the shaft at the set screw location to eliminate this problem. There is also the clamp style.  Clamp style couplings are designed to solve the problems associated with the set screw type. Instead of protruding into the shaft, the screws compress the coupling and lock the coupling into place. The ease of use is maintained with this design and there is no shaft damage. Since the screw compresses the collar and a uniform distribution of the force is imposed on the shaft, this leads to bigger holding power then a set screw couplings. Although the clamp type couplings work well on the relative constant load, shock loads can cause couplings to shift.  Using either set screw type with keyway or clamping style with keyway can solve this problem. Keyways are used as a positive stop that is more resistant to shock loads.

Set Screw Coupling

 

 

Clamp Style Coupling

 

Key Way Coupling

Disc Couplings are designed to transmit torque between the bolts through a series of thin, stainless steel discs assembled in a pack. These bolts connect shaft hubs into one piece construction. Misalignments are absorbed through the flexing of the discs. Disc material and the shape adopt to wide range of transmission capabilities. Disc Couplings have low to zero backlash and therefore are suitable for high positioning and high accuracy applications. Thus, they’re also best suited for high torque servo motor application that require high rigidity and zero backlash. For servo motor applications, it is best to select a coupling with a maximum torque capacity of 5-10 times more of a motor’s peak torque.  They come as a clamp type, keyway type and keyless type, single, double or double with a spacer.

Disc Coupling

Oldham couplings are made of two shaft hubs that are connected via a spacer. Misalignments are absorbed by the sliding and flexing of the spacer. Large allowable misalignment allows for easy coupling installation. Depending on spacer material, couplings have different characteristics. Oldham couplings with aluminum bronze spacers have much higher rigidity compared to coupling with polyacetal or reinforced resin spacers. Oldham couplings are offered for small size shaft diameters, starting from 1mm. Overall these couplings provide good balance in torque transmission, torsional rigidity and misalignment absorption. They also have good electrical insulating properties. One thing to keep in mind is that for couplings using resin spacers allowable torque values and other characteristics are based on operational/usage temperature. If operational temperature is outside of normal range of -20 to +30 degree Celsius allowable values like torque and speed need to be corrected using temperature correction factor.

Oldham Coupling

Bellow couplings’ construction is made of two piece couplings that uses stainless steel and a phosphorus bronze bellow to connect shaft hubs. They have a similar design to the disc type but they have high rigidity due to its more solid construction. Misalignment is absorbed by the flexing of the bellow and has a high moment of inertia and low backlash features.

Bellows Coupling

Pivot type is a three-piece construction or universal joint complex are to be used.  They are commonly used with a drive shaft and have allowable operating angle up to 30 degrees. Shafts are connected to the joint using a pin set or keyway. User also has an option to select a rubber cover. This helps extend life of universal joint by protecting from dirt and contaminants. The opposite can be said of rigid couplings.  They have simple and robust cylindrical body that is easily installed on the shaft but cannot tolerate any misalignment.

Universal Joint Coupling

Some of the selection criteria are dictated by applications, others are user preference – such as how to attached coupling to the shaft.  Coupling usage is straight forward, and most of the design time is spent in selecting a correct type for the application. Engineers are mostly concerned about design requirements: torque ratings, allowable speed, misalignment, and bore sizes. Assembly and maintenance personnel are interested in easy installation, reliability and maintenance costs. During its life span couplings should also be checked for sign of wear and fatigue.  We hope you find your perfect match of couplings with the variety of styles and sizes from MISUMI!

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    The Perfect Pair: Couplings

    Coupling: the pairing of two items.  Couplings are a product that is designed to connect shafts that are manufactured separately such ...
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