A flexible coupling, or shortened to coupling, is a device used to join two shafts together as a way of transmitting power. The main purpose of flexible couplings is to connect two pieces of rotating equipment – one piece of the equipment transmits torque to another while permitting a small amount of misalignment or end movement, or both.
In an easier-to-understand language, a flexible coupling is a mechanical device that join the ends of adjacent objects. Generally speaking, disconnection of shaft is not allowed during operation. However, there is an exception with the use of torque limiting couplings where disconnection is possible when torque limit is exceeded.
Flexible couplings are able to largely reduce maintenance cost and maintenance time. However, misalignment of flexible couplings can result in excessive force on either the drive load or the motor. This may ultimately lead to large amounts of unwanted vibration and the malfunctions of other components, which is why the alignment of the motor and load are regularly inspected when flexible couplings are used.
There are many types of flexible couplings, but can generally be classified into two types: the compression rigid (or clamped) couplings and the flanged rigid couplings.
Compression rigid couplings are consisted of two complementary parts that are fit together around the shafts to form a sleeve. They are known for the superior flexibility than other sleeved models, and can be used on shaft that are stationary. Compression rigid couplings are typically big enough for screws to pass all the way through both couplings to ensure a firm hold.
Flanged rigid couplings, on the other hand, are specifically designed for heavy duty work or industrial equipment. They are generally made up of short sleeved surrounded by perpendicular flange. When in action, each coupling is positioned on each shaft so that both flanges align face to face. The flanges can then be held together by installing a series of screws and bolts on the flanges. Also, they can also bring shafts to line up before they are connected thanks to the superb durability and size.
There are quite a handful of factors you need to take into considerations when selecting the most suitable flexible couples. Many make the mistake by only taking design factors into account, whereas they should also be considering other factors including the size, cost and ease of maintenance. Here are some of the important selection criteria pertaining to coupling designs that you should account for:
● Torque Rating:
One of the key factors is to determine the amount of torque your coupling will transmit. Also, take into consideration of the amount of torque the coupling is supposed to transmit given the size factor. The torque-size ratio is what we call the torque density, or sometimes referred to as the power density.
● Service Factor:
Once you have determined the torque requirement, you need to make sure that it is then able to accommodate torque fluctuations for your lines of applications. This is usually done by applying a service factor (SF) larger than 1.0, which is an indication of the perceived severity of the service. A higher number of SF is equivalent to more severity.
Undesirable weight may lead to deflections in the shaft of the joined equipment, leading to unwanted vibration. Therefore, it is imperative for you to know the effect of the loading on shaft and bearings.
Unbalanced coupling can also create unwanted vibration in the joined equipment. Thankfully, most gear and disc coupling manufacturers will go as far as to improving the operating speed range and balance rating of their product to avoid vibration issue.
● Misalignment Capacity:
As mentioned, although flexible couple allows a certain degree of misalignment, too much is undesirable. That is why coupling manufacturers offer guidelines and recommendations pertaining to the allowable extent of shaft misalignment. It is said that exceeding these allowable limits can greatly reduce the service lives of the coupled equipment.
Other non-design selection factors include:
● Initial Cost:
The initial cost of flexible coupling can vary depending on the equipment. Generally speaking, high torque capacity to size ratio of gear coupling are the least expensive. It is also worth noting that for low speed shaft, gear cost is also the cheapest while the elastomeric ones are the most expensive.
● Replacement Cost:
It is not wise to inconsiderably for go for the lowest cost couplings all the time. Reports have shown that lower cost couplings often lead to more expenses after installation, including replacement cost, maintenance cost, etc.
● Maintenance Factor:
It is important to check with your manufactures with regard to the maintenance outlook upon installation. Some couplings require more frequent maintenance, such as lubrication, than the other. The type of lubrication grease you use is very important to because it could potentially affect the maintenance interval.
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