You may not always find the "perfect" coupling for your motion system, but when it comes time to select a coupling that will best suit your needs, keep these 7 tips in mind to help find the best solution possible.
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The inertial mass of the rotating system has a significant impact on the demands put on your coupling. Both the rate of acceleration and rate of deceleration needs to be factored in. Sudden braking of a high-speed or high-inertia system can cause a large torque spike to pass through the coupling resulting in its damage.
While a machine is running it may be heating up and cooling down or operating at an elevated temperature. When this happens, there can actually be some relative movement between the shafts that had not been considered previously. The reason this is important is that not all coupling solutions are capable of axial compliance, and are considered axially rigid. The risk in this situation is the possibility of putting a large axial load onto the bearings of the motor. The same motor loading can be true with thrust loads.
In a motion control system, there can be loading of the motor bearing if a solid coupling is used and transmits axial loads to the motor before engaging thrust bearings.
Parallel offset is a more difficult misalignment to accommodate than angular misalignment. Couplings generally do a good job at .010 inches and below (parallel offset). Once you start hitting .020 inches or more of offset, you're going to have a harder time finding a coupling that's going to fit in a relatively compact package. If you find a coupling that has a large parallel offset capability be sure and check its speed rating. Usually when a coupling can handle a large parallel offset the product is limited in its maximum RPM rating.
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Keep in mind if the coupling is operating in a harsh environment and/or corrosive atmosphere. Different materials will need to be considered depending on the type of environment. In a severely corrosive environment, specialty materials such as MP35N or Titanium may be the best option. Another environmental consideration is temperature. Certain alloys lose significant strength at elevated temperatures, or for certain elastomeric type couplings, the temperature limits may be limited. In higher temperature applications Inconel may be the material of choice. If the coupling is used in food or medical environments then the material options might be limited to certain stainless steel or Titanium.
Many applications are relatively easy to solve with most couplings if you’re operating at 5,000-10,000 RPM. In today’s high-speed applications it may be possible to reach 25,000 RPM, and sometimes up into 75,000-80,000 RPM. You’ll need to factor in not just torque capacity but how well balanced is the coupling for that type of speed because if it’s not either a balanced coupling or design that’s symmetrical by nature, which makes it a statically balanced part, you’re going to be running into a vibration situation due to the imbalance.
When looking for your best coupling solution, remember there is no governing body for couplings and how their performance is rated. It could be from calculations or the results of testing by the manufacturer. Make sure that you read the data carefully because one manufacturer may have a torque rating and when you look at the footnotes, their torque rating is for static torque which means it’s the torque that will yield the coupling versus another company that uses a dynamic torque, where the rating based upon a high-cycle environment. We actually de-rate the coupling torque capacity based upon realizing that they’re going to be going through millions and millions of revolutions.
Couplings are devices used to connect two shafts together and are divided into two categories: Rigid and Flexible Couplings
Both flexible and rigid couplings are used for the purpose of transmitting power. However, rigid couplings are limited in comparison to flexible couplings.
Flexible couplings are designed to accommodate: angular misalignment, angular offset, axial movement, and various types of load conditions. Rigid couplings are used where shafts are already positioned in precise lateral and angular alignment. Their rigid design does not allow for any misalignment between shafts, but they are generally able to transmit more power than flexible couplings.
With many styles and types of couplings available, your application conditions must be clearly understood.
...Our experienced engineers can solve even the most difficult application issues and find the right coupling solution for your application. SDP/SI offers a wide variety of both rigid and flexible couplings. Many of our precision couplings are designed with our unique Fairloc® Hub design, including the Miniature Fairloc® Bellows Coupling and the Neo-Flex Coupling. Modular Bellows Couplings can be configured to accommodate various shaft diameters and provide backlash-free torque transmission. Magnetic Disk Couplings are built to handle up to 3° of angular offset and operate in temperatures up to 284°F.
Flexible Shafts can be used in applications where the load must be moved in many directions. Flexible Shafts are stiff in torsion and very compliant in bending and lateral misalignments. They are extremely useful where the load is located in a remote position requiring many gear and shafting combinations. The basic design considerations are torque capacity, speed, direction of rotation, bend radii and service conditions.
Metal and Molded Universal Joints
Universal joints are used to transmit power, motion, or both. Cardan joints or Hooke joints are the most common and consists of two yokes, one on each shaft, connected by a cross-shaped intermediate member called the ”spider”. The angle between the shafts is called the operating angle. It is usually constant during operation, but not necessarily so. Good design practice calls for low operating angles, often less than 25°.
Universal joints are available in steel or in thermoplastic body members. For maximum load-carrying capacity, for a given size, a steel universal joint should be selected. Universal joints with thermoplastic body members are used in light industrial applications in which their self-lubricating feature, light weight, negligible backlash, corrosion resistance and capability for high-speed operation are significant advantages.... Ball-jointed universals are used for high-speed operation and for carrying large torques. They are available in miniature and standard sizes.
SDP/SI pioneered the concept of combining the exceptional qualities of selected plastic materials with different metals to create products which incorporate both. “SUPER-PLAST®” designs have the following advantages:
• Inexpensive replacement of metal components
• Dampens vibration
• Corrosive and chemical resistant
• Electrically insulated
SDP/SI offers an extensive selection of inch and metric dimensioned universal joints available in steel, stainless steel, acetal and acetal with brass spider and inserts as standard catalog items.
The molded universal joints can be manufactured to special requirements:
• Bores molded to accommodate square, “D” shape or hexagonal shafts
• Shafts molded into the components
• Zero backlash or components with built-in backlash
• Gears, pulleys or other components molded or assembled to couplings or universal joints
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