Free Webinar: "Flexible Coupling Solutions for OEM Applications"Helical Products Company is partnering with NASA Tech Briefs on March 23rd, 2017 to present a free webinar, "Flexible Coupling Solutions for OEM Applications." Attend this informative webinar to gain insights into specifying a flexible coupling. Designing and manufacturing a custom coupling for OEM applications provides savings by optimizing the size and targeting environmental conditions that would not be met by off-the-shelf products. The incorporation of special attachment features will reduce the time and expense of design, purchase and inventory of multiple parts. In this Webinar, gain insights into specifying a flexible coupling for your application that offers the best combination of value and performance. Attendees will see examples of solutions provided for real world applications that leveraged the engineering support and technical skill of Helical Products. Helical's Domestic Sales Manager, Tom Puerling, will be presenting this webinar. Tom has spent the last 19 years at Helical Products Company training and educating their group of manufacturers' representatives and customers about the versatility of the Helical Flexure. Graduating from the University of California at Santa Barbara, Tom majored in mathematical sciences and brings his years of practical experience to his presentation. Register here for this webinar to attend live or receive the archived link that can be viewed at any time.
"Flexure" concept promotes application versatility.The four red flexures below are identical in outside diameter (OD), inside diameter (ID), length and number of coils. The only difference is the coil thickness. Changing this feature affects torque, angularity (bending moment), parallel misalignment (radial load), torsional rate and compressions spring rate. All of the green colored flexures are identical with the exception of the inside diameters (ID). This change affects the torque, bending moment, radial load, torsional rate and compression spring rate. The only difference between each of the blue colored Heli-cal Flexures is the length of the coil. Changes in flexure length affect angularity, radial load, torsional rate and compression spring rate. However, torque capacity remains constant. The gold Heli-cal Flexures are multi-beam designs. Multi-beam flexures have two or more coaxial beams. The difference between a single beam and multi-beam flexure is analogous to the difference between a single and multi-lead screw. The affect of changing the number of flexure beams, with all other factors remaining constant, will result in changes to all five performance criteria. Variations in the previously mentioned characteristics affect the performance of the flexure in dramatically different ways. Altering these factors results in performance changes that may be linear, by the square, or by the cube. Helical engineers manipulate the various attributes of the flexure to create products that meet the performance criteria specified by the customer. To this point, we have only discussed what can be done by changing the flexure geometry. This is only the beginning. When you consider the additional utility that can be achieved by varying the material from which the flexure is made, or integrating custom end attachments into the product design, you realize there is more to this Heli-cal Flexure concept than originally thought. With all of this versatility, the only missing ingredient is the imagination of the design engineer. The Heli-cal Flexure is definitely more than a means to connect two shafts; it's a mechanical solution!
Selecting the right coupling when electrical isolation is a priority.
Understanding design challenges
Most of the couplings on the market today are either partially, or entirely, made of metal. Since metal is such a good electrical conductor it can be a challenge for the design engineer to find a suitable product to serve as a flexible coupling for transmitting torque and yet not transmitting electrical current.
Looking at options
Randy Kingsbury, Vice President, Sales & Marketing for Helical Products Company explains that there are several options available for specifying an electrically isolating coupling, however there are numerous key factors to consider before making a final selection.
“As with any coupling selection the engineer needs to consider the shaft diameters, attachment method, torque being transmitted, torsional stiffness required for accurate performance, plus the amount of current and/or voltage needing to be isolated,” he says.
Option 1: Non-metallic coupling
“The best choice for total electrical isolation is obviously a non-metallic coupling, but those types are couplings are limited in performance capabilities. The drawback is that because of the low modulus of elasticity and low yield strength of any non-metallic material that would be suitable for a flexible coupling, any part manufactured would have a very low torque rating and high torsional flexibility. These characteristics are usually counter to what a design engineer is seeking.”
“Another major problem with a non-metallic coupling with its inherent low strength, makes fastening the coupling t
he shaft a problem. Threads can easily strip out before enough clamping force can be applied to a cap screw or downward force applied by a set screw, Kingsbury adds.
Option 2: Multi-piece coupling
The next option for specifying a flexible coupling for electrical isolation is to use a multi-piece coupling where the flexing element is non-conductive.
“A good example of this is the jaw type coupling,” says Kingsbury. “In this case the hubs are made of metal (either machined or cast) so the problem with attachment strength is eliminated. An insert between the hubs can be selectedthat is created from a non-metallic elastomeric compound ranging from rubber to urethane or other moldable materials.” “An advantage of this jaw type coupling in electrically sensitive application is that the elastomeric insert can be selected with differing durometers which allow better tuning the coupling to the operational requirements of the application.”
“Although this type of coupling provides good electrical isolation it is still bound by the potential drawbacks of any multi-piece coupling,” Kingsbury warns. “Over time it is possible for the spider to wear and backlash is induced into the system, which will affect its accuracy and smooth operation. Because of this wear this type of coupling would not be a good choice in an operational environment where contamination from wear of the elastomeric spider would be a concern.”
Option 3: Work around the issue
Kingsbury says that the third, and often the best, option for providing electrical isolation with a flexible shaft coupling, is to select the best suited coupling as if electrical isolation is not a concern and then use a non-conductive bushing to isolate either one or both shafts from the coupling.
“This requires ordering the coupling with bores larger than the shafts and then machining, or ordering, a dielectric insert to separate the bore and shaft,” he explains. “To make this work also requires a coupling with a clamp style attachment so the compressive force can prevent slipping between the bushing and shaft, or bushing and coupling.
“Torque transmission may be limited because of the low coefficient of friction between the insert and mating parts. A set screw attachment will not work because it would require so much tightening torque to hold the coupling and bushing that it would likely damage the insert and result in metal to metal contact between the coupling and shaft.” He adds.
Evaluating all the options
In conclusion, Kingsbury says, “Overall there are many ways to solve the challenge of using couplings in electrical isolation applications. These are only a few of the main methods. Whether choosing any of the examples explained above, or attempting any other solutions, it will always be necessary for the design engineer evaluate the option thoroughly, as well as investigate the voltage/current being isolated to ensure there is sufficient resistivity provided by the isolating material selected.