--- title: A Quick Refresher on Converting Coupling Torsional Stiffness description: Describes the unit conversion of torsional stiffness values to assist in the selection of flexible shaft couplings image: https://blog.rw-america.com/hs-fs/file-25151475-jpg/images/shaft_deflection.jpg --- [![logo-RW.png](https://blog.rw-america.com/hs-fs/hubfs/logo%20for%20hubspot.png?width=166&height=76&name=logo%20for%20hubspot.png "logo-RW.png")](http://www.rw-america.com/) Open main menu Close main menu - [About R+W](https://www.rw-america.com/company/) - [Products](https://www.rw-america.com/products/) - [Industries](https://www.rw-america.com/industries/) - [Locations](https://www.rw-america.com/company/locations/) - [News](https://www.rw-america.com/news/) - [Downloads](https://www.rw-america.com/download-section/) - [Contact](https://www.rw-america.com/contact/) [torsional stiffness](https://blog.rw-america.com/blog/topic/torsional-stiffness) [torsionally stiff coupling](https://blog.rw-america.com/blog/topic/torsionally-stiff-coupling) [coupling torsional stiffness](https://blog.rw-america.com/blog/topic/coupling-torsional-stiffness) [torsional rigidity](https://blog.rw-america.com/blog/topic/torsional-rigidity) [torsion resistance](https://blog.rw-america.com/blog/topic/torsion-resistance) [torsion resistant coupling](https://blog.rw-america.com/blog/topic/torsion-resistant-coupling) # A Quick Refresher on Converting Coupling Torsional Stiffness [Niilo Nykanen](https://blog.rw-america.com/blog/author/niilo-nykanen) [Share this blog post on LinkedIn](http://www.linkedin.com/shareArticle?mini=true&url=https://blog.rw-america.com/blog/bid/246460/A-Quick-Refresher-on-Converting-Coupling-Torsional-Stiffness) ![](https://blog.rw-america.com/hs-fs/file-25151475-jpg/images/shaft_deflection.jpg)   [![Bellows Coupling Sizing Program](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/237991/fe7e6334-4274-45e8-b1f6-ae5298b5e019.png)](https://hubspot-cta-redirect-eu1-prod.s3.amazonaws.com/cta/redirect/237991/fe7e6334-4274-45e8-b1f6-ae5298b5e019) **A Quick Refresher on Converting Torsional Stiffness Values ** Customers often ask us for a bit of assistance in comparing torsional stiffness values between flexible couplings made by manufacturers using Imperial versus SI units.  This may be considered rudimentary to some but a quick refresher never hurts.   Any drive component which transmits torque will twist a bit when torque is applied. For example, if a shaft was marked on each end very precisely notched on the same plane along its axis, the notches would move apart as torque is applied.![shaft deflection](https://blog.rw-america.com/hs-fs/file-25151475-jpg/images/shaft_deflection.jpg?width=422&height=253&name=shaft_deflection.jpg)   The amount they move apart is proportional to the applied torque. Therefore, torsional stiffness is called out in units of torque applied per angular unit of twist. North American manufacturers tend to use a variety of different units while Europeans typically use Newton meters per radian.   **The Conversion** I've provided an example conversion below.  In this example calculation, we are converting a given stiffness value of 76000 N\*m/Rad  to in\*lb/Deg , this will require a bit of unit conversion. As many of us may remember from college chemistry, we are allowed to multiply ratios of different units that mathematically equal 1 to cancel unwanted units and convert to the desired ones. Below, we can see that the units in red cancel. In just a couple of steps the desired units emerge. ![equation 1](https://blog.rw-america.com/hs-fs/file-25152627-jpg/images/equation_1.jpg?width=459&height=153&name=equation_1.jpg) ![equation 2](https://blog.rw-america.com/hs-fs/file-25154963-jpg/images/equation_2.jpg?width=460&height=153&name=equation_2.jpg)  Or conversely, converting a value given in in\*lb/Deg to N\*m/Rad: ![equation 4](https://blog.rw-america.com/hs-fs/file-25155947-jpg/images/equation_4.jpg?width=460&height=153&name=equation_4.jpg) ![equation 5](https://blog.rw-america.com/hs-fs/file-25156113-jpg/images/equation_5.jpg?width=460&height=153&name=equation_5.jpg)  **Coupling Stiffness** As we see in the example, if we we’re comparing flexible couplings, the top one would be more torsionally stiff. Having a high level of torsional stiffness is not always desirable, especially if someone wanted to use the flexible coupling to damp vibration.  However in many cases machine builders need to maximize torsional stiffness in order to reduce settling times in highly dynamic applications or to maximize rotational positioning accuracy.  In the case of flexible shaft couplings, [elastomeric (elastic) couplings](http://www.rw-america.com/products/elastomer_couplings/) are typically used for vibration damping, and [metallic couplings](http://www.rw-america.com/products/bellows_couplings/) are used for maximizing torsional stiffness.  This example can be used for comparing any multitude of different units as long as the correct conversion ratios are employed.   [![Bellows Coupling Sizing Program](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/237991/fe7e6334-4274-45e8-b1f6-ae5298b5e019.png)](https://hubspot-cta-redirect-eu1-prod.s3.amazonaws.com/cta/redirect/237991/fe7e6334-4274-45e8-b1f6-ae5298b5e019) ## Related Articles [![flexible disc coupling with clamping hubs](https://blog.rw-america.com/hubfs/LPH%20button.png)](https://blog.rw-america.com/blog/introduction-to-industrial-disc-pack-couplings) [test stand coupling](https://blog.rw-america.com/blog/topic/test-stand-coupling) [torsionally stiff coupling](https://blog.rw-america.com/blog/topic/torsionally-stiff-coupling) [drive couplings](https://blog.rw-america.com/blog/topic/drive-couplings) [lamina coupling](https://blog.rw-america.com/blog/topic/lamina-coupling) [spacer couplings](https://blog.rw-america.com/blog/topic/spacer-couplings) [API 610 coupling](https://blog.rw-america.com/blog/topic/api-610-coupling) [line shaft](https://blog.rw-america.com/blog/topic/line-shaft) [flexible couplings](https://blog.rw-america.com/blog/topic/flexible-couplings) [pump coupling](https://blog.rw-america.com/blog/topic/pump-coupling) [jack shaft](https://blog.rw-america.com/blog/topic/jack-shaft) [high speed coupling](https://blog.rw-america.com/blog/topic/high-speed-coupling) [non keyed coupling](https://blog.rw-america.com/blog/topic/non-keyed-coupling) [torsion resistant coupling](https://blog.rw-america.com/blog/topic/torsion-resistant-coupling) [torque test coupling](https://blog.rw-america.com/blog/topic/torque-test-coupling) [torque sensor coupling](https://blog.rw-america.com/blog/topic/torque-sensor-coupling) [torque tube](https://blog.rw-america.com/blog/topic/torque-tube) [coupling for test stand](https://blog.rw-america.com/blog/topic/coupling-for-test-stand) [Precision shaft couplings](https://blog.rw-america.com/blog/topic/precision-shaft-couplings) [disc pack](https://blog.rw-america.com/blog/topic/disc-pack) [single flex](https://blog.rw-america.com/blog/topic/single-flex) [maintenance free coupling](https://blog.rw-america.com/blog/topic/maintenance-free-coupling) [flexible shaft coupling](https://blog.rw-america.com/blog/topic/flexible-shaft-coupling) [disc coupling](https://blog.rw-america.com/blog/topic/disc-coupling) ### [Introduction to Industrial Disc Pack Couplings](https://blog.rw-america.com/blog/introduction-to-industrial-disc-pack-couplings) ***Maintenance-free and torsionally stiff connecting elements for smooth stable running*** For many decades engineers have turned to the flexible disc pack coupling for... [Andy Lechner](https://blog.rw-america.com/blog/author/andy-lechner) [Read More](https://blog.rw-america.com/blog/introduction-to-industrial-disc-pack-couplings) [![](https://blog.rw-america.com/hubfs/ST%20wastewater.jpg)](https://blog.rw-america.com/blog/wastewater_safety_coupling) ### [R+W Safety Couplings for Wastewater Applications.](https://blog.rw-america.com/blog/wastewater_safety_coupling) # **R+W Safety Couplings for Wastewater Applications.** By popular demand, R+W has developed a standard model of corrosion proof ball detent safety coupling for clarifier... [Sample HubSpot User](https://blog.rw-america.com/blog/author/sample-hubspot-user) [Read More](https://blog.rw-america.com/blog/wastewater_safety_coupling) #### About - Menu Item One - Menu Item Two - Menu Item Three #### Services - Menu Item One - Menu Item Two - Menu Item Three #### News - Menu Item One - Menu Item Two - Menu Item Three Follow us on Facebook Follow us on LinkedIn Follow us on Twitter Follow us on Instagram