Every engineer at some point must decide how to connect their machine shaft to the components that drive it. There are so many options to choose from that it can be an overwhelming proposition.
The popular short-cut way to get close to the right coupling for your application is by matching up the size of your component shafts with the smallest coupling that will handle them. These couplings fit your shafts so now you can sit back, kick up your feet, and drink some coffee, right?
There is so much more that goes into proper coupling selection. While shaft fit is a good place to start, it is only the first step of many on a longer path that must to be taken to decided what coupling will be the optimal choice for your application. Your path depends heavily on the shafts that are being connected and on the machines they are being connected to.
There are as many styles of couplings as there are applications. As hats in your closet perform the right functions for different occasions, couplings also must be selected based on their specific use. Are you playing baseball (ball cap), gardening (long-brimmed-bug-repellent), sledding (warm thermal head sock), or going out for a night on the town (the Fedora is my personal choice)?
In almost all applications, it is advisable to utilize a coupling that has the capacity to handle mis-alignment between the driving and driven shafts due to the near-impossibility of perfectly aligning separate machine components. The type of mis-alignment expected affects selection and a few common descriptions of mis-alignment type are shown in the graphic below.
If the application requires a simple low speed rotation of a belt or screw conveyor that doesn’t need any accurate metering and a little “squishiness” in starting and stopping are no problem, a standard jaw coupling is the most economical choice.
Below is a table that shows a very generalized upper-level view of some coupling applications and styles. All values shown are the maximum for each attribute and type. This is not intended to be an encyclopedia of all the possible couplings that can be used but is intended to be a broad overview of some of the most common styles.
|Application||Coupling Type||Torque Range (lbs)|| Angular|
|Lateral Misalignment (inches)||Axial Misalignment (inches)|
|Low to Medium Torque, Non-Positioning or Backlash Tolerant, Vibration Dampening||Standard Jaw||170,000||1||+/- 0.015||Not Rated|
|Low to Medium Torque, Positioning or Backlash Intolerant, Vibration Dampening||Backlash Free Jaw Couplings||221,000||1.5||+/- 0.024||+/- 0.197|
|High Torque, Backlash Tolerant||Gear Couplings||18,400,000||0.7||Not Rated||0.394|
|Low Torque to Medium Torque, Servo Style, High Torsional Stiffness||Bellows Coupling||88,500||2||+/- 0.016||+/- 0.177|
|Low to High Torque, Overload Protection Required||Torque Limiting Coupling||2,211,800||Style Dependent||Style Dependent||Style Dependent|
The prevalent shaft connection for machines in the US is a keyway and a set screw. Machines have been designed and built this way domestically for the last century, but there are new options available that provide installers and maintenance personnel with more user-friendly options for assembly and disassembly. In addition to unfriendly service, the keyway-set screw combination has the potential to cause imbalance in a rotating load, causing problems especially in high speed applications. There are single and double screw clamp collars that squeeze the shaft, conical clamp collars that provide even distribution of stress around the shaft diameter, and split clamp collars that provide superior ease of installation and removal. The picture below shows an R&W ZAE style split clamp collar unit that allows the coupling to be taken right off the shaft and changed quickly without moving any of the connected equipment.
There are also many special use couplings like one I recently stumbled upon while solving a problem for a more complicated application. This style coupling like the R&W EZV shown here utilizes a clamp collar style shaft connection and can extend and retract as needed for maintenance or tool change out as required. The same clamp collar style that attaches servo style couplings to servo motor shafts also allows quick and easy coupling length adjustment.
All coupling manufacturers rate their couplings for torque capacity while some add horsepower ratings as well. As in selecting any rotating equipment, comparing the application torque and horsepower with appropriate applied service factors to the rated coupling values will provide for the proper coupling selection. I have purposely ignored using shaft size in this selection document for one big reason; It tends to lead you down the wrong path if you start with it. Since many manufacturers utilize different shaft sizes in their motors and gearboxes, aside from NEMA standard motors, utilizing the maximum bore of a coupling will often lead you to an undersized coupling. Sizing is especially important when specifying a torque limiting coupling such as the R&W STE shown here. This coupling is ordered with an adjustable preset torque overload range in addition to handling misalignment. When this type of coupling is specified, simply over-sizing the coupling is just not an option as this could lead to an overload setting that is too high for the application.
Caution should be used while selecting service factors for applications that are very specific to a specialized product line. Unfortunately, there is no manual or chart that guides us in choosing the proper service factor for the EXACT application you might be working on. Component catalogs sometimes publish charts for proper service factors based on very generalized application descriptions, but rarely is your application an exact match and these generally come with a highly amusing disclaimer that reads something like “use this only as a guide, we assume no liability for use of the published service factors”. Engineers, in the end, must draw on our good and bad experiences from the past as a guide for reasonable machine life expectancy.
No matter what your coupling needs may be, the experts at Rumsey can help you find a solution. As a mechanical design engineer, I have found Rumsey’s product offering to provide a multitude of options from economical value to high quality precision value. If you have any questions about coupling selection or any other power transmission needs, please don’t hesitate to contact me or any of Rumsey’s other power transmission experts.