A common question we get is requests for assistance with determining the correct knob to affix to a rotary switch, rheostat, encoder, or other shafted rotary device. This can seem daunting, but there’s help to be had.
First of all, we have our Product Selection Guide for Knobs here on the TechForum. This defines what all the filters in the Knobs section of our website mean, which is a great place to start. When seeking to translate all those filters into a proper knob choice however, there’s a few things to keep in mind.
Let’s use CKN11861-ND as an example to work with - how would we go about finding a knob for this product?
The most important specification for ensuring compatibility between knob and switch is Actuator Type in Rotary Switches, which corresponds to Shaft Size in Knobs. Any device with a rotary shaft should have a description in its filters of the style and diameter of that shaft. For CKN11861-ND, this is listed as a Flatted shaft, 4.75mm diameter. This means you would start by selecting 4.75mm as your value in the Shaft Size filter for knobs, like so.
This ensures the knob is correctly sized for your device’s actuator. But what about the “Flatted” part mentioned by CKN11861-ND? Sadly, this is not something you can search for in our filter system at this time, but in this case “Flatted” is a very common industry standard for any actuator shaft designed to be used with a knob. You can generally presume that most standard knobs will work with flatted shafts. Other actuator types such as knurled are generally meant for finger operation, while round shafts are less expensive or work with specific types of machinery.
You may also need to open up your search a bit depending on your specific switch. Our example switch, CKN11861-ND, has a 4.75mm shaft, but only one knob in our system matches this diameter precisely. To generate more results, we can also select “close enough” values that shouldn’t dramatically impede our system, such as 4.8mm (essentially identical) or 5.08mm, like so:
This still leaves us only three options for this particular switch, but that’s better than one.
The second thing to keep in mind for proper knob selection is Height in Knobs, which roughly corresponds to Actuator Length in Rotary Switches. This one is a little harder, and for best fit in a precision application you’ll want to check the datasheet for any knobs you’re considering, as our system does not list the depth of the shaft well in a knob but rather gives its overall height. As a general rule, you can assume that any knob will have an average shaft depth available of three-quarters of its overall height. 36-8554-ND, for example, has a listed total height of 16.8mm, but its datasheet lists a shaft depth (the ‘B’ dimension, in this case) of 14.5mm. This is significantly more than three quarters of the knob’s total height, but as a rough estimate 75% of total height will ensure you have plenty of overhead for manufacturers who may not be quite as generous as Keystone.
You should also factor in the actual actuator length you have to work with when selecting a knob. CKN11861-ND, our example switch, lists a shaft length of 14.86mm; this is from the tip of the shaft to the base of the mounting bushing. This length is practical in a device like CKN11861-ND which is intended to be mounted straight to the panel, but switches like RTAP31S12W25FS-ND mount to the PCB instead and have a longer shaft. This is because these devices are meant to project their shafts off the board and through a hole in the panel, meaning much of the length of their actuator is behind the panel and thus unavailable to mount a knob to. If RTAP31S12W25FS-ND is mounted 15mm behind the panel, for example, then it effectively has only 10mm of available shaft to mount a knob to.
With these ideas in mind, we can select 1722-1174-ND from Davies Molding as the best fit in DigiKey’s system for CKN11861-ND. Or, if this knob doesn’t work in our application, we now know what to search for in alternative switches that might fit better with a knob we do like.
If you have any further questions, don’t hesitate to contact DigiKey Applications Engineering, and happy part hunting!