Quick Tips for Control Knob Selection

The collet knob offers a number of advantages that make it a superior alternative to traditional set screw and push-on knobs, especially when a secure fit is required

by Judy Willis, Product Manager, Selco Products Co., Orange, California

Choosing the right control knob for your application can make a significant difference in your end product's aesthetics and functionality. For medical equipment or hazardous industrial applications, knob selection has even more important ramifications. Situations can become critical when settings are affected by loose knobs detaching from devices.

Traditionally, designers have made their knob selection from either conventional push-on knobs or set screw style knobs. The set screw style has been the most common choice for medical applications, as it affords a more secure fit over the push-on style. However, the collet knob offers an even more secure fit as well as a number of other advantages over the set screw knob and should be carefully considered for critical applications.

Though somewhat underutilized in the United States, collet knobs are used extensively in Europe and Canada where they are being specified into dialysis machines, ventilators, X-ray systems, meters, pumps and testing equipment.

Collet knob advantages

* Secure fit. Designed for perfectly round shafts, the collet design allows the knob to be clamped down at a full 360 degrees around the shaft. The collet slides on top of the potentiometer, and as the nut is tightened over the collet (using a simple nut driver), the col let grips over the entire 360 degree pot circumference, ensuring a secure fit. Its secure fit is perhaps the biggest advantage over the set screw and push-on knob types. Set screws, if not installed precisely at the first attempt, can be difficult to move as the set screw will try to locate in the original indent in the shaft. And even when installed precisely and initially secure, the set screw can still loosen over time.

The QuadTech 1433 Decade Resistor provides a wide range of precision resistance values, used for calibration and accuracy verification of electronic measurement instrumentation. Photo courtesy of QuadTech, Inc., Maynard, MA.

* Versatile sizes. Available in outside diameter sizes from 10mm up to 47mm, designers can match both large and small collet knobs on the same machine, providing a unified look, not always feasible with size limitations of set screw knobs.

* Easy installation. Mounted from the top where hardware is more easily accessible, the collet knob is installed using a nut driver. This configuration makes production installation easier and faster than side mounting with a screwdriver on the set screw knobs.
* Durable materials. Made from thermoplastic Nylon 6 with brass metalwork, the knobs, caps, pointers and nut covers are very durable over time and can withstand frequent cleaning.

* Pleasing aesthetics. The collet knob provides a matte surface finish. Once the cap is snapped down, the knob conceals all hardware, providing a clean, finished look with no visible hole or screw. Along with the traditional gray color commonly used in medical applications, the collet knob is available in a wide selection of colors. And because the cap, knob and skirt accessories can all be different colors or the same color, the collet knob provides an arsenal of options for the designer.

For more information on collet knobs: Circle 599 - Selco Products Co. or connect directly to their website via the Online Reader Service Program at http://www.1rs.com/005df-599