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Design Considerations for Heavy Duty / Off-road Interior Switches

18 October 2016

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Switches no longer are viewed as a simple electrical component. Today, they are the primary interface when powering devices, triggering relays, selecting functions or enabling a myriad of human/machine interfaces. Depending on the application, the switch may or may not be seen, and may not directly be felt. The design, materials, contact construction, actuation type and mounting all help determine whether the switch will last 50,000 operating cycles or 1 million. C&K continues to deliver innovative switch solutions through appearance, feel, performance, viewing experience and even sound. In the end, the user must continually have a positive experience with the switch’s feel, appearance, performance and result.

One of C&K’s new product introductions is the APB Series of backlit pushbutton switches. With their unique way of illuminating cap symbols, these switches are ideal for a variety of applications including joystick modules and control panels for heavy-duty equipment such as off-road transportation, construction equipment (e.g., front-end loaders, bulldozers or forklifts) and farm machinery. Operators in these competitive businesses can’t afford to neglect safety, driver comfort and driver performance-related items. APB switches, which are illuminated for visibility in the dark, are single pole, single throw (SPST) momentary switches. In momentary-action switches, the operator pushes (in this case the switches are pushbutton but momentary switches can also be toggle) the actuating device and the switch only remains open (or closed) while the switch is held in the appropriate position.

Figure 1. APB switches are backlit switches employing LEDs in either blue or white for visual status or function indication.Figure 1. APB switches are backlit switches employing LEDs in either blue or white for visual status or function indication.Along with the basic functionality of the switch itself, APB switches are backlit switches employing LEDs in either blue or white for visual status or function indication. Further, because of the long life of an LED – on the order of 50,000 hours – LEDs will usually last as long as the switch with which they’re used.

A nice feature of the APB line is that the LEDs are independent of the switching mechanism, so customers can control the LED to indicate whether the switch is in the up (on) or down (off) position and can even control the level of brightness. The rugged, backlit APB series switches feature an extended operating life of up to 1,000,000 cycles.

These switches are commonly used in applications located in poorly lit environments (e.g., dimly lit vehicle cabins), which can make it difficult to identify individual switch function, so the backlit cap graphics will provide operators with a highly visible indication of what they’re activating before they turn it on. They also have distinct feedback that can be felt through thick work gloves.

Contacts are available in a range of ratings (200 mA/24 VDC; 100 mA/50 VDC; 400 mA/22 VAC; 125 mA/125 VAC) and operating life cycles (500,000 and 1,000,000) with an initial contact resistance <50 mΩ. Rated for use in temperatures spanning -40°C to 85°C APB series switches feature a total travel distance of 2.1 mm (±0.2). Travel distance is the specifications indicating the distance required to actuate a switch. The APB family is available with an operating force of either 4 N (±1 N) or 2 N (±0.5 N).

These innovative switch assemblies afford customers flexibility and customization options, depending on their specific needs. C&K Components’ APB product line offers nine symbols as standard parts (See Fig. 2 below). The company also provides a fully customizable painted and laser-etched cap capability that allows it to recreate virtually any symbol on the 12 mm backlit caps. Since these switches are commonly used in poorly lit environments, which can make it difficult to identify individual switch function, the backlit cap graphics will provide operators with a highly visible indication of what they are activating before they turn it on.

Construction, mining, and agriculture sites are not the most forgiving of environments for electrical components. When selecting a switch for an in-cab heavy equipment application, installing a product not designed for harsh environmental conditions can result in dramatically decreased switch life expectancy and even critical switch failure.

Figure 2. Cap symbols for APB switches, Note that all graphics must be positioned as shown. Graphic markings must be lighted with the flat feature on the underside of the cap.Figure 2. Cap symbols for APB switches, Note that all graphics must be positioned as shown. Graphic markings must be lighted with the flat feature on the underside of the cap.For design engineers, every harsh environment presents its own set of challenges stemming from a host of factors, such as extreme temperatures, water, chemicals, dust, oil and other types of contaminants. A key characteristic of heavy equipment environments is that prolonged or particularly obtrusive exposure to contaminants is a certainty. These conditions also often present temperature and vibration concerns, which could affect the fit and performance of the switch.

As such, IP-rated switches are the best choice for heavy equipment applications. IP-rated switches are guaranteed to be resistant to certain environmental factors based on a ratings classification system. The IP code is part of the International Organization for Standardization’s (ISO) IEC60529 standard. This directive specifies the degree of protection of enclosures for low-voltage switches; specifically, concerned with protection of persons against contact with live or moving parts and the prevention of ingress of solid foreign bodies and liquid. The IP code is a specification used internationally and is similar to the National Electrical Manufacturers Association (NEMA) standard. The first digit of the IP code indicates the level of protection that the enclosure provides against access to hazardous parts (e.g., electrical conductors, moving parts) and the ingress of solid foreign objects. The second digit covers liquids.

C&K’s APB switches are available in both a threaded version that features an IP67 panel seal and an IP65 snap-in version without a panel seal. IP67 means that the switch is dust tight and protected against the effects of temporary immersion (up to 1 meter). IP65 switches are dust tight and protected against low-pressure water jets from any direction.

Engineers should select a switch that not only fits size and performance parameters, but is also sufficiently user-friendly. Not everyone that operates heavy machinery is a native speaker of English so cap symbols are important as is the ability to customize these symbols. In the case of APB switches the symbols indicate functions and are laser etched on the button so they are very durable (see Figure 2 above). Similarly, the switches are easy to operate with one hand and robust enough to withstand the repetitive maneuvers related to a typical function.

Several key engineering characteristics must be evaluated when selecting a switch to be sure it can perform the desired functions. Chief among these constraints is electrical performance ratings. The criteria for a switch include the current rating, voltage rating and contact resistance (this last item is the contribution to the total resistance of a system attributed to the contacting interfaces of electrical leads and connections, as opposed to intrinsic resistance, which is an inherent property, independent of the measurement method). The switch manufacturer will specify these ratings, which are determined by such factors as contact size, contact material and contact spacing. Plating erosion is another element that must be considered because it can lead to premature high contact resistance.

The current rating of a switch refers to the maximum current the switch is designed to carry. The current rating should never be exceeded. If it is it may be impossible to open the circuit. For a typical member of the family, the APB4E01TXBE001 the current rating is 125mA @ 125 VAC .

The voltage rating of a switch refers to the maximum voltage allowable in the circuit in which the switch is used. The voltage rating may be given as an AC voltage, a DC voltage, or both. If a voltage higher than the voltage rating of the switch is applied to the switch, the voltage may be able to "jump" the open contacts of the switch. This would make it impossible to control the circuit in which the switch is used. More voltage with the same current will do more damage to the contact and shorten the switch life. Again, using the APB4E01TZBE001, as a typical example, the voltage rating is 125 VAC.

For more information on C&K APB switches visit: http://www.ckswitches.com/products/switches/product-details/Pushbutton/APB



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