Position and Pressure Switches

Postition Pressure Switches Installation, Care, and Diagnostics By Mike Weaver March 2001 Electromechanical switches — from the common toggle and push button switches to the more advanced position (limit) and pressure switches...


Preventative care and maintenance
Generally speaking, electromechanical precision switches are robust mechanisms designed to operate effectively and efficiently, even in the harshest of environments. Provided they are manufactured under controlled conditions, they are more likely to succumb to old age than infant mortality. It is not uncommon for switches to operate flawlessly for hundreds of thousands of cycles. Problems can arise, however, if the specified electrical load, thermal, pressure, or vibration conditions are exceeded. When this happens, circuit resistance anomalies and calibration drift can cause hard or intermittent failures. While many switches for extreme aircraft applications are hermetically-sealed, contamination of critical exposed parts (such as a pressure port) can spell trouble. In addition, if the enclosure of a hermetic switch is damaged, the loss of the seal can lead to internal contamination. Internal contamination causes high circuit resistance, which can give a false open circuit indication. In some of the most modern applications flying today, current levels through precision switches can be as low as two milliamps, thus absolutely clean contact components, gold-plated contacts, and truly hermetic, back-filled enclosures are essential.

Helpful hints
Very little preventive maintenance is needed to keep electromechanical switches operating at peak performance. However, any damage to a switch enclosure can hinder the switch’s performance, therefore it’s important to keep it out of harm’s way. Avoid dropping the component or using it as a ladder step or handhold, and protect all external harnesses that are part of the switch assembly. Always use the proper tools to remove and install a switch to avoid damaging the case. Always be certain that the mounting bolts are torqued correctly in order to reduce damaging vibration and to maintain proper calibration and/or actuation and release. Finally, if the switch is a pressure switch, make sure the port is kept clean and free of foreign material to prevent calibration errors and false readings. Also, remember that some pressure switches have components inside their pressure port. These components are designed to ensure the proper actuation and release over the life of the product and in some cases, act as a pressure snubber. These components should not be adjusted or removed unless the manufacturer’s Component Maintenance Manual (CMM) instructs otherwise.
Any system component can be damaged if the entire system is not operating as specified. If it appears that the system has experienced excessive heat, pressure, vibration, or voltage spikes, it is critical to check the installed switch or switches to rule out any damage that may have occurred to this component as well.

Troubleshooting
When troubleshooting an aircraft’s sub-systems, if a switch is identified as a contributor to a system failure, begin the repair procedure by consulting the CMM or Engine Manual for specific switch part numbers. Additionally, key parameters of the failed switch, required equipment, testing procedures and suggested repair procedures are provided in the CMM. Though most switch testing procedures are not complicated, it is important to adhere to the specified procedures and equipment suggested by the switch manufacturer. Occasionally, the manufacturer will call out special test equipment, such as electrical loads, transient protection, or calibration fixtures, to reduce the risk of damaging a switch under test.
Testing and troubleshooting procedures generally fall into two categories: electrical or mechanical. Electrical testing includes contact resistance and varistor voltage checks, and the testing of dielectric and insulation resistance. Mechanical testing involves the inspection and operation of the actuation mechanism to determine calibration status. Measurements of pre-load, pre-travel, overtravel, and actuation force are key to a proper inspection of the switch. Of course, there’s no substitute for a simple visual check for damage, whether external or internal after teardown.
While it’s important to follow the CMM instructions carefully, there are a few points that should be emphasized to ensure proper testing. First, make sure that the proper test loads are used. Loads in excess of the switch rating can easily cause contact damage. Similarly, overpressure can damage a pressure switch. It’s also important to pay attention to the rate at which voltage or pressure is applied. This is especially true with pressure input, as the port is physically limited and too much pressure too fast will often yield a false reading. Finally, make sure to follow the sequence and procedures provided in the CMM. Failure to do so can yield inaccurate results and cause damage to the switch.

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