Announcement

Collapse
No announcement yet.

OT-Switch Design

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT-Switch Design

    Although not specifically a machining question, this topic does deal with materials. Perhaps some of you have some experience or insight concerning the subject.

    I am in need of some references concerning switch contact materials and design for a slide switch.

    The arrangement of the switch consists of 3 contacts of 260 brass arranged on a plane, one contact being a common contact and the other two being one each a normally opened and normally closed contact.

    The sliding element of the switch is an E shaped conductor where the upper and lower arms are bent to touch the contacts, one always on the common contact, the other touching either the normally opened or normally closed contact. The ends of the legs that touch the contacts are either bent or spoon shaped to provide line or point contact with the fixed contacts, or may have a contact button attached. The middle arm is attached to the actuating device.

    The switch must carry between 1/4 and 1 amp, 12vdc and operate reliably for a million cycles.

    I would perfer to use a 1-piece stamped slider, silver plated, instead of a stamping with silver plated button contacts riveted to it.

    All opinions and ideas are welcome, but I need some sort of references to justify my preferences. Or, good reasons to change my preference.

    Wes

    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
    ~Practitioner of the Electromechanical Arts~

  • #2
    This technology is fully matured and mega cycle, low voltage, low current switches abound in a wide variety of configurations. usually the base material is a spring grade of brass not half hard ASTM 360 and the contacts silver/rhodium alloy in the form of circular contacts for current carrying and crossed gables for signal level.

    If you want to dig into the state of the art of switch technology, I suggest you comb over back issues of Design News and similar publications where new developments are commonly announced usually in full geek format.

    Comment


    • #3
      You will not come close to million cycle reliability with a silver plated slider as a contact. Silver plating makes a poor contact material and will wear away in a very few cycles. Button or edge contacts on a strip of beryllium copper will be required. Rhodium alloy will work but for best reliability palladium is best, possibly with gold plating.

      To ensure million cycle reliability the design should be good for at least 2 million cycles. A slide switch is not a good choice.

      A standard slide switch with silver plated contacts has a rated life of around 30,000 cycles.

      See here:

      http://www.citswitch.com/Products/10...%20Catalog.pdf

      Phosphour bronze with gold or gold/nickle sliding contacts are rated at up to 100,000 actuations.

      http://www.citswitch.com/Products/DL...%20Catalog.pdf

      Even a snap action power rated slide switch rated life is only 20,000 cycles.

      http://www.nkkswitches.com/pdf/cs.pdf

      For million cycle life I would not consider using a slide switch. Some sort of snap action switch would be much more suitable if it has the proper contacts.


      Free software for calculating bolt circles and similar: Click Here

      Comment


      • #4
        Personally, I'd install cherry switches. They are good for a million cycles. On ebay look at arcade and Switches that should give you a decent price and source.

        ------------------

        [This message has been edited by ibewgypsie (edited 11-09-2005).]

        Comment


        • #5
          http://cgi.ebay.com/ws/eBayISAPI.dll...111794692&rd=1

          THIS IS TOO MUCH, ebay has cheaper ones. A good cherry switch should be about .25cents. You can see the cherry switches in there thou.

          I bought two of these joysticks and a dozen pushbuttons for 10.99.. Plan is to install them to operate the air motor/solonoids on the shaper if I ever get around to completing it.



          [This message has been edited by ibewgypsie (edited 11-09-2005).]

          Comment


          • #6
            If I remember platinum is the best switch contact material. That's what Bell Labs used for telephone switching apps. but that pertains to relays and relays must have 'follow' to keep the contacts clean.

            Comment


            • #7
              <font face="Verdana, Arial" size="2">Originally posted by Evan:
              You will not come close to million cycle reliability with a silver plated slider as a contact. Silver plating makes a poor contact material and will wear away in a very few cycles.

              To ensure million cycle reliability the design should be good for at least 2 million cycles. A slide switch is not a good choice.

              For million cycle life I would not consider using a slide switch. Some sort of snap action switch would be much more suitable if it has the proper contacts.

              </font>
              I vote also generally along with the above.

              If you had a "dry" contact, no current, you would have a much better chance with a slide switch.

              Your 1A max will burn off nearly any plating in short order. That is a common mistake in specifying gold, also.
              I believe something around 100 ma will rapidly remove the gold plating on relays, silver is likely similar. Check the Aromat company for tech notes/app notes, I think they have a reference on that.

              You don't specify loading. Resistive loading is the easiest, due to no voltage or current spikes.

              Capacitive loads have a much higher spike of current on "make", which welds contacts.

              Inductive loading has a turn-off arc at potentially much higher than circuit voltage, which will transfer material from one contact to the other. Dampers/snubbers can only do so much.

              If I were you, and had a need for a switch like that, I would take a different approach.

              Since you have the option of designing the contact and housing, and tooling them, you clearly have volume usage. Approach one of the established switch manufacturers with your requirements.

              If your volume is sufficient for you to tool it, they will no doubt be happy with your volume, and will do the design to satisfy you. They know switches far better than we or you do, and will provide a good product.

              I feel confident that what works for GM, GE, Maytag, etc will work for you also...namely, when you need expert design, go to the design experts.

              A couple points:

              1) Do you need million cycle life? Or is that just comfortable beyond the probable usage? A million cycles is 274 cycles per day for 10 years.
              Lifetime is expensive. Million cycle lifetime can be quite expensive. Don't over-spec.

              2) For million cycle life, the mechanicals of the slider and housing are just as important as the electrical performance. Stuff wears out.

              3) Environment is also key. Platings etc are OK, but the environmentals can mess up otherwise good material.
              For instance, german silver is a great contact material, and can be had in spring-type stock. But, sulphur in general pollution, exhaust, cigarette smoke, etc will cause it to corrode, and a "made" contact may actually open as the corrosion builds up.


              BTW:
              In defence of sliding switches, I will note that hand-held Multi-meters, notably Fluke, have very simple slide switches in them as the range selector. It is dry circuit usage, in general.

              The life in cycles obviously depends on the user, since different folks switch the unit differently in terms of full rotation cycles. Probably much less than a million cycles, but significant usage, since the switch is rotated every time the unit is turned on or off.

              I have yet to see a bad one in any meter, and I have bought used meters.

              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                Forrest:
                Too true, the technology is not new, but of neccessity, I have to apply it to some of our products with only a little knowledge (a dangerous situation) and less experience. My situation is not unusual, management regularly expects miracles.
                Evan:
                I mis-stated the life expectancy requirement. The actual customer requirement is 400K cycles, still an awful lot, but I want to test to a million. I have a half dozen parts with the silver plated buttons on test right now that have run 130K so far, 100K at ambient, 30k at 85آ°C. This is just cycling, a relatively easy test. Next, we will need to stop, wait for the inevitable oxide to build up on the contacts then cycle to see if thwe switch will scrub away or penetrate the oxide film.

                IBEW:
                Sorry to say, a snap-action switch is out of the question. Too noisy. The user does not want to hear the "click". Indeed, this design is to replace a snap switch in a similar design. Also, too expensive. In 500K quantities, e-bay would not be a reliable source.

                RobDee:
                Clean contacts are the reason for the slide switch design. Hard as we try, it has been very difficult to maintain a clean environment during manufacture. Anyway, the switch is not enclosed in a sealed case so environmental contaminates are a reality. The sliding design (usually,hopefully) plows away the crud. Unfortunately, the sliding design eventually wears away the contact material. Hopefully, contacts last for the rated life of the part.

                Wes
                Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                ~Practitioner of the Electromechanical Arts~

                Comment


                • #9
                  JTiers:
                  Your comments are well thought out and appreciated.

                  The switch feeds an inductive load, though well suppressed.

                  It has been our experience that major manufacturers of switches, contacts, connectors & etc, consider even 500K volumes almost too small to bother with when special designs are involved, and in some cases we are regarded as competition.

                  Wes
                  Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                  ~Practitioner of the Electromechanical Arts~

                  Comment


                  • #10
                    Wes,

                    Cherry makes snap action microswitches of the required rating that are virtually silent. I have some in my junk box at home. I will see if I can find a Cherry part number on them.
                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #11
                      Evan:
                      A purchased switch is out of the question, quiet or otherwise, purely from a cost standpoint. Also, the switch has to be attached - soldered into place - an extra operation.

                      My original question, buried in the details, may not have been clear:
                      "I would perfer to use a 1-piece stamped slider, silver plated, instead of a stamping with silver plated button contacts riveted to it."

                      The pressures of commerce will probably cause me to arrive at an "adequate" design rather than a good or perfect design.

                      Wes


                      Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                      ~Practitioner of the Electromechanical Arts~

                      Comment


                      • #12
                        Alright. Given those constraints, what metal is the spring portion of the switch going to contact?
                        Free software for calculating bolt circles and similar: Click Here

                        Comment


                        • #13
                          <font face="Verdana, Arial" size="2">Originally posted by Wes1:
                          JTiers:
                          It has been our experience that major manufacturers of switches, contacts, connectors & etc, consider even 500K volumes almost too small to bother with when special designs are involved, and in some cases we are regarded as competition.

                          Wes
                          </font>
                          In such a case, you may want to work with them on a modification, or a "build-in" using their contact and possibly moving components.

                          Talking "low cost" and "million cycles" is probably a problem. Your compromise will probably be reflected in reduced lifetime.

                          Another thought is to use a Mosfet as the switch element, and a switch to drive the gate. You said 12V DC, up to 1A. Mosfets for that rating area are costing as little as a few cents in a surface-mount SOT-23 or similar package.

                          The operations limit on the Mosfet is in the billions....the switch will still limit your opeational lifetime, but it is then virtually only a mechanical and environmental (contact) issue.

                          1601

                          Keep eye on ball.
                          Hashim Khan

                          Comment


                          • #14
                            did you say reed magnetic switch? or optical diode or? hall effect transistor?

                            I personally don't like metal contacts, they are not reliable enough, dirt oil, dust and corrosion is not conductivities friends. That is the reason real limit switches are so sealed up and so expensive, but they usually have cherry switches inside..

                            Comment


                            • #15
                              Evan:
                              My powers of description seem to be under a cloud. I will try to work up a sketch and post it here later.

                              Wes
                              Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                              ~Practitioner of the Electromechanical Arts~

                              Comment

                              Working...
                              X