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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Apr 2020, 5:30 pm | #1 |
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Micro switch reliability?
I got problems with a micro switch in a machine tool. it operates with a satisfying click, but intermittently either the contacts are not making or they're making badly. It operates a small relay with a 24VDC coil, but I don't know what the switched current is.
I thought about drilling a hole in it and squirting some Servisol 10 in, but instead decided to order a new one which isn't cheap. Out of interest I'm just wondering how common this problem is as I've never come across it before. The switch is in an O ring sealed enclosure and isn't exposed to lubricating oil or cutting fluid. It's probably about 20years old. Thanks.
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1st Apr 2020, 7:41 pm | #2 |
Heptode
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Re: Micro switch relaibility?
Looks a good quality switch, I hope your replacement is it's equal. Probably only wants a drop of switch cleaner as it can only be dirty contacts - possible to give it a squirt past the seal?
Alan |
1st Apr 2020, 8:08 pm | #3 |
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Re: Micro switch relaibility?
Sometimes designers of stuff forget that switches need a bit of current to clean the contacts (wetting current). A lot of switches can cope with almost zero current (a microamp or so) for ever, give them a mililamp and after a while they fail, 10mA or so they self clean. A pessimum (as opposed to optimum) current.
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1st Apr 2020, 9:01 pm | #4 |
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Re: Micro switch relaibility?
is it in a particularly accurate application? For example axis referencing? Some of those very small microswitches have very repeatable switch points perfect to a fraction of a millimetre. A standard unsealed V4 microswitch for overtravel or hard limits can bebought for less than a quid.
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1st Apr 2020, 9:30 pm | #5 |
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Re: Micro switch relaibility?
A health and safety report would not let us use micro switches as safety devices on an X-ray set as they were not sufficienty reliable. I forget what the solution was.
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1st Apr 2020, 9:34 pm | #6 | |
Nonode
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Re: Micro switch relaibility?
Quote:
Andy |
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1st Apr 2020, 10:17 pm | #7 |
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Re: Micro switch relaibility?
The switch is exactly like that in post #1 which of course depicts a new one. There's an external spring and roller which goes with it. I count myself lucky that a replacement is still available after more than 20 years. That's not true of the machine's other electrical and electronic components.
The switch detects that an Automatic Tool Changer (ATC) is clamped in the correct position. The operating point is not critical because after power is removed from the ATC's invertor driven three phase motor it runs on due to inertia. A worm gear on the motor shaft drives a gear wheel and there's a shock absorbing arrangement. When the clamping action commences a software timer is started and if operation of the switch isn't detected before the timer expires an Emergency Stop is initiated, disconnecting the mains supply to the invertor. That's what's happening about one time in twenty. I could try to get some contact cleaner into the switch past the grommet round the plunger, but I don't think it's worth it. It might not work and it might dissolve the plastic casing. Being able to use the machine is more important to me than saving money. Apart from that a lot of awkward dismantling is needed to access the switch and I don't want to do the job twice. I'll measure the current through the contacts and report back. There's no wetting current. Thanks for all your advice.
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2nd Apr 2020, 11:47 am | #8 |
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Re: Micro switch relaibility?
Going back to the original question, yes I have come across several uswitches displaying this behaviour over the last 50 odd years. I always thought it was their most common failure mode!
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2nd Apr 2020, 1:51 pm | #9 |
Heptode
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Re: Micro switch relaibility?
Hi Graham,
Industrial automation, machine tools included, routinely use inductive proximity sensors. They are non-contact, solid-state, with reliability way better than that of mechanical solutions. They come in all forms and shape and specifications. See if it fits your purpose and if it is worthwhile to upgrade. Here is just one page describing such sensors: https://fargocontrols.com/sensors/inductive_op.html All the other automation providers have proximity sensors, with other operating principles as well, check those out too. Regards, Peter Last edited by orbanp1; 2nd Apr 2020 at 2:07 pm. |
2nd Apr 2020, 2:47 pm | #10 | |
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Re: Micro switch relaibility?
Quote:
Normally very low current rating, pulling one apart to see how they worked, when the contact snapped shut it then slid across the surface slightly, this keeps them clean. Richard
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2nd Apr 2020, 7:05 pm | #11 |
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Re: Micro switch relaibility?
I wonder how many contact openings/closings the switch expects to see ?
In times-past I managed "Documation M600" 80-column punch-card readers that used a microswitch to sense the clean passage of each card [the data-reading itself being photoelectric] - they did 600 cards a minute - that's ten switch-cycles a second. I don't recall seeing a switch-failure, though paper-dust-accretion on the photocells and dust on the card-feed rollers was a big problem. Last edited by G6Tanuki; 2nd Apr 2020 at 7:17 pm. |
2nd Apr 2020, 7:40 pm | #12 |
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Re: Micro switch relaibility?
Microwave ovens incorporate a crowbar switch to guard against such failures in a safety critical position, so it is a risk factor. Admittedly only consumer grade equipment.
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2nd Apr 2020, 8:13 pm | #13 |
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Re: Micro switch relaibility?
I don't recall seeing many figures for switch life, but I do recall a 1970's new product review of a switch for keyboards which said it was specifically designed for low current operation, with crossed silver wedge contacts and a life of at least one million presses.
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7th Apr 2020, 11:45 am | #14 |
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Re: Micro switch relaibility?
You're probably right. I do recollect replacing another microswitch in a similar machine tool, but can't remember what was wrong with it. It was a push button operated one and may have been physically damaged.
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7th Apr 2020, 11:50 am | #15 | |
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Re: Micro switch relaibility?
Quote:
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7th Apr 2020, 12:00 pm | #16 |
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Re: Micro switch reliability?
I've fitted a new switch which is identical to the old one. Having fitted it I "drip fed" a few tool change commands to the machine tool and the switch operated every time. The switch operates a relay and there's a handy LED wired across its coil to indicate that it's energised.
To give more confidence I wrote a short CNC part program which did nothing but initiate random tool changes at regular intervals. It ran for 15 minutes without failure. The picture shows the switch in-situ before the cover was fitted. As the ATC locks, the inner disc below the switch moves axially operating it. The rotary switch is used to indicate which tool is selected.
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7th Apr 2020, 9:59 pm | #17 |
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Re: Micro switch reliability?
40 years ago I bought my first (used) photocopier. It was an "Oyez" (a MITSUBISHI suitably re-badged for the legal trade) which took a fat roll of paper and chopped it to the correct size. After a month or so, it suddenly went mad, spewing out an uncut length of paper. I rang the firm I bought it from, and was told "Check SW21 and SW23". I don't recall how many microswitches there were, maybe 30, but these two got loads of work relating to the feed and the chopping. I replaced them both and it was good to go, though I think it did fail again later. Shortly after I sold it on ** and had a genuine Mitsubishi from them.
** I MAY have sold it to Chas Miller, He certainly bought one of them from me early in his Radiophile enterprise. Les. |